SIP. IMS uses the IETF's Session Initiation Protocol for session control and signaling. Thus, any device with SIP-compatible software and an SIP address, which is a kind of uniform resource identifier, can participate in IMS sessions.
SIP establishes IP connections and specifies how to initiate, modify, or terminate an interactive user session involving multimedia.
COPS. IMS uses the IETF's Common Open Policy Service protocol to ensure quality of service, which is important for telephony and other traffic that doesn't tolerate latency. COPS enables the communication of QoS and other traffic policy information between a policy server and clients.
Diameter. IMS's home subscriber server (HSS) is the system's master database of information about subscribers, including their names and locations, services they have permission to access, and data to be used with the authentication and authorization processes.
IMS uses the IETF's Diameter protocol to let devices access the HSS and then provide the necessary authentication, authorization, and, for billable communications, accounting services, said Emerio Marchetti, service provider Sprint Nextel's senior director of network engineering operations.
Call session control functions. The three CSCFs process SIP signaling packets and provide control and routing functions for sessions. The serving CSCF handles session control services and application invocation. The interrogating CSCF is the entry point for communications in the home network. The proxy CSCF is the entry point for an outside network and is used when roaming takes place.
SIP application server. This server hosts and executes IMS-based services.
Access gateway. This element provides an interface between the wireless or wireline network used for data transmission and the IP-based network used for processing.
PSTN gateway. The public switched telephone network gateway interfaces with the traditional circuit-switched telephone network by providing signal and coding translation between IP and phone systems.
Breakout gateway control function. The BGCF offers routing functionality based on telephone numbers and is used only in communications between an IMS network and a phone in a traditional circuit-switched system.
Policy decision function. The PDF assigns bandwidth, resources, and packet priorities for sessions based on both QoS requirements and demand on the system.
Foreign and home agents. For mobile users, the foreign agent is a router that serves a device being used away from its home network. For transmissions to the device, the home agent acts as a router on the home network that tunnels packets to a temporary care-of address. The foreign agent picks up the data from this address and delivers it to the device.