ansmission mechanism. This may not work well during vertical handoffs when the wireless links may be subject to a high packet loss rate. Therefore, we have also proposed a new scheme for transporting SIP over PR-SCTP, which chooses the most effective retransmission mechanism between the transport and application layers. Sim- ulation results have been presented to show that the proposed protocol yields excellent performance under various network conditions. References [1] Gustafsson, E., and A. Jonsson. 2003. Always best connected. IEEE Wireless Communication Magazine 10(1): 49-55. © 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks [2] 3GPP. 2005. 3rd Generation Partnership Project; Technical Specifica- tion Group Services and System Aspects; Telecommunication manage- ment; Charging management; Charging principles (Release 5), TS 32.200 version 5.8.0, March. [3] Handley, M., Schulzrinner, H., and Schooler, E. 1999. SIP: Session ini- tiation protocol. IETF RFC 2543, March. [4] 3GPP 2005. 3GPP Systems to Wireless Local Area Network (WLAN) Interworking; System Description (Release 6), 3GPP TS 23.234, March. [5] Ahmavaara, K., H. Haverinen, and R. Pichna. 2003. Interworking archi- tecture between 3GPP and WLAN systems. IEEE Communication Mag- azine 41 (11): 74-81. [6] Song, J., S. Lee, and D. Cho. 2003. Hybrid coupling scheme for UMTS and wireless LAN interworking. In Proceedings IEEE Vehicular Tech- nology Conference, Orlando, FL, 2247-51. October. [7] Perkins, C.E., 2002. IP mobility support for IPv4. RFC 3220. January 41. [8] Stewart R., and Q. Xie. 2001. Stream control transport protocol. IETF RFC2960. October. [9] Haverinen, H., and J. Malinen. 2001. Mobile IP Regional Paging. Inter- net draft, IETF, draft-haverinen-mobileip-reg-paging-00.txt. September. [10] Perkins, C.E., and D.B. Johnson. 2001. Route Optimization in Mobile IP. Internet draft, IETF, draft-ietf-mobileip-optim-11.txt, September. [11] Hsieh, R., Z.G. Zhou, and A. Seneviratne. 2003. S-MIP: A seamless 