Received faxes are commonly encoded in Tagged Image File Format (TIFF). This graphics file format, while not as well known as JPEG or GIF, is not as obscure as one might think. In fact, we suspect your computer (whether you’re running Windows, Linux, or MacOS) will already have the ability to interpret TIFF files built in. While it has become popular to offer PDF as a delivery format for received faxes, we’re not sure this is strictly required, since TIFF is so ubiquitous.
Received faxes will be stored by Asterisk as files. Where those files are stored will depend on several factors, including:
In the dialplan, you will need to build in enough
intelligence to name faxes in such a way that they will be distinct from
each other. There are many channel variables and functions that can be
used for this purpose, such as the STRFTIME() function.
Asterisk can easily handle capturing the fax to a file, but you will need
to make sense out of what happens to that file once it is stored on the
system.
The Tagged Image File Format is not very well known, but it is actually more common than you might realize, and since it is natively supported on Windows, MacOS, and Linux, TIFF files can be viewed on pretty much any computer with the most basic graphics viewer. A subset of the TIFF file format has for a long time been the de facto file format used for faxes.
Since Asterisk will receive and store faxes in TIFF format, there is no post-processing required. Once the incoming fax call has been completed, the resulting TIFF file can be opened directly from the folder where it was stored (or perhaps emailed to the intended user).
Once Asterisk has received a fax, the resulting TIFF file needs a way to get to its final destination: a person.
The key consideration is that unless the
information that Asterisk knows about the fax is sufficiently detailed,
it may not be possible to deduce the intended recipient without having
someone actually read the fax (it is common for a fax to have a cover
page with the recipient’s information written on it, which even the most
capable text recognition software would have a difficult time making
sense of). In other words, unless you dedicate a DID to each user who
might receive a fax, Asterisk isn’t going to be able to do much more
than send all faxes to a single email address. You could code something
in the dialplan to handle this, though, or have an external
cron job or other daemon handle distributing the
received faxes.
A simple dialplan to handle fax to email might look something like this (you will need the mail program mutt installed on your system):
exten => fax,1,Verbose(3,Incoming fax)
; folder where your incoming faxes will initially be stored
same => n,Set(FAXDEST=/tmp)
; put a timestamp on this call so the resulting file is unique
same => n,Set(tempfax=${STRFTIME(,,%C%y%m%d%H%M)})
same => n,ReceiveFax(${FAXDEST}/${tempfax}.tif)
same => n,Verbose(3,- Fax receipt completed with status: ${FAXSTATUS})
; *** This line should not have any line breaks
same => n,System(echo | mutt -a ${FAXDEST}/${tempfax}
-s "received fax" somebody@shifteight.org)
Obviously, this sample would not be suitable for production (for example, it does not handle fax failure); however, it would be enough to start prototyping a more fully featured incoming fax handler.
You may have a dedicated phone number for receiving faxes.
However, with Asterisk, that is not a requirement. Asterisk has the
ability to detect that an incoming call is a fax and can handle it
differently in the dialplan. Fax detection is available for both DAHDI
and SIP channels. To enable it for DAHDI, set the
faxdetect option in
/etc/asterisk/chan_dahdi.conf. In most cases, you
should set this option to incoming. Table 19.1, “Possible values for the faxdetect option in
chan_dahdi.conf” lists
the possible values for the faxdetect option in
chan_dahdi.conf.
Table 19.1. Possible values for the faxdetect option in chan_dahdi.conf
| Value | Description |
|---|---|
incoming
| Enables fax detection on inbound calls. When a fax is
detected, applies the faxbuffers option if it
has been set and redirects the call to the
fax extension in the dialplan. For more
information on the faxbuffers option, see
the section called “Using Fax Buffers in chan_dahdi.conf”. |
outgoing
| Enables fax detection on outbound calls. The dialplan is
not executing on an outbound channel. If a fax is detected, the
faxbuffers option will be applied and the
channel will be redirected and start executing the dialplan at
the fax extension. |
both
| Enables fax detection for both incoming and outgoing calls. |
no
| Disables fax detection. This is the default. |
To enable fax detection for SIP calls, you must
set the faxdetect option in
/etc/asterisk/sip.conf. This option may be set in
the [general] section, or for a specific peer.
Table 19.2, “Possible values for the faxdetect option in sip.conf”
covers the possible values for the faxdetect option
in sip.conf.
Table 19.2. Possible values for the faxdetect option in sip.conf