An Analytical Framework for Path Reliabilities in Mobile Ad Hoc Networks Ash Mohammad Abbas, Bijendra Nath Jain Department of Computer Science and Engineering Indian Institute of Technology, Delhi Hauz Khas, New Delhi - 110016, India abbas, bnj@cse.iitd.ernet.in Abstract An ad hoc network is a collection of mobile nodes con- nected through multi-hop wireless links without the required intervention of any centralized access point or existing in- frastructure. In this work, we have developed an analyti- cal framework for computing the reliabilities of multi-hop paths. We have also investigated the effect of multi-path characteristics on path reliabilities. Among the main pa- rameters considered here are, average link clustering, num- ber of multiple paths, path complexity factor, and hop dif- ference factor. Our approach is unified and general in the sense that it is able to incorporate single as well as multiple paths between a given source and destination pair. 1. Introduction An ad hoc network is a multi-hop wireless network with no stationary infrastructure. A difference from "sin- gle hop"(i.e. cellular) networks, which require fixed base stations interconnected by a wired backbone, multi-hop net- works have no fixed base station nor a wired backbone. The main application for mobile wireless ad hoc networks is rapid deployment and dynamic reconfiguration in scenar- ios where the wireline network is not available or not cost effective e.g. battle field communications, search and res- cue ad hoc networking, etc. In such cases, multi-hop wire- less networks provide a feasible and cost effective means for communications among many mobile hosts. Mobile networks have many unique characteristics that make traditional routing protocols inapplicable. The topol- ogy of a mobile network is often highly dynamic due to the mobile nature of nodes. Whereas a broken link in a wired network is considered as an exception and is likely to occur relatively infrequently, links within wireless networks tend to frequently break as nodes move in and out of transmis- sion range of one another. Furthermore, atmospheric effects (such as rain) and physical objects also play a role in limit- ing the communication between wireless nodes. Additional characteristics of mobile wireless networks include limited power and bandwidth, and high error rates due to the wire- less transmission. Due to the characteristics of wireless transmission, the range of nodes is often limited. It is usually the case that path between source and destination often requires multiple hops. Hence a routing protocol must be able to find multi- hop paths between nodes. Multi-hopping poses several new challenges in the design of ad hoc routing protocols. Many researchers have focused on designing efficient routing protocols for ad hoc networks considering reduc- tion in routing overheads. Broadly, these are classified as " on-demand" protocols e.g. Dynamic Source Routing (DSR) [2], Ad hoc On-demand Distance Vector (AODV) [11], Temporally Ordered Routing Algorithm (TORA) [10]. Unlike, more traditional " proactive" protocols such as link- state or distance vector, on-demand protocols attempt to reduce the routing overheads by initiating route discovery only when a pair of nodes needs to communicate. Previous performance studies [3] have shown better overhead savings for on-demand protocols as opposed to proactive ones. There are problems with on-demand protocols. Since routes are computed only on-demand, route discovery la- tency can add to the end to end delays, unless a previously computed "cached" route is available. Buffering of data packets during the route discovery process can also con- tribute to packet losses due to buffer overflow. With single path routing, this problem becomes severe as the network becomes more dynamic. Frequency of route discovery in- creases with increase in the rate of link failures. Also, since each route discovery incurs substantial packet overheads, its frequency impacts the performance. The frequency can be controlled by computing multiple paths with a single route discovery. This will improve the overall performance. There are protocols like DSR and TORA having built in capability for computing multiple paths. But each of them suffers from a different set of performance problems. DSR uses source routing, by virtue of which it can detect Proceedings of the Eighth IEEE International Symposium on Computers and Communication (ISCC'03) COMPUTER SOCIETY