Performance of TCP over cable modems and ADSL

Abstract

There has been much interest in recent years in using asymmetric transmission systems over existing plant to provide high-speed Internet services using cable modem and Asymmetric Digital Subscriber Line (ADSL) technologies. In this paper analytic expressions for the throughput of TCP sessions on asymmetric links are presented, based on simple models. These are then generalised to account for the effect of transmission errors, and estimates made of the required error-rates in both the downstream and upstream directions. Finally some conclusions are drawn regarding the number of customers which can be supported by typical systems, given a requirement of al least 2 Mbit/s downstream transfer rate to cacti customer. It is found that the two technologies are likely to perform similarly, if the end-to-end delays are similar. However, the threshold at which packet losses become significant depends on the bandwidth-delay product, so lower delay can significantly reduce the effect of errors.