Editor's Note: Welcome to our weekly Reality Check column. We've gathered a group of visionaries and veterans in the mobile industry to give their insights into the marketplace.

Last year the issue of Voice over Long Term Evolution (VoLTE) emerged from the shadows, and after a rigorous debate, concluded in resounding fashion with a positive answer (yes, it's needed) and near unanimous support of a singular approach — it will be SIP. Until 2009, the issue of how service parity for voice and messaging would be delivered over LTE was overshadowed by speeds, spectral efficiency, device-eUTRAN interoperability and Evolved Packet Core (EPC) developments. Despite taking a back seat for some time, how and when voice would be delivered is extremely important as 70¬%-80% of all mobile service revenue comes from voice and messaging services. So, how did we get here?

UMA equipment vendors took an early lead in defining the issue, even pointing out that data-only LTE needs SMS for provisioning and authentication. The transformed UMA technology, called Voice over LTE Generic Access (VoLGA), built early mindshare in defining a solution reusing legacy voice infrastructure and the niche FMC technology. Today, T-Mobile remains the lone service provider backer for VoLGA. Other service providers using UMA for FMC were noticeably absent.

Another option, pushed by 3GPP, called circuit-switched fallback (CSFB), used some initial signaling over the LTE RAN, actually “fell back” to the 2G/3G TDM RAN to establish calls. Many service providers were wary of this approach with a grudging acceptance that this was a solution for non-LTE coverage areas.

During the middle part of the year both Nokia Siemens Networks and Mavenir Systems provided alternative hybrid legacy mobile switching center (MSC)-SIP approaches. This MSC VoIP approach leveraged existing circuit MSCs, but used SIP to deliver the voice to LTE connected devices (or any device on a fixed or mobile IP broadband access network).

The debate came to end in November with the One Voice Initiative from AT&T, Telefónica, Verizon Wireless, Vodafone, and other leading mobile service providers around the globe. While, it's great that vendors advocated for SIP — we all want to sell more equipment — but service providers are the ones that vote with their checkbooks for the architecture of the future. The supporting operators represent over 500 million subscribers and they unequivocally stated that when voice and messaging is delivered over their LTE RANs, it will be SIP-based; it will be IMS. Other all solutions are transitional or non-starters.

The One Voice press release stated: “Open collaborative discussions have concluded that the IP Multimedia Subsystem (IMS) based solution, as defined by 3GPP, is the most applicable approach to meeting the consumers' expectations for service quality, reliability and availability when moving from existing circuit switched telephony services to IP-based LTE services. This approach will also open the path to service convergence, as IMS is able to simultaneously serve broadband wireline and LTE wireless networks.”

Clearly, it's not just about existing voice service parity, but also about access convergence and the multimedia communications — including RCS that can be delivered today — that makes IMS compelling.

Furthering this development in 2010, the GSMA announced this week that it has adopted the work of the One Voice Initiative to drive the global mobile industry towards a standard way of delivering voice and messaging services for LTE using IMS.

As an aside, I've always been perplexed by this incongruity. 3GPP, the standards group for the mobile service providers, defined the next-generation radio access network in LTE. Several years ago, this same standards group also defined the next-generation service delivery architecture that is access dependent and uses a common framework for all communication services: IMS. Yet, the two architectures did not have a speedy marriage. It seemed as if the LTE camp refused to acknowledge the existence of IMS in the same room, despite being put there by the same match maker.

Of course, there is some work to be done to further enhance SIP-based architectures for the era of ubiquitous mobile broadband, such as defining interconnection and international roaming. With the new VoLTE initiative, the GSMA now opens this work to its membership (820+ operators and 220+ vendors) to define protocols needed for LTE voice connectivity. But now, the industry can collectively move forward and work out the when of VoLTE. In fact, Cox and Telefónica have already tested SIP-based voice calls over LTE.

On the question of when, I believe that mobile service providers are putting themselves at risk by not offering voice with their initial LTE “data” offering because over-the-top (OTP) providers will be all over their LTE subscribers. As I said in my last Reality Check column, as 3G speeds continue to increase and 4G is deployed, mobile service providers face a new challenge: how can they avoid becoming a dumb pipe to the richness of the Internet. This is the same challenge that fixed line operators have faced since the advent of broadband. By bundling multiple applications and services over the new mobile broadband connection, mobile service providers can help accelerate the acceptance of LTE, thereby shifting all traffic to IP as rapidly as possible, and ultimately reducing the demand on the circuit-switched RAN.

True, phones with LTE radios will not be available until the 2012 - 2013 time frame, but LTE providers can offer voice softclients on laptops at the outset. If they do not, Skype, Google and other OTP ASPs will swoop in, leaving LTE providers to lose triple/quadruple play revenue from subscribers. This is not just about “me-too” VoIP. Through owning the access infrastructure and EPC along with IMS/SIP-based services core, mobile service providers can differentiate themselves from OTP ASPs with the likes of wideband voice codecs for HD voice, interactive video with QoS and Rich Communication Suite services such as integrated presence and diverse messaging.

The opportunity for differentiation is now. On the eve of LTE deployments, service providers must think beyond the race to have the first 4G RAN. They need to think about driving subscriber ARPU and reducing churn. If 2009 was the beginning and end of the VoLTE debate, and service providers are already building their LTE networks, 2010 needs to be about picking a date for when the call of SIP VoLTE will be heard. The new GSMA VoLTE initiative is the first step in getting us there.

Kevin Mitchell is Director, Solutions Marketing at Acme Packet where he leads wireless solution marketing focusing on wireless access, interconnect and core session routing applications of Acme Packet's Net-Net product family. In addition to developing marketing and event strategy, Kevin serves as the chief spokesperson for Acme Packet's solutions for femtocells, fixed mobile convergence, RCS, 4G voice, VoIP peering and core session routing. Kevin joined Acme Packet in 2005 after eight years with Infonetics Research where he was most recently a Principal Analyst, serving as an overall company director and lead analyst responsible for consulting, analysis, product development and overall strategic direction of multiple technology coverage areas.

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