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Wireless Local Area Network (WLAN) 445

video, background and best‐effort transmissions. Each class, with its queue, has the
following variable parameters:
The minimum number of slots that a device has to wait for before it is allowed to
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transmit a frame (Arbitration Interframe Space Number, AIFSN).
Shortest Contention Window (CWmin): The minimum number of slots that can be
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selected with the random number generator.
Longest Contention Window (CWmax): Maximum number of slots from which the
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random number generator can select a value.
Transmit Opportunity (TXOP): Maximum transmission time. The granularity of the
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parameter is 32 microseconds.
Admission Control: Indicates whether devices have to request permission to use a
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transmission class. Further details are discussed below.
Figure 6.27 shows an example of how these values could be set in practice for the
different priority classes. Speech frames, for example, have very stringent requirements
concerning delay and jitter. Therefore, it is important in this QoS class that such frames
can be sent after a short backoff time and are hence preferred over other frames. This is
achieved by configuring a short waiting time (AIFSN), for example, two slots, and set-
ting the contention window size to three slots. The maximum waiting time is hence
only five slots. This way, voice frames will always be transmitted before other devices
(using best‐effort frames) have a chance to access the air interface. This is the case in the
scenario shown in Figure 6.27 – best‐effort frames have to wait at least for seven slots
before their contention window begins. The values for CWmin, CWmax and TXOP are
variable and many vendors allow the user to change these values via the user interface
of the AP. If WMM is activated, the parameters are broadcast in the WMM parameter

SIFS

Voice 2 0 – 3

Video 2 0 – 7 slots

Background 3 slots 0 – 15 slots

Best Effort 7 slots 0 – 15 slots

Minimum wait Minimum contention window
(AIFSN) (CWmin)
Figure 6.27 WMM priority classes with example values for CWmin, CWmax and TXOP.