# How does fax work?

**URL:** <https://forum.sierrawireless.com/t/how-does-fax-work/2919>\
**Category:** Around the water cooler\
**Created:** [February 11, 2009, 2:12pm UTC](https://forum.sierrawireless.com/t/how-does-fax-work/2919 "2009-02-11T14:12:19Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![awneil](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@awneil](https://forum.sierrawireless.com/u/awneil)\
**Post date:** [February 11, 2009, 2:12pm UTC](https://forum.sierrawireless.com/t/how-does-fax-work/2919/1 "2009-02-11T14:12:19Z")

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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)?

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**Author:** ![wismo\_coder](https://avatars.discourse-cdn.com/v4/letter/w/6bbea6/32.png) [@wismo\_coder](https://forum.sierrawireless.com/u/wismo_coder)\
**Post date:** [February 11, 2009, 3:06pm UTC](https://forum.sierrawireless.com/t/how-does-fax-work/2919/2 "2009-02-11T15:06:35Z")

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> [@awneil](#):
>
> If the call comes from an ordinary fax machine on an ordinary phone line, it is indistinguishable from a voice call.

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,

wismo\_coder

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**Author:** ![awneil](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@awneil](https://forum.sierrawireless.com/u/awneil)\
**Post date:** [February 11, 2009, 3:38pm UTC](https://forum.sierrawireless.com/t/how-does-fax-work/2919/3 "2009-02-11T15:38:16Z")

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> [@wismo\_coder](#):
>
> 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.

Quite so.

> [@](#):
>
> 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, according to the AT Commands manual, the modem should do that itself:

> [@AT Commands Interface Guide for Open-AT Firmware v7.2](#):
>
> 2.8.2 Receive a Fax Class 2  
> AT+FCR=1  
> OK
> 
> AT+FLID=“LocalFax”  
> OK
> 
> RING – Incoming call  
> ATA – Answer  
> +FCON – Connection OK  
> [+FTSI: “RemoteFax”]  
> +FDCS:0,3,0,2,0,0,0,0

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.