Characterization of performance of TCP/IP over PPP and ATM over asymmetric links

The objective of this paper is to characterize and analyze the performance of TCP/IP when used over point-to-point protocol (PPP) and asynchronous transfer mode (ATM) over asymmetric links. This will allow us to gain insight into this new protocol architecture used in the asymmetric digital subscriber line (ADSL) access network and investigate the related quality of service (QoS) issues. Using simulation, we verified the effects of asymmetry on the performance of TCP and additional throughput degradation caused by segmentation at the ATM layer. This study was done for unidirectional and bi-directional data transfer using different traffic mixes. We also quantified the improvement in the downstream throughput obtained by delaying the TCP acknowledgements and using TCP/IP header compression. Although these techniques are effective for unidirectional TCP/IP data transfer over asymmetric links, they do not prove as effective for bi-directional traffic, and the problem further exacerbates when ATM enters the scenario. Hence, there is a need for additional enhancements in such scenarios. Further, we modified the existing protocol stack model to implement PPP encapsulation over the ATM adaptation layer (AAL5). We characterized and analyzed the effect of the additional PPP overhead on system performance in terms of throughput degradation and additional delay. We also evaluated the use of TCP/IP header compression for improving performance in presence of PPP encapsulation.