Energy Efficiency and Capacity for TCP Traffic in Multi-Hop Wireless Networks

We study the performance metrics associated with TCP-regulated traffic in multi-hop, wireless networks that use a common physical channel (e.g., IEEE 802.11). In contrast to earlier analyses, we focus simultaneously on two key operating metrics—the energy efficiency and the transport-layer (TCP) throughput. Using analysis and simulations, we show how these metrics are strongly influenced by the radio transmission range of individual nodes. Due to tradeoffs between the individual packet transmission energy and the likelihood of retransmissions, the total energy consumption is a convex function of the number of hops (and hence, of the transmission range). On the other hand, the throughput of a single TCP session decreases with a decrease in the transmission range. The overall achievable TCP throughput in an ad-hoc network thus involves a tradeoff between the reduced throughput of an individual flow and the greater degree of spatial reuse possible. As a consequence of this tradeoff, the overall network capacity turns out to be a concave function of the transmission range. We analyze how parameters such as the node density and the radio transmission range affect the overall network capacity under different operating conditions. Our analysis shows that capacity metrics at the TCP layer behave quite differently from the capacity results previously presented in literature. We then extend the work and examine the sensitivity of the TCP-layer capacity to the speed of the nodes and the number of TCP connections in an ad hoc network. By incorporating the notion of a minimal acceptable QoS metric (loss) for an individual session, we show why the QoS-compliant capacity is a more accurate metric for studying the capacity of TCP traffic in an ad hoc network. Finally, we study the dependence of capacity on the source application (Telnet or FTP traffic) and on the choice of the ad hoc routing protocol (AODV, DSR or DSDV).