CITS3002 Computer Networks  
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Ethernet's Contention Algorithm

Each station wanting to transmit, listens to the ether and on finding it silent begins transmission.

On detecting a collision a station:

  • 'backs-off' for a random period which is a multiple of the 802.3 slot time. (This time is chosen based on the longest allowable path being 2.5km, and is set at 51.2microseconds).

  • After the first collision each station backs-off for 0 or 1 slot times before trying again.

    If there is a second collision, a station backs-off for 0, 1, 2 or 3 slot times.

  • In general, a station will back-off from 0 to 2i-1 slot times after the ith collision.

    This continues for a maximum of 10 collisions (1023 back-offs), after which the station stays at 1023 for 6 more collisions.

  • After 16 collisions the station considers the 'ether' severed and reports back to the Networking Layer.

This method, termed binary exponential back-off, ensures a short delay for each station when a small number of stations collide and a reasonable delay when many stations collide.

CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs and WLANs), p12, 20th March 2024.