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The access point implements QoS using the Wi-Fi Multimedia (WMM) standard.
Using WMM, the access point is able to prioritize traffic and optimize performance
when multiple applications compete for wireless network bandwidth at the same
time. WMM employs techniques that are a subset of the developing IEEE 802.11e
QoS standard and it enables the access point to inter operate with both WMM-
enabled clients and other devices that may lack any WMM functionality.
Access Categories — WMM defines four access categories (ACs): voice, video,
best effort, and background. These categories correspond to traffic priority levels
and are mapped to IEEE 802.1D priority tags (see Table 5-1). The direct mapping of
the four ACs to 802.1D priorities is specifically intended to facilitate inter operability
with other wired network QoS policies. While the four ACs are specified for specific
types of traffic, WMM allows the priority levels to be configured to match any
network-wide QoS policy. WMM also specifies a protocol that access points can use
to communicate the configured traffic priority levels to QoS-enabled wireless clients.
Access Category WMM Designation Description
AC_VO (AC3)
Voice
AC_VI (AC2)
Video
AC_BE (AC0)
Best Effort
AC_BK (AC1)
Background
WMM Operation — WMM uses traffic priority based on the four ACs; Voice, Video,
Best Effort, and Background. The higher the AC priority, the higher the probability
that data is transmitted.
When the access point forwards traffic, WMM adds data packets to four independent
transmit queues, one for each AC, depending on the 802.1D priority tag of the
packet. Data packets without a priority tag are always added to the Best Effort AC
queue. From the four queues, an internal "virtual" collision resolution mechanism
first selects data with the highest priority to be granted a transmit opportunity. Then
the same collision resolution mechanism is used externally to determine which
device has access to the wireless medium.
For each AC queue, the collision resolution mechanism is dependent on two timing
parameters:
• AIFSN (Arbitration Inter-Frame Space Number), a number used to calculate the
minimum time between data frames
• CW (Contention Window), a number used to calculate a random backoff time
Table 5-1. WMM Access Categories
Highest priority, minimum delay. Time-sensitive
data such as VoIP (Voice over IP) calls.
High priority, minimum delay. Time-sensitive data
such as streaming video.
Normal priority, medium delay and throughput.
Data only affected by long delays. Data from
applications or devices that lack QoS capabilities.
Lowest priority. Data with no delay or throughput
requirements, such as bulk data transfers.
5
Radio Interface
802.1D Tags
7, 6
5, 4
0, 3
2, 1
5-59
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