ween SIP client and SIP client: classical communi- cation over IP, when the TRIP can localize the GW through the LS or DNS: 1 + 2 + 5 + 7 + 13 + 14. Communication SIP client and PSTN client: in this communication the NAPTR records with the E2U service (sip+E2U or tel+E2U) and makes the conversion SIP-PSTN or PSTN-SIP:1+2+3+4+5+7+10+ 11. To access IN services by a PSTN client: classical connection: 11 + 10 + 12 + 9. To access IN services by a SIP client: this is the contribution of this work. Figure 21.6 illustrates this access. The SIP client connects to the SCP as it is already shown. If the asked service is accepted, the IN broadcasts the request to the SIP client caller, the SIP client caller will be found with DNS or TRIP through the LS. The diagram to access IN services by SIP client is described in Figure 21.7. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Telecommunications Services Security with SIP 21.4.1 Threats of the Intelligent Network As all elements of a network can be distributed geographically and that ele- ments of system IP are totally opened, several security threats can rise and attack these different elements. The interconnection IN is the process to exe- cute a demand of service IN on the part of the user through at least two different autonomous areas. Typically, each area represents a system IN sep- arated with different legal entities and entities of resource IN. The IN service is only to be executed, if the two different areas cooperate by exchanging management, control, and services of data on the basis of a legal contract (co-contract of operation). In this competition, each area applies these clean mechanisms to provide the integrity, the availability, and the confidentiality of service user. If the two distribution operators cooperate, they have to apply the same totality of mechanisms to exchange data between their SCPs and SDPs. The main security problems posed by multimedia communications and tele- phony over IP (in an IN under IP system