Optimizing the end-to-end performance of reliable flows over wireless links

Pure end-to-end error recovery fails as a general solution to optimize throughput when wireless links form parts of the end-to-end path. It can lead to decreased end-to-end throughput, an unfair load on best-effort networks, and a waste of valuable radio resources. Link layer error recovery over wireless links is essential for reliable flows to avoid these problems. We demonstrate this through an analysis of a large set of block erasure traces measured in different real-world radio environments, with both stationary and mobile hosts. Our analysis is based on a case study of the circuit-switched data service implemented in GSM. We show that the throughput on this wireless channel can be increased by using a larger (fixed) frame size for the reliable link layer protocol. This yields an improvement of up to 25% when the channel quality is good and 18% even under poor radio conditions. Our results suggest that adaptive frame length control could further increase the channel throughput. Finally, we discuss link and transport layer error control mechanisms and their interactions with end-to-end congestion control schemes. For reliable flows, we argue in favor of highly persistent error recovery and lossless handover schemes implemented at the link layer.