System information

Fax Pass-Through
In theory, it should be possible to connect a traditional fax machine to an FXS port of
some sort and then pass incoming faxes to that device (see Figure 19-1). This concept
is attractive for a few reasons:
1. It allows you to integrate existing fax machines with your Asterisk system.
2. It requires far less configuration in the dialplan.
Unfortunately, fax pass-through is not the home run we would like it to be. The analog
carrier signal that two fax machines use to communicate is a delicate thing, and any
corruption of that signal will often cause a transmission failure. In an Asterisk system
performing pass-through, internal timing issues, coupled with signal attenuation, can
create an environment that is unstable for fax use, especially for larger (multipage)
faxes.
Figure 19-1. Typical fax pass-through
If you are using fax on a casual basis (mostly noncritical, one-page faxes), this sort of
setup can work well. If faxing is critical to your business, or you are often expecting
multipage faxes, we must reluctantly recommend that you connect your fax machines
directly to the PSTN and leave Asterisk out of it.
Using Fax Buffers in chan_dahdi.conf
Many of the problems with fax pass-through are caused by inconsistent timing. Since
faxes are more tolerant of latency than voice calls (a fax has to be able to travel halfway
around the world, which takes a few dozen milliseconds), the introduction of a buffer
in DAHDI (which is strictly used for faxes) has reportedly corrected many of the prob-
lems that have plagued fax pass-through.
As of this writing, this is a fairly new configuration option. The currently preferred
setting is as follows:
faxbuffers => 12,half
454 | Chapter 19: Fax