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October 2007 | Volume 2 / Number 5
Feature Articles

Interworking with Legacy TDM Networks

By Richard “Zippy” Grigonis
When the NGN started its ascent, one great hope of reconciling it with the TDM was SIGTRAN, an IETF working group that devised a family of protocols (IUA, M2PA, M2UA, M3UA, SUA, V5UA), and in particular SCTP (Stream Control Transmission Protocol) that could carry PSTN signaling over IP. Ironically, while SCTP is now used for datagram services that have little to do with its original purpose.

Indeed, SIGTRAN can't deal with everything, mostly because the NGN/IMS architecture doesn't specify it, says Patrick FitzGerald, Vice President of Marketing at AppTrigger ( AppTrigger offers service providers and network operators their Application Session Controller, a device housing an interesting combination of media, signaling, call control, and a family of APIs for multi-network, converged application deployments.

We talk with service providers quite a bit about TDM because interworking with it has always been a challenge for the service provider, says FitzGerald. It's been a challenge inside the core of the network. We hear about a variety of issues over and over again from service providers. We at AppTrigger focus on the interworking protocol mediation issue at the application layer. A key challenge providers have at that layer involves the IMS network build-out. The beauty part of IMS is that if fosters a unified IP/NGN/IMS infrastructure that allows you to create and deploy applications quickly, and you can monetize the services. But the providers say, 'I already have invested millions of dollars in 10 or 20 large-scale TDM/IN applications such as, say, prepaid, that currently generate RPU [Revenue Per User] from 15 or 20 million subscribers. I'm building out an IMS core network. The business case doesn't justify me buying brand-new, large-scale 20 million subscriber applications without any additional revenue from an RPU perspective. Since I don't want to buy new applications, how do I get my existing applications to interwork with my IMS build-out so I can repurpose the applications to work with both kinds of networks?' I would argue that the marketplace in general is struggling with this particular issue.

Many providers come to us with this problem, says FitzGerald, and we work with them relative to how to help them interwork - to do protocol mediation for IN-based applications such as Free Phone, into the IMS architecture. Today, in the standards, you have the IMSSF [IP Multimedia Services Switching Functions] that are very focused on 3GSM (News - Alert) and CAMEL [Customized Applications for Mobile Networks Enhanced Logic], but what's missing are all of the wireline protocols as well as additional wireless protocols, that are able to mediate and go into an all-SIP/DIAMETER environment.

AppTrigger's Vice President of Sales Engineering, Jose Deras, says, The IMS networks being built today by the large service providers are essentially 'parallel networks' to the current large-scale, basic PSTN service that they maintain today. They can't do a quick cut-over. They have to use parallel networks, which is in fact the correct thing to do. IMS networks are just starting up, which means that initially they're very small. IMS thus affects a fraction of the operators' overall subscriber base. But, simultaneously, the provider must be able to offer the same capabilities and applications that the larger user population enjoys within the IMS environment.

Ironically, some basic TDM applications are required to work with IMS which are not even money-making applications, says Deras. They're just basic network necessities. A good example is LNP [Local Number Portability]. If a portion of the user population is moving to IMS, the operator or provider must assure that those subscribers can still port their numbers, which is mandated by different regulatory bodies in specific countries. So the operator must figure out a way to bring the legacy platform that does LNP over to the IMS world and interconnect it with their SIP-based architecture. If that's not possible, then the operator needs more CapEx to replicate the existing functionality in the new IMS network, where the operator may not necessarily make more money, but the functions are a necessity.

SCIM-ing TDM Networks

Tekelec (News - Alert) ( is a network applications company that helps service providers make the huge transition to IMS networks.

Steve French, Tekelec's Manager of Product Marketing, says, Like many companies, we came up with an IMS strategy. From the beginning, we've always talked about the migration to IMS and IP. Our Eagle 5 integrated signaling system works in the core of the signaling network handling multi-protocol global signaling and real-time transaction-oriented applications. We work within TDM and also play a part in the NGN, so we naturally felt it was pretty critical to leverage whatever was out there. Of course, we are still pitching an IMS story, and we talk about our Call Session Control functionality. We've got partnerships with BEA (News - Alert) and HP. That's all part of one of our strategies for getting carriers to move to IMS.

But we see a couple of other near-term trends relating to different migration strategies, says French. One is the concept of SCIM (Service Capability Interaction Manager). We all think of SCIM being closely associated with IMS, but we're actually seeing more of a need for SCIM in the existing world, where operators are trying to do things such as CAMEL mediation and mix services, such as Local Number Portability, Prepaid, Calling Name, and so forth, using existing SCPs [Service Control Points] within the TDM network and wanting to be able to combine these services via service interaction and mediation between the different protocols to create bundles of mixed services for their customers.

Since there can be voice application servers in the NGN, an operator or service provider would want to do some interaction among those and to have them communicate with more conventional technology, too, says French. Now that you've put down a network for the NGN, you can extend the SCIM to this, and so you can conduct interactions between the TDM/IN network and the NGN network. Of course, eventually that SCIM technology will evolve and continue to move across to cover the IMS network, so you'll have some sort of interaction of the different applications and the SCPs, application servers, or whatever you want to call them, and there will be mediation between and among the different protocols and technologies.

So that's one concept of SCIM that we definitely believe is here today, says French. Companies use different terms for this, such as 'application control switches', 'service mediation', 'service interaction' and we think this is a very near-term phenomenon that allows operators to leverage their existing equipment - which is what they all want to do - and it allows them to start making some money from that existing equipment and incrementally build out the new network and start to leverage it immediately. So SCIM in general is a big topic.

Dan Bantukul, Tekelec's Director of Product Marketing, says, If you imagine the operation of a hybrid network, you first have the legacy network, and now the NGN, and then at some in the future there will be an overlay IMS network. These three networks will be coexisting within a single physical network operation. For the last few years we have concentrated our efforts on interworking these technologies. For example, between the SS7 TDM and NGN you already have softswitches that help manage connections for VoIP calls between the TDM and NGN. We're also concentrating on getting voice calls to travel between SS7 and IMS and the NGN and IMS. A critical point that people have to look at, beyond just getting the call connected, is to figure out a way for the services to move across network boundaries at will. Enhanced services functionality in an SS7 world must be accessible in an NGN or IMS network. These services must carry across or else they must be dropped and a new type of service established. The industry as a whole must look very carefully at this and what to do.

At Tekelec our strategy in transitioning to this new world focuses on the signaling information that allows operators to make happen seamless services across the network boundaries, says Bantukul. We at Tekelec have developed a unified control layer using TekSCIM to interact and mediate applications and capabilities so that value-added services can be delivered to subscribers of services, regardless of the type of access network or device.

Instead of implementing SCIM as a function in an IMS application server or as a function in a CSCF (Call Service Control Function) platform, Tekelec favors creating a standalone functional node (though not a totally independent network element) on the network call control layer. By placing this TekSCIM-powered unified control layer functionality in between the access and application layer, the technology is in an optimum location for handling service orchestration and session management by communicating with the CSCF and SIP router, or the STPs (Signal Transfer Points) in a TDM network, and in the process dealing with such disparate technologies such as GSM, NGN VoIP and IMS as well as the mediation of multiple protocols like CAMEL, BICC and SIP.

Companies such as AppTrigger and Tekelec are building Star Trek-like universal translators for the world's networks. Thanks to their efforts, the protocol babble that could have stopped IMS dead in its tracks won't be such a problem after all.

Richard Grigonis is Executive Editor of TMC's (News - Alert) IP Communications Group.

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