low down and fall short of the requirements for real-time traffic. Voice cannot transmit over a radio inter- face unless the network knows which codec is used, with what port num- bers, IP addresses, and so on. Only then can the network apply techniques such as error protection and header compression to make the voice trans- fer efficient. These techniques also improve the perceived quality of voice received by a terminal beyond anything that could be delivered over the packet-switched domain. Herein lies the main reason to couple access and multimedia service provision in the cellular environment. IP Multimedia Architecture SIP is used as the principal signalling protocol within the IP multimedia domain. All 3G terminals contain a SIP User Agent (UA), and IP multimedia network nodes consist of SIP proxies. However, as we explained before, these are not actually called SIP proxies. Instead, they're referred to as Call/Session Control Functions (CSCFs). The 3GPP architecture defines three types of CSCFs, each with a differ- ent role. They are the Proxy CSCF (P-CSCF), Serving CSCF (S-CSCF), and Interrogating CSCF (I-CSCF). The P-CSCF is the point of contact between the network and the terminal. Both outgoing and incoming SIP messages traverse the P-CSCF. A P-CSCF acts as an outbound SIP proxy because all SIP requests are sent to it regardless of their final destination. The S-CSCF provides services to the user. When a terminal REGISTERs, it is associated with an S-CSCF, which provides it with services that the user is subscribed to. Both outgoing and incoming sessions will traverse the S-CSCF associated with the terminal. This way, an S-CSCF can provide services on both types of sessions. The role of the I-CSCF is to find the proper S-CSCF for a particular user. When an I-CSCF receives a request, it routes to the S-CSCF that has to Building Applications with the SIP Toolkit handle the session. For incoming sessions, the I-CSCF is the point of contact within a provider's network; it receives requests for a user with