me time, thus avoiding the transport of stale messages in the networks. Network resources are therefore used more efficiently. Comparing SIP to traditional SCTP or SIP over UDP, our proposed SIP over PR-SCTP is more effective in retransmitting lost SIP messages. Accord- ing to [34], loss recovery is faster using SCTP than SIP when SIP traffic is high. Therefore, when SIP traffic is low, loss recovery at the SIP layer is a better choice. The proposed scheme coordinates the loss recovery at both the RP-SCTP and SIP layers SO that the faster PR-SCTP loss recovery takes over if SIP loss recovery fails. This coordination also prevents duplicating retrans- missions over both layers. The effect of this coordination is that when SIP traffic is low, loss recoveries are mainly performed by SIP, whereas when SIP traffic is high, loss recoveries are mainly performed by PR-SCTP. Thus, the proposed SIP over PR-SCTP scheme is able to employ the most appropriate loss recovery mechanism in the networks. 11.4.3 Simulation Results and Discussions Our simulation experiments for SIP signalling are designed to study the per- formance of the proposed SIP over PR-SCTP scheme. In our simulations, SIP messages with an exponentially distributed inter-arrival time are passed to the transport layer. Figure 11.5 shows a simple network topology that is used to evaluate the performance differences among different schemes. Two SIP agents are connected through two routers. The overall link propagation delay is set to 45 ms, which approximates the propagation delay between two 15 ms delay 15 ms delay SIP UA SIP server 15 ms delay 15 ms delay 15 ms delay Traffic Traffic sources sources FIGURE 11.5 A simple network topology in simulations. © 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks entities located in North America and Europe. The capacities of the links in the network are chosen SO that the only bottleneck in the network is the link between two routers. We simulate scenarios where SIP traffic 