Network Router User Manual

System Configuration
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6
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
After a collision detection, a backoff wait time is calculated. The total wait time is the
sum of a minimum wait time (Arbitration Inter-Frame Space, or AIFS) determined
from the AIFSN, and a random backoff time calculated from a value selected from
zero to the CW. The CW value varies within a configurable range. It starts at CWMin
and doubles after every collision up to a maximum value, CWMax. After a
successful transmission, the CW value is reset to its CWMin value.
Table 6-1. WMM Access Categories
Access Category WMM Designation Description 802.1D Tags
AC_VO (AC3) Voice Highest priority, minimum delay. Time-sensitive
data such as VoIP (Voice over IP) calls.
7, 6
AC_VI (AC2) Video High priority, minimum delay. Time-sensitive data
such as streaming video.
5, 4
AC_BE (AC0) Best Effort Normal priority, medium delay and throughput.
Data only affected by long delays. Data from
applications or devices that lack QoS capabilities.
0, 3
AC_BK (AC1) Background Lowest priority. Data with no delay or throughput
requirements, such as bulk data transfers.
2, 1