The Wi-Fi Alliance provides prioritization of data packets to clear the way on wireless networks.
As audio, video and voice applications proliferate on wireless networks, quality-of-service mechanisms are needed to make sure applications with low tolerance for latency and jitter are treated with higher priority than less-sensitive data applications. QoS is critical because wireless LANs have less bandwidth and are less tolerant of packet collisions than wired LANs.
The IEEE is finalizing a wireless QoS standard, 802.11e , that is expected to be approved this year. Eager to spur interoperable QoS among different vendors’ wireless gear, the Wi-Fi Alliance developed a subset of 802.11e called the Wi-Fi Multimedia (WMM) specification. Makers of wireless access points as well as wireless client devices such as laptops, phones and consumer electronics products are incorporating support for WMM into their wares.
WMM is designed to support consumer and corporate applications and works with all three 802.11 wireless physical layer standards – 802.11a, 802.11b and 802.11g. The specification provides basic prioritization of data packets based on four categories – voice, video, best effort and background. In its current form, WMM doesn’t provide mechanisms for scheduling or controlling network access to improve QoS, but these enhancements may be added in future releases.
Prioritization is based on the original Carrier Sense Multiple Access/Collision Avoidance Protocol in the initial 802.11 standard. A mechanism called Distributed Coordination Function (DCF) uses a simple listen-before-talk algorithm to minimize the chance of packet collisions caused by more than one device accessing the wireless medium at the same time. A client must wait for a randomly selected time period and then “listen” to find whether any other device is communicating before starting to transmit. The random back-off period gives all devices a fair opportunity to transmit.
However, WMM stipulates different fixed and random wait times for the four prioritization categories to provide more favorable network access for applications that are less tolerant of packet delays. Devices that have less time to wait have a better chance of being able to transmit than those that have a longer wait. In order of highest priority, the access categories are voice, video, best effort and background. These WMM prioritization categories map to Ethernet 802.1d prioritization tags to allow consistent QoS across wireless and wired network segments.
The back-off timing for each access category consists of a fixed period called the Arbitrary Inter-Frame Space Number followed by a random period called the Contention Window (CW), both specified in multiples of the slot time (see graphic below). For 802.11b, one slot is 20 microsec and for 802.11a and 802.11g, a slot is 9 microsec. The CW maintains the DCF random back-off component to help avoid collisions of packets from the same access category. The CW range doubles each time there is a collision and is reset to its minimum value after a successful transmission.
WMM prioritization achieves the goal of minimizing delay in wireless networks for time-sensitive applications. The ability to use wireless networks for data, voice and multimedia applications will continue to drive widespread adoption.
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Guderian is vice president of Market Strategies and Industry Relations for SpectraLink. He can be reached at bguderian@spectralink.com.
