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Voice Over LTE & the 'IMS Gap'

Mobile voice is a phenomenon. Together with SMS, it accounts for 85 percent of the world's $800 billion per year mobile services market, according to the latest estimates in the Global Mobile Data Forecast from Pyramid Research (Heavy Reading's sister company).

But for operators evaluating the investment case for Long Term Evolution (LTE) mobile broadband networks, there's concern that this next-generation technology doesn't yet adequately support their most critical and profitable circuit-switch applications: voice and SMS.

The risk is that without solving this, LTE could be delayed or compromised commercially. To discuss the challenge of how operators can support the rich, optimized set of circuit-switched services in an LTE environment, on Wednesday, July 1, I will be introducing Solving the Voice & SMS Over LTE Problem, a custom Webinar featuring Franz Seiser of T-Mobile International AG , on behalf of the VoLGA Forum.

You are more than welcome to join us! To register, click here.

Recent Heavy Reading research shows that operators, as a group, are uncertain about how to proceed on LTE voice. Broadly, there are three options:

  • IP Multimedia Subsytem (IMS) – the 3rd Generation Partnership Project (3GPP) -endorsed approach to LTE voice; virtually all the major industry players see it as the preferred outcome. The catch is the expense and immaturity of IMS technology.

  • Circuit Switch (CS) Fallback – where the mobile terminal is forced off the LTE network onto 2G/3G for voice calls. A kludgy solution that no one really loves, it acts as the lowest common denominator.

  • Circuit Switch-over-Packet – where circuit-switch domain services are offered over LTE access. There are two options: the Voice over LTE Generic Access (VoLGA) approach currently seeking support for inclusion into 3GPP, and various proprietary schemes.


Some operators see voice as a prerequisite for the LTE investment case, while others are happier to launch a data-only service and defer the decision. Even then, however, data-only services often need SMS for key functions such as SIM configuration messages.

Much will depend on timing and how operators and vendors perceive the risk of a gap developing between IMS and LTE development; and if so, what should be done to bridge it. With the industry keen to avoid fragmentation of 3GPP standards, IMS would ideally come through at a tearing pace in time for LTE. Could it be that simple?

— Gabriel Brown, Senior Analyst, Heavy Reading

clark-peng 12/5/2012 | 4:01:23 PM
re: Voice Over LTE & the 'IMS Gap' null
Clear-Q 12/5/2012 | 3:59:42 PM
re: Voice Over LTE & the 'IMS Gap'

VoLGA has one huge advantage over IMS: being motivated by commercial interests to solve a specific business problem.   Both IMS, and the LTE service architecture, have been 'developed' by career delegates to 3GPP, with precious little exposure to the grubby business of exploiting spectrum for profit.    No surprise then at IMS being built around SIP, despite the initial motivation for SIP being to connect over the IP network for bypassing phone company tolls.   And no suprise either that LTE designers, in their rush to deliver an improbable bit rate to a single user below the cell-site, should forget the 1000s of other subscribers, each paying good money for calls to their cellphone.

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