Tuesday, April 17, 2007

Beware of IMS Service Architecture Prejudices!

It is very easy to wrongly describe the IMS service architecture as an IP replicat of the Intelligent Network architecture.

The analogy can be the following:
- SIP is a call control protocol like ISUP
- The Serving Call/Session Control Function (S-CSCF) is a softswitch
- IMS applications are the equivalent of IN Service Control Points. Actually, two of the IMS application servers types, the IM SSF (gateway to CAMEL) and the OSA/Parlay gateway (typically used as an IN extension), explicitly refer to IN.
- The application servers are invoked through triggers stored in the HSS (equivalent to the HLR). Though these triggers are called "Initial Filter Criterias" (iFCs), they include so-called Service Trigger points (STPs).
- There is an "IMS Service Control" (ISC) interface between the S-CSCF and the application server, which is the equivalent of INAP or CAP.

A first argument against this analysis is that SIP is very different from ISUP, and can support much more than voice or even session control.

A second one is that the Initial Filter Criterias that form service profiles are very different from IN triggers. These are actually generic rules that can apply to any (existing or future) SIP messages and analyze the direction of the message (did it originate from or is it addressed to the user?), the method (is it an INVITE, a SUBSCRIBE, a MESSAGE?), the existence of headers, and the value of these headers (including the possibility to use wildcards).

Finally, the most important aspect is that ISC is not a control protocol that permits an application server to instruct the S-CSCF to do this or that. ISC is just SIP, and its role is to extend SIP reachability to IMS application servers.

When the S-CSCF receives a SIP message and applies iFCs to it, the potential consequence is that this SIP message may be forwarded to one or more application servers. Similarly, when an application server generates a SIP message, the role of the S-CSCF will be to route it to another SIP entity (application server or device). In this interaction, the S-CSCF only acts as a proxy between two SIP entities. The real service control is what may happen between these two SIP entities.

Consequently, in the IMS service architecture, when you combine normal SIP routing and iFC -based routing, the S-CSCF simply acts as a SIP service router, which can support SIP-based interactions between IMS clients and IMS application servers. These interactions may relate to session control, but not necessarily.

The potential for this service architecture is huge, and I will come back on it over and over.

Christophe

56 comments:

Zebo said...

I often hear from equipement vendors that they don't want to lose supervision of call control - that in the case of IMS, call control is moving out of the "switch". Especialy those migrating from traditional solutions (switch/softswitch), that today also comprise a lot of service logic.

Any opinion?

BTW, very interesting blog!

Christophe Gourraud said...

Hi Zebo,

It is clear that equipment vendors want to keep as much control as possible. Many of them propose more intelligent CSCFs, that can host service logic or that can support more intelligent service routing than what the standard provides.

I personally think that the future of services (if any) belongs to standard and open IT, instead of being embedded in telco black boxes. For me, it is important to have a clear interface between the core network, fully standardized, efficient, but not too smart, and a very flexible and dynamic application layer, implemented outside of it.

However, there is a clear question of market segmentation here. Telco suppliers think that the more they place intelligence in the core network, the more they can control future IMS-related revenues.

Christophe

Anonymous said...

Or course there are very real similarities between IN and IMS service triggering. It is true that we don't see INAP or state machine-based tight coupling that you see between the CS switch and the SCF. However, The fact that operators controls the CSCF and therefore access to the application servers (i.e services) is a crucial and very unfortunate similarity.

IN died from, amongst other things, a lack of creativity in services brought about by too much control by operators and manufactuers who are singulalry unable to come up with interesting applications. From this point of view IMS=IN.

Stéphane Fortier said...

Christophe, this is one of the best technical blog I have seen, very well done! I'm in the process to read all your posts (starting from the oldest one so I can absorb your way of thinking), and until now, they are very informative and well written.

In my mind, the separation between the call control layer and the application layer is the most important characteristic IMS brings, detaching you from the Network Equipment Provider (Nortel, ALU, Ericsson, ...) roadmaps when it comes to bringing new apps.

This also allows to break the legacy "one service - one black box" silo approach. But there is still a lot of work to do at the app layer in order to make sure we have not just pushed the silo approach to that layer. Your UOA is a good beginning.

Good work!

Christophe Gourraud said...

Hi Stephane,

I have nothing to add. I totally agree with you.

Thanks for the nice words.

Christophe

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