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WLAN QoS configuration
The terms AP and fat AP in this document refer to A-MSR900 and A-MSR20-1X routers with IEEE 802.11b/g
and A-MSR series routers installed with a SIC WLAN module.
An 802.11 network offers contention-based wireless access. To provide applications with QoS services, IEEE
developed 802.11e for the 802.11-based WLAN architecture.
While IEEE 802.11e was being standardized, Wi-Fi Alliance defined the WMM standard to allow QoS
provision devices of different vendors to interoperate. WMM makes a WLAN network capable of providing
QoS services.
Terminology
1.
WMM
WMM is a wireless QoS protocol designed to preferentially transmit packets with high priority. Thus, it
guarantees better QoS services for voice and video applications in a wireless network.
2.
EDCA
EDCA is a channel contention mechanism designed by WMM to preferentially transmit packets with high
priority and allocate more bandwidth to such packets.
3.
AC
AC is used for channel contention. WMM defines four access categories. They are AC-VO (voice) queue,
AC-VI (video) queue, AC-BE (best-effort) queue, and AC-BK (background) queue, in the descending order of
priority. When contending for a channel, a high-priority AC queue preempts a low-priority AC queue.
4.
CAC
CAC limits the number of clients that are using high-priority AC queues (including AC-VO and AC-VI queues)
to guarantee sufficient bandwidth for existing high-priority traffic.
5.
U-APSD
U-APSD is a new power saving mechanism defined by WMM to enhance the power saving capability of
clients.
6.
SVP
SVP is a voice priority protocol designed by Spectralink to guarantee QoS for voice traffic.
WMM protocol overview
The DCF in 802.11 stipulates that APs and clients use the CSMA/CA access mechanism. APs or clients listen
to the channel before they hold the channel for data transmission. When the specified idle duration of the
channel times out, APs or clients randomly select a backoff slot within the contention window to perform
backoff. The device that finishes backoff first gets the channel. With 802.11, all devices have the same idle
duration and contention window. Therefore, they are equal when contending for a channel. In WMM, this
fair contention mechanism is changed.