Reliable transport protocols such as TCP are tuned to perform well in traditional networks where packet losses occur mostly because of congestion. However, networks with wireless and other lossy links also suffer from significant losses due to bit errors and handoffs. TCP responds to all losses by invoking congestion control and avoidance algorithms, resulting in degraded end-to-end performance in wireless and lossy systems. In this paper, we compare several schemes designed to improve the performance of TCP in such networks. We classify these schemes into three broad categories: end-to-end protocols, where loss recovery is performed by the sender; link-layer protocols, that provide local reliability; and split-connection protocols, that break the end-to-end connection into two parts at the base station. We present the results of several experiments performed in both LAN and WAN environments, using throughput and goodput as the metrics for comparison. Our results show that a reliable link-layer protocol that is TCP-aware provides very good performance. Furthermore, it is possible to achieve good performance without splitting the end-to-end connection at the base station. We also demonstrate that selective acknowledgments and explicit loss notifications result in significant performance improvements.

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There is general belief that networks based on wireless technologies have much higher error rates than those based on more traditional technologies such as optical fiber, coaxial cable, or twisted pair wiring. This difference has motivated research on new protocol suites specifically for wireless networks. While the error characteristics of wired networks have been well documented, less experimental data is available for wireless LANs. In this

This paper describes the design and performance of a link-layer protocol for indoor and outdoor wireless networks. The protocol is asymmetric to reduce the processing load at the mobile, reliability is established by a combination of automatic repeat request and forward error correction, and link-layer packets are transferred appropriately during handoffs. The protocol is named AIRMAIL (AsymmetrIc Reliable Mobile Access In Link-layer). The asymmetry is needed in the design because the mobile terminals have limited power and smaller processing capability than the base stations. The key ideas in the asymmetric protocol design consist of placing bulk of the intelligence in the base station as opposed to placing it symmetrically, in requiring the mobile terminal to combine several acknowledgments into a single acknowledgment to conserve power, and in designing the base stations to send periodic status messages, while making the acknowledgment from the mobile terminal event-driven. The asym...

This article discusses the problems arising when the TCP/IP protocol suite is used to provide Internet connectivity over existing and emerging wireless links. Due to the strong drive towards wireless Internet access through mobile terminals, these problems must be carefully studied in order to build improved systems. We review wireless link characteristics using Wireless LANs and Cellular Communications systems as examples. We then outline the performance problems of the TCP/IP protocol suite when employed over those links, such as degraded TCP performance due to mistaking wireless errors for congestion. We present various proposals for solving these problems and examine their benefits and limitations. Finally, we consider the future evolution of wireless systems and the challenges that emerging systems will impose on the Internet protocol suite.

Wireless links can exhibit high error rates due to attenuation, fading, or interfering active radiation sources. To make matters worse, error rates can be highly variable due to changes in the wireless environment. Researchers and developers have explored a wide range of solutions to optimize communication in this difficult error environment, including traditional end-to-end solutions, link-layer solutions, and solutions involving layer four processing inside the network. A significant challenge is ensuring that systems with multiple layers of error control avoid compromising performance by duplication of effort. We argue

It is well-known that TCP performance may degrade over paths that include wireless links, where packet losses are often not related to congestion. We examine this problem in the context of the GSM digital cellular network, where the wireless link is protected by a reliable link layer protocol. We propose the use of multi-layer tracing as a powerful methodology to analyze the complex protocol interactions between the layers. Our measurements show that TCP throughput over GSM is mostly ideal and that spurious timeouts are extremely rare. The multi-layer tracing tool we developed allowed us to identify the primary causes of degraded performance: (1) inefficient interactions with TCP/IP header compression, and (2) excessive queuing caused by overbuffered links. We conclude that link layer solutions alone can solve the problem of “TCP over wireless links”. We further argue that it is imperative to deploy active queue management and explicit congestion notification mechanisms in wide-area wireless networks; which we expect will be the bottleneck in a future Internet.

this paper we evaluate error control strategies for a wireless LAN. Based on low-level packet traces of WaveLAN, we first show that forward error correction (FEC) is effective in recovering from bit corruptions and that packet length adjustment can reduce packet truncation. However, as expected, fixed error control policies can perform very poorly, because they either introduce too much overhead in "good" environments or are not aggressive enough in "bad" environments. We address this problem through adaptive error control, i.e., error control policies that adapt the degree of FEC redundancy and the packet size to the environment. The effectiveness of adaptive error control depends on the characteristics of the error environment, e.g., the type of errors and the frequency with which the error environment changes. Our evaluation shows that adaptive error control can improve throughput consistently across a wide range of wireless LAN error environments. The reason for this effectiveness is that changes in the error environment are often caused by human mobility-related events such as the motion of a cordless phone, which take place over seconds, while adaptation protocols can respond in tens of milliseconds. Evaluating adaptive error control in a wireless environment is challenging because repeatable experiments are difficult: the wireless environment cannot easily be isolated and the adaptation process itself changes the environment, which may make trace-based evaluation difficult. We introduce a trace-based evaluation methodology that deals appropriately with changes in packet content and size.

This article discusses the problems that arise when standard Internet protocols such as TCP are used over wireless links. We review wireless link characteristics with case studies drawn from commercial Wireless LANs and Cellular Telephony systems. We discuss problems with Internet protocols when employed over these systems, such as degraded TCP performance when wireless errors are interpreted as congestion losses. We survey various proposed approaches to mitigating such problems and examine their applicability. Finally, we look at the future of wireless systems and the new challenges that they will create for Internet protocols, and state some goals for further protocol enhancement and evolution, pointing out the need for better protocol integration across layers. 1 Introduction The Internet has historically expanded its reach over new communications systems not long after each became available, so it is not surprising that existing and emerging wireless systems are no excepti...

We propose a progressively reliable transport protocol for delivery of delay-sensitive multimedia over Internet connections with wireless access links. The protocol, termed "Leaky" ARQ, initially permits corrupt packets to be leaked to the receiving application and then uses retransmissions to progressively refine the quality of subsequent packet versions. A Web server would employ Leaky ARQ to quickly deliver a possibly corrupt first version of an image over a noisy bandlimited wireless link for immediate display by a Web browser. Later, Leaky ARQ's retransmissions would enable the browser to eventually display a cleaner image. Forwarding and displaying corrupt error-tolerant image data: (1) lowers the perceptual delay compared to fully reliable packet delivery, and (2) can be shown to produce images with lower distortion than aggressively compressed images when the delay budget only permits weak forward error correction. Leaky ARQ supports delaying of re-transmissions so that initial packet transmissions can be expedited, and cancelling of retransmissions associated with "out-of-date" data. Leaky ARQ can be parameterized to partially retransmit audio and video. We propose to implement Leaky ARQ by modifying Type-II Hybrid/"code combining" ARQ.