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CS 313 Introduction to Computer Networking & Telecommunication

Learn about the channel allocation problem, multiple access protocols, CDMA, broadcast networks, and the importance of the MAC sublayer in LANs, wireless, and satellite networks.

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CS 313 Introduction to Computer Networking & Telecommunication

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

  2. Topics • Introduction • Channel Allocation Problem • Multiple Access Protocols • CDMA

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

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

  7. Dynamic Channel Allocation • 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 • 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 • 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 • 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 • ALOHA • Carrier sense multiple access protocols (CSMA) • CSMA w/ collision detection (CSMA/CD) • Collision-free protocols • Limited-contention protocols

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

  13. Pure ALOHA • Let users transmit whenever they have data to be sent • Colliding frames are destroyed • 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 • Discrete time • Agreed slot boundaries • Synchronization needed • Performance • Which ALOHA has a shorter medium access delay? • Which ALOHA has a higher throughput?

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

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

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

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

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

  21. CSMA/CD Model

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

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

  24. Collision-Free Protocol Token ring Token Station Direction of transmission

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

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