Verizon Wireless (NYSE: VZ), AT&T Mobility (NYSE: T) and T-Mobile US' (NYSE:TMUS) implementations of Voice over LTE demonstrate significant improvements in voice quality over legacy circuit-switched voice, with Verizon leading the way in terms of call quality, according to a new benchmarking test by network testing firm P3 Communications.

P3 tested VoLTE calling for all three carriers in the Washington, D.C., area, the first such independent, cross-carrier test in the U.S. Sprint (NYSE: S) has not yet launched VoLTE.

Although the test looked at only one market, it does provide some insight into the benefits of VoLTE and the difference in implementation between the carriers. P3 found that while VoLTE is delivering on its promise of faster call set-up times and improved HD voice quality, carriers still have plenty of work ahead to optimize their LTE networks and provide the VoLTE-to-VoLTE calling between carriers and the international VoLTE roaming.

During two weeks of intensive testing in late April and early May, P3 engineers drove more than 1,300 miles on and inside the I-495 Capital Beltway in two specially-equipped measurement vehicles. All voice test calls were mobile to mobile and set up from one P3 test vehicle to the other. P3 engineers rode on carefully selected routes to measure VoLTE performance from the customer's perspective, and made around 7,000 voice calls and collected more than 50,000 speech samples to compare the real-world performance of legacy circuit-switched networks and the new VoLTE networks of major carriers in Washington, D.C.

P3 focused on call quality to determine the clarity of the audio, call set-up times and how reliable the VoLTE calls were and how long they stayed connected.

Speech quality was evaluated using an international measurement standard (POLQA is the ITU-defined voice quality testing technology that P3 used) to derive what P3 calls a "Mean Opinion Score" (MOS) on a scale between 1 and 4.75. A higher score means better voice quality.

AT&T and Verizon, which do not use AMR wideband or HD Voice technology in their circuit-switched networks, showed the most dramatic improvement in clarity with VoLTE.

Verizon Wireless came away with the highest MOS score, at 3.60, while AT&T (3.55) and T-Mobile (3.54) were close behind. Sprint, which tied T-Mobile for the best circuit-switched voice quality performance, has potential for improvement when it deploys VoLTE, P3 said. Sprint and T-Mobile had circuit-switched voice scores of 2.77 while AT&T had a score of 2.35 and Verizon had 2.28.

Source: P3 Communications





















































P3 CEO Dirk Bernhardt told FierceWireless the difference in VoLTE call quality "was very, very minor and statistically not really relevant and not really perceptible by the end user."

"The improvements are really remarkable, especially if you look at where some of the carriers come from," he said. "Anything beyond a 0.5 increase is really recognizable by humans."

In comparison to major international cities, the carriers' legacy voice networks in Washington were inferior in quality, Bernhardt said, but the addition of VoLTE catapulted Washington back ahead.

Circuit-switched voice connections for the three carriers tested took an average of 6.1 to 7.8 seconds to initiate, Bernhardt said. With VoLTE, those figures dropped down to 2.1 to 3.8 seconds, on average, or less than half the time. Anything less than five seconds is considered fast. Bernhardt added. All of the carriers were relatively similar in this metric, though T-Mobile was slightly slower than AT&T and Verizon, P3 said.

All the carriers showed solid performance in sustaining ongoing VoLTE calls, although AT&T and T-Mobile hand off ongoing VoLTE voice calls to their legacy networks using Enhanced Single Radio Voice Call Continuity (eSRVCC) when a caller travels outside of their LTE footprints. As a result, they drop fewer VoLTE calls than Verizon, which requires customers to be in LTE signal areas during the entire call, otherwise it drops. "It forces Verizon to have a much better contiguous network coverage," Bernhardt said, adding that the carriers "have slightly different philosophies about rolling out VoLTE."

Overall, the VoLTE channel call set-up success and dropped call ratios were on "impressive levels for such a new technology," P3 said, adding that "as VoLTE evolves there will be room for further network optimization and improvement."

Bernhardt said that "if we use the other carriers as an indication of what Sprint can achieve they should also be able to get up there" in terms of VoLTE call quality.

In the first half of next year P3 plans to expand the geographical scope of its tests significantly, and also test more kinds of voice and data performance, Bernhardt said.

Verizon and T-Mobile offer VoLTE nationwide, while AT&T is still rolling it out market by market, and offers VoLTE in all or part of 23 states. U.S. Cellular (NYSE:USM) is planning three VoLTE trial markets for launch later this year, though customers will not be able to sign up for the service. Sprint has said it will eventually launch VoLTE but has not given a timeframe for doing so.

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Article updated June 17 at 4:30 p.m. to clarify the nature of U.S. Cellular's VoLTE market trial.