Cross-layer aware transport protocols for wireless networks

This dissertation addresses the problem of reliable file transfer over single-hop and multi-hop shared-media wireless networks which are generally characterized by fluctuating bandwidth and error characteristics. Traditional reliable file transport protocols such as TCP assume relatively slow-varying links and were not generally designed to deal with interference problems of shared media wireless networks. The large performance gap between unreliable UDP and reliable TCP motivates the investigation of new transport protocols that might achieve significantly faster file transfer than TCP on wireless media. CLAP - a Cross Layer Aware transport Protocol has been developed as a general solution for reliable file transfer, with decoupled flow control and error control to accommodate time-varying links. Error control in CLAP was designed to minimize interference and round-trip time estimation. Flow control in the proposed transport protocol leverages MAC status information via a novel cross-layer software framework (CLF), developed to provide systematic access to intra-node and inter-node status information. Single hop evaluations, which consider an 802.11b wireless LAN with wired backhaul, were carried out using both NS2 simulations and ORBIT test-bed experiments. In time-varying, high loss scenarios, TCP shuts down operation without MAC retries, while an early CLAP version (CLAP-beta) achieves over 68% of upper-bound UDP performance. In noise-free scenarios, a "skip-ACKs" TCP modification to reduce interference achieves limited gains since TCP flow control depends on regular ACKs, while CLAP-beta approaches peak UDP performance by fully using the bandwidth available. Multi-hop evaluations with NS2 simulations consider a 3-hop primary path in a 4x4 wireless mesh over 802.11b single-channel interfaces. Occasional background flows and on-off channel noise injection produce bandwidth and error fluctuations. These simulations expose the general multi-hop wireless problem where self interference in the forward path significantly reduces end-to-end bandwidth. Increasing interference and random packet losses tend to degrade TCP performance even more significantly than in 1-hop scenarios. Here, CLAP-final with improvements (relative to CLAP-beta) to reduce dependence on RTT estimation achieves over 90% of UDP performance in a variety of time-varying conditions. This thesis demonstrates the efficacy of reliable file transfer using CLAP to address interference and time-varying links in both single- and multi-hop wireless network scenarios. Future research opportunities include cross-layer techniques for error control, efficient inter-node protocols for CLF, and tighter integration with mesh network routing protocols.Ph.D.Includes bibliographical references (p. 108-113).by Sumathi Gopa