Nortel 120 Using Manual page 86

86 System configuration management
WMM Access Categories
Access Category
AC_VI (AC2)
AC_BE (AC0)
AC_BK (AC1)
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:
•
•
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
Number, or AIFSN) determined from the AIFSN, and a random backoff time
calculated from a value selected from zero to the CW. The CW value varies
Copyright © 2006, Nortel Networks
.
WMM Designation
Video
Best Effort
Background
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
Using the Nortel Business Access Point 120
NN47921-301 01.01 Standard
1.0 August 2006
Description
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.
BAP120
802.1D Tags
5, 4
0, 3
2, 1
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