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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 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
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 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.
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.
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