Table 4: Wmm Access Categories - SMC Networks E21011 User Manual

| Wireless Settings
C 8
HAPTER
Quality of Service (QoS)
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.

Table 4: WMM Access Categories

Access WMM
Category Designation
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
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.
– 96 –
Description
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.
802.1D
Tags
7, 6
5, 4
0, 3
2, 1