[asterisk-users] T.38 faxing with spandsp and Grandstream HT.486

[Henning Holtschneider]

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Johann Steinwendtner schrieb:

> I 'm trying T.38 faxig with spandsp using rxfax/txfax as fax terminal.
> As another endpoint I 'm using Grandstream HT 486 ATA and a fax machine.
> Has anybody success with the HT486 as T.38 terminal ?

I've successfully faxed via T.38 using these combinations:

fax machine -> HT486 -> SIP server -> HT486 -> fax machine

fax machine -> HT486 -> SIP server -> Thomson ST620 -> fax machine

fax machine -> Thomson ST620 -> SIP server -> HT486 -> fax machine

fax machine -> AVM Fritzbox 7050 -> SIP server -> HT486 -> fax machine

fax machine -> AVM Fritzbox 7050 -> SIP server -> Thomson ST620 -> fax
machine

(note: T.38 is not officially supported on the AVM Fritzbox. It only
works with older firmware versions and only when sending. When receiving
a fax tone from the remote end, the Fritzbox does not issue a T.38
re-invite).

I also just recently tested the following cases:

fax machine -> HT486 -> OpenPBX with spandsp T.38 -> RxFAX

fax machine -> Thomson ST620 -> OpenPBX -> RxFAX

TxFAX -> OpenPBX -> HT486 -> fax machine

TxFAX -> OpenPBX -> Thomson ST620 -> fax machine

> ATA as originator: I managed only onetimes a successfull T.38 fax
> session. The other times the HT486 did not initiate a re-invite with
> T.38 parameters. Or shall the Terminator inititate a re-invite ?

When I tested T.38 with OpenPBX, it never detected the fax tone from the
 fax machine and thus did not issue a T.38 re-invite. This might be due
to a configuration problem because I'm not overly familiar with OpenPBX
and there are not many user reports available on the net. If you point
me to the necessary patches for Asterisk, I will be glad to repeat my
tests ;-)

The fax tone detection and T.38 re-invite is always performed by the
receiving terminator or gateway. I haven't read the corresponding RFC
thoroughly enough but this is the way it works on all ATAs and gateways
I've worked with so far. I also noticed that many modems which are
connected to or built into PCs do not send a proper fax tone (CNG) so
make sure you are using a good old fax machine. Thirdly, make sure that
your fax machine does not support V.34 (class 2.1 aka "Super G3")
because T.38 is limited to 14400 bps on the modem side. I only tested
one V.34 fax machine but its modem would not negotiate with the HT486
properly.

> txfax as originator: T.38 fax exchange takes place but the transmission
> is not successful, txfax reports errorcode 60 (Disconnected after
> permitted retry).

I haven't been able to send more than a simple page using TxFAX with
T.38 support yet. And even that did not work reliably. Either, the
single page wasn't transmitted at all or fax machine on the HT486/ST620
recognized the end of the page but failed to negotiate the end of the
transmission correctly.

Once again, I only tried OpenPBX with T.38 termination support, yet, so
your problem may be different.

> Can someone recommend a T.38 able ATA which is working with spandsp ?

T.38 support in spandsp is still work-in-progress so I think it's a
litte early to  make any recommendations. All I can say is that the T.38
support on the HT486 works reliably if the remote end does not stretch
the specifications too much.

Cheers,
Henning Holtschneider
- --
LocaNet oHG - http://www.loca.net
Lindemannstrasse 81, D-44137 Dortmund
tel +49 231 91596-25, fax +49 231 91596-55
sip 25 at voip.loca.net
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