 from applications and create standard charging streams that could be forwarded to billing systems. The overall security is reduced when opening up between applications and core net- works. Therefore some kind of firewall is necessary to protect the core networks from attacks from unauthorized users of the service network. All applications must therefore first become authenticated. Only authorized applications are permitted at the border. Before an application can use the network functionality made available through the service capability servers, authentication between the application and the framework is needed. After authentication, the discovery service feature enables the application to find out which network service capability features are provided by the service capability servers. The registration support service enables a service capability feature (e.g. user location) to register its existence. The discovery support service enables applications (end-user services) to identify all the service capability features that have been registered in a service capability server. 9.6.5 APPLICATION SERVICES Application services (end-user services), for example VPN, conferencing, and location- based applications, are implemented in one or more application servers. The end-user service is provided by executing the service logic on the application server. Service logic and data for end-user services reside on the application server. Application control servers provide the control for the application and application con- tent servers provide the content for the application (e.g. text, video clips etc.) 9.6.6 SUMMARY The overall architecture SO far can be presented as a stack. See Figure 9.7. 9.6.7 THE WAP/MExE/SAT LAYER The physical enablers are sometimes considered not to be SCS. Therefore they are treated as a separate layer here. The name service enabler is still correct. Two important enablers Understanding Changing Telecommunications - Building a Successful Telecom Business Applications executing on an Application 