The first specification of IMS is from 2003 and was created by the 3rd Generation Partnership Project committee (3GPP). The original area of application of the IP Multimedia Subsystem was the service network of the 3G UMTS mobile network. IMS is now also used in 4G LTE networks. The standard describes how multimedia connections and services are controlled by the IMS core network. Services can be provided over open interfaces via application server. The standard specifies transport, security mechanisms and signaling for this purpose. Whilst IMS was only defined for mobile networks, the standard was soon expanded to fixed-line networks.
Key requirements for a IP Multimedia Subsystem are linking classic mobile or fixed-line networks with IP-based networks and providing additional services. This includes IP-based real time services such as voice, video-telephony, machine-to-machine communication, as well as games. Thanks to cross-provider function and defined IMS standards new services and products can be established within a short amount of time. All communication between the subscribers uses packet-switched connections which are able to guarantee defined qualities of service (QoS) end-to-end. IMS allows services to operate independently from the access network and can be charged according to service.
One key component of IMS is the Session Initiation Protocol, or SIP for short, primarily known as a protocol for signaling and establishing a connection in Voice-over-IP telephony (VoIP telephony). In the IP Multimedia Subsystem, SIP governs communication between the individual network elements, as well as between the various users.