[asterisk-dev] Fax with Asterisk internal flow (conversion)

[Scott Griepentrog]

To add some details to your call flow for clarity, I would write it as:

Sending FAX machine (T.30) ==POTS==> Local CO ==PRI/G711/T.30==> (PSTN) ==>
VoIP Gateway with T.38 support ==IP/SIP/RTP/T.38==> (INTERNET) ==>
Asterisk (with T.38 capable fax modem software) => .TIFF file

I'm using ==protocol/stack==> to indicate the signalling/media methods, and
(network) to indicate a large complicated routing network that isn't
changing the datastream.

In this diagram, note that:

1) POTS Analog to Digital (G711) conversion is done by the CO's SLIC

2) The VoIP gateway when using T.38 does the G711/T.30 "Modem" conversion
to T.38.  A way to understand this is that it's like the serial port on the
back of the old style modems, only the data is carried in RTP packets and
called T.38.  Portions of the T.30 decoding still have to be done by a
software program inside or alongside Asterisk.

The same sequence where T.38 is  not used - just a conventional T.30 fax
over G711:

Sending FAX machine (T.30) ==POTS==> Local CO ==PRI/G711/T.30==> (PSTN) ==>
VoIP Gateway  ==IP/SIP/RTP/G711/T.30 ==> (INTERNET) ==> Asterisk (with fax
modem software) => .TIFF file

Note that the "asterisk fax modem software" has to include the entire T.30
stack including all the V.xx protocols, plus T.38 if needed.



On Thu, Jan 9, 2014 at 5:38 AM, Chandramouli P <crazymoonboy at yahoo.com>wrote:

>  Hi,
>
> I have some basis queries about the internal flow (conversion) and
> architecture of FoIP. Please note that I have done enough research on
> Internet to clarify my queries. But, I didn't get proper information. I
> thought of sending you guys an email to clarify my queries:
>
> I want to implement a real time fax service with Asterisk (FoIP with
> Asterisk) using T38 protocol. For this, first I would like to clear with
> the basic internal flow of sending fax over IP.
>
> *Send Fax -> Fax Modem -> Asterisk -> Receive Fax *
>
> 1) Please correct me, If I am wrong with the above flow.
> 2) Can we consider "opalvoip" as Fax modem?
> 3) Can we consider "SpanDSP" as Fax modem?
> 4) Can we consider "HylaFax" as Fax modem?
> 5) Can we consider "Fax Modem" as "T38 Gateway" in the above flow? If not,
> where I need to put T38 Gateway in the above flow? Can you please rewrite
> the flow?
> 6) Can I use "opalvoip" as Fax modem in the above flow?
> 7) T38 Gateway contains T38 Receiver and T38 Sender. Is it correct?
>
> Hence request you to find some time to reply and also reply in inline.
>
> I look forward to hearing from you.
>
> Thanks in advance.
>
> Regards,
> Chandra.
>
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*Scott Griepentrog*
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