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

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

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

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

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

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

7 Dynamic Channel Allocation l Key assumptions 1. Station model  Independent  Work is generated constantly  One program per station  Station is blocked once a frame has been generated until the frame has been successfully transmitted 2. Single channel assumption

8 Dynamic Channel Allocation l Key assumptions 3. Collision Assumption  Collision: Two frames are transmitted simultaneously, overlapped in time and resulting signal garbled  Can be detected by all stations  No other errors

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

10 Dynamic Channel Allocation l Carrier sense ("carrier" refers to electrical signal): either Y or N  Yes  A station can check channel before transmission  If busy, station idle  Wired LANs  No  “Just do it"  Can tell if transmission successful later  Wireless networks, cable modems

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

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

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

14 Pure ALOHA Where are the collisions?

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

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

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

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

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

20 CSMA w/ Collision Detection (CSMA/CD) l Conceptual model  3 states  Contention  Transmission  Idle l Minimum time to detect collision determines time slot  Depends on propagation delay of medium

21 CSMA/CD Model

22 CSMA/CD Algorithm Source:

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

Collision-Free Protocol Token ring Station Direction of transmission Token

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

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