ervices as well as the multimedia services. SIP is the signaling protocol used between the MH or User Equipments (UE) and the IMS as well as with its internal components. As far as the SIP signal- ing is concerned, the main component of the IMS involved is the Call Session Control Function (CSCF), which is basically a SIP server. The CSCF performs a number of functions such as multimedia session control and address transla- tion (i.e., evolution of digit translation function). In addition, the CSCF must perform switching function for services, voice coder negotiation for audio com- munication, and handling the subscriber profile (analogous to the VLR). The CSCF play three roles, viz. the Proxy CSCF (P-CSCF) role, the Interrogating CSCF (I-CSCF) role and the Serving CSCF (S-CSCF) role. P-CSCF is the mobile's first point of contact with the IMS network; I-CSCF is responsible for selecting the appropriate S-CSCF based on load or capability; and S-CSCF is responsible for mobile session management. The other access network technology considered is IEEE 802.11-based WLAN that consists of several access points (AP) providing the radio access to the MH. The APs are connected to the backbone IP network with an eth- ernet switch. A Dynamic Host Configuration Protocol (DHCP) [10] server is used to assign an IP address to a visiting MH. We assume that an MH moving between a UMTS network and WLAN has separate network interfaces to connect to these networks. The MH, after moving to a UMTS network or a WLAN, switches to the respective interface in order to attach to the corresponding access network infrastructures. The switching-over operation is initiated by the reception of the GPRS pilot signal in a UMTS network and the characteristics beacon in a WLAN. 1) In-session or Mid-call handoff with SIP: For pre-call mobility, as des- cribed before, the MH re-registers its new IP address with its "home" by sending a REGISTER message, while for mid-call mobility, the terminal needs to intimate the correspondent host (CH) or the host 