ime refers to silence periods. In the case of data, the ON time refers to the download time and the OFF time refers to data reading time. When video is considered, only the ON time is necessary. To provide a means for comparing different implementations and SIP based platforms, this section provides a set of parameters and expressions. Based on these definitions, in one flow of a multimedia TCP session, the following © 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance parameters apply in defining SIP QoS performance following the example of Hassan [1]. Consider that arrival time for the traffic can be modeled as a Poisson process with the following parameters [1]: D: Session rate per second (session/sec) N: Session load (number of simultaneous sessions) T: Average mean duration of one session (sec) Average rate of one session (kbps) L: Average rate of one flow (kbps) Np: Average number of ON-OFF patterns in one session Ton: Average duration of ON period (sec) Toff: Average duration of OFF period (sec) Ron: Average rate in ON period (kbps) Q: Average file size in ON period (kb) The ON period for data transfer is a function of the data rate and the average file size used during the ON period. This is given by the expression (22.1) Several authors have modelled the ON and OFF times. The ON and OFF durations for Web and FTP sessions have Pareto distributions. Normally the download rate is a function of the quality of the network or the level of activity in the network. At some point, the download speed will be very high and at other times could grind to a very low value when the network is busy. Hence the average download rate is used. The average download rate can be calculated based on a statistical average of per minute rates accumulated over a length of time. Consider that a session consists of several ON and OFF patterns. That being the case, the average data transferred during a session is given by the product of the data transfer rate and the number of ON and OFF patterns: a=Q