 u is chosen by the user). 11.3.4 Simulations Results and Discussions Figure 11.4 shows the delay performance for two vertical handoffs managed by S-SIP and SIP individually. Here the Y axis shows the sequence numbers of packets received at the time given by the X axis. In this Figure, the MN leaves domain A at time 4 S. The bandwidth of domains A and B are 1.5 Mbps and 1 Mbps, respectively. From the Figure, we can see that the general SIP-based hard-handoff scheme causes a handoff delay of around 1.2 S, where the old connection breaks before the new connection is set up when the MN attaches to domain B after it loses the signal from domain A. The handoff delay of the general SIP scheme exceeds 1 S, which is unacceptable to real-time multi- media traffic. Comparatively, it is evident that the proposed S-SIP scheme supports seamless roaming between domains. At time 2.65 S, the MN predicts a future movement out of the current domain. So it attaches to domain B, obtains a new domain/IP address and joins the session with the new address at about time 3.85 S. Once the MN receives the packets through the new C 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks S-SIP scheme General SIP scheme Connection via old interface Connection via new interface Duplicated packets Handoff delay New connection established Activate handoff Leave domain Received time (s) FIGURE 11.4 Handoff events. interface, the MN terminates the old connection and leaves domain A at time 4 S. Virtually no packets are lost during this handoff procedure. 11.4 Efficient Transport of SIP Traffic over SCTP SIP is expected to be the dominant signalling protocol for multimedia ses- sions over both wireless and wired networks in the future. Efficient trans- port of SIP traffic becomes increasingly important. Currently, SIP traffic is mainly transported over UDP due to latency constraints. For reliability, SIP employs its own retransmission mechanisms. Recent study [30] on transport- ing SIP traffic over SCTP show