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The back off procedure

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• Back-off after a collision
  • Back off algorithm of IEEE 802.11:

    A node picks a random number the nodes must defer the (random) back off period before re-try the transmission

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  • Backoff protocol used in 802.11:

    802.11 uses a similar binary exponential backoff algorithm of 802.3 (Ethernet)

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• The 802.11 binary exponential back off algorithm
  • The initial range of random values used in 802.11 is:

    [ 0 .. CWmin ] CWmin = 31 in 802.11b CWmin = 15 in 802.11g

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  • The maximum range of random values used in 802.11 is:

    [ 0 .. 1023 ] (CWmax = 1023)

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  • Doubling algorithm:

    CW = CWmin; // Initialize the range of Congestion Window values if ( CW < CWmax (= 1023) ) CW = 2×CW + 1; // Doubling algorithm Example: CW = 15 CW = 2×CW + 1 = 2×15 + 1 = 31 CW = 2×CW + 1 = 2×31 + 1 = 63 and so on