G726 for fax

Unanswered Question
Jul 13th, 2007

I know we use G729 and G711 for fax in most cases. Can we actually use G726 with either relay or pass through?Here is the scenario. Thanks

fax-PBX-r0-G726-r1-PBX-fax

I have this problem too.
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jaregalado Thu, 07/19/2007 - 15:39

Hi,

any codec which uses compression tehcniques (g729, g726, g723) is unsuitable for fax pass through as the tones representing each sample on the original bitmap from the fax needs to reach the other end of the connection exactly as sent, and the only codec cabable of this is g711.

You can configure g729 codec on ATA and IOS devices for voice calls and switch to G711 as needed (called upspeeding on some documentation) but that doesn't mean your faxes are being carried by a g729 codec which is in fact impossible because this codec works upon the fact that the human hear is incapable of interpeting all the frequencies on the audio spectrum which on a regular phone line are limited to the 4 khz upper range, but fax machines do use precise tones which are turned into garbage by lossy codecs like G.729.

Hope this helps.

Regards.

ciscoforum Fri, 07/20/2007 - 10:45

Thanks. Actually I did verift the g729 upspeed to g711 and worked fine. What I am trying to ask now is that G726 should also be able to upspeed to g711 right?

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