wifi - What is the reason CSMA/CD can't be used on a wireless network?

24
2014-07
  • Ian Muscat

    I'm new to wireless networking and wondering why the same Collision Detection mechanisms used for Ethernet can't be applied to WiFi. I think I'm starting to understand, but not sure if I've got it:

    The physical characteristics of WiFi make it impossible and impractical for the CAMA/CD mechanism to be used. This is due to CSMA/CD’s nature of ‘listening’ if the medium is free before transmitting packets. Using CSMA/CD, if a collision is detected on the medium, end-devices would have to wait a random amount of time before they can start the retransmission process. For this reason, CSMA/CD works well for wired networks, however, in wireless networks, there is no way for the sender to detect collisions the same way CSMA/CD does since the sender is only able to transmit and receive packets on the medium but is not able to sense data traversing that medium. Therefore, CSMA/CA is used on wireless networks. CSMA/CA doesn’t detect collisions (unlike CSMA/CA) but rather avoids them through the use of a control message. Should the control message collide with another control message from another node, it means that the medium is not available for transmission and the back-off algorithm needs to be applied before attempting retransmission.

    Am I on the right track or is there something else I should be considering?

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