Expected data rate: an accurate high-throughput path metric for multi-hop wireless routing

Abstract

We present a new metric, Expected Data Rate (EDR), for accurately finding high-throughput paths in multi- hop ad hoc wireless networks. Our metric is based upon a new model for transmission interference which is a critical factor in determining path throughput. We construct a realistic and practical transmission interference model by (1) determining transmission contention degree of each link as a function of the wireless link loss, (2) quantifying the impact of the wireless link loss on medium access backoff, and (3) considering possible concurrent transmissions when two links do not interfere with each other. Our transmission interference model also takes the non-optimality of IEEE 802.11 medium access scheduling into account. Using extensive ns-2 simulations of IEEE 802.11 ad hoc networks, we find that EDR can accurately determine the achievable data rates of ad hoc paths, thereby significantly outperforming the other existing metrics. I. INTRODUCTION Multi-hop ad hoc wireless networks provide a flexible solution to applications where wireless users, mobile or not, wish to communicate with each other without a fixed wired infrastructure. Efficient ad hoc routing protocols are necessary to extract optimal performance from these networks. There has been a lot of research on designing and developing efficient ad hoc routing protocols. Some of these efforts have extended existing wired network routing protocols to the wireless sce- nario. More recently, research on ad hoc routing has been focusing on understanding the characteristics of the shared wireless medium, and on incorporating these characteristics in determining ad hoc path performance metrics for finding best ad hoc paths. Although interesting, the existing research on finding suitable path metrics only provides a good beginning. It fails to fully address some of the fundamental properties of multi-hop ad hoc wireless networks and hence the existing path metrics are not very accurate. A simple metric for finding ad hoc paths in multi-hop wireless networks is minimum hop count. There are many existing routing protocols based on this metric including Dynamic Source Routing (DSR) (1), and Ad Hoc On-Demand Distance Vector Routing (AODV) (2). These approaches use shortest-path routing, implicitly assuming that links either work perfectly or do not work at all. They do not consider the wireless link loss. The shortcomings of shortest-path routing using only the minimum hop count metric have been investi- gated recently (3)-(5). The results of (3)-(5) consistently show that the wireless link loss must also be considered in the path metric for finding high performance ad hoc paths. In this paper, we develop a new path metric that we call Expected Data Rate or EDR in short. Our metric accurately determines the data rates of ad hoc paths in multi-hop ad hoc wireless networks. In order to develop EDR, we use an accurate understanding for transmission interference in the shared wireless medium. Transmission interference is a fun- damental property of shared wireless networks. It is a critical factor in determining ad hoc path throughput. We find that transmission interference behavior is highly dependent upon the wireless link loss rates. Existing path metrics, including even those that consider wireless link loss, do not address this dependence. Interestingly, we find that the transmission interference does not only depend upon the wireless link loss rates, but also on the ordering of link loss rates along the ad hoc path. We construct a realistic and practical transmission interference model in the context of the IEEE 802.11 medium access control protocol (6) by (1) determining transmission contention degree of each link as a function of the wireless link loss, (2) quantifying the impact of the wireless link loss on medium access backoff, and (3) considering possible concurrent transmissions when two links do not interfere with each other. Our transmission interference model also takes the non-optimality of IEEE 802.11 medium access scheduling into account. Using extensive ns-2 simulations, we find that our new path metric EDR, can fairly accurately determine the achievable data rates of ad hoc paths. Unlike existing path metrics, EDR can find best ad hoc paths in almost all cases that we study. The remainder of this paper is organized as follows. In the next section, we present the existing work. In Section III, we motivate our ideas to demonstrate the limitation of the existing metric. Section IV describes our problem setting. In Section V, we develop the transmission interference model in the presence of lossy links. Our new metric EDR is also presented in the section. Issues about incorporating EDR into existing ad hoc routing algorithms are discussed in Section VI. In Section VII we evaluate the performance of EDR and demonstrate its superior performance over the existing path metrics. We conclude our work in Section VIII.