H3C Technologies H3C MSR 50 - Manual (Page 72)

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Configuring WLAN QoS
The terms AP and fat AP in this document refer to MSR 900, MSR 930, and MSR 20-1X routers with IEEE
802.11b/g and MSR series routers installed with a SIC WLAN module.
Overview
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 Wi-Fi Multimedia (WMM)
standard to allow QoS provision devices of different vendors to interoperate. WMM makes a WLAN
network capable of providing QoS services.
Terminology
• WMM—A wireless QoS protocol designed to preferentially transmit packets with high priority, thus
guaranteeing better QoS services for voice and video applications in a wireless network.
• Enhanced distributed channel access (EDCA)—A channel contention mechanism designed by
WMM to preferentially transmit packets with high priority and allocate more bandwidth to such
packets.
• Access category (AC)—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.
• Connection admission control (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.
• Unscheduled Automatic Power-Save Delivery (U-APSD)—A new power saving mechanism defined
by WMM to enhance the power saving capability of clients.
• SpectraLink voice priority (SVP)—A voice priority protocol designed by the SpectraLink company to
guarantee QoS for voice traffic.
WMM protocol
The distributed coordination function (DCF) in 802.11 stipulates that access points (APs) and clients use
the carrier sense multiple access with collision avoidance (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.