H3C Technologies H3C WA2600 Series WLAN Access Points - Manual (Page 53)

8-2
4) CAC
Connection admission control (CAC) limits the number of clients that are using high-priority ACs
(AC-VO and AC-VI) to guarantee sufficient bandwidth for existing high-priority traffic.
5) 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.
6) 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 the 802.11 protocol 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 AP that finishes backoff first gets the channel.
With 802.11, all APs 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.
EDCA parameters
WMM assigns the data packets in a basic service set (BSS) to four ACs. By allowing a high-priority AC
to have more channel contention opportunities than a low-priority AC, WMM offers different service
levels to different ACs.
WMM define a set of EDCA parameters for each AC, covering the following:
z Arbitration inter frame spacing number (AIFSN): Different from the 802.11 protocol where the idle
duration (set using DIFS) is a constant value, WMM can define an idle duration per AC. The idle
duration increases as the AIFSN value increases (see Figure 8-1 for the AIFS durations).
z Exponent form of CWmin (ECWmin) and exponent form of CWmax (ECWmax) determine the
average backoff slot. The average backoff slot increases as the two values increases (see Figure
8-1 for the backoff slots part).
z Transmission opportunity limit (TXOPLimit) indicates the maximum time for which a user can hold
a channel after a successful contention. The greater the TXOPLimit is, the longer the user can hold
the channel. The value 0 indicates that the user can send only one packet each time it holds the
channel.