AN EFFICIENT MULTIACCESS PROTOCOL FOR WIRELESS NETWORKS Benjamin W. Wah and Xiao Su Department of Electrical and Computer Engineering and the Coordinated Science Laboratory University of Illinois at Urbana-Champaign Urbana, IL 61801, USA E-mail: wah, xiao-su @manip.crhc.uiuc.edu URL: http://manip.crhc.uiuc.edu ABSTRACT In this paper, we propose and evaluate an efficient mul- tiaccess protocol for cell-based wireless networks. Our pro- tocol addresses the problems in existing random-access pro- tocols for wireless networks: long-term fairness as well as short-term fairness in accessing a shared channel and the detection of hidden and exposed collisions. Our proposed protocol is a limited contention protocol in which the set of contending mobiles are chosen based on a global com- mon contention window maintained by every mobile station. The contention window is adjusted based on three possible channel states: no transmission, success, and collision. We assume that the channel state at the end of each contention slot is broadcast by a base station in a control channel. We show analytically that the time interval between two succes- sive accesses to the channel by any station is geometrically distributed, and that each station has equal chance to access the channel in every contention period. This is significantly better than existing random-access protocols based on the bi- nary exponential backoff algorithm, which results in large variances in inter-access delays. Our experimental results also show that the number of contention slots to resolve col- lisions is constant on the average, independent of the number of contending stations. 1. INTRODUCTION The design of an efficient and scalable medium access con- trol (MAC) protocol is extremely important for wireless net- works, where bandwidth is a precious and scarce resource. Existing work on wireless medium access control protocols can be classified into two categories: ordered-access and random-access. Ordered-access protocols, such as token- based and polling schemes, rely on knowledge of the network configuration in order to predetermine the use of a shared channel. They are usually very efficient when the network configuration is static, requiring constant overhead to resolve the transmission order. However, they do not work well in Research supported by National Aeronautics and Space Administra- tion Grant NAG 1-613 and by National Science Foundation Grant MIP 96- 32316. Proceedings of 1998 International Symposium on Internet Technology, Taipei, Taiwan, April 1998 mobile networks in which stations can join and leave dynam- ically. For this reason, we study random-access schemes in this paper. One of the popular random-access schemes used in mo- bile networks today is DFWMAC, a CSMA/CA protocol se- lected as the IEEE 802.11 draft standard [4]. Collisions in this protocol are resolved by a binary exponential backoff algorithm, similar to that used in Ethernets. There are two problems associated with the use of the backoff algorithm. First, although the algorithm is fair in the long term so that every station has equal access on the average, it is not fair in the short term because it does not give equal access to all the stations competing for the channel. Oftentimes, a station that has just transmitted has a higher chance to access the chan- nel again in the near future. This behavior may cause large variations in inter-channel access delays, an undesirable phe- nomenon in systems wishing to provide certain qualify of service in access. Second, the protocol does not operate effi- ciently in the presence of hidden and exposed terminals [8]. The backoff counters are updated incorrectly for stations in- volved, and do not reflect the local contention level. Our proposed wireless window protocol (WWP) is a lim- ited contention protocol in which the set of contending mo- biles are chosen based on a global common contention win- dow maintained by every mobile. The contention window is adjusted based on three possible channel states: no trans- mission, success, and collision. We assume that the channel state at the end of each contention slot is broadcast by the base station in the downlink. Initially, each station gener- ates a random contention parameter between zero and one based on a uniform distribution. Each station then derives a window with the goal of isolating exactly one parameter in the window. Since all stations derive the window bound- aries using identical information and the same algorithm, the windows at all stations are synchronized. Depending on the state of contention (collision, idle, success) broadcast by the base station, stations update their windows in a synchronized fashion. Eventually, only one station is isolated in the win- dow and transmits the message to the base station, which may forward it to another mobile in the same cell. Our protocol addresses the two problems associated with DFWMAC. Our analytical and experimental results demon- strate WWP’s channel efficiency as well as its long-term and 173