3Com 8760 User Guide - Page 85

Radio Interface 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 WMMenabled 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. 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 WMM Access Categories 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 7, 6 over IP) calls. AC_VI (AC2) Video High priority, minimum delay. Time-sensitive data such as streaming 5, 4 video. AC_BE (AC0) Best Effort Normal priority, medium delay and throughput. Data only affected by long delays. Data from applications or 0, 3 devices that lack QoS capabilities. AC_BK (AC1) Background Lowest priority. Data with no delay or throughput requirements, such as bulk 2, 1 data transfers. 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 4-45