Skip to content

Random Medium Access Control

CSMA Protocol

  • Carrier Sense Protocol
  • Increases performance
  • Chances of collision exists because of propagation delay. Because a frame sent by a station may not be received, causing the medium to be "free".

1 - Persistent CSMA

  • If the channel is busy, listen continously until it becomes free, then transmit immediately.

Non-Persistent CSMA

  • If the channel is busy, back off and wait a random amount of time before trying again.
  • Might lead to lower channel utilisation. If the random backoff time is long, the channel might sit idle even though a station is ready to transmit.

P-Persistent CSMA

  • If the channel is busy, wait until it is free. When it is free, transmit with a certain probability. Once the channel is idle, it transmits with a probability p
  • Basically randomising the the decision to transmit.
  • After first collision, they get to choose a re transmission time between 0-1
  • After each collision, the window size increases exponentially 2^k-1.

CSMA/CD

  • If two stations sense the channel to be idle and begin transmitting simultaneously they will both detect the collision almost immediately.
  • Rather than finishing transmitting their frames, they should abruptly stop transmitting as soon as the collision is detected.
  • Saves time and bandwidth.
  • Has alternating contention and transmission periods, with idle periods occuring in between. Pasted image 20251027215507.png

Collision

We can detect collision if Transmission Time > 2 * Propagation Time