H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 164)

152
Configuring WLAN QoS
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.
When IEEE 802.11e was being standardized, Wi-Fi Alliance defined the Wi-Fi Multimedia (WMM)
standard to allow QoS provisioning of different vendors' devices to interoperate. WMM makes a WLAN
network capable of providing QoS services.
Terminology
• WMM
WMM is 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.
• EDCA
Enhanced distributed channel access (EDCA) is a channel contention mechanism designed by
WMM to preferentially transmit packets with high priority and allocate more bandwidth to such
packets.
• AC
Access category (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. AC also stands for access controller in
this document. Identify the acronyms through the context.
• CAC
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.
• U-APSD
Unscheduled automatic power-save delivery (U-APSD) is a new power saving mechanism defined
by WMM to enhance the power saving capability of clients.
• SVP
SpectraLink voice priority (SVP) is a voice priority protocol designed by the Spectralink company
to guarantee QoS for voice traffic.
WMM protocol overview
The distributed coordination function (DCF) in 802.11 stipulates that access points (APs) and clients
employ 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