Prof. Younghee Lee 1 1 Computer Networks u Lecture 10: Data Link - LANs Prof. Younghee Lee * Some part of this teaching materials are prepared referencing.

1Prof. Younghee Lee1

Computer Networks Lecture 10: Data Link - LANs

Prof. Younghee Lee

* Some part of this teaching materials are prepared referencing the lecture note made by F. Kurose, Keith W. Ross(U. of Massachusetts)


Point to Point Data Link Control

one sender, one receiver, one link: easier than broadcast link:– no Media Access Control– no need for explicit MAC addressing– e.g., dialup link, ISDN line

popular point-to-point DLC protocols:– PPP (point-to-point protocol)– HDLC: High level data link control (Data link used

to be considered “high layer” in protocol stack!


HDLC Connection-oriented error-correcting data link protocol

– Early international protocol standard (original 1974, frequent updates) Designed to transmit data frames across (potentially) high error-rate

data links– Improved on older character-based data link protocols

Standard defines– Framing scheme and frame format(s)– Recommended window sizes and approved checksum lengths– Primitives (commands)– Procedures for connection set-up and release, data transfer, error detection

and error correction– Connectionless mode of operation

Basis of many Link Layer protocols developed since that time– Including those for X.25, FR, ISDN, PPP, modems

HDLC frames are delimited by flags: 01111110 Stuff bits if pattern appears in data:

– 01111110 ; flag– Data 01111110 -> 011111010


PPP

Point-to-point Protocol Originally for User-network connection Now being used for router-router connection also PPP is used when you connect to an internet

service provider (ISP) via modem PPP is a variation of HDLC Uses flags like HDLC Uses byte stuffing in stead of bit stuffing


PPP Data Frame

Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fi

elds Protocol: upper layer protocol to which frame delivered (eg,

PPP-LCP, IP, IPCP, etc) info: upper layer data being carried check: cyclic redundancy check for error detection


PPP is character-oriented version of HDLC Flag is 0x7E (01111110) Control escape 0x7D (01111101) Any occurrence of flag or control escape inside of frame is replaced

with 0x7D followed by original octet XORed with 0x20 (00100000)

Byte-Stuffing in PPP

Data to be sent

41 7D 42 7E 50 70 46

After stuffing and framing

5D 42 7D 5E 50 70 467E 41 7D 7E


PPP PhasesHome PC to Internet Service Provider1. PC calls router via modem2. PC and router exchange LCP packets to

negotiate PPP parameters3. Check on identities4. NCP packets exchanged to configure the

network layer, e.g. TCP/IP ( requires IP address assignment)

5. Data transport, e.g. send/receive IP packets6. NCP used to tear down the network layer

connection (free up IP address); LCP used to shut down data link layer connection

7. Modem hangs up

Two-layer protocol that encapsulates multiple packet types over data link

– Link Control Protocol (LCP) initializes and configures the data link

– Network Control Protocol (NCP) controls packet encapsulation

Dead

Establish

Authenticate

Network

Terminate

Open

Failed

Failed

1. Carrier detected

2. Optionsnegotiated

3. Authentication completed4. NCP

configuration

6. Done

7. Carrierdropped

5.


ATM protocol architecture ATM protocol architecture

– ITU-T SG 13, SG 11– ATM Forum– Connection-oriented packet-switched network– Used in both WAN and LAN settings– Signaling (connection setup) Protocol: Q.2931– Packets are called cells: 5-byte header + 48-byte payload– Commonly transmitted over SONET (but not necessarily)


Cell Network

Why Cell?– Taking advantages of the characteristics of both packet and bit --> good for multimedia communications– Interference

o interference at gigabit speed?


ATM architecture

adaptation layer: only at edge of ATM network– ATM Adaptation Layer (AAL): “adapts” upper layers

AAL present only in end systems, not in switches– data segmentation/reassembly– roughly analagous to Internet transport layer

ATM layer: “network” layer– cell switching, routing

physical layer


ATM: network or link layer?

Vision: end-to-end transport: “ATM from desktop to desktop”– ATM is a network

technology

Reality: used to connect IP backbone routers – “IP over ATM”– ATM as switched link

layer, connecting IP routers


ATM LayerService: transport cells across ATM network analogous to IP network layer very different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?


ATM Layer: Virtual Circuits

VC transport: cells carried on VC from source to dest– call setup, teardown for each call before data can flow– each packet carries VC identifier (not destination ID)– every switch on source-dest path maintain “state” for each passing c

onnection– link,switch resources (bandwidth, buffers) may be allocated to VC: to

get circuit-like perf.

Permanent VCs (PVCs)– long lasting connections– typically: “permanent” route between to IP routers

Switched VCs (SVC):– dynamically set up on per-call basis


ATM Physical Layer

Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a contai

ner carrying bits); – bit synchronization; – bandwidth partitions (TDM); – several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps;

OC48 = 2.45 Gbps, OC192 = 9.6 Gbps

TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps

unstructured: just cells (busy/idle)


IP Forwarding IP over ATM

– LANE: ATMF, to make an ATM LAN appear as a set of logical shared medium LANs interconnected via router.

– IPOA: IETF, group of ATM stations are divided into Logical IP Subnet(LIS), interconnected via router. Each LIS has an ATM ARP server for address resolution

– NHRP(Next Hop Resolution Protocol): IETF, to locate an exit point in the ATM cloud closest to the destination and to obtain ATM address.

– Multiprotocol over ATM(MPOA): ATMF, to provide internetworking service such as IP, IPX, and AppleTalk over an ATM network: an extension of LANE. Uses NHRP.

IP switching: Ipsilon Tag switching: Cisco Cell Switched Router: Toshiba Aggregate Route Based IP Switching(ARIS): IBM MPLS: IETF(~Tag switching)


IP over ATM

CLIP(Classical IP over ATM): – group of ATM stations are divided into Logical IP Subnet(LIS), interconn

ected via router. Each Logical IP subnetwork(LIS) has an ATM ARP server for address resolution

– All members(IP end system) in the same LIS must use the same IP address prefix

LIS1

LIS3 LIS4 LIS5LIS6

LIS2

RouterRouter

Router RouterRouter

ATM network


IP Switching How to use ATM hardware without the software.

– ATM switches are very fast data switches– software adds overhead, cost

The idea is to identify flows at the IP level and to create specific VCs to support these flows.– flows are identified on the fly by monitoring traffic– flow classification can use addresses, protocol types, ...– can distinguish based on destination, protocol, QoS

Once established, data belonging to the flow bypasses level 3 routing.– never leaves the ATM switch

Interoperates fine with “regular” IP routers.– detects and collaborates with neighboring IP switches


IP Switching

Flows– any sustained traffic between end points across a network

» consist of a collection of flows with different characteristics, including different traffic profiles and different QoS requirements

» flows: based on the applications running over TCP/IP, UDP/IP or raw IP.– How to know which flows would benefit from delivery across ATM virtual

channels, bypassing the router function ?» Application declare the traffic profile and QoS requirements of the flow

IPv4 provides only limited capability IPv6 flow label field is well suited but not widely deployed RSVP provides the way to identify specific flow but not widely deployed

» To do flow analysis classify traffic on the basis of its likely flow characteristics “Relatively small percentage of flows are of long duration and carry a

high percentage of packets.”


An Alternative: Tag Switching Instead of monitoring traffic to identify flows to op

timize, use routing information to guide the creation of “switched” paths.– Switched paths are set up as a side effect of filling in fo

rwarding tables

Generalize to other types of hardware. Also introduced stackable tags.

– Made it possible to temporarily merge flows and to demultiplex them without doing an IP route lookup

– Requires variable size field for tag

A

B

A

B

A

B

C

C


IP Switching versus Tag Switching

Flows versus routes.– tags explicitly cover groups of routes– tag bindings set up as part of route establishment– flows in IP switching are driven by traffic and detected

by “filters”» Supports both fine grain application flows and coarser grain flow

groups

Stackable tags.– provides more flexibility

Generality– IP switching focuses on ATM– not clear that this is a fundamental difference


MPLS

Goal of MPLS group in IETF: To develop a standard for integration of layer 2 switching with layer 3 routing in order to improve price/performance of network layer routing, scalability of network layer and provide great flexibility in providing new routing service

Core MPLS components– Basic Routing Approach: use OSPF, BGP without change– Labels: semantics, granularity, assignment, stack/forwarding

operation– Encapsulation: Label swapping: encoded in a layer 2 header


MPLS capable routers

a.k.a. label-switched router forwards packets to outgoing interface based

only on label value (don’t inspect IP address)– MPLS forwarding table distinct from IP forwarding

tables signaling protocol needed to set up forwarding

– RSVP-TE– forwarding possible along paths that IP alone would

not allow (e.g., source-specific routing) !!– use MPLS for traffic engineering

must co-exist with IP-only routers


Elements of a wireless network

network infrastructure

base station typically connected to wir

ed network relay - responsible for se

nding packets between wired network and wireless host(s) in its “area”– e.g., cell towers 802.

11 access points


Characteristics of selected wireless link standards

384 Kbps384 Kbps

56 Kbps56 Kbps

54 Mbps54 Mbps

5-11 Mbps5-11 Mbps

1 Mbps1 Mbps

802.15

802.11b

802.11{a,g}

IS-95 CDMA, GSM

UMTS/WCDMA, CDMA2000

.11 p-to-p link

2G

3G

Indoor

10 – 30m

Outdoor

50 – 200m

Mid rangeoutdoor

200m – 4Km

Long rangeoutdoor

5Km – 20Km



network infrastructure

infrastructure mode base station connects

mobiles into wired network

handoff: mobile changes base station providing connection into wired network



Ad hoc mode no base stations nodes can only transmit

to other nodes within link coverage

nodes organize themselves into a network: route among themselves


MAC MAC Problems in wireless network: to use CSMA/CD

– Collision Detection(CD) does not work– CS might not work in some case( if a terminal is “hidden”)

Hidden terminal problem– Nodes A and C cannot hear each other

» Node A : currently transmitting to B» Node C : wants to transmit to B» Transmissions by nodes A and C can collide at node B

• Nodes A and C are hidden from each other

A B C


MAC Exposed terminal problem

– Node C cannot send to D due to carrier of B sense » Node B : currently transmitting to A» Node C : wants to transmit to D» Carrier of C doesn’t interfere A’s reception, Carrier of B doesn’t interfere D’s

reception Waiting is not necessary

» But C is waiting since it sense carrier of B

– C is exposed to B

A B C D


CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M


CDMA: two-sender interference


IEEE 802.11 Wireless LAN

802.11b– 2.4-5 GHz unlicensed

radio spectrum– up to 11 Mbps– direct sequence spread

spectrum (DSSS) in physical layer

» all hosts use same chipping code

– widely deployed, using base stations

802.11a – 5-6 GHz range– up to 54 Mbps

802.11g – 2.4-5 GHz range– up to 54 Mbps

All use CSMA/CA for multiple access

All have base-station and ad-hoc network versions


802.11 LAN architecture

wireless host communicates with base station– base station = access poi

nt (AP) Basic Service Set (BSS) (aka

“cell”) in infrastructure mode contains:– wireless hosts– access point (AP): base st

ation– ad hoc mode: hosts only

BSS 1

BSS 2

Internet

hub, switchor routerAP

AP


802.11: Channels, association 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at

different frequencies; 3 non-overlapping– AP admin chooses frequency for AP– interference possible: channel can be same as that chosen by

neighboring AP!

host: must associate with an AP– scans channels, listening for beacon frames containing AP’s

name (SSID) and MAC address– selects AP to associate with; initiates association protocol– may perform authentication [Chapter 8]– will typically run DHCP to get IP address in AP’s subnet


IEEE 802.11: multiple access Like Ethernet, uses CSMA:

– random access– carrier sense: don’t collide with ongoing transmission

Unlike Ethernet:– no collision detection – transmit all frames to completion– acknowledgment – because without collision detection, you don’

t know if your transmission collided or not

Why no collision detection?– difficult to receive (sense collisions) when transmitting due to we

ak received signals (fading)– can’t sense all collisions in any case: hidden terminal, fading

Goal: avoid collisions: CSMA/C(ollision)A(voidance)


MAC IFS: interframe space: depend on the type of frame to transmit

– SIFS: Short IFS» High priority frame before contending for channel» ACK frame, CTS frame, data transfer of a segmented MSDU, frames from station that are respon

ding to a poll from an AP, any frame from an AP during CFP

– PIFS: PCF-IFS» Used by PCF to gain priority access to the medium at the start of a CFP(Contention Free Period)

– DIFS: DCF-IFS» Used by the DCF to transmit data and management MPDU

DCF : CSMA/CA– Initial MAC PDU: medium idle for a period DIFS or greater -> transmit

» Medium busy -> wait random backoff time to schedule a reattempt

– Reattempt: decrement a counter each time an idle contention slot transpire – After successful frame transmission -> backoff procedure to transmit next frame

Busy Medium SIFS

PIFS

DIFSContentionWIndow

Next

Frame


IEEE 802.11 MAC Protocol: CSMA/CA

802.11 sender

1 if sense channel idle for DIFS then

- transmit entire frame (no CD)

2 if sense channel busy then

- start random backoff time

- timer counts down while channel idle

- transmit when timer expires

- if no ACK, increase random backoff interval, repeat 2

802.11 receiverif frame received OK

- return ACK after SIFS (ACK needed due to hidden terminal problem)

sender receiver

DIFS

data

SIFS

ACK


RTS/CTS

idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames

optional; not typically used sender first transmits small request-to-send (RTS) packets to A

P using CSMA– RTSs may still collide with each other (but they’re short)

AP broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes

– sender transmits data frame– other stations defer transmissions

Avoid data frame collisions completely using small reservation packets!


Collision Avoidance: RTS-CTS exchange

APA B

time

RTS(A)RTS(B)

RTS(A)

CTS(A) CTS(A)

DATA (A)

ACK(A) ACK(A)

reservation collision

defer


hub or switch

AP 2

AP 1

H1 BBS 2

BBS 1

802.11: mobility within same subnet

router

H1 remains in same IP subnet: IP address can remain same

switch: which AP is associated with H1?– self-learning (Ch. 5):

switch will see frame from H1 and “remember” which switch port can be used to reach H1


Mradius ofcoverage

S

SS

P

P

P

P

M

S

Master device

Slave device

Parked device (inactive)P

802.15: personal area network

less than 10 m diameter replacement for cables

(mouse, keyboard, headphones)

ad hoc: no infrastructure master/slaves:

– slaves request permission to send (to master)

– master grants requests

802.15: evolved from Bluetooth specification– 2.4-2.5 GHz radio band– up to 721 kbps


Mobile Switching

Center

Public telephonenetwork, andInternet

Mobile Switching

Center

Components of cellular network architecture

connects cells to wide area net manages call setup (more later!) handles mobility (more later!)

MSC

covers geographical region base station (BS) analogous to 802.11 AP mobile users attach to network through BS air-interface: physical and link layer protocol between mobile and BS

cell

wired network


Cellular networks: the first hop

Two techniques for sharing mobile-to-BS radio spectrum

combined FDMA/TDMA: divide spectrum in frequency channels, divide each channel into time slots

CDMA: code division multiple access frequency

bands

time slots


Cellular standards: brief survey

2G systems: voice channels IS-136 TDMA: combined FDMA/TDMA (north america) GSM (global system for mobile communications): combi

ned FDMA/TDMA – most widely deployed

IS-95 CDMA: code division multiple access

IS-136 GSM IS-95GPRS EDGECDMA-2000

UMTS

TDMA/FDMADon’t drown in a bowlof alphabet soup: use thisoor reference only



2.5 G systems: voice and data channels for those who can’t wait for 3G service: 2G extensions general packet radio service (GPRS)

– evolved from GSM – data sent on multiple channels (if available)

enhanced data rates for global evolution (EDGE)– also evolved from GSM, using enhanced modulation – Date rates up to 384K

CDMA-2000 (phase 1)– data rates up to 144K– evolved from IS-95



3G systems: voice/data Universal Mobile Telecommunications Service (UMTS)

– GSM next step, but using CDMA CDMA-2000

….. more (and more interesting) cellular topics due to mobility (stay tuned for details)

Prof. Younghee Lee 1 1 Computer Networks u Lecture 10: Data Link - LANs Prof. Younghee Lee * Some part of this teaching materials are prepared referencing.

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