H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 563)

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.