This paper has two main contributions. First the extremely high latency of the GSM link is revealed which through measurements is determined to have a magnitude usually only known from satellite links. This greatly impacts the link configuration time of packet framing protocols for serial links like the point-to-point protocol (PPP). The key idea of the proposed solution-Quickstart-PPP-is to break the strict sequential order of the different signalling phases while ensuring that protocol semantics are not violated and interoperability with existing PPP implementations is preserved. As a result the link configuration delay can be completely eliminated. The second contribution uncovers a fundamental problem which is that the GSM circuit-switched data service is not capable of satisfying different reliability requirements simultaneously. More precisely it is not possible to use applications requiring the GSM link to operate in reliable mode together with loss-tolerant but delay-sensitive applications at the same time. The developed solution-Link Sniffer-suggests a mechanism that can be added to the implementation of GSM's reliable link layer protocol which allows to "sniff" on packet headers to determine the reliability mode to be used. The key advantage of this solution is that the link layer protocol itself does not have to be changed.

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.