Table 2: Wmm Access Categories - SMC Networks SMC2890W-AN Management Manual

Table 2: 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.
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.
– 87 –
Chapter 6
| Wireless Settings
Wi-Fi Multimedia (WMM)
802.1D
Tags
7, 6
5, 4
0, 3
2, 1