September 2004

Migrating from TDM to IP
Real-World Business and Technology - The Mobile Persepctive

BY BROUGH TURNER

Ringback — A New Service in an Old Network
In the world of mobile telephony, new services seem to start in Asia, come to Europe and then later to the United States. At least this has happened with SMS, downloadable ringtones and camera phones. Custom ringback is a new service that’s taken off in Asia. Custom ringback allows the subscriber to choose what people calling them will hear instead of the conventional ring, ring sound — typically music, a comedy bit or sound effects.

Deployment today is with a pure IP-based data center application to support subscriber self-provisioning, but the actual service itself is delivered via IN control of traditional circuit-switched mobile switching centers or MSCs (Figure 1).

A key point here is that the Intelligent Peripheral, while it resides in the TDM and SS7 network, is an IP-based system internally, as described in more detail below. As a result, the system easily migrates as the mobile operator’s network evolves to VoIP.

A typical way to start migrating a mobile network to VoIP is to deploy a VoIP tandem network between the existing circuit-switched MSCs. Some forward-looking operators are taking this step today. A mobile network is somewhat more complex than a fixed network. The tandem softswitch in a mobile network must perform gateway MSC functions — it must communicate with the called party’s Home Location Register to determine where the subscriber is currently located. Then it must route the call to the MSC serving the subscriber’s current location.

When the ringback service architecture shown above is deployed in a packet tandem network, some labels change (e.g., SCP to Application Server; Intelligent Peripheral to Media Server) and some interfaces change (e.g., SS7 to SIP), but the application is unchanged. Also, the content files are unchanged and the subscriber data is stored in the same fashion. Even the N+1 media servers are the same but for their network interfaces which now provide VoIP over Ethernet rather than 64 kbps PCM over T1/E1 trunks.

In the longer term migration to 3GPP Release 5, the labels change again and some of the protocols change, but the investment is preserved. The key is to use a next generation IP-based services platform, even in today’s TDM mobile network.

Next Generation Services Platform
Figure 2 illustrates a high- level view of such a platform. The media services subsystem may be as simple as an audio file player for the ringback application, or it can include rich media services to support video conferencing, speech recognition and/or scripting services. The important thing is that call establishment is abstracted from the rest of the services platform.

Looking at this system in more detail, it’s scalable from separate software components in one chassis to separate blades or chassis interconnected with (usually redundant) Ethernet networks. Furthermore many of the components can be acquired from independent suppliers and/or deployed only as needed.
This model (Figure 3) works whether the gateway services are connecting to a VoIP network using SIP over Ethernet or to a TDM mobile network using separate blades for the media gateway and the SS7 signaling server. Notice that this model also supports a range of service creation possibilities, including traditional APIs, protocols, and scripting interfaces.

Conclusion
The mobile sector lags fixed telephony in embracing VoIP, in part because of the added complexity of mobile telephony, but more significantly, because there’s more payback from deploying new services than from optimizing traditional voice infrastructure. Luckily, new service platforms can leverage IP-based design concepts to deliver services into both old and new networks.

It’s quite feasible to support enhanced services for traditional telephony, softswitch-based VoIP, and SIP-based VoIP with a single architecture, even on a single platform. The key is to leverage decomposed network designs from the Internet community and abstract those elements most likely to differ, for example development environments or, more dramatically, call control.

The result is a platform that can support new services and new revenue streams immediately while preserving traditional business and traditional connectivity.

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Brough Turner is senior vice president at NMS Communications. For more information, visit the company online at www.nmscommunications.com.

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