Application-Assisted Adaptation of Real-Time Streams over Wireless Links Margaritis Margaritidis and George C. Polyzos Center for Wireless Communications and Computer Systems Laboratory Department of Computer Science and Engineering University of California, San Diego La Jolla, CA 92093-0114 Abstract – We introduce MobiWeb, a new proxy-based network architecture that enhances the performance of adaptive real-time streams over wireless Internet links. MobiWeb includes a priority scheme that preserves media smoothness despite short-term fluctuations of the link, as well as a filter- based scheme to adapt to long-term link changes. The two schemes are shielded from each other with the use of two timers which permit the continuous exploration of the link resources, increasing the utilization of resources when they are abundant, while forcing enhanced streams to back-off to more moderate resource usage when resources are scarce. MobiWeb uses admission control for real-time traffic in order to provide a base level of quality, while remaining fair and transparent to unaware, best-effort traffic. I. INTRODUCTION The design of the first and second generation mobile communication systems aimed at the transmission of voice, which made their integration with data networks a difficult task. Since then, new technologies and devices emerged, providing users the opportunity to have decent access to the Internet. However, support for interactive, real-time continuous media is lacking. The performance of real-time applications appears to be significantly impacted by wireless channel impairments, much more so than that of other Internet traffic. Real-time continuous media demand significant amounts of bandwidth and pose rather tight delay and delay jitter requirements. These demands are difficult to satisfy because of the frequent and unpredictable fluctuations in the quality of wireless links. Even though continuous media can cope with occasional infrequent transmission errors, their performance can be devastated by bursts of them. In addition, wireless mobile communication is often subject to handoffs as users move among cells. The different characteristics of the channel after the handoff might cause contention between the newly arrived streams brought to this cell (because of the mobile’s movement) and the preexisting traffic. The traditional Internet protocols lack appropriate protection mechanisms and will probably lead to all applications suffering performance degradation or backing off (the ones using TCP), which will mostly disrupt the real-time ones. MobiWeb introduces a new solution to the above problems by means of an inter-stream priority scheme, to address the short term fluctuations of the wireless link, and by supporting adaptive applications, which can gradually adjust their traffic and performance to the long-term changes of the channel quality. The priority scheme classifies packets according to the importance of the stream they belong to in the current environment. Priorities can change dynamically to reflect the relative importance of this stream, as new streams are initiated and old ones are terminated or as the focus of the users and their actions change. The adaptation interface allows applications to indicate their preferred adaptation method in the form of filters to be applied onto a stream to change its traffic (and presentation) characteristics. MobiWeb utilizes quality-of-service information provided by lower network layers (e.g., as in [1]) in order to adjust the traffic and the application expectations according to the resources that the network can provide at a certain moment. The remainder of this paper presents the MobiWeb architecture in more detail and it is organized as follows. Section II discusses the features that such a scheme must have to effectively accommodate real-time traffic over wireless channels. Section III describes the MobiWeb architecture and justifies some decisions we had to make during the design of specific features. Section IV presents related work from other important projects in this area. Finally, Section V provides a summary and conclusions. II. THE NEED FOR PRIORITIES AND ADAPTATION Traditional Internet applications, such as ftp, telnet, http etc., can be operated fairly seamlessly from a mobile host. On the other hand real-time traffic has a hard time to perform well over wireless channels with the current Internet protocols and