Wi-Fi Multimedia (Wmm) Configuration; Wmm Operation - Enterasys RoamAbout RBT-4102 Configuration Manual

Wi-Fi Multimedia (WMM) Configuration

Wireless networks offer an equal opportunity for all devices to transmit data from any type of
application. Although this is acceptable for most applications, multimedia applications (with
audio and video) are particularly sensitive to the delay and throughput variations that result from
this "equal opportunity" wireless access method. For multimedia applications to run well over a
wireless network, a Quality of Service (QoS) mechanism is required to prioritize traffic types and
provide an "enhanced opportunity" wireless access method.
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.
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 (refer
to Table 4‐6). 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.
Table 4-6 WMM Access Categories
Access
Category
AC_VO
(AC3)
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:
• 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
WMM
Description
Designation
Voice Highest priority, minimum delay. Time-sensitive data such
as VoIP (Voice over IP) calls.
Video High priority, minimum delay. Time-sensitive data such as
streaming video.
Best Effort Normal priority, medium delay and throughput. Data only
affected by long delays. Data from applications or devices
that lack QoS capabilities.
Background Lowest priority. Data with no delay or throughput
requirements, such as bulk data transfers.
RoamAbout RBT-4102 Wireless Access Point Configuration Guide 4-65
Radio Interface
802.1D
Tags
7, 6
5, 4
0, 3
2, 1