802.11e brings QoS to WLANs

As users experience the convenience of wireless connectivity, they are beginning to demand support for the same applications they run over today's wired networks. Because wireless bandwidth availability is restricted, quality of service is increasingly important in 802.11 networks.

Enter 802.11e, a proposed IEEE standard to define QoS mechanisms for wireless gear that gives support to bandwidth-sensitive applications such as voice and video.

The original 802.11 media access control protocol was designed with two modes of communication for wireless stations. The first, Distributed Coordination Function (DCF), is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), sometimes referred to as "listen before talk." A station waits for a quiet period on the network and begins to transmit data and detect collisions. DCF provides coordination, but it doesn't support any type of priority access of the wireless medium.

An optional second mode, Point Coordination Function (PCF), supports time-sensitive traffic flows. Wireless access points periodically send beacon frames to communicate network identification and management parameters specific to the wireless network. Between the sending of beacon frames, PCF splits the time into a contention-free period and a contention period. With PCF enabled, a station can transmit data during contention-free polling periods. However, PCF hasn't been implemented widely because the technology's transmission times are unpredictable.

Because DCF and PCF do not differentiate between traffic types or sources, the IEEE is proposing enhancements in 802.11e to both coordination modes to facilitate QoS. These changes would let critical service requirements be fulfilled while maintaining backward-compatibility with current 802.11 standards.

The proposed enhancement to DCF - Enhanced Distribution Coordination Function (EDCF) - introduces the concept of traffic categories. Each station has eight traffic categories, or priority levels. Using EDCF, stations try to send data after detecting the medium is idle and after waiting a period of time defined by the corresponding traffic category called the Arbitration Interframe Space (AIFS). A higher-priority traffic category will have a shorter AIFS than a lower-priority traffic category. Thus stations with lower-priority traffic must wait longer than those with high-priority traffic before trying to access the medium.

To avoid collisions within a traffic category, the station counts down an additional random number of time slots, known as a contention window, before attempting to transmit data. If another station transmits before the countdown has ended, the station waits for the next idle period, after which it continues the countdown where it left off.

No guarantees of service are provided, but EDCF establishes a probabilistic priority mechanism to allocate bandwidth based on traffic categories.

Another way 802.11e aims to extend the polling mechanism of PCF is with the Hybrid Coordination Function (HCF). A hybrid controller polls stations during a contention-free period. The polling grants a station a specific start time and a maximum transmit duration.