Top Banner
1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 e Material in these slides from J.F Kurose and K.W. Ross material copyright 1996-2007
53

1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

Dec 20, 2015

Download

Documents

physical layer

network layer slide

systems layer

presentation layer

numbers layer

data link layer difference

data link layer moves

protocol layering layer

Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

1

CSCD 439539Wireless Networks and Security

Lecture 2Ethernet and 80211

Fall 2007

Some Material in these slides from JF Kurose and KW RossAll material copyright 1996-2007

2

Introduction

bull Study the OSI model

bull Focus lower two layers Link and Physical

bull Review how Ethernet is implemented ndash Will help better understand 80211 and how it

differs from wired

bull Begin to talk about 80211

3

OSI Model and Protocol Layering

bull Late 1970rsquos ndash International Organization for Standards (ISO) released a proposal for a network modelndash OSI Reference Model

bull Open Systems Interconnectionndash Based on protocol layeringndash Each layer has small defined jobndash Together layers called protocol stackndash Original OSI model includes 7 layers

4

OSI Model and Protocol Layering

bull Layer 7 ndash Application Layerbull Actual applications that use the communication

channel

bull Layer 6 ndash Presentation Layerbull How data elements are represented for

transmission ndash order of bits and bytes in numbers

bull Layer 5 ndash Session Layerbull Coordinates sessions between machines ndash helping

to initiate and manage them

5

OSI Model and Protocol Layering

bull Layer 4 ndash Transport Layerbull Reliable communication stream between two

systems

bull Layer 3 ndash Network Layerbull Responsible for moving data from one system

through routers to destination system

bull Layer 2 ndash Data Link Layerbull Moves data across one hop of the network

bull Layer 1 ndash Physical Layerbull Transmits bits over physical medium ndash copper

fiber radio link or any other medium

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 2: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

2

Introduction

bull Study the OSI model

bull Focus lower two layers Link and Physical

bull Review how Ethernet is implemented ndash Will help better understand 80211 and how it

differs from wired

bull Begin to talk about 80211

3

OSI Model and Protocol Layering

bull Late 1970rsquos ndash International Organization for Standards (ISO) released a proposal for a network modelndash OSI Reference Model

bull Open Systems Interconnectionndash Based on protocol layeringndash Each layer has small defined jobndash Together layers called protocol stackndash Original OSI model includes 7 layers

4

OSI Model and Protocol Layering

bull Layer 7 ndash Application Layerbull Actual applications that use the communication

channel

bull Layer 6 ndash Presentation Layerbull How data elements are represented for

transmission ndash order of bits and bytes in numbers

bull Layer 5 ndash Session Layerbull Coordinates sessions between machines ndash helping

to initiate and manage them

5

OSI Model and Protocol Layering

bull Layer 4 ndash Transport Layerbull Reliable communication stream between two

systems

bull Layer 3 ndash Network Layerbull Responsible for moving data from one system

through routers to destination system

bull Layer 2 ndash Data Link Layerbull Moves data across one hop of the network

bull Layer 1 ndash Physical Layerbull Transmits bits over physical medium ndash copper

fiber radio link or any other medium

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 3: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

3

OSI Model and Protocol Layering

bull Late 1970rsquos ndash International Organization for Standards (ISO) released a proposal for a network modelndash OSI Reference Model

bull Open Systems Interconnectionndash Based on protocol layeringndash Each layer has small defined jobndash Together layers called protocol stackndash Original OSI model includes 7 layers

4

OSI Model and Protocol Layering

bull Layer 7 ndash Application Layerbull Actual applications that use the communication

channel

bull Layer 6 ndash Presentation Layerbull How data elements are represented for

transmission ndash order of bits and bytes in numbers

bull Layer 5 ndash Session Layerbull Coordinates sessions between machines ndash helping

to initiate and manage them

5

OSI Model and Protocol Layering

bull Layer 4 ndash Transport Layerbull Reliable communication stream between two

systems

bull Layer 3 ndash Network Layerbull Responsible for moving data from one system

through routers to destination system

bull Layer 2 ndash Data Link Layerbull Moves data across one hop of the network

bull Layer 1 ndash Physical Layerbull Transmits bits over physical medium ndash copper

fiber radio link or any other medium

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 4: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

4

OSI Model and Protocol Layering

bull Layer 7 ndash Application Layerbull Actual applications that use the communication

channel

bull Layer 6 ndash Presentation Layerbull How data elements are represented for

transmission ndash order of bits and bytes in numbers

bull Layer 5 ndash Session Layerbull Coordinates sessions between machines ndash helping

to initiate and manage them

5

OSI Model and Protocol Layering

bull Layer 4 ndash Transport Layerbull Reliable communication stream between two

systems

bull Layer 3 ndash Network Layerbull Responsible for moving data from one system

through routers to destination system

bull Layer 2 ndash Data Link Layerbull Moves data across one hop of the network

bull Layer 1 ndash Physical Layerbull Transmits bits over physical medium ndash copper

fiber radio link or any other medium

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 5: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

5

OSI Model and Protocol Layering

bull Layer 4 ndash Transport Layerbull Reliable communication stream between two

systems

bull Layer 3 ndash Network Layerbull Responsible for moving data from one system

through routers to destination system

bull Layer 2 ndash Data Link Layerbull Moves data across one hop of the network

bull Layer 1 ndash Physical Layerbull Transmits bits over physical medium ndash copper

fiber radio link or any other medium

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 6: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

6

Current Internet Model

bull Current Internet maps to bottom 5 layers of the OSI modelndash Wraps the top 3 layers into application layerndash Everything above transport layer is application

responsibilityndash Looks like this hellip

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 7: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

7

Current Internet Modelbull Hybrid OSI Architecture

ndash Combines TCPIP standards at layers 3-5 withndash OSI standards at layers 1-2

OSI Hybrid TCPIP-OSI

Application

Presentation

Session

Application

Transport Transport

Network Internet

Data Link Data Link

Physical Physical

TCP

IP

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 8: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

8

Data Link and Physical

bull IEEE 802 specs focus on lower two layers of OSI modelndash Incorporate both physical and link

componentsndash All 802 networks have both a MAC (Media

Access Control) and a physical (PHY) component

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 9: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

9

Data Link Layer

bull Review data link layer specifics including Ethernetndash Protocol defines packet format typically called

frames where each frame encapsulates one packet from network layer

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 10: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

10

Data Link Layer

bull Difference in responsibility between ndash Network layer

bull Has end-to-end job bull Moves transport-layer packets from source to

destination hosts

ndash Link layer bull Has node-to-node job of moving network-layer

packets over a single link in the pathbull Multiple protocols accomplish thisbull Both Ethernet and wireless for example

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 11: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

11

Link Layer Servicesbull Framing link access

ndash Encapsulate datagram into frame adding header trailerndash Channel access if shared mediumndash ldquoMACrdquo addresses used in frame headers to identify

source dest bull different from IP address

bull Reliable delivery between adjacent nodesndash Seldom used on low bit-error link (fiber some twisted

pair)ndash But Wireless links much high error ratesndash Why

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 12: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

12

Link Layer Services (more)bull Flow control

ndash Pacing between adjacent sending and receiving nodes

bull Error detectionndash Errors caused by signal attenuation noise ndash Receiver detects presence of errors

bull Signals sender for retransmission or drops frame

bull Error correctionndash Receiver identifies and corrects bit error(s) without

resorting to retransmission

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 13: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

13

Where is the link layer implemented

bull In every hostbull Link layer implemented

in ldquoadaptorrdquo network interface card

bull Ethernet card PCMCI card 80211 card

bull implements link physical layer

bull Attaches to system buses ndash hardware software and firmware

controller

physicaltransmission

cpu memory

host bus (eg PCI)

network adaptercard

host schematic

applicationtransportnetwork

link

linkphysical

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 14: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

14

Adaptors Communicating

bull Sending sidendash encapsulates datagram in

framendash adds error checking bits

rdt flow control etc

bull Receiving sidendash looks for errors rdt flow

control etcndash extracts datagram passes to

upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 15: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

15

Multiple Access Links and ProtocolsTwo types of ldquolinksrdquobull Point-to-point

ndash PPP for dial-up accessndash point-to-point link between Ethernet switch and

host

bull Broadcast (shared wire or medium)bull old-fashioned Ethernetbull upstream HFCbull 80211 wireless LAN

shared wire (eg cabled Ethernet)

shared RF (eg 80211 WiFi)

shared RF(satellite)

humans at acocktail party

(shared air acoustical)

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 16: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

16

Multiple Access protocolsbull Shared broadcast channel

ndash Must have way of sharing mediumndash Two or more simultaneous transmissions by nodes

bull Interference collision if node receives two or more signals at the same time

Multiple access protocolbull Distributed algorithm that determines how nodes

share channel ie determine when node can transmit

bull Communication about channel sharing must use channel itself ndash No out-of-band channel for coordination

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 17: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

17

Multiple Access protocols

bull As humans we have following rulesndash Give everyone a chance to speakndash Donrsquot speak until you are spoken tondash Donrsquot monopolize the conversationndash Raise your hand if you have a questionndash Donrsquot interrupt when someone is speakingndash Donrsquot fall asleep when someone is talking

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 18: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

18

Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1 When one node wants to transmit it can send at rate R

2 When M nodes want to transmit each can send at average rate RM

3 Rully decentralizedndash no special node to coordinate transmissionsndash no synchronization of clocks slots

4 Simple

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 19: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

19

MAC Protocols a taxonomy

Three broad classesbull Channel Partitioning

ndash Divide channel into smaller ldquopiecesrdquo (time slots frequency code)

ndash Allocate piece to node for exclusive use

bull Random Accessndash Channel not divided allow collisions

ndash ldquorecoverrdquo from collisions

bull ldquoTaking turnsrdquondash Nodes take turns but nodes with more to send can

take longer turns

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 20: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

20

Channel Partitioning MAC protocols TDMA

TDMA time division multiple access bull Access to channel in rounds bull Each station gets fixed length slot (length = pkt trans

time) in each round bull Unused slots go idle bull Example 6-station LAN 134 have pkt slots 256

idle

1 3 4 1 3 4

6-slotframe

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 21: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

21

Channel Partitioning MAC protocols FDMA

FDMA frequency division multiple access ndash Channel spectrum divided into frequency

bandsndash Each station assigned fixed frequency bandndash Unused transmission time in frequency bands

go idle fr

equ

ency

bands time

FDM cable

134 - data

256 - idle

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 22: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

22

Random Access Protocolsbull When node has packet to send

ndash transmit at full channel data rate Rndash no a priori coordination among nodes

bull two or more transmitting nodes ldquocollisionrdquobull Random access MAC protocol specifies

ndash how to detect collisionsndash how to recover from collisions (eg via delayed retransmissions)

bull Examples of random access MAC protocolsndash slotted ALOHAndash ALOHAndash CSMA CSMACD CSMACA

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 23: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

23

Slotted ALOHAAssumptionsbull All frames same size

ndash time divided into equal size slots (time to transmit 1 frame)

ndash nodes start to transmit only slot beginning

ndash nodes are synchronized

ndash if 2 or more nodes transmit in slot all nodes detect collision

Operationbull When node obtains fresh

frame transmits in next slotndash if no collision node

can send new frame in next slot

ndash if collision node retransmits frame in each subsequent slot with prob p until success

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 24: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

24

Slotted ALOHA

Prosbull Single active node can

continuously transmit at full rate of channel

bull Highly decentralized only slots in nodes need to be in sync

bull Simple

Consbull Collisions wasting slotsbull Idle slotsbull Nodes may be able to

detect collision in less than time to transmit packet

bull Clock synchronization

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 25: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

25

Slotted Aloha efficiency

bull suppose N nodes with many frames to send each transmits in slot with probability p

bull prob that given node has success in a slot = p(1-p)N-

1

bull prob that any node has a success = Np(1-p)N-1

bull max efficiency find p that maximizes Np(1-p)N-1

bull for many nodes take limit of Np(1-p)N-1 as N goes to infinity gives

Max efficiency = 1e = 37

Efficiency long-run fraction of successful slots (many nodes all with many frames to send)

At best channelused for useful transmissions 37of time

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 26: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

26

CSMA (Carrier Sense Multiple Access)

CSMAbull Listen before transmitbull If channel sensed idle transmit entire framebull If channel sensed busy defer transmission

bull Human analogyDonrsquot interrupt others

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 27: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

27

CSMA collisions

Collisions can still occurPropagation delay means two nodes may not ldquohearrdquoeach otherrsquos transmissionCollisionEntire packet transmission time wasted ndash Get damaged frames

spatial layout of nodes

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 28: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

28

CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA

ndash Collisions detected within short timendash Colliding transmissions aborted reducing channel

wastage

bull Collision detection bull Easy in wired LANs measure signal strengths

compare transmitted received signalsbull Difficult in wireless LANs received signal

strength overwhelmed by local transmission strength

Human analogythe polite conversationalist

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 29: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

29

CSMACD collision detection

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 30: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

30

ldquoTaking Turnsrdquo MAC protocolsChannel partitioning MAC protocols

ndash share channel efficiently and fairly at high loadndash inefficient at low load delay in channel access 1N

bandwidth allocated even if only 1 active node

Random access MAC protocolsndash Efficient at low load single node can fully utilize

channelndash High load collision overhead

ldquotaking turnsrdquo protocolsLook for best of both worlds

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 31: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

31

ldquoTaking Turnsrdquo MAC protocolsPolling bull master node ldquoinvitesrdquo

slave nodes to transmit in turn

bull typically used with ldquodumbrdquo slave devices

bull concernsndash polling overhead ndash latencyndash single point of failure

(master)

master

slaves

poll

data

data

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 32: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

32

ldquoTaking Turnsrdquo MAC protocolsToken passingbull control token passed

from one node to next sequentially

bull token messagebull concerns

ndash token overhead ndash latencyndash single point of failure

(token)

T

data

(nothingto send)

T

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 33: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

33

Summary of MAC protocolsbull channel partitioning by time frequency or

codendash Time Division Frequency Division

bull random access (dynamic) ndash ALOHA S-ALOHA CSMA CSMACDndash carrier sensing easy in some technologies (wire)

hard in others (wireless)ndash CSMACD used in Ethernetndash CSMACA used in 80211

bull taking turnsndash polling from central site token passingndash Bluetooth FDDI IBM Token Ring

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 34: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

34

EthernetldquoDominantrdquo wired LAN technology bull Cheap $20 for NICbull First widely used LAN technologybull Simpler cheaper than token LANs and ATMbull Kept up with speed race 10 Mbps ndash 10 Gbps

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 35: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

35

Ethernet History

bull The IEEE 8023 standard is for a CSMACD LANndash Ethernet is a specific product that almost

implements this standard (Ethernet differs from standard in one header field)

ndash Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium

ndash Who founded Ethernet

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 36: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

36

Ethernet Operation

bull Common cable providing communication channel was likened to the ether and it was from this reference that the name Ethernet was derived

bull Ethernet stations communicate with each other by sending each other data packets

bull As with other IEEE 802 LANs each Ethernet station is given a single 48-bit MAC address which is used both to specify the destination and the source of each data packet

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 37: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

37

Star topologybull Bus topology popular through mid 90s

ndash All nodes in same collision domain (can collide with each other)

bull Today star topology prevailsndash Active switch in centerndash Each ldquospokerdquo runs a (separate) Ethernet protocol (nodes

do not collide with each other)

switch

bus coaxial cable star

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 38: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

38

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

Preamble bull 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011bull used to synchronize receiver sender clock rates

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 39: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

39

Ethernet Frame Structure (more)bull Addresses 6 bytes

ndash if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol

ndash otherwise adapter discards framebull Type indicates higher layer protocol (mostly IP but

others possible eg Novell IPX AppleTalk)bull CRC checked at receiver if error is detected frame

is dropped

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 40: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

40

Ethernet Unreliable connectionless

bull Connectionless No handshaking between sending and receiving NICs

bull Unreliable receiving NIC doesnrsquot send acks or nacks to sending NICndash Stream of datagrams passed to network layer can

have gaps (missing datagrams)ndash Gaps will be filled if app is using TCPndash Otherwise app will see gaps if based on UDP

bull Ethernetrsquos MAC protocol unslotted CSMACD

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 41: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

41

Ethernet CSMACD algorithm1 NIC receives datagram

from network layer creates frame

2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits

3 If NIC transmits entire frame without detecting another transmission NIC is done with frame

4 If NIC detects another transmission while transmitting aborts and sends jam signal

5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from 012hellip2m-1 NIC waits K512 bit times returns to Step 2

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 42: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

42

Ethernetrsquos CSMACD (more)Jam Signal make sure all

other transmitters are aware of collision 48 bits

Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec

Exponential Backoff bull Goal adapt retransmission

attempts to estimated current loadndash heavy load random wait

will be longerbull first collision choose K

from 01 delay is K 512 bit transmission times

bull after second collision choose K from 0123hellip

bull after ten collisions choose K from 01234hellip1023

Seeinteract with Javaapplet on AWL Web sitehighly recommended

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 43: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

43

CSMACD efficiencybull Tprop = max prop delay between 2 nodes in LAN

bull ttrans = time to transmit max-size frame

bull efficiency goes to 1 ndash as tprop goes to 0

ndash as ttrans goes to infinity

bull better performance than ALOHA and simple cheap decentralized

transprop ttefficiency

51

1

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 44: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

44

8023 Ethernet Standards Link amp Physical Layers

bull Many different Ethernet standardsndash Common MAC protocol and frame formatndash Different speeds 2 Mbps 10 Mbps 100 Mbps

1Gbps 10G bpsndash Different physical layer media fiber cable

applicationtransportnetwork

linkphysical

MAC protocoland frame format

100BASE-TX

100BASE-T4

100BASE-FX100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layercopper (twisterpair) physical layer

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 45: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

45

bull 80211 History and Brief Facts

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 46: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

46

History of Wireless

bull 80211 working group ndash Established in 1990 by IEEE Executive

Committeendash Goal was to create a wireless local area

network (WLAN) standardndash Standard specified an operating frequency in

the 24GHz ISM (Industrial Scientific and Medical) band

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 47: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

47

History of Wireless80211 Specifications

bull Pre-standard solution to wireless LANndash Introduced in 1993ndash Based on frequency hopping spread spectrum

technology in the 24 - 2483 GHz bandndash Uncompressed data rate of 16 Mbps and 800

Kbps fallback ndash Multiple channels can support up to 15

wireless LAN connections

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 48: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

48

History of Wireless80211 Specifications

80211 (basic)

bull Seven years after 80211 working group is formed (1997) hellipndash Group approved IEEE 80211 as worlds first

WLAN standard with data rates of 1 and 2 Mbps

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 49: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

49

History of Wireless80211 Specifications

80211 (ldquoardquo and ldquobrdquo)bull In 1999 the working group approved two

extensions to 80211ndash 80211a - 5GHz band

bull Operates at 54 Mbps (due to higher frequency) bull Only allow access to clients within 40 ndash50 feet

due to power limits enforced by the FCC

ndash 80211b - 24GHz ISM band bull Operates at 11 Mbps bull Allows client access up to well over 1000 feet

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 50: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

50

History of Wireless80211 Specifications

80211 (ldquogrdquo)

bull Introduced in June 2003 (though early adoption began in January 2003)ndash Utilizes Orthogonal Frequency Division

Multiplexing (OFDM similar to the operation of 80211a) to acheive 54Mbs connection rates

ndash Backwards compatible with 80211b clients

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 51: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

51

History of Wireless80211 (alphabet soup)

bull IEEE 80211 - The original 2 Mbits 24 GHz standard bull IEEE 80211a - 54 Mbits 5 GHz standard (1999)bull IEEE 80211b - Enhancements to 80211 to support 55 and 11

Mbits (1999)bull IEEE 80211d - New countriesbull IEEE 80211e - Enhancements QoS including packet burstingbull IEEE 80211f - Inter-Access Point Protocol (IAPP)bull IEEE 80211g - 54 Mbits 24 GHz standard (backwards

compatible with b) (2003)bull IEEE 80211h - 5 GHz spectrum Dynamic ChannelFrequency

Selection (DCSDFS) and Transmit Power Control (TPC) for European compatibility

bull IEEE 80211i - Enhanced securitybull IEEE 80211j - Extensions for Japanbull IEEE 80211n - Higher throughput improvements

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 52: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

52

80211 Protocols

bull 80211 vs 8023ndash 80211 is comparatively complex compared

with traditional Ethernetndash Radio waves as physical layer compared with

wired medium creates a more complicated transmission mechanism

ndash Must account for more unreliable physical medium

ndash Details forthcoming hellip

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish
Page 53: 1 CSCD 439/539 Wireless Networks and Security Lecture 2 Ethernet and 802.11 Fall 2007 Some Material in these slides from J.F Kurose and K.W. Ross All material.

53

Finish

Next time bull Student presentations of wireless topics and URLrsquos

  • CSCD 439539 Wireless Networks and Security
  • Introduction
  • OSI Model and Protocol Layering
  • Slide 4
  • Slide 5
  • Current Internet Model
  • Slide 7
  • Data Link and Physical
  • Data Link Layer
  • Slide 10
  • Link Layer Services
  • Link Layer Services (more)
  • Where is the link layer implemented
  • Adaptors Communicating
  • Multiple Access Links and Protocols
  • Multiple Access protocols
  • Slide 17
  • Ideal Multiple Access Protocol
  • MAC Protocols a taxonomy
  • Channel Partitioning MAC protocols TDMA
  • Channel Partitioning MAC protocols FDMA
  • Random Access Protocols
  • Slotted ALOHA
  • Slide 24
  • Slotted Aloha efficiency
  • CSMA (Carrier Sense Multiple Access)
  • CSMA collisions
  • CSMACD (Collision Detection)
  • CSMACD collision detection
  • ldquoTaking Turnsrdquo MAC protocols
  • Slide 31
  • Slide 32
  • Summary of MAC protocols
  • Ethernet
  • Ethernet History
  • Ethernet Operation
  • Star topology
  • Ethernet Frame Structure
  • Ethernet Frame Structure (more)
  • Ethernet Unreliable connectionless
  • Ethernet CSMACD algorithm
  • Ethernetrsquos CSMACD (more)
  • CSMACD efficiency
  • 8023 Ethernet Standards Link amp Physical Layers
  • Slide 45
  • History of Wireless
  • History of Wireless 80211 Specifications
  • Slide 48
  • Slide 49
  • Slide 50
  • Slide 51
  • 80211 Protocols
  • Finish

Related Documents
CSCD 303 Essential Computer Security Fall 2010

CSCD 303 Essential Computer Security Fall 2010

Category: Documents
September 24 th, 2013 CS1652 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights.

September 24 th, 2013 CS1652 The slides are adapted from the...

Category: Documents
1 CSCD 330 Network Programming Winter 2014 Some Material in these slides from J.F Kurose and K.W. Ross All material copyright 1996-2007 Lecture 4 Application.

1 CSCD 330 Network Programming Winter 2014 Some Material in....

Category: Documents
CSCD 434 Spring 2012

CSCD 434 Spring 2012

Category: Documents
By: K.W. Reed .

By: K.W. Reed .

Category: Documents
Web- and Agent-based CSCD · ACHI’11/Tutorial/ntegrated Web- and Agent-based CSCD/2011-02© B. Kwintiana Ane, IRIS, Universität Stuttgart, 2011 1. Introduction: Today’s CSCD

Web- and Agent-based CSCD · ACHI’11/Tutorial/ntegrated.....

Category: Documents
CS1652 September 5 th, 2013 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights.

CS1652 September 5 th, 2013 The slides are adapted from the....

Category: Documents
Networking CS 1652 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved.

Networking CS 1652 The slides are adapted from the...

Category: Documents
CS 1652 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved Jack Lange.

CS 1652 The slides are adapted from the publisher’s...

Category: Documents
CS1652 September 12th, 2013 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights.

CS1652 September 12th, 2013 The slides are adapted from the....

Category: Documents
Data Communication & Computer Networks CS 1652 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W.

Data Communication & Computer Networks CS 1652 The slides...

Category: Documents
Computer Networks CS1652 The slides are adapted from the publisher’s material All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved.

Computer Networks CS1652 The slides are adapted from the...

Category: Documents