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April 20, 2016

T-Mobile Goes Beyond HD Voice to EVS

In the U.S., T-Mobile has continually pushed the envelope of services and pricing in the wireless industry. The (un) carrier's latest move is to introduce Enhanced Voice Services (EVS), going beyond stock VoLTE and HD voice to a new level of voice quality.  The new service/code also delivers better reliability in areas of weaker signal.

EVS comes out of the 3GPP working group for Voice over LTE (VoLTE), and is a codec offering a number of advantages over the current AMR-WB codec used for HD voice in VoLTE, 2G, and 3G networks.  It was formalized as a standard in late 2014.

From a frequency perspective, AMR-WB delivers from around 300 Hz to 7000 Hz while EVS delivers a far wider range of audio from 50 Hz to over 14,000 Hz.  To get the wider, richer range of audio, EVS uses 24.4 kbps.  AMR-WB needs anywhere from 12 kbps to 24 kbps, so EVS is delivering a better experience at the same bandwidth as fully turned-up AMR-WB. In addition, EVS provides AMR-WB and AMR modes for backwards compatibility with the bonus of delivering the stock HD voice experience in less bandwidth, from around 5.95 kbps to 7.36 kbps.

T-Mobile says EVS has been implemented as the default codec for customers capable of using it (more on that in a minute) regardless of if the called party supports HD voice or not.  Previously, if a customer was making an outbound calling using AMR-WB, if the called party didn't support it, the call was switched to stock AMR narrowband, resulting in a downgrade in call quality regardless where the call goes. Calls coming from non-EVS customers will get transcoded in T-Mobile's network and delivered in EVS format, so there should be no loss of quality coming from the PSTN or other cellular networks.

One other bonus to EVS is its ability to provide error correction.  Out at the edge of the network, EVS will be more resilient than AMR-WB while delivering HD voice quality in less bandwidth.  A T-Mobile spokesperson says in certain EVS modes the codec can deliver the same audio quality as AMR-WB while encountering three times the error conditions.

Currently, T-Mobile supports EVS on the LG G5 at its April launch and on the Samsung Galaxy S7 and S7 Edge via an over-the-air software update.   With EVS being pushed as the next big thing for VoLTE, nearly all smart phones shipping in the future should be capable to support it.

The final interesting tidbit to T-Mobile's EVS deployment is "patent pending."  Exactly what that means, we'll have to wait and see.  T-Mobile says it has implemented EVS between all access networks it has, including GSM, UMTS, VoLTE, and Wi-Fi calling.  Other carriers are planning EVS deployments and you are likely to see announcements come out of South Korea sometime in the near future.

Don't expect EVS from other U.S. carriers anytime in the near future.  AT&T is the one most likely to deploy EVS in the future, but it may do so as much as to reduce bandwidth load – you always heard someone complaining about bandwidth usage as part of the reason why it wasn't in a hurry to deploy AMR-WB – than for a better voice experience.  Both Verizon and Sprint have to worry about getting their legacy CDMA networks shut down, so enhancements to baseline VoLTE are probably lower on the list as much as getting VoLTE up and getting everything touching 2G/3G systems turned off in a timely fashion. 

Edited by Maurice Nagle

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