Wireless/Redundant Edge Services xl Module Management and Configuration Guide WS.01.03 or greater

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Wireless Local Area Networks (WLANs)
Traffic Management (QoS)
Prioritization with WMM
WMM improves QoS by dividing traffic into priority queues, one for each of
four access categories (ACs). The higher the AC, the higher QoS the traffic
requires.
Table 3-5 shows the queues into which RPs and wireless stations can divide
traffic.
Table 3-5. WMM Priority Queues
Each queue is defined by different parameters, which include:
■ the IFS—now called the arbitration IFS number (AIFSN)
■ the minimum contention window (CW Min)—the maximum value for the
initial random backoff time
■ the maximum contention window (CW Max)—the maximum value for the
random backoff time in a network experiencing collisions
■ the transmit opportunity (Transmit Ops)—the continuous time during
which a device that has won control of the radio can retain control
When devices use different parameters to transmit different types of traffic,
the most time-sensitive traffic receives the QoS that it needs. For example, the
queue for voice traffic uses a smaller contention window, so VoWLAN devices
on average choose smaller backoff times and win control of the medium more
quickly.
Wireless devices place traffic into the four queues according to the QoS mark
in frames’ headers, which can be either:
■ an 802.1p mark in the VLAN tag
■ Differentiated Services Code Point (DSCP) in the IP header
Table 3-6 shows how priorities map to the queues.
Queue Number AC
1 Background
2 Best effort
3Video
4Voice