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5.5 WMM
5.5.1 Overview
WMM is a wireless QoS protocol used to ensure that packets with high priorities are transmitted
first. This ensures better experience of voice and video service over WiFi networks.
WMM involves the following terms:
Enhanced Distributed Channel Access (EDCA): It is a channel competition mechanism to ensure
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 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.
ACK Policies
WMM specifies the Normal ACK and No ACK policies.
According to the No ACK policy, no ACK packet is used during wireless packet transmission to
acknowledge packet reception. This policy is applicable to scenarios where interference is mild
and can effectively improve transmission efficiency. In case of strong interference, lost packets
are not sent again if this policy is adopted. This leads a higher packet loss rate and reduces the
overall performance.
According to the Normal ACK policy, each time a receiver receives a packet, it sends back an
ACK packet to acknowledge packet reception.
EDCA Parameters
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 devices
to fairly compete for channels. All the services implemented over WiFi 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 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.
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