Enhanced SIP for Reducing IMS Delay under WiFi-to-UMTS Handover Scenario

After acquiring a new Internet protocol (IP) address due to a vertical handover, IP multimedia subsystem (IMS) subscriber has to send session initiation protocol (SIP) messages for registration and session setup to keep an ongoing session. In this paper, we call this registration and session setup delay ´IMS delay.´ We show that the IMS delay is unacceptable for supporting real-time applications in vertical handover from Wireless Fidelity (WiFi) to Universal Mobile Telecommunications System (UMTS). To reduce the IMS delay in case of moving to UMTS from WiFi, we propose an enhanced SIP that makes it possible to apply make-before-break handover scheme under IMS. Using the proposed enhanced SIP, SIP messages are sent via WiFi interface before breaking WiFi connection. We define two new headers in SIP and how these new headers work. Numerical results show that the minimum IMS delay with traditional SIP is 0.609936s whilst the maximum IMS delay with enhanced SIP is 11.666ms.