Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
5-10
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 5 Cisco Unified Wireless QoS
Wi-Fi Multimedia
EDCA
The EDCA process is illustrated in
.
Figure 5-9
EDCA Example
The EDCA process follows this sequence:
1.
While Station X is transmitting its frame, three other stations determine that they must send a frame.
Each station defers because a frame was already being transmitted, and each station generates a
random backoff.
Each station defers because a frame was already being transmitted, and each station generates a
random backoff.
2.
Because the Voice station has a traffic classification of voice, it has an arbitrated interframe space
(AIFS) of 2, and uses an initial CWmin of 3, and therefore must defer the countdown of its random
backoff for 2 slot times, and has a short random backoff value.
(AIFS) of 2, and uses an initial CWmin of 3, and therefore must defer the countdown of its random
backoff for 2 slot times, and has a short random backoff value.
3.
Best-effort has an AIFS of 3 and a longer random backoff time, because its CWmin value is 5.
4.
Voice has the shortest random backoff time, and therefore starts transmitting first. When Voice starts
transmitting, all other stations defer.
transmitting, all other stations defer.
5.
After the Voice station finishes transmitting, all stations wait their AIFS, then begin to decrement
the random backoff counters again.
the random backoff counters again.
6.
Best-effort then completes decrementing its random backoff counter and begins transmission. All
other stations defer. This can happen even though there might be a voice station waiting to transmit.
This shows that best-effort traffic is not starved by voice traffic because the random backoff
decrementing process eventually brings the best-effort backoff down to similar sizes as high priority
traffic, and that the random process might, on occasion, generate a small random backoff number
for best-effort traffic.
other stations defer. This can happen even though there might be a voice station waiting to transmit.
This shows that best-effort traffic is not starved by voice traffic because the random backoff
decrementing process eventually brings the best-effort backoff down to similar sizes as high priority
traffic, and that the random process might, on occasion, generate a small random backoff number
for best-effort traffic.
7.
The process continues as other traffic enters the system. The access category settings shown in
and
are, by default, the same for an 802.11a radio, and are based on formulas
defined in WMM.
Note
refers to the parameter settings on a client, which are slightly different from the settings for
an AP. The AP has a larger AIFSN for voice and video ACs.
132601
AIFS[0]
Best effort
Voice
Frame
Frame
Defer
Frame
Defer
AIFS[0]
AIFS[0]
AIFS[0]
AIFS[6]
AIFS[6]
AIFS[6]
AIFS[6]
Defer
Defer
Defer
Table 5-3
WMM Client Parameters
AC
CWmin
CWmax
AIFSN
TXOP Limit
(802.11b
(802.11b
TXOP Limit
(802.11a/g)
(802.11a/g)
AC_BK
aCWmin
aCWmax
7
0
0
AC_BE
aCWmin
4*(aCQmin+1)-1
3
0
0