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Entering the Point Coordinated Function operational mode

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• Preliminary: the "Network Allocation Vector"
  • Network Allocation Vector:

    Network Allocation Vector = a state variable maintained by a wireless node (e.g., laptop) for IEEE 802.11 protocol When a period in the Network Allocation Vector is "allocated": The wireless will not initiate a data frame transmission on the IEEE 802.11 network on its own.

    Example:

    Explanation:

    There is a reservation in the NAV from now until 7:30 This node will not start a data frame transmission on its own during the reservation period The node will still transmit ACK frames (because an ACK frame must be sent ASAP !!!) The node will only transmit a data frame when it is polled (= given permission to do so)

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• Entering into the PCM operational mode
  • Initial (default) operation:

    Initially, the IEEE 802.11 network operates in the DCF mode

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  • Entering the PCM mode:

    The Access Point (AP) (coordinator) defers (= listens) for PIFS (= PCF interframe spacing time and transmits a beacon frame The beacon frame will contain a PCF duration value that is the length of the contention-free period All nodes that are associated with the base station (and in its range) will receive the beacon frame Graphically: Note: Because a data frame transmission must defer DIFS time (and PIFS < DIFS): The beason frame has a higher transmission priority than data frames !!!         Graphically: (The contention period duration parameter is called CFPMaxDuration) When stations (e.g., laptops) receive the beacon frame: The node will reserve the given duration in their Network Allocation Vector (NAV) Graphically: The time reservation made in the Network Allocation Vector will: Prevent a node (station) from transmitting during the reserved period (More on the NAV when we discuss DCF)

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  • Result:

    Nodes will not transmit by its own initiative Nodes will only transmit data frames when the node receives a permission The permission is given as a poll frame (originating from the Access Point)