IMS: Core Network or Service Framework?

Is IMS essentially a new telecommunications core network complemented with an application layer, like the circuit-switched core network is complemented with IN application servers? Or is IMS rather a framework to support services, for which the so-called "core network" acts as a "service bus" or a "network middleware"?

In other terms, which component of IMS (core network, application layer) is the water boy of the other?

I obviously opt for the "service framework" option.

In this post I will support the idea that IMS is currently mostly considered as a new telco core network, and that this perception has a very negative impact on the ability for the industry to evaluate its potential.

The 3GPP full name of IMS is IP Multimedia Core Network Subsystem. The IMS architecture specification (TS 23.228) presents IMS as a core network and includes a few chapters about the IMS service architecture and its components. Similarly, there is no 3GPP specification that describes the specifics of the IMS Service Control (ISC) interface between the IMS core and the application layer, or even a dedicated chapter in a specification: the description of ISC-related aspects is totally embedded in the description of IMS core network procedures (TS 24.229).

Overall, if you carefully study IMS specifications (or IMS books) you can understand how the IMS service architecture works, but you have no clue what it can be used for. It is just an extension of the IMS core network.

Telco companies (both operators and suppliers) have a clear separation between technical organizations taking care of access and core networks (e.g. cellular access, xDSL access, circuit-switched core, IP core), and technical organizations taking care of applications (e.g. IN, portal, messaging, IPTV). The cultures in these organizations tend to differ a lot, as they deal with different problems, different protocols, different platforms, different innovation cycles, etc. Interactions between these organizations are usually limited and technically based on few and well-defined interfaces.

There is a technical and cultural fence between network and application organizations, and IMS fell on the network side of this fence.

Specified as a core network, IMS is "owned" by core network organizations of both vendor and operator companies. When you ask these organizations with a traditional telco core network culture what IMS is good for, the most natural answer is "voice". Then: "voice added value services".

Seen as a core network, IMS looks complex and costly, more especially when you do not know how this apparent complexity may benefit applications, and when you compare it to simpler IETF SIP architectures or P2P architectures ala Skype. Then the question is: do we really need to deploy such a complex network when lighter and cheaper alternatives can support the same service (i.e. voice)? You can find arguments for using IMS for first line telephony (e.g. security, QoS, support of regulatory services) but are they strong enough when voice revenues are steadily declining?

What about application layer organizations?

IN and Messaging organizations can see their role as re-creating pre-IMS services on top of IMS.

On the other hand, organizations that look for ideas to evolve the application layer will tend to have little interest for IMS, a new core network, and SIP, a new core network protocol. After all, who "owns" IMS in the company?

A problem with IMS is that current telco culture and organizations, directly derived from the pre-IMS circuit-switched centric network, are an obstacle to the understanding of its service potential. You can basically meet two kinds of technical people in telco companies: those who care about IMS but are unable (or simply not mandated) to perceive its potential, and those who should investigate and exploit its potential, but could not care less.

One of the most prominent IMS vendors on the market has recently made a big reorganization, which led to the definition a huge unit responsible for networks and another one responsible for "multimedia". All of this company's IMS products, both core network and application layer, are now owned by the network organization. Can you turn to this vendor to ask how IMS can change the world?

I sometimes imagine the following scenario: one day, telco operators would discover that Internet companies start to provide a new generation of communication services, much richer than those from today, and these operators would start to lament about their decision to go for IMS... without realizing that all these new services and others could have been delivered in a better way through IMS. Maybe some of the Internet companies would use SIP as the generic control protocol for these new services, and the telco industry would then underline the irony that this protocol has the same name as the one IMS was developed around.

Christophe