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Abstract:

The General Packet Radio Service (GPRS) is being deployed by GSM network operators world-wide, and promises to oer users \always-on " data access at bandwidths comparable to that of conventional xed-line telephone modems. Unfortunately, many users have found the reality to be rather dierent, experiencing very disappointing performance when, for example, browsing the web over GPRS. In this paper we investigate what causes the HTTP protocol and its underlying transport TCP to underperform in a GPRS environment. We examine why certain GPRS network characteristics interact badly with TCP to yield problems such as: link under-utilization for short-lived ows, excess queueing for longlived ows, ACK compression, poor loss recovery, and gross unfairness between competing ows. We also show that many web browsers tend to be overly aggressive, and by opening too many simultaneous TCP connections can aggravate matters. We present the design and implementation of GPRSWeb { a mobile HTTP proxy system that mitigates many of the performance problems with a simple software update to a GPRS mobile device. The update is a `client proxy ' that sits in the mobile device, and communicates with a `server proxy ' located at the other end of the GPRS link close to the wired-wireless border. The dual proxy architecture collectively implements a number of key enhancements { an aggressive caching scheme that employs content-based hash keying to improve hit rates for dynamic content, a preemptive push of web page support resources to mobile clients, resource adaptation to suit client capabilities, delta encoded data transfers, DNS lookup migration, and a UDP-based reliable transport protocol that is specically optimized for use over GPRS. We show that these enhancements result in signi-cant improvement in overall WWW performance over GPRS.

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