IM-SSF an architectural framework

IP Multimedia Subsystem

The IP Multimedia Subsystem (IMS) is an architectural framework for delivering internet protocol (IP) multimedia to mobile users. It was originally designed by the wireless standards body 3rd Generation Partnership Project (3GPP), and is part of the vision for evolving mobile networks beyond GSM. Its original formulation (3GPP R5) represented an approach to delivering "Internet services" over GPRS. This vision was later updated by 3GPP, 3GPP2 and TISPAN by requiring support of networks other than GPRS, such as Wireless LAN, CDMA2000 and fixed line.
To ease the integration with the Internet, IMS as far as possible uses IETF (i.e. Internet) protocols such as Session Initiation Protocol (SIP). According to the 3GPP[1], IMS is not intended to standardise applications itself but to aid the access of multimedia and voice applications across wireless and wireline terminals, i.e. aid a form of fixed mobile convergence (FMC). This is done by having a horizontal control layer that isolates the access network from the service layer. Services need not have their own control functions, as the control layer is a common horizontal layer.
Alternative and overlapping technologies for access and provision of services across wired and wireless networks depend on the actual requirements, and include combinations of Generic Access Network, soft switches and "naked" SIP. This makes the business use of IMS less appealing. It is easier to sell services than to sell the virtues of "integrated services". But, services for IMS have not been prolific.
Since IMS was conceived years ago, it is becoming increasingly easier to access content and contacts using mechanisms outside the control of traditional wireless/fixed operators, and so those operators are likely to reconsider their strategies[2]. Although it is expected that eventually IP will be available on all mobile phones and operators, it is not clear how much of the 3GPP/3GPP2/TISPAN IMS as it exists today will be deployed. "Early IMS" might be used in IMS implementations that do not yet support all "Full IMS" requirements, although it's not clearly defined what differences there might be (IPv4 support instead of IPv6 is often mentioned).