Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
5-9
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 5 Cisco Unified Wireless QoS
Wi-Fi Multimedia
WMM Queues
shows the queuing performed on a WMM client or AP. There are four separate queues, one
for each of the access categories. Each of these queues contends for the wireless channel in a similar
manner to the DCF mechanism described previously, with each of the queues using different interframe
space, CWmin, and CWmax values. If more than one frame from different access categories collide
internally, the frame with the higher priority is sent, and the lower priority frame adjusts its backoff
parameters as though it had collided with a frame external to the queuing mechanism. This system is
called enhanced distributed channel access (EDCA).
manner to the DCF mechanism described previously, with each of the queues using different interframe
space, CWmin, and CWmax values. If more than one frame from different access categories collide
internally, the frame with the higher priority is sent, and the lower priority frame adjusts its backoff
parameters as though it had collided with a frame external to the queuing mechanism. This system is
called enhanced distributed channel access (EDCA).
Figure 5-7
WMM Queues
shows the principle behind EDCF, where different interframe spacing and CWmin and
CwMax values (for clarity CwMax is not shown) are applied per traffic classification. Different traffic
types can wait different interface spaces before counting down their random backoff, and the CW value
used to generate the random backoff number also depends on the traffic classification. For example, the
CWmin[3] for Voice is 2
types can wait different interface spaces before counting down their random backoff, and the CW value
used to generate the random backoff number also depends on the traffic classification. For example, the
CWmin[3] for Voice is 2
3
-1, and CWmin[5] for Best effort traffic is 2
5
-1. High priority traffic has a small
interframe space and a small CWmin value, giving a short random backoff, whereas best-effort traffic
has a longer interframe space and large CWmin value that on average gives a large random backoff
number.
has a longer interframe space and large CWmin value that on average gives a large random backoff
number.
Figure 5-8
Access Category Timing
132600
Application
Background
Best effort
Video
Voice
132598
7 slots
CWmin[4]
3 slots
CWmin[5]
CWmin[3]
2 slots
2 slots
CWmin[5]
Background
Best effort
Video
Voice
SIFS