o allow the visited domain to serve the user. Security parameters are therefore transferred to the visited network for roaming sub- scribers. This enables authentication in the visited network. Typically, the AAA server is in charge of initially authenticating a user in an access system in an administrative domain, then authorizing him to access some specific services with certain QoS charac- teristics, in accordance with his profile. See also the AAA info in the reference point in Figure 15.7. PDP context has both QoS and security tasks, such as setting up tunnels. For interworking between UMTS and WLAN, the UMTS HSS capabilities can be used for authentication in both networks. Figure 15.12 also illustrates the concept of break-out point. This is the point where the call leaves the originating network. HomePLMN network Subscription IPSec tunnel Roaming addressing hop-by-hop Re-attach Visiting PLMN Figure 15.12 Two security features: fetching subscription data and setting up a tunnel between gateways Understanding Changing Telecommunications - Building a Successful Telecom Business 15.10 SIGNALLING INTERWORKING 15.10.1 A MOBILE STATION-MOBILE STATION CALL IN UMTS Let us say that subscriber A wants to talk to subscriber B when both of them are in a visited network. Figure 15.13 depicts the concepts in an end-to-end scenario. The payload can still follow the optimal (shortest) path between visited networks to allow transmission efficiency and an optimal QoS. The signalling has to pass several routers. Now we have to go back to Chapter 14 to check the tasks of nodes or functions P-(proxy), I-(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 mobil