The IMS was originally designed for the mobile industry to add a common session layer on top of the IP network layer to support many different applications. The applications can range from real-time call control applications to multi-media streaming applications for content services.
The IMS was standardised for the mobile industry by an organisation called 3GPP (3G Partners Program). Recently the standard has been augmented for fixed networks by ETSI TISPAN adding authentication, authorisation and accounting (AAA) and quality of service (QoS) control for fixed access networks.
The principle advantages of an IMS are:
Access agnostic: having the potential to run over fixed, mobile, WiFi and WiMax (in the future)
Intrinsically mobile: supporting nomadic users and terminal mobility
User centric: the user's profile is stored once for all of the applications he/she uses
Provides a common API to all of the applications that run on top of an IMS.
An IMS is primarily about the services and applications it supports, allowing both traditional communications services but also a new range of next generation network services. The unifying architecture of the IMS allows a more flexible environment for the development of applications.