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How IEEE 802.11 counts down a back off period

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• The back off count down counter (timer)
  • Back off count down timer:

    Back off count down timer = a couting variable that contains the time remaining before a node will transmit a frame

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  • Counting down procedure for the back off timer:

    The back off count down timer is initialized to a random value The back off count down timer is then decremented (more later) When the back off count down timer becomes 0 (zero), the node transmits a frame

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• How to update the count down timer
  • How the back off count down timer is decrement:

    The count down process illustrated:

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    Explanation:

    A node must first listen for DIFS duration before counting down the count down timer If the channel is idle for DIFS sec: Start counting down (= decrement) the count down timer Otherwise: Wait for the channel to become idle Repeat the waiting for DIFS duration Important fact: The node will only decrement the count down timer when the channel is idle after listening for DIFS sec !!! Furthermore: the count down can be interrupted be a transmission: When the node hears a transmission, it will stop the count down The counter is frozen (i.e., not reset !!!) The node will wait for the transmission channel to become clear again.... When the channel become idle: The node will listen (defer) for DIFS again Then: If the channel is idle for DIFS sec: Continue counting down the count down timer Otherwise: Wait for the channel to become idle Repeat listening for DIFS duration