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162 Chapter 6
Table 6.11 : WMM access category descriptions
Access category
WMM voice priority
WMM video priority
WMM best effort priority
WMM background priority
Description
Highest priority. Allows multiple concurrent VoWLAN calls, with low
latency and quality equal to a toll voice call.
Prioritizes video traffic above lower categories. One 802.11g or
802.11a channel can support 3 to 4 standard definition TV streams or
1 high definition TV stream.
Traffic from legacy devices, from applications or devices that lack QoS
capabilities, or traffic such as internet surfing that is less sensitive to
latency but is affected by long delays.
Low priority traffic, such as a file download or print job, that does not
have strict latency or throughput requirements.
Voice
Video
DIFS34 μs
SIFS2 slots
DIFS34 μs
SIFS2 slots
Backoff 0–3 slots
Backoff 0–7 slots
Best effort
AIFS[2]43 μs
SIFS3 slots
Backoff 0–15 slots
Background
AIFS[3]79 μs
SIFS7 slots
Interframe spacing per traffic class
Backoff period traffic class
Figure 6.8 : AIFS and back-off timing per WMM traffic class
Backoff 0–15 slots
also requires the access point to make certain assumptions regarding data packet
sizes, data transmission rates and the need to reserve surplus bandwidth for
transmission retries.
The Wi-Fi Alliance adopted a subset of the 802.11e standard in advance of the
IEEE’s September 2005 approval. This subset, called Wi-Fi multimedia (WMM),
describes four access categories as shown in Table 6.11 , with EDCF timings as
shown in Figure 6.8 .
The prioritization mechanism certified in WMM is equivalent to the EDCF coordination
mode defined in 802.11e but did not initially include the scheduled access capability
available through HCF and HCCA. This and other 802.11e capabilities are planned to be
progressively included in the Wi-Fi Alliance’s WMM certification program.
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