ding set up delay, signaling delay, call set up delay, queuing delay, message transfer delay, session setup delay, handoff delay, disruption time and throughput. The SIP-T signaling mechanism is used to facilitate the interconnection of PSTN with carrier-class VoIP network. Its major attributes are the promises for scalability, flexibility, and inter-operability with PSTN. It also provides the call control function of a Media Gateway Controller required for set up, tearing down and managing VoIP calls in carrier-class VoIP networks. The performance of the SIP-T signaling system has a direct impact in optimizing network QoS. The QoS performance attributes include the queue size, mean of queuing delay and the variance of queuing delay. To represent the SIP-T signaling system, Wu [2] assumed an M/G/1 queue with non-pre-emptive pri- ority assignment. Formulae for queuing size, queuing delay and delay variation for the non-pre-emptive priority queue from queuing theory, respectively were presented and readers are urged to check this work. The number of SIP-T signaling messages in the queue is called the queuing size and the queuing delay is the average delay of messages in the queue. Kueh [3] also evaluated the performance of SIP-based session setup over satellite-UMTS (S-UMTS) taking into account the larger propagation delay over a satellite link as well C 2009 by Taylor & Francis Group, LLC SIP Security and Quality of Service Performance as the contribution of the UMTS radio interface. In the following paragraphs we discuss similar works in more details. 22.2.2.1 Queuing Delay Performance Queuing delay is a result of queuing size and is one of the several sources of delay in SIP operations. This was analyzed by Wu and Wang [2] using imbed- ded Markov chain and semi-Markov processing while queuing delay and delay variation was analyzed using the Laplace-Stieltjes transform (LST) [4]. Queu- ing size is defined as the number of SIP-T messages in the system. Table 22.1 and Table 22.2 give the comparative views between t