In the age of email,and pdf there are still large number of companies relying in fax to transmit documents.
The reasons are many, but the problems implementors face are common.
Bad quality of faxes with "missing" or malformed lines, large number of failed faxes etc.
Most of this issues come from two facts.
The first is that we are trying to use a medium that was primarily designed for human voice to carry high speed analog modem signals and second,that fax standards are not so "standard".
Putting asterisk to handle faxes can be both a blessing and a curse for those two reasons.
Asterisk has the ability to detect if an incoming (or outgoing) call is a fax and do a number of things, depending if its acting as a "pass-through" or receive the fax.
The first thing Asterisk does is try to detect if the call is a fax call and shutdown the echo canceler on that channel.
For this to work, asterisk listens for the CNG tone emitted by the calling fax and if found, disables the echo canceler in zaptel.
The CNG is in the CCITT (ITU-T) Recommendation T.30 and describes how FAX calls are established.
During the call setup/establishment process, there are two tone signals that are send:
CNG and CED
The CNG signal *may* be sent from the Originating FAX machine after dialing is complete.
The CNG signal consists of the transmission of 1100 Hz for 1/2 second, followed by a 3 second silent (2100 Hz OFF) period.
The CED signal *may* be sent by the Terminating FAX machine anywhere between 1.8 to 2.5 seconds AFTER answering the call. The CED signal consists of a 2100 Hz tone that is from 2.6 to 4 seconds in duration. The CED tone is useful for disabling any echo cancellers on the line.
(notice the *may*, don't you love clearly defined standards ;)
Asterisk then tries to redirect the call flow to the 'fax' extension, if its present in the context that is currently executing in the dialplan.
There you can use an asterisk application to receive faxes or redirect to another port etc.
Recently we faced a problem with one of our clients that is using an E1 for voice and fax calls.
The calls come in through the E1 and the faxes are connected to an 8 port analog card
Some of the faxes received had quality issues and we tried to figure out what the problem was.
After a lot of head scratching and countless tries we realized two things.
The PRI card was somehow "loosing" some frames and that the echo canceler, OSLEC in this case, was not shutting down when a fax was received.
The first issue was easy to solve, as we discovered that the PRI card was sharing the same IRQ as the network card, so switching the pci slot and making sure that the PRI did not share IRQ's with any other device.
The tricky part was to figure out why the echo canceler was not shutting down when the call was a fax call.
The problem with an E1 (or T1) is that the calls don't use "fixed" channel numbers as in a pstn card for example, where you know that number X is on channel Y.
The provider is sending the call to the first available channel and its up to the dialplan to determine where the call would be routed.
Shutting down the echo canceler manually for all of the E1 channels, solved the issue of the faxes but created 'random' problems of echo to the voice channels.
So i started looking in detail (i.e the code) how asterisk detects faxes.
What i realized is that asterisk needs a 3-5 seconds *after* the call is answered to determine if this is a fax call,shutdown the echo canceler and then jump to the 'fax' priority.
If you "bridge" the call to another zap channel (or to sip channel of an ATA for example) before that, then the EC stays on.
And this was exactly what we were doing in our case.
As soon as the call come in, we bridged it to the analog card, not giving enough time to asterisk to shutdown the EC.
So the problem can be solved by adding a delay of 5 seconds and let asterisk plenty of time to determine is this is a fax call or if your extension is a fax only extension (i.e you don't expect to receive any voice calls on that number) explicitly disable the echo canceller using the zapec(off) command in the dialplan.
exten => _XXXX,1,NoOP(Incoming call)
exten => _XXXX,n,answer()
;give the caller something to hear while we wait.
exten => _XXXX,n,Ringing
exten => _XXXX,n,Wait(5)
This was not a fax but a normal call so let's answer it
exten => _XXXX,n,Dial(SIP/
exten => _XXXX,n,Hangup()
;This is were we land if asterisk detects a fax call
exten => fax,1,Goto(in_fax,s,1)
;turn the echo canceler off for this channel
exten => s,1,ZapEC(off)
;call the zap channel the fax is connected to
exten => s,n,Dial(ZAP
exten => s,n,Hangup
One drawback of having a single line handling both fax and voice is that detection can not be 100% accurate.
This can be either because Asterisk did not detect correctly the CNG or the that calling fax was not sending a CNG at all.
A quick note here is that all of the above are true for zaptel based channels (pri and pstn cards) plus bristuffed (bri) zaptel.
I am not 100% sure how other channels (Sangoma for example) handle this.
And one last advice.
To make the quality of of faxes even better make sure that you have the ECM mode of the fax turned on both for transmition and reception.