Supporting QoS in IEEE 802.11e wireless LANs over fading channel Marzieh Varposhti, Naser Movahhedinia * Department of Computer Engineering, University of Isfahan, Hezar-Jerib Ave., Isfahan 81744, Iran article info Article history: Received 14 May 2008 Received in revised form 17 December 2008 Accepted 21 December 2008 Available online 31 December 2008 Keywords: Wireless LAN 802.11e EDCA Quality of service abstract Transferring real-time traffic such as voice and video over wireless LAN networks (WLAN) requires strin- gent delay and jitter requirements. Recently IEEE 802.11e standard has been emerged to support QoS in WLAN. One of the methods to provide QoS in this standard is Enhanced Distributed Channel Access (EDCA) which benefits form the concept of traffic categories. However, EDCA is a contention based method; therefore it can not guarantee strict QoS required by real-time services without proper network control mechanisms. In this paper, we analyze the effect of loss and delay caused by fading channel on EDCA performance. Then, we propose a modification to the media access scheme, called CAFD (Collision Avoidance with Fading Detection) to elevate the performance against channel failures. Moreover an adjustment for the maximum number of retransmissions is proposed to maintain the delay and jitter requirements of the real-time traffic. The performances of the proposed methods are evaluated by simulations. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction One of the channel access mechanisms in IEEE 802.11 is Dis- tributed Coordination Function (DCF). DCF is based on CSMA/ CA. 1 In this mechanism, a station waits for a quiet period in wire- less media, and then begins to transmit data while detecting col- lisions. The time lapse between successive carrier senses, when channel is occupied, is given by a back-off counter which has an initial random value within a predetermined range. DCF does not support any type of priority access to the wireless medium. As a consequence, DCF provides only best-effort service, and there is no mechanism to provide better service for real-time traffic. An- other drawback against QoS provisioning is that the 802.11 MAC does not specify any admission control mechanism. This implies that under heavy traffic load the performance deteriorates in an uncontrollable manner. To address these deficiencies, the ‘‘IEEE 802.11 Task Group E” recently has proposed a new contention- based channel access method called EDCA for the IEEE 802.11e standard [1]. Many research efforts have been done to study the IEEE 802.11 DCF performance, by both of analysis and simulation. Most of them assume the ideal channel condition, which means that the packet corruptions are only due to collisions [2–4]. In [5], based on the IEEE 802.11 DCF, a novel scheme named DCF f is proposed to improve the performance of Wireless Local Area Network (WLAN) in fading channel. Several priority studies have been reported in the literature for EDCA. In [6] three approaches are proposed; first, dynamic ReAllo- cative Priority (ReAP) scheme, wherein the priorities of packets in the MAC queues are not fixed, but keep changing dynamically. It uses the laxity and the hop length information to decide the prior- ity of the packet. Second, Adaptive-TXOP 2 (A-TXOP), that involves modifying TXOP interval dynamically based on the packets in the queue. Third, TXOP-sharing, that uses TXOP to transmit to multiple receivers in order to utilize the TXOP interval completely. The method mentioned in [7] is based on the network condi- tion. It gradually lessens the contention window, instead of reset- ting the initial window value to CW min directly, to avoid continuous collisions. The authors of [8] have proposed a method called MEDCF. 3 This method adjusts the contention window dynamically according to the average collision rate, which is an indication of the traffic load. The proposed model in [9] distinguishes internal collision from external collision. It also differentiates the two cases when the backoff counter decreases, i.e. an arbitration interframe space (AIFS) period after a busy duration and a time slot after the AIFS period. The authors of [10] have proposed a three-dimensional Markov chain model for the EDCA mode, and based upon this Markov mod- el, they have computed the throughput that different traffic classes can sustain, and the distribution of the channel-access delay that each head-of-line (HOL) packet experiences when the network is heavily loaded. 0140-3664/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2008.12.034 * Corresponding author. Tel.: +98 311 7934103; fax: +98 311 7932670. E-mail address: naserm@eng.ui.ac.ir (N. Movahhedinia). 1 Carrier Sense Multiple Access with Collision Avoidance. 2 Transmission Opportunity. 3 Modified EDCF. Computer Communications 32 (2009) 985–991 Contents lists available at ScienceDirect Computer Communications journal homepage: www.elsevier.com/locate/comcom