Cellular carriers seek smooth handoffs from LTE to 3G

Mobile engineers have successfully demonstrated the handoff of a voice call from LTE to 3G, a capability that may prove critical in carriers' plans to put voice on their new, fast data networks.

Engineers from Telefonica Deutschland used SRVCC (Single Radio Voice Call Continuity) to move a call without disruption from an LTE to a 3G network. The test took place in a lab and was carried out over equipment from at least six different vendors in an effort to emulate mixed real-world networks, according to Telefonica.

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Keeping a subscriber's call going as they move out of an LTE coverage area will probably be important to the deployment of VoLTE (voice over LTE), a technology that breaks a voice call into packets and transmits it as data traffic. Carriers that can't fill their whole coverage area with LTE will need a way to make the handoff. SRVCC, part of the 3GPP family of standards that underlies GSM and LTE, has wide support to become that mechanism. Products from Ericsson, Huawei Technologies, Nokia Siemens Networks, Acme Packet, Qualcomm and Sony Mobile were included in Telefonica's demonstration network.

However, the need for SRVCC is expected to trail demand for VoLTE itself, which is only slowly emerging. Even most carriers that have extensively deployed LTE are still transmitting voice over their older 3G networks, which are expected to remain online for many years.

A major limitation of SRVCC, in some parts of the world, is that it doesn't work with CDMA, the form of 3G used by Verizon Wireless, Sprint Nextel, MetroPCS, South Korea's SK Telecom, and some other carriers. It's strictly for sending calls over to 3G networks based on the GSM family of standards, which include those used by AT&T and T-Mobile USA as well as most carriers in Europe and Asia, according to Ovum analyst Sara Kaufman.

Early implementations

Carriers in Asia are moving quicker on VoLTE than those in North America and Europe, Kaufman said. In the U.S., Verizon Wireless, AT&T and Sprint Nextel all have said they plan to use it, but none of those deployments is imminent. On Friday, Verizon Wireless said it plans to start offering VoLTE late this year or early next year; Sprint Nextel would not name a date.

MetroPCS, a regional U.S. carrier, is already using VoLTE in some areas. Asked about handoffs to 3G, which still makes up much of MetroPCS's network, the company provided an assurance without details.

"MetroPCS has implemented VoLTE in a way that is transparent to the user on performance and services," MetroPCS said in a statement attributed to Ed Chao, senior vice president of corporate engineering and network operations.

Even when VoLTE is deployed, it's likely to benefit service providers more than subscribers. With voice calls on the data network, carriers ultimately will be able to run just one network and reuse the 3G spectrum for LTE. Calls on LTE also make more efficient use of spectrum. The potential benefits to subscribers include higher call quality and Rich Communications Suite, a set of services that could encompass voice and video calls and text chatting.

Stephen Lawson covers mobile, storage and networking technologies for The IDG News Service. Follow Stephen on Twitter at @sdlawsonmedia. Stephen's e-mail address is stephen_lawson@idg.com

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