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The p-persistent CSMA protocol

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• p-persistent CSMA:
  • The p-persistent CSMA protocol:

    The p-persistent CSMA protocol tries to fix the "guaranteed" collision problem in the p-persistent CSMA protocol When a node can transmit, the node will: Flip a coin         If coin comes up head, then The node will transmit         Otherwise The node will sense again         (I call this protocol the gambler...)

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  • Flow chart:

    Explanation:

    If the node senses that the carrier is idle, then: The node will transmit with probability p           Graphically: Reason: There may be multiple nodes sensing at the same time: Then: one node may decide to transmit, while the other node may decide to back off: If the node senses that the carrier is busy, then: Wait until the current transmission ends Transmit with probability p Graphically: Reason: There may be multiple nodes sensing at the same time: Then: one node may decide to transmit, while the other node may decide to back off:

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• What's value should we use for p ????

  • Selecting the p value in p-persistent CSMA:

    p must be set to a value such that: It is very likely that exactly one node will pick a random number x such that: x < p I.e.: exactly 1 node should transmit its message

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  • Result from probability theory:

    If n nodes are sensing, then the probability: 1 p = --- n will maiximize the likelihood that exactly one node will transmit (and (n-1) nodes will back off)

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  • Example:

    If there are 2 nodes waiting for the current transmission to finish, then use: p = 0.5 On the other hand, if there are 3 nodes waiting for the current transmission to finish, then use: p = 0.33333333 (1/3) And so on...

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    Conclusion:

    p must be dynamic

    It's hard to know how many nodes are busy...

    So:

    The p-persistent CSMA protocol is not implemented as a real life protcol.