Common approach to voice over LTE
Leading operators, network equipment vendors and handset developers have agreed on a common method for handling voice and SMS traffic over LTE. The operators and vendors have formed the One Voice initiative to promote their preferred specifications for providing interoperable voice over LTE.
The group hopes to hand over its recommendations for adoption within industry standards bodies – and Mobile Europe has been told that discussions are in place with the GSMA and 3GPP.
Although LTE networks are likely to be primarily marketed as high speed data networks, it has become clear that they will still need to be able to carry voice and SMS traffic, and interoperate with other LTE networks.
Competing approaches
The issue for the industry is that there are competing approaches to handling voice, until now a circuit-switched technology in GSM networks, over LTE’s all-IP infrastructure. There have been three main competing approaches – CS Fallback (a system which would instruct an LTE handset to “fall back” to the GSM network when a voice call is being made/received), VoLGA (claimed by supporters as offering a transition path to full IMS that would work by packetising voice via an extra element at the voice switching site) and full IMS Telephony – using the IMS for call set-up and session control. Nokia Siemens Networks had also announced a fourth method it termed “Fast Track” voice over LTE – another way of introducing IMS-like functionality to soft switches to handle voice over LTE networks.
Now the operators and vendors have combined to list the minimum specifications required to ensure voice over LTE interworking and roaming – using 3GPP specifications for IMS. The specification include E-UTRAN, evolved packet core, user equipment and IMS core features that the group considers essential to interoperable IMS voice.
As even the 3GPP specifications included several different ways in which different functions for call set-up could be implemented, the group has recommended one specification in each instance.
The companies behind the One Voice initiative are AT&T, Orange, Telefonica, TeliaSonera, Verizon, Vodafone, Alcatel-Lucent, Ericsson, Nokia Siemens Networks, Nokia, Samsung and Sony Ericsson.
What does One Voice mean for operators without IMS?
Sandro Tavares, Converged Core Marketing Manager at Nokia Siemens Networks, said that he expected other companies to sign up to the One Voice profile once it had been adopted by industry standards forums.
He also said he thought the “weight” of the players behind the One Voice profile would make it “unlikely parallel non-standards substitute would come onto the scene.”
The specification means that the operators and vendors involved have taken it as read that operators moving to LTE and SAE in the RAN and core network, will also move to IMS in the control plane. But what of operators who cannot make the business case for IMS investment in parallel with LTE?
In that case, Tavares said, operators could migrate by using a method such as NSN’s Fast Track, which introduces IMS functionality to non-IMS elements. Tavares said that Fast Track was completely compliant with the One Voice profile.
Operators who wish to use LTE purely as a data network could still use CS Fallback, Tavares said, to handle voice and SMS traffic to LTE devices. Such an approach essentially keeps the LTE network itself for data.