Ece453 – Introduction to Computer Networks Lecture 7 – Multiple Access Control (I)



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ECE453 – Introduction to Computer Networks

  • Lecture 7 – Multiple Access Control (I)


Broadcast vs. PPP

  • Broadcast channel = multiaccess channel = random access channel

  • Broadcast

    • LAN
    • Satellite network
  • PPP

    • WAN (router-router lease line)
    • Home user  ISP


Delay

  • Packet experiences delay from end to end

  • Nodal delay = processing delay + queuing delay + transmission delay + propagation delay



The Channel Allocation Problem

  • Static Channel Allocation in LANs and MANs

    • FDM or TDM
    • Problems
      • Fewer than N users
        • A valuable chunk of time (TDM) or bandwidth (FDM) is wasted
      • More than N users
        • Some users are denied (even if another user is idle)
      • Exactly N users
  • Dynamic Channel Allocation in LANs and MANs



Five Assumptions for Dynamic Channel Allocation

  • Station Model

    • N independent stations generating frames
    • Once a frame is generated, the station is blocked until the frame has been transmitted
  • Single Channel Assumption

  • Collision Assumption

    • If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled. This event is called a collision.
    • All stations can detect collisions.
  • Time Assumption

    • Continuous time
    • Slotted time
  • Carrier assumption

    • Carrier Sense (LAN)
    • No Carrier Sense (Satellite)


Multiple Access Protocols

  • ALOHA

  • Carrier Sense Multiple Access Protocols

  • Collision-Free Protocols

  • Limited-Contention Protocols

  • Wireless LAN Protocols



Some Terminologies

  • Contention systems

    • Systems in which multiple users share a common channel in a way that can lead to conflicts
  • Throughput

    • The maximum continuous traffic rate that a device can handle without dropping a single packet.
    • Measured in terms of the number of frames per second at a given frame size
  • Frametime

    • Time used to transmit a frame (frame_size/data rate)


ALOHA

  • Pure ALOHA

    • 1970, Norman Abramson, U. of Hawaii
    • Users transmit whenever they have data
    • When collision occurs, wait for a random amount of time and send again
  • Slotted ALOHA

    • 1972, Roberts
    • Divide time into discrete intervals
    • A computer is not permitted to send until the beginning of the next slot
    • Doubles the maximum throughput


*ALOHA – Performance Analysis

  • Poisson process (1837)

    • Simplest model for arrivals into a queue system
    • the probability of an arrival in a small interval of time depends only on the size of the interval
    • The probability that k frames are generated during a given frame time follows the Poisson distribution


CSMA – Carrier Sense Multiple Access (LAN)

  • 1-persistent

    • When idle, transmit a frame
    • When busy, continuously sensing
    • When collision, wait for a random amount of time
  • Non-persistent

    • When idle, transmit a frame
    • When busy, wait for a random amount of time
    • When collision, wait for a random amount of time then re-sense
  • p-persistent

    • Slotted channels
    • When idle, transmit with a probability p, defers until the next slot with probability 1-p
  • CSMA with collision detection (CSMA/CD)



A Worst Case Scenario



Collision-Free Protocols

  • Assumption

    • N stations with a unique address from 0 to N-1
    • Propagation delay is negligible
  • The bit-map protocol

    • A reservation protocol: the desire to transmit is broadcast before the actual transmission
  • Binary countdown



Limited-Contention Protocols

  • ALOHA

  • Collision-free protocols

    • Low load: high delay
    • High load: improved efficiency
  • New protocol? – limited contention protocol

    • Uses contention at low load
    • Uses collision-free at high load
    • Adaptive tree walk protocol


Wireless LAN Protocols

  • (a) The hidden station problem (C  B)

  • (b) the exposed station problem (C  D)

  • Activity around the receiver vs. activity around the sender



Wireless LAN Protocols

  • MACA – Multiple Access with Collision Avoidance (Karn, 1990)



Multiple Access Protocols

  • ALOHA

    • Pure ALOHA
    • Slotted ALOHA
  • Carrier Sense Multiple Access Protocols (CDMA)

    • 1-persistent
    • Non-persistent
    • p-persistent
    • CDMA/CD (Ethernet)
  • Collision-Free Protocols

    • Bitmap protocol
    • Binary countdown
  • Limited-Contention Protocols

  • Wireless LAN Protocols



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