H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 562)

48-2
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 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.
EDCA parameters
WMM assigns 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:
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 48-1 for the AIFS durations).
Exponent form of CWmin (ECWmin) and exponent form of CWmax (ECWmax) determine the
average backoff slots, which increases as the two values increase (see Figure 48-1 for the backoff
slots).
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.