ansfer, videoconference and voice messaging. However, the growing globalization and liberalization of the telecommuni- cations market necessitates a more global IN infrastructure to satisfy multina- tional subscribers. A lot of these services are offered on current, but specialized, systems. The IN concept is to provide these services on a coherent and stable basis [2]. Some security functions have already been introduced in current sys- items, but the end-user is constrained by private lines, proprietor equipment, and software algorithms. There is a major difference between todays limited service and the original goals at the introduction of IN. IN originally offered open public access to user groups to communicate across open yet secure networks. Therefore, the security services provided on the current network will not sufficient for IN. New security functions need to be publicly usable, economi- cally feasible and insured. Implementing Intelligent Network in VoIP Applications with SIP 21.3.1 Interworking SIP and IN Applications The IN functional architecture identifies several entities such as the service switching function (SSF), the service control function (SCF), and the call control function (CCF) that model the behavior of the IN network. The CCF represents the normal activities of call and connection processing in traditional switch, for two clients. The SSF models additional functionality required for interacting with a service logic program executed by the SCF. The CCF and the SSF are generally co-located in specific switching centers, known as SSP, while the SCFs are hosted in dedicated computers known as SCP. Communi- cation between the different nodes uses the INAP protocol. Accessing IN services from VoIP network requires that one or more entities in these networks can play the role of an SSP and communicate with existing SCPs via the INAP protocol. To realize services with SIP it is important that the SIP entity is able to provide features normally provided by the traditional switch, including C 2