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MAC Protocols

Random Access Protocols

Randomly sends the frames, no priority

Controlled Access Protocols

A station cannot send unless it has been authorised by other stations.

  • Reservation, Polling, Token

Channelisation Protocol

Available bandwidth is shared among the stations in time, space and frequency.

ALOHA

  • Pure Aloha - When a station sends data it waits for an ack, if ack doesn't come within the allotted time then the station wait for random amount of time called back-off time and re-sends the data. Since the different stations wait for different times, collision probability decreases. Uniform frame length guarantees less collision.

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Formula

Vulnerable Time = 2 * T Throughput = G * e ^ -2e G = 1/2, efficiency = 18.4%

Slotted ALOHA

  • The time of the shared channel is divided into discrete time intervals called slots
  • Sending of data is allowed only at the beginning of the time slots.

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Formula

Vulnerable Time = T Throughput = G * e ^ -e G = 1, efficiency = 36.8%