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UNIT-II

(Wireless) Medium Access Control

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Dr. S V N SREENIVASU

Professor, Dept. of CSE

NARASARAOPETA ENGINEERING COLLEGE

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Contents

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  1. Motivation for Specialized MAC
    • Hidden and Exposed Terminals
    • Far and Near Terminals
  2. Access Methods
    • SDMA
    • FDMA
    • TDMA
    • CDMA

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1. Motivation for Specialized MAC

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MULTIPLEXING

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  • A fundamental mechanism in communication system and networks
  • Enables multiple users to share a medium
  • For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency and code

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Access Methods

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2. Space Division Multiple Access (SDMA)

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  • Channels are assigned on the basis of space(but operate on same frequency)
  • The assignment makes sure that the transmission do not interfere with each (with a guard band in between)

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3. FDMA: Frequency Division Multiple Access

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  1. Frequency domain is subdivided into several non- overlapping frequency bands
  2. Each channel is assigned its own frequency band (with

guard spaces in between)

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3. TDMA : Time Division Medium Access

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  • A channel is given the whole bandwidth for a certain amount of time
    • All senders use the same frequency, but at different

point of time

  • Synchronization can be done by using
    • Fixed Allocation Scheme or
    • Dynamic Allocation Scheme

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Reservation Algorithms

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Packet Reservation Multiple Access (PRMA)

/ Implicit Reservation

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Carrier Sense Multiple Access Protocols (CSMA)

  • In this each terminal on the network is able to monitor the status of the channel before transmitting information
  • Variations:
    • 1-persistent CSMA
    • non-persistent CSMA -
    • p-persistent CSMA
    • CSMA/CA
    • Elimination yield – non-preemptive multiple access (EY-NMPA)

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Multiple Access with Collision Avoidance (MACA)

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POLLING

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Inhibit Sense Multiple Access (ISMA)

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4. CDMA: Code Division Multiple Access

  • separation of channels achieved by assigning each channel its own code
  • guard spaces are realized by having distance in code space (e.g. orthogonal codes)
  • transmitter can transmit in the same frequency band at the same time, but have to use different code
  • Provides good protection against interference and tapping
  • but the receivers have relatively high complexity.

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Spread Aloha Multiple Access (SAMA)

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Comparisons of S/T/F/CDMA

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Thank you