JOURNAL OF COMPUTING, VOLUME 2, ISSUE 8, AUGUST 2010, ISSN 2151-9617 HTTPS://SITES.GOOGLE.COM/SITE/JOURNALOFCOMPUTING/ WWW.JOURNALOFCOMPUTING.ORG 13 Efficient Contention Window Control with Two-Element Array Ali Balador, Ali Movaghar, and Sam Jabbehdari Abstract—in wireless networks, the sharing channel has limited communication bandwidth. So designing efficient Medium Access Control (MAC) protocol with high performances is a major focus in distributed contention-based MAC protocol research. IEEE 802.11 MAC protocol is the most famous standard in this area. But, this standard has a problem with adopting its backoff range based on channel status. It causes some problems in throughput and fairness in a real situation. In this paper, we propose a simple algorithm that maximizes the throughput and fairness among competing nodes. We have divided nodes into four section of our backoff range. Numerical results show improvement in all performances except end to end delay. Index Terms—Backoff Algorithm, Contention Window, IEEE 802.11 DCF, Mobile Ad-Hoc Network, MAC layer ——————————  —————————— 1 INTRODUCTION He rapid development in wireless communication and the growth of mobile communication and com- puting devices like cell phones, PDAs or laptops cause Mobile Ad-Hoc Networks (MANETs) as an impor- tant part of future ubiquitous communication. MANETs are the important part of mobile communications because they are infrastructure-less, low cost and quick deployed. This technology is useful for ubiquitous environment in offices, hospitals, campuses, airports, factories and battle- field communications. In wireless networks, the channel is shared among network’s nodes. A node should compete with other nodes that have packet in their buffers to transmit. Be- cause of that, Medium Access Control (MAC) plays an important role in controlling channel access among nodes. Study group 802.11 was formed and introduced IEEE 802.11 standard. This standard provides detailed MAC and Physical layer (PHY) specification for wireless LANs. The MAC incorporates two different medium access methods: the compulsory Distributed Coordina- tion Function (DCF) and the mandatory Point Coordina- tion Function (PCF). 802.11 can operate both in DCF mode and PCF mode. Every 802.11 node should imple- ment DCF mode, which is contention-based method and supports asynchronous data transfer on a best effort ba- sis. On the other hand, the implementation of PCF is not mandatory in IEEE 802.11. [1] The DCF in IEEE 802.11 is based on a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) technique, which uses a combination of the CSMA and MACA schemes. In CSMA-based schemes, sender first senses the channel to check whether it is idle or busy. If they found channel idle, transmit their own packets. On the other hand, other nodes that could not transmit their packets should defer their own transmissions when the medium is busy. IEEE 802.11 DCF uses this routine to prevent a collision with other competing nodes.Although, collision occurs at receiving nodes. There are two known problems in wireless networks: hidden node problem and exposed node problem. [2] As illustrated in Fig. 1, node C wants to transmit to node D but mistakenly thinks that this will interfere with B’s transmission to A, so C refrains from transmitting. This problem is referred to “exposed node problem” leads to loss in efficiency. A more serious problem is known as the hidden node problem. Assume that node A is sending data to node B. a terminal C is “hidden” when it is far away from the data source A but it is close to the destination B. without the ability to detect the ongoing data transmission, C will cause a collision at B if C starts transmitting a packet. Fig. 2 shows this problem. MAC schemes are designed to overcome these problems. For preventing these problems IEEE 802.11 DCF uses a method to reserve the medium that other neighboring nodes can distinguish the transmission. IEEE 802.11 uses Request-To-Send (RTS) / Clear-To-Send (CTS) mechan- ————————————————  Ali Balador is with the Computer Engineering Department, Science and Research Branch, Islamic Azad University, Tehran, Iran.  Ali Movaghar is with the Department of Electrical and Computer Engi- neering, Sharif University of Technology, Tehran, Iran.  Sam Jabbehdari is with the Computer Engineering Department, North Tehran Branch, Islamic Azad University, Tehran, Iran. T Fig. 1. Hidden node problem © 2010 Journal of Computing Press, NY, USA, ISSN 2151-9617 http://sites.google.com/site/journalofcomputing/