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EDCA parameters
WMM assigns data packets in a BSS to four AC queues. By allowing a high-priority AC queue to have more
channel contention opportunities than a low-priority AC queue, WMM offers different service levels to
different AC queues.
WMM define a set of EDCA parameters for each AC queue, covering the following:
AIFSN: Different from the 802.1 1 protocol where the idle duration (set using DIFS) is a constant value,
WMM can define an idle duration per AC queue. The idle duration increases as the AIFSN value
increases (see
ECWmin and ECWmax determine the average backoff slots, which increases as the two values
increase (see
TXOPLimit indicates the maximum time for which a user can hold a channel after a successful
contention. The greater the TXOPLimit is, the longer the user can hold the channel. The value 0 indicates
that the user can send only one packet each time it holds the channel.
Figure 24 Per-AC channel contention parameters in WMM
Busy Medium
CAC admission policies
CAC requires that a client get permission of the AP before it can use a high-priority AC queue for transmission,
thus guaranteeing bandwidth to the clients that have gained access. CAC controls real time traffic (AC-VO
and AC-VI traffic), but not common data traffic (AC-BE and AC-BK traffic).
If a client wants to use a high-priority AC queue, it must send a request to the AP. The AP returns a positive
or negative response based on either of the following admission control policy:
Channel utilization-based admission policy: the AP calculates the total time that the existing
high-priority AC queues occupies the channel in one second, and then calculates the time that the
requesting traffic occupies the channel in one second. If the sum of the two values is smaller than or
equal to the maximum hold time of the channel, the client can use the requested AC queue. Otherwise,
the request is rejected.
Users-based admission policy: if the number of clients using high-priority AC queues plus the clients
requesting for high-priority AC queues is smaller than or equal to the maximum number of high-priority
AC queue clients, the request is accepted. Otherwise, the request is rejected. During calculation, a
client is counted once even if it is using both the AC-VO and AC-VI queues.
Figure 24
for the AIFS durations).
Figure 24
for the backoff slots).
AIFS[AC-BK]
AIFS[AC-BE]
AIFS[AC-VI]
Backoff slots
AIFS[AC-VO]
Backoff slots
DIFS
Backoff slots
Backoff slots
Backoff slots
Contention Window
56
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