Fair Access Provisioning through Contention Parameter Adaptation in the IEEE 802.11e Infrastructure Basic Service Set

Abstract

We present the station-based unfair access problem among the uplink and the downlink stations in the IEEE 802.11e infrastructure Basic Service Set (BSS) when the default settings of the Enhanced Distributed Channel Access (EDCA) parameters are used. We discuss how the transport layer protocol characteristics alleviate the unfairness problem. We design a simple, practical, and standard-compliant framework to be employed at the Access Point (AP) for fair and efficient access provisioning. A dynamic measurement-based EDCA parameter adaptation block lies in the core of this framework. The proposed framework is unique in the sense that it considers the characteristic differences of Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) flows and the coexistence of stations with varying bandwidth or Quality-of-Service (QoS) requirements. Via simulations, we show that our solution provides short-and long-term fair access for all stations in the uplink and downlink employing TCP and UDP flows with non-uniform packet rates in a wired-wireless heterogeneous network. In the meantime, the QoS requirements of coexisting real-time flows are also maintained. The IEEE 802.11 standard [1] defines Distributed Coordination Function (DCF) as a contention-based Medium Access Control (MAC) mechanism. The 802.11e standard [2] updates the MAC layer of the former 802.11 standard for Quality-of-Service (QoS) provisioning. In particular, the Enhanced Distributed Channel Access (EDCA) function of 802.11e is a QoS enhancement of the DCF. The EDCA scheme (similarly to DCF) uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) and slotted Binary Exponential Backoff (BEB) mechanism as the basic access method. The major enhancement to support QoS is that EDCA differentiates packets using different priorities and maps them to specific Access Categories (ACs) that use separate queues at a station. Each AC i within a station (0 ≤ i ≤ 3) contends for the channel independently of the others. Levels of services are provided through different assignments of the AC-specific EDCA parameters; Contention Window (CW) sizes, Arbitration Interframe Space (AIFS) values, and Transmit Opportunity (TXOP) limits.