Medium Access Control: Stochastic Methods - CSMA

In this lesson, we'll study the carrier sense multiple access protocol.

Carrier Sense Multiple Access (CSMA)


  • ALOHA and slotted ALOHA can easily be implemented, but using them in anything but a lightly loaded network will be extremely inefficient. Designing a network for a very low utilization is possible, but it clearly increases the cost of the network.

  • To overcome the problems of ALOHA, many Medium Access Control mechanisms have been proposed which improve channel utilization.

  • Carrier Sense Multiple Access (CSMA) is one of these and is a significant improvement compared to ALOHA.

How It Works

CSMA requires all nodes to listen to the transmission channel to verify that it’s free before transmitting a frame. When a node senses the channel to be busy, it defers its transmission until the channel becomes free again.


The pseudocode below provides a more detailed description of the operation of CSMA.

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