An Overlay Algorithm to Improve the Support of Multi-Hopping in the IEEE 802.11 WLANs Hanem A. Eladly Dept. of Electrical Engineering Southern Methodist University PO Box 750338 Dallas, TX, USA heladly@engr.smu.edu Thomas M. Chen Dept. of Electrical Engineering Southern Methodist University PO Box 750338 Dallas, TX, USA tchen@engr.smu.edu Abstract— Previous studies have argued that the performance of the IEEE 802.11 protocol is poor when used as a platform to implement multi-hop ad hoc networks. In this paper, we analyze the negative behavior caused by having multiple overlapping IBSSs (Independent Basic Service Sets) operating at the same frequency channel in an 802.11 ad hoc network. We propose an overlay algorithm on top of 802.11 to help discipline the interaction of overlapping IBSSs. Our algorithm yields noticeable improvement in the aggregate normalized throughput value per IBSS as well as the stability of the system. Keywords-Ad hoc; WLANs; IEEE 802.11; MANETs I. INTRODUCTION Today we see a great expansion in the production of technology to support mobile computing. Not only are the computers themselves getting more and more capable, but also many new applications are being developed and wireless data communications products are becoming available that are much improved over those available in the past. Such rapid advancement in portable computing platforms and wireless communication technology has led to significant interest in the design and development of instantly deployable, wireless networks often referred to as "ad-hoc networks". Mobile ad hoc networks (MANETs) have no fixed routers; all nodes are capable of movement and can be connected dynamically in an arbitrary manner. They have several advantages such as: • On demand setup: MANETs don't rely on wired base stations and therefore are capable of being deployed in places with no existing infrastructure. • Fault tolerance: In a cellular system, a malfunction in the base station will impair all mobiles in its cell. In MANETs, a malfunction in one node can be easily overcome through network reconfiguration. • Unconstrained connectivity: In a wired network the physical cabling is done a priori restricting the connection topology of nodes. This restriction is not present in the wireless domain, provided that two nodes are within hearing distance of each other; an instantaneous link between them is automatically formed. However, the benefits of MANETs come with some new challenges. Lack of any centralized control and possible node mobility give rise to many issues at the network, medium access and physical layers, which have no counterparts in the wired networks like the Internet, or infrastructure-based wireless networks like cellular networks. In this paper, we focus on the medium access control (MAC) issues that affect ad hoc networks. A number of standards and products that allow the development of small-scale ad hoc networks have already emerged. Wireless local area products (e.g. IEEE 802.11) are now widespread and provide in-building wireless access [1]. The IEEE 802.11 MAC protocol dominates today’s WLAN market. It is already used in almost all of the test beds and simulations for wireless ad hoc network research. The IEEE 802.11 platform, though being widely used as a platform to implement MANETs, was not designed to be used in multi-hop wireless links. It may work well in small enterprises or homes where a single hop network may exist but not in a large-scale network where multi-hopping is a necessity. Its behavior has been studied and it has been proven that it does not perform well in multi-hop networks [2][4][5][6][7]. In this paper, we are going to analyze the negative behavior caused by operating the 802.11 in a multi-hop environment. Then we are going to present an overlay algorithm that will discipline this behavior. This algorithm will essentially lay out the framework to implement a multi-hop ad hoc network on top of 802.11 that spans over the coverage area of multiple IBSSs. This paper is organized as follows; Section II analyzes the negative behavior encountered in a simple multi-hop network. In section III we propose an overlay algorithm that counteracts such behavior and finally section IV presents the simulation results. II. DIFFICULTIES IN 802.11 MULTI-HOPPING In the IEEE 802.11, an ad-hoc network is named an IBSS (independent basic service set). An IBSS enables two or more IEEE 802.11 stations to communicate directly without requiring the intervention of either a centralized access point or an infrastructure network. Hence, the IBSS can be considered as the support provided for mobile ad hoc networking by the IEEE Communications Society 0-7803-8533-0/04/$20.00 (c) 2004 IEEE 3817