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October 2009 | Volume 12 / Number 10
Feature Story

Do the Math: How IMS Figures in to LTE

By: Paula Bernier (News - Alert)

Large incumbent telephone companies have been working to realize their vision of a single, service-independent network as opposed to multiple service-specific networks. The idea is to employ the IMS architecture to collapse multiple wireline and wireless networks while decoupling the service layer from the transport layer. That is expected to facilitate the delivery of various blended services.

The introduction of IMS into core networks on the wireline side of the house has already begun. So it makes sense to ask: Just what role will IMS play on the wireless sides of some of these same service providers as they move to 4G LTE.

As Greg Collins, vice president of the carrier IP telephony and set-top box market research effort at Dell (News - Alert)’Oro Group Inc. notes, the LTE specification is about more than just a next-generation radio interface. It also calls for something referred to as the system architecture evolution, which includes an IMS core to process phone calls and other applications like presence and location-based services.

Dell’Oro recently issued a forecast indicating that the nascent IMS equipment market is expected to generate $743 million in revenue by 2013, driven by LTE (News - Alert) deployment, application delivery and location services.

Meanwhile, Natasha Takaskar, vice president of product marketing for GENBAND (News - Alert), says many believe LTE will accelerate telcos’ move to IP-based IMS cores. In the white paper “Long-Term Evolution: The Path to Deployment,” Takaskar writes: “There are operators, especially in the Asia Pacific Region, that have already make heavy investments in IMS and will deploy it in early LTE rollouts.”


Ed Elkin, director of IMS marketing for Alcatel-Lucent (News - Alert)’s carrier business, a key LTE supplier for Verizon Wireless, notes that IMS enables massive application scaling and allows for the creation of mashups including any mix of voice, video and data traffic.

Rose Miller, director of LTE solution management with Alcatel-Lucent, adds that Verizon (News - Alert) Wireless has been working with IMS for some time and mentioned it as one of three key components – the others being the enhanced packet core and radio access network, RAN – of the Verizon Wireless LTE effort when in made its vendor announcements earlier this year.

“We first plan to use IMS to support VoIP or EVDO Rev A for applications that might require voice and data simultaneously,” Jeffrey Nelson, a spokesman for Verizon Wireless, tells Internet Telephony (News - Alert). “Initial launch of LTE will utilize the IMS core for subscriber database and authentication functions. VoIP over LTE will come later, with additional IP applications after that.

“IMS to support SMS over LTE is a launch requirement,” he adds. “While SMS over IMS needs to be supported, I expect we'll provide details around that later this year.”

One “controversy,” as Collins terms it, now swirling around the industry is how to support voice on LTE networks.

The first round of 4G rollouts – whether they are WiMAX (News - Alert)- or LTE-based – will focus on data-only services. However, despite the excitement around these new mobile broadband services, keep in mind that voice and SMS are the cash cows for mobile network operators today, so it’s not a stretch to believe that they’d like to bring such services into the LTE mix, and quickly.

Right now there appear to be three options to deliver voice services to customers subscribing to LTE-based services.

One option is to run all traffic, whether it’s voice, video or data, on the LTE network and related IMS backbone. A second choice is to set up a mechanism to enable the LTE network to hand off voice calls to 2G or 3G wireless networks. And a third possibility is to encapsulate voice so it rides over the LTE connection as a packet service.

The first option is attractive for those telcos that are early adopter types and are comfortable with the ample investment and high learning curve IMS can entail. But in light of economic realities, it seems realistic to believe that the majority of wireless providers are more likely to go with one of the other two options.

The option in which the call is handed off to a GSM or UMTS network is commonly referred to as the fallback method. In this scenario, put forward by the Third Generation Partnership Project, which is called 3GPP, the LTE network acts like a 3G SGSN, which means operators can reuse existing servers both for 4G requirements and for the 4G-to-2G/3G handover.

The choice is known as 3GPP TR 23.879. It preserves the voice call control, provisioning and charging procedures found in existing wireless networks, which is attractive because it allows service providers to leverage their existing – and recent – investments in 2G and 3G technology. The Voice over LTE via Generic Access, or VoLGA, forum is promoting this option, which is being moved forward by such names as Ericsson and T-Mobile (News - Alert).

Just which of these options future LTE operators like AT&T and Verizon in the U.S., and other carriers abroad, settle on in the short- and long-term is still anybody’s guess.

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