PSTN codecs

[carl_townshend]

Hi All

When you go from the CUCM voip phone and out to the PSTN, can anyone tell me what codec it normally gets transcoded to when sending out to the PSTN?

[R0g22]

By PSTN if you mean TDM or maybe an FXO, there is no concept of codec there. Codec is a IP thing, remains confined on the call leg b/w CUCM and voice router on the IP side. DSP's on the router take over, to decode the RTP packets and send the media in-band out the bearer channels.

There would be transcoding in an IP to IP call flow if both the call leg tries to/negotiates a different codec.

[carl_townshend]

Hi

So do the DSPs convert from the RTP stream from IP to analogue voice going to the PSTN? Is this not called transcoding then ?

[R0g22]

Nope, this is not transcoding. Transcoding is fixing a codec mismatch by inserting a media resource which is a transcoder which utilises DSP's. IP to TDM is more on the lines of multiplexing/sampling and it requires DSP's to be able to unpacketize the media from the RTP packets which are IP only.

[Jaime Valencia]

https://www.webopedia.com/DidYouKnow/Internet/VoIP_POTS_Difference_Between.asp

https://www.plumvoice.com/resources/blog/how-public-switched-telephone-network-pstn-works/

https://www.ramcomminc.com/ram-telecom-101/

[Chris Deren]

The only PSTN connection where codec would be at play would be SIP trunk, if you do in fact have SIP trunk rather than TDM PSTN then the codec is whatever the SIP provider supports, they typically will prefer codec you agree upon i.e. g711 but will allow other codecs i.e. g729, with that being said they will NOT transcode anything for you so if you have an application that supports only one codec i.e. CCX installed with G711, but call arrives as  G729 then you will be responsible for the transcoding.