5.2 ppt

1

Summary of MAC protocols

What do you do with a shared media Channel Partitioning by time frequency

bull Time Division Frequency Division Random partitioning (dynamic)

bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)

hard in others (wireless)bull CSMACD used in Ethernet

Taking Turnsbull polling from a central site token passing

2

Link Layer

51 Introduction and services

52 Error detection and correction

53Multiple access protocols

54 Link-Layer Addressing

55 Ethernet

56 Hubs and switches 57 PPP 58 Link Virtualization

ATM

3

Ethernet

Connection topology Frame structure CSMACD (Exponential Backoff) Link and physical layers

4

CSMA (Carrier Sense Multiple Access)

CSMA listen before transmit - Carrier Sensing

If channel sensed idle transmit entire frame If channel sensed busy defer transmission

Human analogy donrsquot interrupt others

5

CSMA collisions

collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted

spatial layout of nodes

notedistance amp propagation delay causes possible collision

d propagation delay

6

CSMACD (Collision Detection)

CSMACD carrier sensing deferral

as in CSMA collision detection

during transmission collisions could be

detected within short time

colliding transmissions aborted reducing channel wastage

human analogy the polite conversationalist

7

Exercise with Ethernet CSMACD

The protocol Analyze the time sequence of events

Time is measured asBit time the time to send one bitCalculation 01 microsecbit for 10 Mbps

Ethernet What about 100Mbps Ethernet

8

Ethernet uses CSMACD

No slots adapter doesnrsquot transmit if it senses that

some other adapter is transmitting that is carrier sense

transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection

Before attempting a retransmission adapter waits a random time that is random access

Adapt the length of random time to reduce collisions exponential backoff

9

Time is measured asBit time the time to send one bitCalculation 01 microsecbit for 10 Mbps Ethernet What about 100Mbps Ethernet

10

Ethernet CSMACD algorithm1 Adaptor receives datagram from net layer amp creates frame2 If adapter senses channel idle (lasting for 96 bit times) it starts

to transmit frame If it senses channel busy waits until channel idle and then transmits

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

4 If adapter detects another transmission while transmitting aborts and sends jam signalJam Signal make sure all other transmitters are aware of collision 48 bits

5 After aborting adapter enters a random defer period called exponential backoff

exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2 The maximum m can be 10

Bit time 01 microsecbitThe longest backoff time is at K=1023 about 50 msec

11

More on Ethernetrsquos CSMACD

Exponential Backoff first collision choose K from 01 delay is K 512 bit

transmission times after second collision choose K from 0123 after ten collisions choose K from 01234hellip1023

heavy load random wait will be longer advantage adapt retransmission attempts to

estimated current load and reduce collisions

Seeinteract with Javaapplet on AWL Web sitehighly recommended

12

Exercise with Ethernet CSMACD

Events at one transmitting node given a K = 10 how long the node wait

Events at two transmitting nodes Examples trace the protocol Analyze the time sequence of events

13

Ethernet

Connection topology Frame structure CSMACD (Exponential Backoff) Link and physical layers

14

Ethernet

ldquodominantrdquo wired LAN technology first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps

Metcalfersquos Ethernetsketch

5 DataLink Layer 5-15

Star topology bus topology popular through mid 90s

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

today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol

(nodes do not collide with each other)

bullswitch

bullbus coaxial cable bullstar

16

Not an atypical LAN (IP network)

Dedicated

Shared

17

Ethernet Frame Structure

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

Preamble 7 bytes with pattern 10101010 followed by one

byte with pattern 10101011 used to synchronize receiver sender clock

rates

18

Ethernet Frame Structure (more)

Addresses 6 bytes if adapter receives frame with matching destination

address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol

otherwise adapter discards frame

Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)

CRC checked at receiver if error is detected the frame is simply dropped

19

Unreliable connectionless service Connectionless No handshaking between

sending and receiving adapter Unreliable receiving adapter doesnrsquot send

acks or nacks to sending adapter stream of datagrams passed to network layer can

have gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see the gaps

Ethernetrsquos MAC protocol unslotted CSMACD

23

CSMACD efficiency

Tprop = max propagation delay between 2 nodes in LAN (eg 400m)

ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0

Goes to 1 as ttrans goes to infinity Much better than ALOHA but still decentralized simple and cheap

transprop tt 51

1efficiency

5 DataLink Layer 5-24

8023 Ethernet Standards Link amp Physical Layers

many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100

Mbps 1Gbps 10G bps different physical layer media fiber cable

bullapplicationbulltransportbullnetwork

bulllinkbullphysical

bullMAC protocolbulland frame format

bull100BASE-TX

bull100BASE-T4

bull100BASE-FXbull100BASE-T2

bull100BASE-SX bull100BASE-BX

bullfiber physical layerbullcopper (twisterbullpair) physical layer

25

Manchester encoding

Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to

synchronize to each other no need for a centralized global clock among nodes

Hey this is physical-layer stuff