T.38 Fax with Transcoder

[whenusayloveme]

Hi All,

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

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

BR

WINAI

[Jonathan Schulenberg]

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.

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[whenusayloveme]

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

[Ayodeji Okanlawon · Accepted solution]

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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