[rescue] Static IP! Now what?

John Hudak jjhudak at gmail.com
Wed Apr 5 19:52:00 CDT 2017

In summary, there are two problems: lossy audio compression ( a
characteristics of the codecs), and the timing relationships of the
signals, aka jitter.
The codecs remove some of the audio content of the signal during the
compression (for the theoretical treatment of signal integrity and
information content, look up papers by Claude Shannon), and the jitter
comes about from the fact that packet stream arrival is irregular..
Yes, T.38 is the more popular ITU standard and if you are interested, two
open source packages that implement T.38 are AvantFAX and Asterisk the open
source PBX which uses the SpanDSP library to implement faxing. It has
modules to handle T.38.
A (IMO) good FAX program is Hylafax and its spinoff Hylafax+.  If you go
this route, keep in mind that Hylafax and a number of other FAX programs do
not work well with soft modems....but thats another story.....

On Wed, Apr 5, 2017 at 5:26 PM, Andrew Jones <andrew at jones.ec> wrote:

> On 04/05/2017 02:11 PM, John Hudak wrote:
> > a FAX signal being carried by VOIP  is a very bad combination.  Don't do
> > it.  FAX protocol dictates that the timing relationships between and
> among
> > tone signals and handshaking are tight and deterministic.  Attempting to
> > run the FAX protocol over a temporal
> >  non-deterministic transport is asking for trouble.  There are times when
> > the communication will 'work'.  Most of the time, it will not. By this is
> > mean, yea, you *may* get one page to FAX successfully, you won't get a
> doc
> > of >1 to work.
> Getting into the weeds a bit, VOIP audio codecs are designed around a
> psycho-acoustic model, so they remove noise you can't hear very well.
> A fax machine counts on using every iota of available audio bandwidth in
> a conventional telephone line.  That is to say, nearly all of its signal
> falls under the category "noise you can't hear very well."
> If you want to do fax-over-IP, you need a fax-over-IP gateway.  The
> relevant ITU standards are T.37 and T.38.  Lots of VOIP gear supports
> T.38.  I've never seen a T.37 implementation.
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