Chi-Cheng Lin, Winona State University CS412 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  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  No  “Just do it"  Can tell if transmission successful later  LANs: carrier sense  Satellites -> not carrier sense (why?)

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 Figure 13.4 Procedure for ALOHA protocol

15 Pure ALOHA Where are the collisions?

16 Pure ALOHA Frames are assumed to have the same size (same frame time) for analysis

17 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?

18 Performance of ALOHA l Slotted ALOHA can double the throughput of pure ALOHA

19 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

20 Figure 13.5 Collision in CSMA

21 Figure 13.6 Persistence strategies

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23 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

24 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

25 CSMA/CD Model

CSMA/CD procedure

27 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

28 Bit-Map Protocol

29 Figure Token-passing network

30 Figure Token-passing procedure

31 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 :)

32 Code Division Multiple Access (CDMA) l In FDMA, the bandwidth is divided into channels. l In TDMA, the bandwidth is just one channel that is timeshared. l In CDMA, one channel carries all transmissions simultaneously.

33 Figures & Chip sequences and encoding rules The chip sequences must be orthogonal! A  B = A  C = A  D = B  C = B  D = C  D = 0 A  A = B  B = C  C = D  D = length of chip sequence

34 Figure CDMA multiplexer

35 Figure CDMA demultiplexer

36 CDMA (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal

37 Summary of Channel Allocation Methods/Systems