IP has been extended to enable modification and termination of Wireless access networks Wired access networks Circuit mobile network WiMax IMS network Signalling Transporting Cable IP/MPLS core network IP network FIGURE 11.1 A next generation network structure. © 2009 by Taylor & Francis Group, LLC SIP and Vertical Handoffs in Heterogeneous Wireless Networks multimedia sessions as well as delivery of instant messages and handling of subscriptions to events. Based on SIP, IMS supports QoS provisioning, and service charging, as well as service integration. Due to the popularity of WLAN technologies, 3GPP has also released spec- ifications on the integration of the Universal Mobile Telecommunication Sys- tem (UMTS) and WLAN [4]. Two approaches called loose coupling and tight coupling [5] are specified. In tight coupling, WLAN access routers connect to the UMTS core network like other radio access networks (RAN) and data from WLAN are delivered to the external networks via the UMTS core net- work. In this case, the WLAN serves as an alternate RAN and the roaming between these two systems is similar to the inter-RAN roaming. In a loose coupling architecture, WLAN delivers data traffic to the Internet directly, while some signalling messages are transmitted to the IMS of UMTS. In a tight coupling architecture, both UMTS and WLAN belong to the same operator in order to allow the WLAN data to cross the UMTS core net- works. The Serving GPRS Support Node (SGSN) and Gateway GPRS Sup- port Node (GGSN) should also be configured to accommodate the heavy traffic load from the WLAN. On the contrary, the loose coupling architec- ture allows for independent traffic management of both UMTS and WLAN, which is more applicable in situations where different operators cooperate to provide service. Therefore, the loose coupling architecture is generally regarded as a mainstream integration architecture [5]. However, loose cou- pling of UMTS and WLAN also incurs some technical challenges. For example, although both networks are operated 