[asterisk-users] T.38 and Linksys SPA8000

[David Backeberg]

2009/12/29 Vinícius Fontes <vinicius at canall.com.br>:
> Hello everyone.
>
> I'm trying to set up a SIP DID on a customer, which uses T.38 for faxing. Voice is working great, but I never configured anything using T.38 in Asterisk so I'm kinda lost.

So you're trying to use a SPA8000 to act as a gateway, then you're
trying to put asterisk somewhere. And there's a real fax machine
involved. I don't think you've given enough information for anybody to
actually tell you what went wrong with your 'transmission error'.

So here are my suggestions:
* if you're not already using the latest 1.6, you should be. A lot of
T.38 and faxing-in-general improvements have been released.
* are you sure the T.38 is actually negotiating?
* if you leave T.38 disabled, everything should try to pass
traditional audio over the VoIP codec of your choosing, and if you
have a fairly reliable network, you MAY be able to accomplish faxing
in that manner.
* And now for my personal opinion... if you need reliable faxing, you
shouldn't be using a SIP trunk into the premise, but rather a real
traditional phone line. Fax is a hack to play tones to represent an
image. VoIP is a hack to try to reduce the quality of a voice call
without a human noticing. Put those together over the internet, and
you shouldn't expect reliable faxing. Fax machines are much less
forgiving than a human. If you lose data in the middle of a fax, the
fax machine gets confused and can drop a fax that was mostly received
okay.
* I do think fax over voip has good usages. That means if you have a
dedicated line to your voip provider with reliable quality, you would
probably be okay. If you have a dedicated LAN that you are using
internally for voip fax you are also probably okay.
* if this whole project is an effort to save money and avoid the
Brazil telco monopoly, you should consider paying for one of those
services that allows you to send and receive faxes using a
third-party, and not try to receive faxes at the premise.