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Cac admission policies, U-apsd power-save mechanism, Figure 48-1 – H3C Technologies H3C WX6000 Series Access Controllers User Manual

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48-3

Figure 48-1 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 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, it needs to 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 ACs occupies 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. Otherwise,

the request is rejected.

Users-based admission policy: if the number of clients using high-priority ACs plus the requesting

clients is smaller than or equal to the maximum number of high-priority AC clients, the request is

accepted. Otherwise, the request is rejected. During calculation, a client is counted once even if it

is using both AC-VO and AC-VI.

U-APSD power-save mechanism

U-APSD improves the 802.11 APSD power saving mechanism. When associating clients with ACs, you

can specify some ACs as trigger-enabled, some ACs as delivery-enabled, and the maximum number of

data packets that can be delivered after receiving a trigger packet. Both the trigger attribute and the

delivery attribute can be modified when flows are established using CAC. When a client sleeps, the

delivery-enabled AC packets destined for the client are buffered. The client needs to send a

trigger-enabled AC packet to get the buffered packets. After the AP receives the trigger packet, packets

in the transmit queue are sent. The number of sent packets depends on the agreement made when the

client was admitted. ACs without the delivery attribute store and transmit packets as defined in the

802.11 protocol.