T.38 Fax with Transcoder

Answered Question
Jun 29th, 2012

Hi All,

Is it possible to do fax over IP which topology below?

Fax <<>> VG+transcoder <<<<<H323 G729>>>>>> 3rdParty PBX <<>> Fax

BR

WINAI

I have this problem too.
0 votes
Correct Answer by Ayodeji oladipo... about 2 years 9 months ago

No, you cant use xcoders with T.38 fax. T.38 Fax does not use G729 or G711. It will use the bandwidth required for the fax e.g 14.4kbps fax rate. When the voice call is setup using either g729 or G711, the gateway will then switch to T.38 for fax transmission. So you can configure your voice gateway to use G729 to setup the call and then swicth over to T.38 for fax. You cant use a xocder and you dont need one.

Fax passthrough is a different fax mechanism.

In fax passthrough mode, gateways do not distinguish a fax call from a voice call. Fax communication between

the two fax machines is carried in its entirety in-band over a voice call. It only support g711.

NB: That there is a difference between faxpassthrough and fax-passthrough..

Fax Passthrough” can refers to the more specific passthrough transport method using an NSE switchover to pass faxes over G.711. “Fax pass-through” details a switchover to passthrough using the call control protocol.

Yes, for T.38 to work, your 3rd party vendor/device must support it.

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Jonathan Schulenberg Fri, 06/29/2012 - 20:32

No. T.38 is used instead of G.729 and will consume more bandwidth to properly relay the T.30 signals. It is possible to negotiate T.38 over the H.323 trunk as long as both systems support it.

Please remember to rate helpful responses and identify helpful or correct answers.

whenusayloveme Sat, 06/30/2012 - 05:26

Hi Jonathan,

-You mean we can use T.38 Fax protocol with transcoder involve right? (Transcoder do the transcoding between g711<>G729)

- But can't use fax passthrough isn't it?

- With T.38 scenario, 3rd party PBX must support also?

BR

Winai

Correct Answer
Ayodeji oladipo... Sat, 06/30/2012 - 16:06

No, you cant use xcoders with T.38 fax. T.38 Fax does not use G729 or G711. It will use the bandwidth required for the fax e.g 14.4kbps fax rate. When the voice call is setup using either g729 or G711, the gateway will then switch to T.38 for fax transmission. So you can configure your voice gateway to use G729 to setup the call and then swicth over to T.38 for fax. You cant use a xocder and you dont need one.

Fax passthrough is a different fax mechanism.

In fax passthrough mode, gateways do not distinguish a fax call from a voice call. Fax communication between

the two fax machines is carried in its entirety in-band over a voice call. It only support g711.

NB: That there is a difference between faxpassthrough and fax-passthrough..

Fax Passthrough” can refers to the more specific passthrough transport method using an NSE switchover to pass faxes over G.711. “Fax pass-through” details a switchover to passthrough using the call control protocol.

Yes, for T.38 to work, your 3rd party vendor/device must support it.

Please rate useful posts

"For the love of God is broader than the measure of man's mind And the heart of the Eternal is most wonderfully kind"

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