Packet Size Distribution: An Aside?

For multimedia traffic like VBR video, knowledge of the average loss probability is not sufficient to determine the impact of loss on the perceived visual quality and on the possible ways of improving it, for example by forward error correction (FEC) and error concealment. In this paper we investigate how the packet size distribution affects the packet loss process, the distribution of the number of packets lost in a block of packets and the related FEC performance. We present an exact mathematical model for the loss process of an MMPP + M/Er/1/K queue and compare the results of the model to simulations performed with various other packet size distributions (PSDs), among others, the measured PSD from an Internet backbone. We conclude that the packet size distribution affects the packet loss process and thus the efficiency of FEC. This conclusion is mainly valid in access networks where a single multimedia stream might affect the multiplexing behavior. The results show that analytical models of the PSD matching the first three moments (mean,variance and skewness) of the empirical PSD can be used to evaluate the performance of FEC in real networks. We also conclude that the exponential PSD, though it is not a worst case scenario, is a good approximation for the PSD of today’s Internet to evaluate FEC performance.