Wmm Setup; Overview - Tenda i9 User Manual

7.4 WMM Setup

7.4.1 Overview

802.11 networks offer wireless access services based on the Carrier Sense Multiple Access
with Collision Avoidance (CSMA/CA) channel competition mechanism, which allows all
wireless clients to fairly compete for channels. All the services implemented over wireless
networks share the same channel competition parameters. Nevertheless, different services
usually have different requirements for bandwidth, delay, and jitter. This requires wireless
networks to offer accessibility based on the services implemented over the networks.
WMM is a wireless QoS protocol used to ensure that packets with high priorities are
transmitted first. This ensures better voice and video service experience over wireless
networks.
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WMM involves the following terms:
Enhanced Distributed Channel Access (EDCA): It is a channel competition mechanism to
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ensure that packets with higher priorities are assigned more bandwidth and transmitted
earlier.
Access Category (AC): AC: The WMM mechanism divides WLAN traffic by priority in
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descending order into the AC-VO (voice stream), AC-VI (video stream), AC-BE (best effort),
and AC-BK (background) access categories. The access categories use queues with
different priorities to send packets. The WMM mechanism ensures that packets in queues
with higher priorities have more opportunities to access channels.
According to the 802.11 protocol family, all devices listen on a channel before using the
channel to send data. If the channel stays idle for or longer than a specified period, the devices
wait a random backoff period within the contention window. The device whose backoff period
expires first can use the channel. The 802.11 protocol family applies the same backoff period
and contention window to all devices across a network to ensure that the devices have the
same channel contention opportunity.
 EDCA Parameters
WMM changes the contention mechanism of 802.11 networks by dividing packets into four
ACs, among which the ACs with higher priorities have more opportunities to access channels.
The ACs help achieve different service levels.
WMM assigns each AC a set of EDCA parameters for channel contention, including:
Arbitration Inter Frame Spacing Number (AIFSN): Different from the fixed distributed
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inter-frame spacing (DIFS) specified in the 802.11 protocol family, AIFSN varies across ACs.
A greater AIFSN indicates a longer backoff period. See AIFS in the following figure.
Contention window minimum (CWmin) and contention window maximum (CWmax)
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specify the average backoff period. The period increases along with these two values. See
the backoff slots in the following figure.
Transmission Opportunity (TXOP): It specifies the maximum channel use duration after
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successful channel contention. The duration increases along with this value. The value 0
indicates that a device can send only one packet through a channel after winning
contention for the channel.
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