Operators worldwide are investing in the core network technology that enables them to deliver Voice over LTE (VoLTE) services to their customers. EE is the latest UK operator to announce its intentions of a nationwide service, with its availability chalked for later this year. Bringing VoLTE to market is a priority for operators given its benefits, offering consumers superior voice and video quality and faster set-up times. For operators it delivers increases in spectrum capacity, promising major network cost savings.
The commercial benefits of VoLTE are clear. However, grasping the opportunity relies on an efficient transition of additional voice services on the legacy circuit switch network, to the IP Multimedia Subsystem (IMS) network so that they are also available for VoLTE. This is proving extremely challenging for many operators, and it has become apparent that an evolutionary transformation of the operators’ service layer is needed.
Despite the move to LTE and the deployment of IMS for delivering communication services, operators cannot afford to dispense with their 2G and 3G network capabilities, at least for the time being. This is because a great deal of legacy services, which are still generating revenue and adding value to customers, are dependent on 2G and 3G network infrastructure. A prime example of this would be inbound roaming, which represents a substantial income for operators. This means that network operators can only retire their 2 and 3G networks at the speed of the slowest operator to adopt LTE unless they are prepared to forego the roaming revenues. This network transition therefore looks likely to take several take years to complete.
As a result, operators will be running both circuit switched networks and IMS networks concurrently to deliver their voice and video services for some considerable time. During this protracted transition period they need to ensure that their customers receive the same set of service regardless of the network they are on – 2G, 3G or 4G/LTE. Failure to do so will result in an incoherent experience for subscribers.
In a circuit switch network the switch provides basic call handling features, such as call barring and call forwarding. Over the years, operators have added new services and additional functionality to the network, developed on Intelligent Network (IN) platforms rather than on the switch. These services make up the fabric of an operators’ core service offering and are taken for granted from consumers who expect them available at all times. Such services include voicemail, number translation, least-cost routeing and mobile roaming.
Most importantly operators have developed premium revenue generating business services for the enterprise market. There is a plethora of such service variations that are widely used and valued by enterprises and consumers alike. The challenge is therefore how should an operator provide such “service parity” effectively and efficiently for both circuit-switched and IMS networks concurrently for a considerable period?
In the IMS network the ‘switch’ is just a router and does not provide any of the call handling capabilities delivered by the switch in circuit switched telephony. In an IMS Network, the model is completely different, with the Multimedia Telephony Telecom Application Server (MMTel TAS) providing the equivalent call handling functionality. The call handling capabilities of the MMTel TAS is set by the GSMA IR.92 VoLTE standard, which currently simply replicates the basic functions currently provided by a mobile network switch. It does not address any of the added services and functionality that has been developed on IN platforms on legacy networks. As a result, whilst users on LTE will have voice calling with basic features equivalent to those of 2 and 3G, they will not have any of the additional Value-added Services (VAS) provided on the IN platform.
One approach that operators are considering for service parity is to systematically re-implement all of the services on the legacy network onto the IMS network. However, this process takes considerable time and is costly; it is not feasible for operators that will be feeling the pressure to bring their VoLTE services to market and migrate their customers to the IP network before their competitors. Re-implementation of all their voice and video services prior to any significant migration of subscribers onto the IMS is a financial and commercial risk, and in turn would be an impediment to their network transformation and the promised re-farming of spectrum. Services can eventually be re-implemented but for now a more urgent, risk-averse approach is needed.
Fortunately there is another approach that some operators have identified that takes the risks out of VoLTE service parity. They are opting to evolve their service layers through the simple implementation of an IMS service switching function (IM-SFF) which sits between the legacy and core network. It enables operators to re-use the call-handling intelligence that already exists in the circuit switch IN infrastructure where needed, so that users have access to the legacy services. This reverse capability can also be adopted so that new IMS services can be made available to legacy network (2G and 3G) users. The result is that operators can re-use their legacy services while also focussing efforts on developing new VoLTE services on the IMS network.
This simple but effective method can accelerate the rollout of VoLTE without any negative effect on subscribers – they just continue to use the services as they always have. We can expect many operators to start looking at service layer evolution as a fundamental part of a successful and smooth VoLTE deployment.