Re: SIP fax sending software?

[John Osmon <josmon () rigozsaurus com>]

On Thu, May 31, 2018 at 12:54:05AM -0400, John R. Levine wrote:
> > You *can* get a fax across a G.711 connection if your throughput,
>
> My SIP provider supports T.38.  How much difference does that make?

It can make a great deal of difference.  


You change the dynamics from:
           
   fax   ------------   fax
  modem <->  VOIP  <-> modem
         ------------


Into something more like:

               T.38 service
  fax   -----------------------------   fax
 modem <->  modem <-> IP <-> modem <-> modem
        -----------------------------


Fax communications have a data envelope that don't play well with
anything that has latency/jitter/loss issues that don't act/feel like
a POTS system.  Also, the envelope doesn't recover well from errors.
So --- faxes fail.

So, you terminate the envelop at each end, and plug them together with 
and IP connection.  You may still have problems, but a single issue
doesn't necessarily mean that you have to terminate the transmission and 
start over.

More (and better/more accurate) details with the article I linked
earlier:   http://www.soft-switch.org/foip.html