CiiT International Journal of Wireless Communication, Vol 2, No 9, September 2010 0974-9756/CIIT–IJ-0943/05/$20/$100 © 2010 CiiT Published by the Coimbatore Institute of Information Technology 276 Abstract--- Providing Quality of Service (QoS) for real time applications in mobile Ad hoc networks is a challenging task, due to the characteristics of the wireless links and networks. These Characteristics include: dynamic nature, infrastructure less architecture, and time varying unstable links and topology. Features as: low cost, ease of deployment, increased coverage, and enhanced capacity make IEEE 802.11 distributed coordination function (DCF) more popular in wireless applications. However, DCF is unsuitable for real time applications which have strict demands on QoS. IEEE 802.11e Medium Access Control (MAC) was introduced to support QoS in Wireless Local Area Networks (WLANs). Many researches have been proposed to enhance IEEE 802.11e such as Gentle Decrease, and SCW. In this paper, we propose a new approach called “Dynamic Adaptive Approach for enhancement of EDCA” (DAA_EDCA). The proposed approach was inspired from the Gentle Decrease Scheme. Keywords—Ad hoc Networks, MANET, QoS, DCF, EDCA, Gentle Decrease, SCW, DAA-EDCA. I. INTRODUCTION mobile Ad hoc network consists of a collection of mobile nodes forming a dynamic autonomous network. Nodes communicate with each other over the wireless medium without the intervention of centralized access points or base stations. Due to limited transmission range of wireless network interfaces, multiple hops may be needed to exchange data. When using of a wired or an infra structured wireless network is either impractical or expensive we need to use Mobile Ad hoc networks. Conferencing, Home Networking, Emergency Services, Sensor Fields, Military Battle Site Networks are examples of Applications, where mobile Ad hoc networks can be deployed. Nowadays, real time applications using wireless links is getting more attention. Such kind of applications demands specific characteristics on QoS, such as throughput, delay, jitter, and error rate. [1] IEEE 802.11 is the most deployed wireless MAC protocol, but it gives only best effort support for the applications. This means that IEEE 802.11 deals equally with all applications. IEEE 802.11e was introduced as an extension of IEEE 802.11 to give support to applications, which have demands on QoS. Manuscript received on August 18, 2010, review completed on September 08, 2010 and revised on September 17, 2010. Yasser A. Dahab, Hesham N. El mahdy, and Iman A. Saroit are with the Information technology Department, Faculty of computers and information, Cairo University, Cairo Egypt. (e-mail: ydahab@aswanonline.com ,ehesham@mailer.eun.eg , isaroit@fci-cu.edu.eg ). Digital Object Identifier No: WC092010004. A. IEEE 802.11 IEEE 802.11 standard defines two medium access mechanisms: the Distributed Coordination Function (DCF), and the Point Coordination Function (PCF). DCF is based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) mechanism, to transmit data. The access mechanism of the PCF is contention free central polling, where the Access Point (AP) controls all transmissions. According to DCF, a station senses the medium before a transmission of a packet. The station starts sending the packet, after sensing the medium Idle for the duration of DIFS (Distributed Interframe Space). Otherwise, the transmission is deferred and a backoff process starts. Each station stores a variable called CW (Contention Window). A backoff timer is set to the value of the CW, initially the timer is initialized with the value of CWmin. During the backoff period, the backoff timer is decremented by one for each time slot of the idle medium. When the backoff timer reaches zero, the packet is sent out. A collision occurs when two or more stations begin to transmit at the same time. Once an error occurs, the packet has to be retransmitted. The CW is incremented exponentially with each attempt to retransmit the packet. CW is reset to CWmin after the successful transmission of the packet. [3] DCF has not any mechanism to differentiate different flow types. B. IEEE 802.11e IEEE 802.11e introduces a new coordination function called Hybrid Coordination Function (HCF). HCF combines the aspects of DCF and PCF with enhanced QoS mechanisms. The Enhanced Distributed Channel Access (EDCA) is the contention based channel access mechanism of HCF. The contention free channel access mechanism of HCF is called Hcf Controlled Channel Access (HCCA). The transmission opportunity (TXOP) was introduced by IEEE 802.11e. It is defined as the time period during which a station has the right to transmit. EDCA introduces four Access Categories (ACs) for different types of data traffic. Service differentiation is achieved through assigning different parameters to each AC. The parameters are AIFS, maximum/minimum contention window, and TXOP. These parameters are referred as EDCA parameters. Frames from different types are mapped into different ACs according to the QoS of their applications. Improving the Performance of IEEE 802.11e using A Dynamic Adaptation Approach Yasser A. Dahab, Hesham N. El Mahdy and Iman A. Saroit A