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Edca parameters, Cac admission policies – H3C Technologies H3C MSR 50 User Manual

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EDCA parameters

WMM assigns data packets in a basic service set (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:

Arbitration inter-frame spacing number (AIFSN)—Different from the 802.11 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

Figure 28

for the AIFS durations).

Exponent form of CWmin (ECWmin) and exponent form of CWmax (ECWmax)—Determine the

average backoff slots, which increases as the two values increase (see

Figure 28

for the backoff

slots).

Transmission opportunity limit (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 28 Per-AC channel contention parameters in WMM

CAC admission policies

CAC requires that a client obtain permission of the AP before it can use a high-priority AC queue for

transmission, 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 policies:

Channel utilization-based admission policy—The AP calculates the total time that the existing
high-priority AC queues occupy the channel in one second, and then calculates the time that the

requesting traffic will occupy 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

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