Techniques for modeling and simulating channel conditions play an essential role in understanding network protocol and application behavior. In [11], we demonstrated that inaccurate modeling using a traditional analytical model yielded significant errors in error control protocol parameters choices. In this paper, we demonstrate that time-varying effects on wireless channels result in wireless traces which exhibit non-stationary behavior over small window sizes. We then present an algorithm that divides traces into stationary components in order to provide analytical channel models that, relative to traditional approaches, more accurately represent characteristics such as burstiness, statistical distribution of errors, and packet loss processes. Our algorithm also generates artificial traces with the same statistical characteristics as actual collected network traces. For validation, we develop a channel model for the circuit-switched data service in GSM and show that it: (1) more closely approximates GSM channel characteristics than a traditional Gilbert model and (2) generates artificial traces that closely match collected traces' statistics. Using these traces in a simulator environment enables future protocol and application testing under different controlled and repeatable conditions.

The Eifel detection algorithm allows a TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily. It requires that the TCP Timestamps option defined in RFC 1323 be enabled for a connection. The Eifel detection algorithm makes use of the fact that the TCP Timestamps option eliminates the retransmission ambiguity in TCP. Based on the timestamp of the first acceptable ACK that arrives during loss recovery, it decides whether loss recovery was entered unnecessarily. The Eifel detection algorithm provides a basis for future TCP enhancements. This includes response algorithms to back out of loss recovery by restoring a TCP sender's congestion control state. Terminology

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We analyze two alternative retransmission timers for the Transmission Control Protocol (TCP). We first study the retransmission timer of TCP-Lite which is considered to be the current de facto standard for TCP implementations. After revealing four major problems of TCP-Lite's retransmission timer, we propose a new timer, named the Eifel retransmission timer , that eliminates these. The strength of our work lies in its hybrid analysis methodology. We develop models of both retransmission timers for the class of network-limited TCP bulk data transfers in steady state. Using those models, we predict the problems of TCP-Lite's retransmission timer and develop the Eifel retransmission timer. We then validate our model-based analysis through measurements in a real network that yield the same results.

We present the results of a performance evaluation of link layer error recovery over wireless links. Our analysis is based upon a case study of the circuit-switched data service implemented in GSM digital cellular networks. We collected a large set of block erasure traces in different radio environments and used a measurement-based approach to derive real-world models of the wireless link. We show that the throughput of the GSM circuit-switched data channel can be improved by up to 25 percent by increasing the (fixed) frame size of the reliable link layer protocol. Our results also suggest that adaptive frame length control could further increase the channel throughput. In general, our case study shows that pure end-to-end error recovery fails to optimize throughput when wireless links form parts of the end-to-end path. In many cases, it leads to decreased end-to-end throughput, an unfair load on a best-effort network, such as the Internet, and a waste of valuable radio resources (e.g., spectrum and transmission power). In fact, we show that link layer error recovery over wireless links is essential for reliable flows to avoid these problems.