Filter
Top Products
Radio Interface
BreezeMAX Wi² and BreezeACCESS Wi² System Manual 97
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 WiFi 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.
Access Categories — 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 (see Table 4-5). 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.
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
Table 4-5: 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