IP Multimedia Subsystem The IP Multimedia Subsystem (IMS) is a standardised Next Generation Networking (NGN) architecture for telecom operators that want to provide mobile and fixed multimedia services. It uses a Voice-over-IP (VoIP) implementation based on a 3GPP standardised implementation of SIP, and runs over the standard Internet Protocol (IP). Existing phone systems (both packet-switched and circuit-switched) are supported. The aim of IMS is not only to provide new services but all the services, current and future, that the Internet provides. In this way, IMS will give network operators and service providers the ability to control and charge for each service. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals, IMS uses open standard IP protocols, defined by the IETF. So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols. Architecture The IP Multimedia Core Network Subsystem is a collection of different functions, linked by standardized interfaces. A function is not a node (hardware box) : an implementer is free to combine 2 functions in 1 node, or to split a single function into 2 or more nodes. Each node can also be present multiple times in a network, for load balancing or organizational issues. Access Network The user can connect to an IMS network using various methods, all of which are using the standard Internet Protocol (IP). Direct IMS terminals (mobile phones, PDAs, computers, ...), can register directly into an IMS network, even when they're roaming in another network or country (the visited network). The only requirement is that they can use IPv6 (also IPv4 in 'Early IMS') and are running SIP User Agents. Fixed access (e.g., DSL, cable modems, Ethernet, ...), mobile access (W-CDMA, CDMA2000, GSM, GPRS, ...) and wireless access (WLAN, WiMAX, ...) are all supported. Other phone systems like the POTS (the old analogue telephones), H.323 and non IMS-compatible VoIP systems are supported through gateways