Understanding Voice over LTE

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Voice over LTE (VoLTE) is the ascendant method for delivering standardized voice and Short Message Service (SMS) text messages over next-generation 4G wireless broadbandLong-Term Evolution (LTE) networks.

Voice over LTE began as the One Voice Initiative, established by a select group of mobile operators and suppliers that wanted to ensure interoperable LTE voice and messaging. The One Voice creators based their LTE voice work on IP Multimedia Subsystem (IMS) specifications developed by 3rd Generation Partnership Project (3GPP), a telecom body developing and ratifying LTE standards. IMS is widely used to carry voice over IP (VoIP) in fixed-line networks. The One Voice VoLTE technical profile covers the device, the LTE access network, the Evolved Packet Core network and IMS.

Initially the One Voice Initiative was a collaboration among AT&T, Orange, Telefonica, TeliaSonera, Verizon Wireless, Vodafone, Alcatel-Lucent, Ericsson, Nokia,

 Nokia Siemens Networks, Samsung and Sony Ericsson. Its purpose was to use current open standards to define the minimum mandatory functionalities for interoperable IMS-based voice and SMS over LTE.

In early 2010, the GSM Association, a global mobile industry association, endorsed the One Voice Initiative as the method it will pursue for carrying voice over LTE. It has expanded the initiative to incorporate end-to-end requirements for seamless voice over LTE transport. In particular, GSMA is developing roaming, interconnect and customer-to-network interfaces. Building to the VoLTE standard, mobile operators, network equipment vendors and handset makers should be able to ensure compatibility of LTE voice solutions.

Why you need to know about Voice over LTE

The GSMA is the mobile industry's kingpin, with at least 80% of mobile subscribers worldwide receiving service over member networks, said Stéphane Téral, a principal analyst with Infonetics Research. That makes the GSMA's backing of the IMS-based LTE voice standard no small thing. Mobile operators now have IMS as a technology directive for supporting voice and SMS over next-generation, IP networks and will need to begin plotting their voice to LTE evolutions. Likewise, network vendors and handset suppliers have early specifications to support.

What you need to know about Voice over LTE

While more than 20 mobile services providers plan to have LTE networks up and running by the end of this year, according to the Global mobile Suppliers Association (GSA), voice is not an initial concern for most, Téral said. Rather, their motivation for deploying LTE is to offload heavy data users and relieve congestion on 3G networks. For the next five years or so, most mobile services providers will continue carrying voice and SMS over traditional 2G circuit-switched networks, leaving 4G innovation mostly for data services, he said.

Mobile operators building new LTE networks, however, will be able to support Voice over LTE from the very beginning. "If IMS is deployed at the core of the LTE network, then it's ready for voice," said Téral, noting that NTT DoCoMo and Verizon Wireless are two carriers that fall into this category. "The question is, 'Will they choose to run voice over IMS on the LTE network?'"

At this point, the answer is unclear. Verizon Wireless, for example, has been aggressively building and promoting its 4G LTE network and participated in the One Voice Initiative. Verizon has not yet indicated when voice over LTE support will be forthcoming on that network or carried over the cost-effective 2G network for the time being. In five years, the voice over LTE work underway at GSMA will be mature and 4G networks able to handle LTE voice without issue.

Voice over LTE interfaces and alternate options. Specifically, the GSMA is working on three sets of interfaces:

  1. The User Network Interface (UNI) between the customer's equipment and the operators network
  2. The Roaming Network Network Interface (R-NNI) between the home and visited network of a subscriber that is not attached to their normal home network
  3. The Interconnect Network Network Interface (I-NNI) between the networks of the two parties making a call.

Handling pre-standardized Voice over LTE. Until Voice over LTE specifications are finalized, adopted and tested, the mobile industry must come to terms with how it will handle voice and text calls that cross from the 2G/3G to the LTE world.

  • The GSMA supports the circuit-switched fallback (CSFB) standard, which allows users with LTE handsets that support voice to switch between an LTE session and a 2G/3G call.
  • An alternative -- Voice over LTE via Generic Access (VoLGA), supported by the VoLGA Forum -- enables the voice call to use both the LTE and the 2G/3G networks simultaneously to establish an end-to-end voice connection. VoLGA has no backing from the standards-setters, and the majority of providers have left it by the wayside. VoLGA supporters include Alcatel-Lucent (which also supports VoLTE), Deutsche Telekom, Huawei, Kineto Wireless, Motorola, Sonus Networks, Cisco's Starent and ZTE.

"The large majority of service providers say they'll support CSFB," Téral said, "So we'll see a lot of circuit-switched fallback to carry voice until there is enough LTE footprint and until the dust settles regarding standards, certification, testing and the device ecosystem so that everything can work."

About the author: Beth Schultz is an IT writer and editor based in Chicago.

This was first published in June 2010

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