(interrogating) and S-(serving) CSCF. By means of the explanations in Chapter 14 the call signalling according to Figure 15.13 can be understood. Observe the straight traffic path, which does not have to pass the home networks. The next example gives further clarification. 15.10.2 TRAFFIC CASE GPRS-ISDN WITH SIP SIGNALLING AND IMS CONTROL LAYER Figure 15.14 shows a SIP call from a mobile terminal to a fixed ISDN phone. The mobile terminal is already attached to GPRS in the SGSN server. We can also assume that the terminal has performed a SIP registration to the home network, that is, a S-CSCF server has been allocated to the terminal, and data relating to the user profile has been fetched from the HSS and stored in the S-CSCF server. When the mobile user wants to initiate a SIP call while roaming in another network, the GPRS bearer must first be established. The network does this by activating a PDP context. When the GPRS tunnel has been established, a SIP message is sent to the P-CSCF server in the visited network. The P- CSCF server is able to resolve the address of the subscriber's home network and forward the SIP message. User A A's vistied network Required on A's home network registration, optional on session establish S-CSCF P-CSCF I-CSCF I-CSCF Optional Signalling traffic User traffic User B I-CSCF I-CSCF P-CSCF Required on S-CSCF registration, optional on session establish B's home network B's visited network Figure 15.13 A call between two visited areas in UMTS Interconnection Services Interrogating CSCF server Serving CSCF server Home network server Proxy CSCF server Visited network RANAP ISDN gateway GTP-C network Access ISDN, PSTN networks GTP-U networks Connectivity layer Figure 15.14 Example of a SIP call from a mobile terminal to the ISDN The SIP message enters the home network in an I-CSCF server, whose task is to find the S-CSCF server in which the subscriber is currently registered. The I-CSCF server does this by sending a query to the location service in the HSS. When it receives a reply from t