Modeling the DOCSIS 1.1/2.0 MAC protocol

We have developed a model of the Data over Cable (DOCSIS) version 1.1/2.0 MAC and physical layers using the ‘ns’ simulation package. In this paper we present the results of a performance analysis that we have conducted using the model. The main objective of our study is to examine the performance impact of several key MAC layer system parameters as traffic loads are varied. We focus on the DOCSIS best effort service. We measure the level of ACK compression experienced by a downstream TCP connection and show that even under moderate load levels DOCSIS can cause TCP acknowledgement packets to compress in the upstream direction leading to bursty (and lossy) downstream dynamics. We explore the use of downstream rate control on network performance and find that it does reduce the level of ACK compression for moderate load levels compared to an equivalent network scenario without rate control. However, as the number of active subscribers on a given channel increase, the level of ACK compression grows implying that other mechanisms should be looked at to improve performance during periods of high usage. KeywordsBroadband Access; DOCSIS; TCP Performance;