(Image Credit: iStockPhoto/Martin Dimitrov)
Most industry analysts agree that mobile operators will operate both circuit-switched and IP Multimedia Subsystem (IMS) networks concurrently for many years while rolling out LTE and introducing Voice over LTE (VoLTE). Circuit-switched networks delivering 2G and 3G services remain essential to serve users who have not yet upgraded to 4G, to fill coverage gaps where IP connectivity is weak, and to support roaming subscribers.
During this transition, operators face the challenge of providing a consistent set of communication services across all network generations—commonly called “service parity.” If operators fail to achieve parity, subscribers will have an inconsistent experience, receiving different services depending on whether they are on a 2G/3G or an LTE connection.
A legacy of services in the circuit-switched network
In a traditional circuit-switched network, the switch handles basic call-control functions such as call forwarding and call barring. Over time, however, operators added many more services and capabilities by deploying Intelligent Network (IN) platforms in the service layer. These IN platforms enabled a wide range of value-added services that complement core telephony: voicemail, call screening, number translation, routing optimization, roaming support, and many enterprise-focused features. Enterprise customers in particular often rely on dozens of such services—many networks contain fifty to one hundred enterprise-oriented IN services that are used frequently and considered valuable.
In an IMS network, the “switch” behaves like a router and does not inherently offer the call-specific handling capabilities provided by the circuit-switched switch. Equivalent call control in IMS is delivered by the Multimedia Telephony Application Server (MMTel TAS). The GSMA IR.92 VoLTE standard defines a basic set of call-handling functions that an MMTel TAS must support, but it does not address the wide range of additional services implemented on circuit-switched IN platforms. As a result, IR.92 MMTel on its own does not deliver full service parity for VoLTE users, many of whom depend on those legacy IN services when connected to the circuit-switched network.
Building from the ground up
One path to service parity is to re-implement every legacy service natively within the IMS environment. While this approach can ultimately deliver full convergence, it is time-consuming and expensive. Operators racing to migrate subscribers to IMS and to realize the benefits of spectrum refarming often cannot afford the time or upfront investment required to re-create every IN service before meaningful subscriber migration. Re-implementing all services—including enterprise-focused capabilities—under tight deadlines raises costs and complicates transformation efforts. Operators need cost-effective options that allow them to introduce VoLTE quickly while minimizing upfront spend and preserving flexibility for phased service migration.
Getting a head start
A more pragmatic approach, being adopted by some operators, is to enable IMS users to access the services already hosted in the circuit-switched network as an interim step before full native re-implementation. This can be achieved via an IMS Service Switching Function (IM-SSF), a bridging or adaptation layer between the IMS core and the circuit-switched network. The IM-SSF allows IMS-based calls to leverage the call-control intelligence and service logic of existing IN platforms, delivering a consistent user experience across network domains. The adaptation layer also works in reverse, making IMS-native services accessible to users still on legacy networks. By using this bridging strategy, operators can re-use proven service assets across both IP and circuit-switched domains and roll out enhanced VoLTE capabilities at their own pace without disrupting subscriber migration to IMS.
Operators around the world are deploying VoLTE to take advantage of improved spectral efficiency, higher voice quality, and faster call setup times. Realizing the full potential of VoLTE, however, depends on a smooth and cost-effective transition that preserves the service experience subscribers expect. Introducing an adaptation layer between the networks’ call-control elements and the service-control layer simplifies that transition today and supports future service convergence, enabling operators to protect existing investments while evolving to an all-IP future.
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