80211e

IEEE 802.11e is an amendment to the IEEE 802.11 standard that introduces quality of service (QoS) features for wireless local area networks. Ratified in 2005, it specifies MAC layer enhancements intended to support real-time and interactive applications such as voice over IP and video conferencing by prioritizing traffic and reducing latency.

The core of 802.11e is the Enhanced Distribution Channel Access (EDCA), a contention-based mechanism that divides traffic into four Access Categories: voice (AC_VO), video (AC_VI), best effort (AC_BE), and background (AC_BK). Each category uses its own arbitration parameters: AIFS (arbitration interframe space) value, minimum and maximum contention window (CWmin, CWmax), and a TXOP (transmission opportunity) limit. This allows higher-priority traffic to access the channel sooner and for longer periods when appropriate, while lower-priority traffic yields.

In addition, 802.11e formalizes a Hybrid Coordination Function (HCF) that includes a centralized, polling-based mechanism called HCF Contention-Based Channel Access (EDCA) and HCF Controlled Channel Access (HCCA) for scheduled access via a Point Coordinator. HCCA enables the coordinator to poll stations and grant TXOPs, providing predictable service for QoS streams. The standard also introduces traffic specification (TSPEC) for admission control and resource reservation in networks that implement HCCA.

Impact and adoption: 802.11e QoS concepts became foundational for later wireless standards (e.g., 802.11n/ac/ax) which maintain QoS mechanisms, and are widely implemented in enterprise networks to support multimedia and voice applications. Backward compatible with non-QoS devices, 802.11e enables mixed deployments with QoS capable devices.