This paper evaluates the performance of TCP over ATM by simulation studies to clarify its applicability to high-speed WANs. We compared the performance of TCP over ABR with that of TCP over UBR, and TCP over UBR with Early Packet Discard (EPD). As for TCP over UBR, TCP has all responsibilities for end-to-end performance. In this case, cell loss at the ATM layer degrades TCP performance. Optimum tuning of TCP parameters may mitigate this degradation problem, but cannot solve it. Using EPD with UBR can fairly reduce useless transmission of corrupted packets and improve TCP performance, but still have the problem on fairness. As a result, TCP over ABR was proved to be the most effective as long as it suppressed cell loss. It was also proved that, if we want to extract best performance by TCP over ABR, we need to choose TCP parameters such as window size or timer granularity, so that ABR rate control does not interact with TCP window control and retransmission control.

We investigate performance of TCP protocol over ATM networks by using a simulation technique. As the ATM layer, we consider (1) rate-based control of the ABR service class and (2) an EPD (Early Packet Discard) technique applied to the UBR service class and (3) and EPD with per-VC accounting for fairness enhancement applied to the UBR service class. In comparison, we adopt a multi-hop network model where the multiple ATM switches are interconnected. In such a network, unfairness among connections is a possible cause of the problem due to differences of the number of hops and/or the round trip times among connections. Simulation results show that the rate-based control method of ABR achieves highest throughput and best fairness in most circumstances. However, the performance of TCP over ABR is degraded once the cell loss takes place due to the inappropriate control parameter setting. To avoid this performance degradation, we investigate the appropriate parameter set suitable to TCP on ABR service. As a result, parameter tuning can improve the performance of TCP over ABR, but limited. We therefore consider TCP over ABR with EPD enhancement where the EPD technique is incorporated into ABR. We last consider the multimedia network environment, where the VBR traffic exists in the network in addition to the ABR/UBR traffic. By this, we investigate an applicability of the above observations to a more generic model. Through simulation experiments, we find that the similar results can be obtained, but it is also shown that parameters of the rate-based congestion control must be chosen carefully by taking into account the existence of VBR traffic. For this, we discuss the method to determine the appropriate control parameters.