T-Mobile: Voice Discord Threatens LTE

If operators can't agree on how to deliver voice services over Long Term Evolution (LTE) networks, then the development of the next-generation mobile broadband technology could be thwarted, according to T-Mobile International AG .

The carrier stated concerns about the lack of support in LTE for legacy voice and SMS services at the LTE World Summit in Berlin last week, and called for a unified approach. (See T-Mobile, Orange Open Up on LTE.)

"There's a big risk if we don't decide how to go forward with our cash cow," said Franz Seiser, head of core network architecture at T-Mobile. "If we don't get this right, we could put the whole of LTE at risk."

LTE, as an all-IP, packet-based network, does not support legacy circuit-switched services, and there are several options for delivering these services. The various technology choices for voice over LTE include IP Multimedia Subsystem (IMS), circuit-switch (CS) fallback (which is basically using the legacy 2G or 3G networks for voice), and the relatively new initiative called Voice over LTE via Generic Access (or VoLGA, which essentially sends the CS traffic from the mobile switching center through an IP tunnel across the LTE network).

A rift has already emerged, as T-Mobile is VoLGA's most fervent -- only, actually -- operator supporter, while Verizon Wireless and France Telecom SA (NYSE: FTE)'s Orange favor the CS fallback approach.

Even though many in the industry assume LTE will be deployed for data services initially, these first signs of fragmentation for voice service delivery have some operators worried, particularly in the context of roaming. (See T-Mobile, Orange Open Up on LTE.)

T-Mobile believes that without support for voice services from day one, the promised cost-per-bit efficiency of LTE will not be realized fully. Also, T-Mobile's assumption, according to Seiser, is that the majority of classic 2G/3G services will also be expected of LTE.

"It's difficult to position LTE as a data-only network," said Seiser. "We need a good solution for voice and SMS as soon as possible."

He said that IMS was not yet ready for delivering mobile voice services. "[IMS] is working in fixed networks, but there's a huge amount of work to do voice on mobile… There is no IMS ecosystem in place for yet for IMS roaming and interconnect. No one believes IMS can deliver in the 2011-2012 timeframe. There will be no IMS for the next five years."

And Seiser dismissed the CS fallback approach as a "quick and dirty solution to get something done very quickly. And it's not a solution that costs nothing."

"We can't wait for IMS to do voice," he said. "We don't believe CS fallback is the answer. VoLGA is the most elegant approach to solve the issue."

While the Voice over LTE via Generic Access (VoLGA) Forum has the support of nearly all the major mobile infrastructure vendors -- except for Nokia Siemens Networks -- T-Mobile is the lone operator supporter right now. The initiative will need more operator backing to get its specifications accepted by the 3rd Generation Partnership Project (3GPP) standards body.

The 3GPP is not working on any specifications related to VoLGA right now. And the standards body recently rejected a technology forerunner to VoLGA, dubbed circuit-switch-over-packet-switch, because there was not enough industry support.

"3GPP rejected the proposal because of a concern with a split in the industry between operators who thought in terms of 'LTE with voice now' versus 'LTE with voice eventually,' " says Stephen Hayes, 3GPP systems and architecture chair. "If everyone agreed on launching LTE with voice now, we’d probably do something like [VoLGA]." He notes that VoLGA "wasn’t endorsed by 3GPP because most of the community didn’t see voice as critical and didn’t want to see several different approaches."

The only 3GPP-endorsed solution for voice over LTE is IMS, according to Hayes.

— Michelle Donegan, European Editor, Unstrung