 is a must in the USA premium rate information services with fixed charging. televoting universal access number A number of other IN services have been placed in 'maintenance mode'. The com- plicated IN-VPN service, with company internal telephony traffic at reduced rate as the main application, has often been replaced by special agreements with carriers. At least the successful IN services are migrating to IP based networks. The No 7 signalling system is a key enabler for IN, therefore VoIP systems must support and interoperate with SS7 in order to take advantage of what is already there. Interac- tions between IN and IP-based networks are subject to international standardization. See Section 14.9. 14.4 A SIMPLE TARGET CONTROL NETWORK SIGNALLING A target network example can look simple, if the network is homogeneous. See Figure 14.5. Control Network Service network IMS = IP multimedia subsystem SIP client IP network Figure 14.5 The future is simpler than the present convergence phase Audio & Video Coders H.323 IP v4, IP v6 Figure 14.6 Position of the SIP protocol The session initiation protocol (SIP) is a key standard in the IP telephony architecture, backed up by for example IETF and 3GPP. See Figure 14.6. H.323 is an alternative. In this book SIP gets most focus. Both SIP and H.323 are signalling and control standards for Internet telephony. H.323 is an ITU recommendation, and SIP is developed by IETF. Both protocols provide a similar set of services, but SIP is often considered simpler with less logical components. H.323 is an ITU standard. In H.323 architecture, each client belongs to a zone, and there is a gatekeeper in each zone. All the clients of a zone are registered to its gatekeeper. The gatekeeper provides address translation (allowing aliases to be used), admission control and bandwidth control. 14.4.1 SESSION INITIATION PROTOCOL (SIP) The IETF Multiparty Multimedia Session Control (MMUSIC) working group has specified an IP telephony architecture based on SIP. The two basic protocols are the session