Contention Window Optimization: an enhancement to IEEE 802.11 DCF to improve Quality of Service

Wireless communication technology has gained widespread acceptance in recent years. Wireless Local Area Networks (WLANs) are becoming more and more popular and widely used, since they provide high data rates. The main characteristics of this technology are simplicity, flexibility and cost effectiveness. Recent applications include media streaming, interactive collaboration, videoconferencing and multimedia messaging. However, multimedia applications require a certain quality of service (QoS) support such as guaranteed bandwidth, reduced delay, high throughput and error rate. Guaranteeing those QoS requirements in the existing WLANs characterized by mobile stations with a low bandwidth and high rate error is a challenging task. Distributed Coordination Function (DCF) is the basic Medium Access Control (MAC) method. It is based on the Carrier Sense Multiple Access (CSMA) that works as Listen-before talk scheme. After collisions during competitive access to the channel in DCF scheme, packets must be retransmitted. These retransmissions consume enough bandwidth and increase the end-to-end delay for these packets. To address this, we propose to extend DCF with a new calculation method to increment contention window (CW) that enables each station to access the channel after a small number of attempts. Otherwise, it declares failure case early, if collision still occurs. The purpose of our scheme is to reduce delay and packet loss ratio and increase the efficiency of the transmission channel. To show the performance of the proposed solution, simulations were conducted under Network Simulator (NS2) to measure the traffic control and packet loss ratio under various constraints