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Asterisk Fax T.38 Support Explained

Posted on 2009-04-08
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Could someone explain this to me:

"Asterisk 1.6 support G.711 and T.38 FAX origination and termination. T.38 gateway features are still in development. "

What is gateway features?  What will Asterisk 1.6 actually support with faxing, in layman terms?  Real world scenario examples would be helpful.  Thank You!
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Question by:jkockler
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ai_ja_nai earned 500 total points
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A fax gateway is a facility that collects all the in/outcoming traffic and routes it to the right places, instead of letting the clients do it in their own place.

In practical scenarios, a T.38 fax call has at least part of the call being carried over PSTN, although this is not required by the T.38 definition, and two T.38 devices can send faxes to each other. This particular type of device is called Internet-Aware Fax device, or IAF, and it is capable of initiating or completing a fax call towards the IP network.

The typical scenario where T.38 is used is - T.38 Fax relay - where a T.30 fax device sends a fax over PSTN to a T.38 Fax gateway which converts or encapsulates the T.30 protocol into T.38 data stream. This is then sent either to a T.38 enabled end point such as fax machine or fax server or another T.38 Gateway that converts it back to PSTN PCM or analog signal and terminates the fax on a T.30 device.
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by:jkockler
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Great explanation!  Thanks!

Now just a bit more information and I should be good.

I am running Asterisk 1.4.23 which supports T.38 pass through.  Is this enough to get 2 fax machines to talk to each other?  Or should I upgrade to Asterisk 1.6, which supports origination and termination?  I would just test it but the fax machine I have right now doesn't work real well in general, so it is not a good test of function for the Asterisk box.  Thanks Again!
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by:ai_ja_nai
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From version 1.4, Asterisk supports T.38 negotiation for SIP users, and the related passthrough of UDPTL T.38 data. This allows many T.38 nodes to communicate through an Asterisk box. Asterisk 1.4 does not, however, understand the T.38 protocol. It cannot terminate T.38 calls, or act as a T.38 PSTN gateway without external support - i.e. by passing the T.38 data to something which can perform those functions.
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by:jkockler
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Ok but What does terminate T.38 calls mean?  What is T.38 origination and termination?
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by:ai_ja_nai
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it's the capability of creating and destroying telephone sessions
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by:jkockler
ID: 24109496
Oh ok, so if the fax machine originates and terminates the call, then there should be no problem with 2 fax machines talking across an asterisk 1.4 box, right?  
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by:ai_ja_nai
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yes, because 1.4 can passthrough the calls, but not start and terminate on its own
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by:jkockler
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That's awesome.  So Asterisk 1.6 can actually send a fax document directly to a fax machine, from itself.  That is really cool.  Thanks man!
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