As test and measurement specialists evaluate voice over LTE (VoLTE) systems ahead of commercial service launches planned for later this year, they find that challenges remain with interoperability and device support.

Despite those hurdles, though, the view from several test companies is that there isn't a major technical glitch lurking that would prevent commercial 4G voice services from going ahead. Rather, it's a matter of timing. (See When Will Operators Bolt to VoLTE?.)

VoLTE relies on IP Multimedia Subsystem (IMS) elements to deliver packet voice services over all-IP LTE networks. And the driving force behind VoLTE is Verizon Wireless , which is aiming to offer the 4G voice service nationwide in early 2013, as Light Reading Mobile has reported. (See VZ Plans Nationwide VoLTE in 2013 .)

But one of the issues with the IMS-based technology is that its implementation in the network and on devices is open to interpretation, which can cause interoperability problems when carriers use equipment from many different vendors and, longer term, when the services are expected to roam among LTE networks.

The GSM Association (GSMA) has provided some recommendations regarding the IMS elements that are needed for VoLTE, but these are not standards or technical specifications.

Lost in translation?
According to Nigel Wright, product marketing VP at Spirent Communications plc , there are potential interoperability issues with VoLTE because it has so many optional specifications. For example, the SIP messaging protocol can be written in either abbreviated or long-form format, he said, which is significant as that is the enabling protocol for IMS.

"The GSMA guideline helps narrow down IMS [options] but doesn’t remove all the ambiguity of implementation," said Wright.

In September 2011, the MultiService Forum hosted a VoLTE interoperability test that involved 25 vendors, including Acme Packet Inc. (Nasdaq: APKT), Alcatel-Lucent (NYSE: ALU), Cisco Systems Inc. (Nasdaq: CSCO), Codenomicon Ltd. , D2 Technologies Inc. , EXFO (Nasdaq: EXFO; Toronto: EXF), Huawei Technologies Co. Ltd. , Metaswitch Networks , Samsung Corp. , Stoke Inc. and VSS Monitoring Inc. . (See MSF Updates on VoLTE.)

EXFO's Wayne Davis, senior product manager for service assurance, said that at the start of the MSF's interoperability test there were some "rough patches" due to "some misinterpretations of the VoLTE specs." By the end, though, interpretation among all the players was "really good," he said. (See EXFO Aids VoLTE Tests.)

"From a network element point of view, we didn't see any obstacles for a commercial rollout," said Davis. "But there were plenty of issues that came up in the testing." These he described as "simple things to fix."

But for Ixia (Nasdaq: XXIA)'s Joseph Zeto, senior manager for market development, interoperability is critical because it will affect operator's ability to deliver a good quality voice service. (See Ixia Adds VoLTE Test.)

"The big issue is that voice needs a guaranteed quality of service to be toll grade, high quality," he said. "For VoIP to work on LTE and be accepted by the masses, it needs to be of that quality. It has to deliver packets with very little loss ... [across] backhaul, core, IMS, policy servers, base station from different vendors."

Device dilemmas
Along with these interoperability concerns, there is also an issue with device support for VoLTE.

According to Spirent's Wright, smartphones have a "way to go, without doubt" when it comes to supporting VoLTE, and device manufacturers are "pretty nervous" about it. "It's the first time they've had to interact with the core network at a detailed level."

Indeed, MetroPCS Inc. (NYSE: PCS) pinned the blame for its VoLTE launch delay this year squarely on Qualcomm Inc. (Nasdaq: QCOM), because the company's chipset was not ready. (See MetroPCS Blames 4G Voice Wait on Qualcomm}.)

But Verizon's ambitious LTE and VoLTE plans have set the pace for vendors to follow. As Wright puts it, "everyone's going hell for leather after the Verizon deployment."

And with only eight or nine months away from a commercial launch, there is "still quite a bit of work to do," he said.

— Michelle Donegan, European Editor, Light Reading Mobile