Since ATM is specified by ITU-T as the unique technology for the future broadband ISDN (B-ISDN), ATM has also been expected to be the underlying network of Internet. Meanwhile, a number of asymmetric technologies such as ADSL and cable modem are developed to introduce the residents to access the Internet. So, it becomes essential to study the performance of TCP traffic over ATM in such asymmetric environment. In this paper, we build up a simulation model while explicitlyconfigure the backward path. We also define the "normalized asymmetry"as the ratio of the transmission time of ACKs on the reverse path to that of packets on the forward path, which directly affects the throughput. Through the simulation study, we find that the throughput f TCP traffic is heavily deteriorated by the slow backward link when TCP is over plain UBR. However, the key reasons for the deterioration of the throughput are not only the bandwidth asymmetry but also the fragmentation problem in ATM networks. Thus, our main finding is that the Early Packet Discard (EPD) scheme used in the backward buffer can effectively improve the performance of through-put in the asymmetric environment.
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