How does fax work?

More specifically, how does an ME know that an incoming call is a fax call?

The example at the end of the AT Commands Guide shows a +FCON response after ATA - but that doesn’t happen if I just call from an ordinary fax machine on an ordinary phone line.

If the call comes from an ordinary fax machine on an ordinary phone line, it is indistinguishable from a voice call.

So how does one place a fax call to a GSM ME?
Does it have to go via some specific interworking facility in the network?
Does the SIM have to have a specific Fax number allocated (as for Data calls)?

An ordinary fax machine can only distinguish between a fax call and a voice call by detecting the continuous tone sent by the emitting fax machine. Once this tone is detected there is some sort of a handshake between the two machines to agree upon the transmission speed etc.
So unless there are some digital signals sent along with the call on the GSM network I am inclined to think that you would have to use the DTMF decoder to recognize a fax call. But don’t ask me what to do after that…

Best regards,


Quite so.

But, according to the AT Commands manual, the modem should do that itself:

There’s no Open-AT application there; nothing about DTMF - it should just “magically” know that it’s a fax call, and give the +FCON result.

In fact, my unit doesn’t give the +FCON result - which makes me think that you would have to go through a specific interworking function in the network, so that the network would specifically identify the call as a Fax call to the receiving ME.

Originating a fax call would seem to be similar - the example in the Manual also doesn’t work.