Abstract

Since the TCP protocol uses the loss of packets as an indication of network congestion, its performance degrades over wireless links, which are characterized by a high bit error rate. Different solutions have been proposed to improve the performance of TCP over wireless links, the most promising one being the use of a hybrid model at the link level combining Forward Error Correction (FEC), Automatic Repeat Request with Selective Repeat (ARQ-SR), and an in-order delivery of packets to IP. The drawback of FEC is that it consumes some extra bandwidth to transmit the redundant information. ARQ-SR consumes extra bandwidth only when packets are lost, its drawback is that it increases the round-trip time (RTT), which may deteriorate the performance of TCP. Another drawback of ARQ-SR is that a complete packet can be retransmitted to correct a small piece of errored data. We study in this paper the performance of TCP over a wireless link implementing hybrid FEC/ARQ-SQ. The study is done by simulating and modeling long-lived TCP transfers over wireless links showing Bernoulli errors. We are motivated by how to tune link-level error recovery, e.g. amount of FEC, persistency of ARQ, so as to maximize the performance of TCP. We provide results for different physical characteristics of the wireless link (delay, error rate) and for different traffic loads (number of TCP connections).

Author Keywords: Link-level hybrid; FEC/ARQ-SR; TCP traffic

Article Outline

1. Introduction

2. The model

3. Simulation study

3.1. Simulation scenarios

3.2. First set of results: link throughput for 10 connections

3.2.1. FEC alone

3.2.2. ARQ-SR alone

3.2.3. Hybrid FEC/ARQ-SR

3.3. Second set of results: link throughput for one connection

4. Analytical study

4.1. Analytical model

4.2. Computation of the packet loss rate

4.3. Computation of the maximum throughput α

4.4. Computation of the average round-trip time

4.4.1. General expression

4.4.2. Case of large bandwidth-delay product wireless networks

4.4.3. Case of medium bandwidth-delay product wireless networks

4.4.4. Correction of our model for the delay introduced by the resequencing module

4.5. Validation of the model

4.6. Optimization of the model

5. Conclusions

Appendix A. Computation of

References

Vitae

Corresponding author. Tel.: +41-21-693-6601; fax: +41-21-693-6610.

*1 The simulation results in this paper have appeared in the Proceedings of WiOpt’03: Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks, Sophia Antipolis, France, March 2003. The analytical model is an original work.

¹ This work was done while the author was doing his DEA internship in the Planète Research Group at INRIA, Sophia Antipolis.