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Jamming to ensure collision detection

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• Duration of a collision
  • The duration of a collision:

    Suppose node A starts a transmission: Node B starts to transmit at Δ time before A's transmission arrives at B: Suppose node B will stop transmitting when B hears A's transmission: Then: the duration of the collision is equal to: Δ sec

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• The duration of collision can be very short
  • Fact:

    If a node (B) starts a transmission just before a transmission arrives: the duration of collision will be very short:

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  • Fact:

    Short collision periods generates few bit errors and may escape detection !!!

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• Improving collision detection
  • Making sure that a collision is easily detectable:

    When a transmitting node has detected a collision, it will: Transmit a (short) jam signal Stop transmitting (The jam signal is used to help the other transmitting nodes detect the collision)

    Example:

    Node B starts just before A's transmission arrives: Node B has detected the collision: Node B transmits a (short) jam signal to improve collision detection:

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• The final CSMA/CD procotol (chart)
  • The final CSMA/CD flow chart: