CS 313 Introduction to Computer Networking Telecommunication Medium

CS 313 Introduction to Computer Networking & Telecommunication Medium Access Control Sublayer Chi-Cheng Lin, Winona State University

Topics l Introduction l Channel Allocation Problem l Multiple Access Protocols l CDMA 2

Introduction l Broadcast networks q. Key issue: who gets to use the channel when there is competition q. Referred to as ØMultiaccess channels ØRandom access channels l MAC (Medium Access Control) sublayer q. LANs q. Wireless networks q. Satellite networks 3

Channel Allocation Problem l Channel Allocation q. Static q. Dynamic l Performance factors q. Medium access delay ØTime between a frame is ready and the frame can be transmitted q. Throughput Ø#frames can be transmitted in unit time interval 4

l FDM Static Channel Allocation q. Bandwidth divided into N equal sized portions for N users q. Problems Ø#senders large Ø#senders continuously varies Øbursty traffic q. Discussion: #users > N ? <N ? =N ? q. N times worse than all frames queued in one big queue 5

Static Channel Allocation l TDM q. Each user is statically allocated every Nth time slot q. Same problems as FDM l Under what circumstances are static channel allocation schemes efficient? 6

Dynamic Channel Allocation l Key assumptions 1. Station model ØIndependent ØWork is generated constantly ØOne program per station ØStation is blocked once a frame has been generated until the frame has been successfully transmitted 2. Single channel assumption 7

Dynamic Channel Allocation l Key assumptions 3. Collision Assumption ØCollision: Two frames are transmitted simultaneously, overlapped in time and resulting signal garbled ØCan be detected by all stations ØNo other errors 8

Dynamic Channel Allocation l Key assumptions 4. Time: either continuous or discrete (slotted) ØContinuous § Frame transmission can begin at any instant § No "master clock" needed ØSlotted § Time divided into discrete intervals (slots) § Frame transmissions begin at the start of a slot § #frames contained in a slot: 0 ? 1 ? >1 ? 9

Dynamic Channel Allocation l Carrier sense ("carrier" refers to electrical signal): either Y or N q. Yes ØA station can check channel before transmission ØIf busy, station idle ØWired LANs q. No Ø“Just do it" ØCan tell if transmission successful later ØWireless networks, cable modems 10

Multiple Access Protocols ALOHA l Carrier sense multiple access protocols (CSMA) l CSMA w/ collision detection (CSMA/CD) l Collision-free protocols l Limited-contention protocols l 11

ALOHA l Applicable to any contention system q. System in which uncoordinated users are competing for the use of a single shared channel l Two versions q. Pure ALOHA q. Slotted ALOHA 12

Pure ALOHA Let users transmit whenever they have data to be sent l Colliding frames are destroyed l Sender can always find out destroyed or not l q. Feedback (property of broadcasting) or ACK ØLANs: immediately ØSatellites: propagation delay (e. g. , 270 msec) q. By listening to the channel q. If frame is destroyed wait a random amount of time and retransmit (why "random"? ) 13

Pure ALOHA Where are the collisions? 14

Slotted ALOHA Discrete time l Agreed slot boundaries l Synchronization needed l Performance l q. Which ALOHA has a shorter medium access delay? q. Which ALOHA has a higher throughput? 15

Performance of ALOHA l Slotted ALOHA can double throughput of pure ALOHA Throughput versus offered traffic for ALOHA systems. 16

Carrier Sense Multiple Access (CSMA) Protocols Stations can listen to the channel (i. e. , sense a carrier in the channel) l Types l q 1 -persistent CSMA q. Nonpersistent CSMA qp-persistent CSMA 17

Performance of MAC Protocols Comparison of the channel utilization versus load for various random access protocols. 18

CSMA w/ Collision Detection (CSMA/CD) l Can listen to the channel and detect collision q. Stop transmitting as soon as collision detected Widely used on LANs (e. g. , Ethernet) l Collision detection l q. Analog process q. Special encoding is used 19

CSMA w/ Collision Detection (CSMA/CD) l Conceptual model q 3 states ØContention ØTransmission ØIdle l Minimum time to detect collision determines time slot q. Depends on propagation delay of medium 20

CSMA/CD Model 21

CSMA/CD Algorithm Source: http: //www. 10 gea. org/gigabit-ethernet/ 22

Collision-Free Protocols l Model q. N Stations: 0, 1, . . . , (N-1) l Question q. Which station gets the channel after a successful transmission? l Protocols q. Bit-map (i. e. , reservation) protocol q. Token passing protocol ØExample: Token ring 23

Collision-Free Protocol Token Station Direction of transmission Token ring

Performance of Contention and Collision-Free Protocols l Contention q. Low load => low medium access delay : ) q. High load => low channel efficiency : ( l Collision-Free q. Low load => high medium access delay : ( q. High load => high channel efficiency : ) 25

Summary of Channel Allocation Methods/Systems * * * * 26 * | Token Passing | Contention-free protocol |