Wmm Settings; Overview - Tenda 11AC User Manual

7.4

WMM Settings

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.
WMM involves the following terms:
Enhanced Distributed Channel Access (EDCA): It is a channel competition mechanism to ensure
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that packets with higher priorities are assigned more bandwidth and transmitted earlier.
Access Category (AC): The WMM mechanism divides WLAN traffic by priority in descending order
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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.
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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 inter-frame
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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) specify the
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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 successful
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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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