LONDON — The Voice over LTE via Generic Access (VoLGA) Forum has published the second version of its architecture specifications in an effort to introduce an alternative way to deliver voice services over Long Term Evolution (LTE) networks.

The group says it is now close to its goal of creating a new set of specifications that will be ready for submission to the 3rd Generation Partnership Project (3GPP) standards body by the end of the year.

The VoLGA development is at the heart of a divisive issue for mobile operators as they plan LTE deployments.

The problem is that LTE, as an all-IP mobile broadband technology, does not support legacy circuit-switched voice and SMS services, and operators are undecided about the best way to deliver these services.

Thus the 3GPP standards body has envisioned that this functionality would be solved by the use of IP Multimedia Subsystem (IMS) architecture. Unfortunately, this is taking longer to roll out than was originally envisaged.

There are three distinct ways to deliver voice services over LTE: IP Multimedia Subsystem (IMS), which, as noted, may be too immature to rely on for early LTE services; Circuit-switch (CS) fallback, in which the 2G or 3G networks are relied on for voice services; and Circuit-switch-over-packet, where the CS voice or SMS traffic is tunneled over LTE.

VoLGA is one option for the CS-over-packet approach, along with various proprietary solutions.

Some of the challenges facing the operators are outlined by Heavy Reading senior analyst Gabriel Brown in a recent column : Voice Over LTE & the 'IMS Gap'.

The Forum, comprising a group of handset and equipment vendors joined forces in March to promote the technology. The initiative was led by T-Mobile International, who, unfortunately, remains the only operator publicly to have backed the effort so far.

T-Mobile International has been arguing that LTE needs to support voice services from the start. The operator said that since LTE is supposed to give operators the most spectrally efficient networks, it would only make sense to put all traffic on that network and not rely on less efficient 3G networks. Such support, the operator argues, would also entice manufacturers to make LTE phones that will be more appealing to consumers than dongles and data cards.

And Kineto Wireless, one of the initiative's backers, pointed out in a recent blog that a lack of native SMS over LTE is set to doom even LTE dongle services because operators rely on SMS for back office, customer care, provisioning and management of HSPA-based dongle services.

The same systems need to be available for LTE dongle and data card services at launch, Kineo insits.

Other backers include heavyweights such as Alcatel-Lucent, Ericsson, Huawei, LG Electronics, Motorola, Samsung, Starent, and ZTE.

The biggest absentee on the network equipment OEMS' side is Nokia Siemens Networks, which is understood to be developing its own proprietary solution.

The big worry for the industry is that operators will adopt different approaches for voice-over-LTE services, which would threaten the industry's biggest sources of service revenue generation -- voice and SMS.

The Forum hopes to have the third stage of the specifications completed before the end of the year so that, with enough industry and operator support, VoLGA can be submitted to the 3GPP for a work item in Release 10 of the standard body's specifications work.