Performance of TCP over ATM with time-varying available bandwidth

Abstract

While both ABR and UBR services can be used to support data traffic in ATM, many ATM switch vendors consider UBR to be more attractive because of its low implementation cost in comparison with ABR. In this paper, we present a simulation study on the performance of TCP over ABR and UBR services in ATM networks. The motivation for our work is to understand the degree of improvement in TCP performance by using a more expensive congestion control scheme in ABR compared with a low-cost EPD mechanism in UBR. We focus our study on congested LAN environments where the available bandwidth for ABR and UBR can be time-varying. Our simulation results show the following: (1) In a single congested node and symmetric configuration, persistent TCP connections suffer some degradation in throughput fairness over UBR with EPD; (2) In a congested multi-hop configuration, highly bursty TCP connections over UBR with EPD can suffer severe degradation in throughput fairness even with a small number of active virtual connections; (3) With an effective rate-based congestion control scheme in ABR, TCP achieves good performance in terms of fairness, link utilization, and buffer requirement.