The PSTN Transition to VoIP: What About Codecs?

Scalable Codec Interoperability is Key to PSTN Retirement

One of the big advantages of VoIP vs. PSTN voice is VoIP’s capability to offer high definition or wideband voice-- providing a fuller and richer sound quality that is similar to the leap made between standard definition video and high definition video.    The PSTN’s limitation for sound quality stems from a history that included analog transmissions and a corresponding limited amount of available bandwidth. 

Even when the PSTN went to an all-digital network and sounds were converted to data packets, the PSTN’s voice quality didn’t change much.  As audio processing evolved, the encoding and decoding of voice sound waves also evolved, with standards such as the ITU-T G.711 and G.729 standards offering a more efficient way to digitize voice, but not necessarily offering significantly improved sound quality over the old analog transmissions.  The proliferation of wireless voice services, where radio spectrum is limited, also focused on offering a premium on efficiency and not necessarily on premium audio quality. 

The introduction of VoIP, plus the availability of more affordable bandwidth offered experts the opportunity to offer “wideband” audio, and the industry took of the opportunity by offering a variety of wideband standards and codecs, such as ITU-T G.722, G.729.1, and Microsoft Real-time audio (RTA). 

As the old joke goes, the good news about standards is that there are so many of them to support.  However, the proliferation of standards that include multiple wideband and narrowband voice options also means that something in the middle needs to provide interoperability between different codecs.  In some cases today, a session border controller or a gateway offers this functionality—most frequently in an enterprise setting.  In other cases, the PSTN offers itself as one big gateway, offering “lowest common denominator” sound quality when one side of the conversation supports standard voice quality while the other side supports high definition voice. 

As the PSTN is eventually  replaced with a VoIP-only infrastructure, the optimists can always hope that the industry will agree on and implement one standard codec for HD voice.  However, historical experience and the significant investments already made in existing (but different) codecs suggest that somebody-- or something, will need to offer interoperability before the PSTN as we know it can be scrapped.  This  interoperability will be needed to connect residential and business VoIP codecs, cable and telco VoIP codecs, narrowband and wideband codecs, wireless and wireline codecs, and even video-call audio and voice call audio. And that doesn't even count WebRTC voice and video codecs, although that is fodder for a future edition. 

Anybody up to the challenge? 

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