Voice Service with LTE: Can you Still Hear me now?

By Michael Keeley On January 1, 2011
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There’s a lot of misinformation regarding voice service delivery on LTE networks. Voice seems trivial, but when some 3G technologies first rolled out, voice was problematic. Subscribers weren’t happy when new phones with upgraded service plans couldn’t make a phone call. In reality, the way North American operators are deploying voice is a little surprising, so please read on.

For a long time any discussion of LTE was data-centric; we hardly ever mentioned voice. In fact, we as an industry were very much unsure of how voice would eventually be delivered. Proposals came from different camps, all of them in seeming opposition to the others.

The most intuitive method, described in 3GPP 23.272, is Circuit-Switched Fallback (CSFB), which basically hands voice calls over to legacy networks. While it makes sense to use existing networks, testing showed that the call setup can take up to 7 seconds. Most subscribers would have an issue with setup times like that.

The next big idea came as an industry specification released in September, 2009. Voice over LTE using Generic Access (VoLGA) grew out of an existing technology called the Generic Access Network (GAN). Where GAN defines seamless handovers between cellular and 802.11 (WiFi) networks, VoLGA replaces WiFi with a secure tunnel on an LTE bearer. VoLGA had a lot of support, having been endorsed by such heavyweights as Alcatel-Lucent, Huawei, Deutsche Telekom and Ericsson, among others.

VoLGA seemed cost-efficient since it did not require an IMS network. However, many operators had other reasons to deploy IMS and this was a sunk cost, so VoLGA was not as cost-efficient as some had claimed.

Still, VoLGA was the only clearly-defined direction supported by industry leaders. That changed on November 5th, 2009, when a consortium including Verizon Wireless, AT&T, Alcatel-Lucent, Ericsson, Vodafone and others announced the “One Voice” initiative.

“One Voice” is neither a standard nor a specification. It is an IMS-based profile built with existing 3GPP-defined components. To seemingly seal the deal, the GSMA announced its support of “One Voice by publicly announcing the Voice over LTE (VoLTE) initiative. VoLTE is not just an endorsement of One Voice. It has already added some clarity in the area of roaming. More importantly, VoLTE is the GSMA’s commitment to further defining how voice and SMS will work in the future.

So what are the North American operators doing? Here’s a hint: what does it cost AT&T or Verizon Wireless to deploy 2G/3G service? Answer: nothing … it’s already there. So they’re using CSFB? Not quite… the latency was not acceptable. The solution is to use devices with two radios… essentially a voice phone and data terminal in a single enclosure. Of course, this isn’t as simple as it sounds. It requires massive testing to ensure that neither technology interferes with the other at any layer of the protocol stack. But it is deployed right now, and so far seems to be effective and efficient.


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