IMS (IP multimedia subsystem) is an architecture that is deployed to deliver IP-based multimedia services.

It enables multimedia based sessions between two or more end devices (both mobile and fixed devices) and is deployed to support the introduction of more services on cellular networks in addition to the voice and sms.

More services delivered on the network (for example, VoLTE or mobile video services) needed more sessions to be initiated concurrently and IMS provides a common IP interface so that signalling, traffic, and application development are greatly simplified.

Key components of the IMS are the presence server, which initiatives a session based on user presence (as in the push-to-talk service on cellular) and the SIP protocol that establishes, manages and terminates sessions on IP networks