IMS (IP Multimedia Subsystem) allows operators to deliver multimedia services based on IP and Session Initiation Protocol (SIP). This means a standard signaling mechanism for all IMS-based services and communication sessions. The IMS standard has been defined by the Third-Generation Partnership Project (3GPP/3GPP2) and is being embraced by other standards bodies including ETSI/TISPAN for addition of more wireline-related issues.

In essence, IMS helps bring internet applications such as presence, buddy lists and peer-to-peer communications into the telecom world. It not only allows these types of new IP-based multimedia communication services, it also enriches existing circuit-switched services. Over time, it will enable the same set of services to be available to users across different access networks and device types, with consistent look and feel, and ease of use.

IMS’s horizontal architecture lets operators move away from vertical "stovepipe" implementations of new services. This eliminates the costly and complex traditional network structure of overlapping functionality for charging, presence, group and list management, routing, operation, and maintenance and provisioning. Along with Push-to-Talk-standard services for mobile implementations, there are (or soon will be) IMS-based IP Telephony, IP Centrex and supplementary services for wireline broadband implementations. Other multimedia features such as Combinational Services, combining voice and different media streams, will follow.

IMS can also bring significant cost savings and performance improvements, by allowing the integrated delivery of multimedia communications services, the unification of fixed and mobile operations, and the creation of a single infrastructure for voice and data. Above all, IMS provides a future-proof architecture that simplifies and speeds up the service creation and provisioning processes.

The standard supports multiple access types - including GSM/GPRS/EDGE, WCDMA, CDMA2000, wireline broadband access and wireless LAN. IMS can also function in an access-aware network, so that services are adapted to users' current access network and bearer characteristics. IMS ensures that the calling and called parties' service preferences interact to provide the best common experience for each session.