by GPRS or some other means, e.g. by DHCP, can get used for (but not limited to) the following [7]: the exchange application level signalling (e.g. registration, CC) with the serving CSCF from the access network currently used; application level registration to an IP MM CN subsystem as an address used to reach the UE; an address used to reach the UE for multimedia calls. In GPRS, we associate the terminal with an IP address when we activate the primary PDP context. This IP address used for the purpose described above can be: the IP address obtained by the UE during the activation of a primary PDP context (e.g. if the UE does not have any existing PDP context active or desires to use a dif- ferent IP address); the IP address of one of the already active PDP contexts. Figure 9.11 illustrates the order in which we execute the registration procedure and how the IP address gets allocated. IP MM CN Subsystem 1. Bearer Level Registration: GPRS 2. PDP Context Activation 3. CSCF Discovery 4. Application Level Registration Figure 9.11 Registration of the IP address. The steps performed include: 1. bearer level registration (e.g. after a MS gets switched on or upon explicit user de- mand); 2. when the PDP context gets activated, the UE has two options: activate a primary PDP context and obtain a new IP address (e.g. if the UE does not have any existing PDP context active or desires to use a different IP address), activate a secondary PDP context and re-use the IP address of one of the al- ready active PDP contexts; 3. UE performs the CSCF discovery procedure to select the CSCF to register with. The UMTS Network and Radio Access Technology The procedures can have time gaps between them, e.g. the UE may perform PDP context activation and the CSCF discovery, but not the application level registration. The UE may use the activated PDP context for other types of signalling, e.g. for CSCF discovery [7]. the UE performs application level registration by providing the IP address obtained at step 2 to the CSCF selected at step 3. In