Fax modem?

Patrick Smears patrick.smears at ensoft.co.uk
Fri Nov 2 19:01:28 CET 2007

On Fri, 2 Nov 2007, Jonathon Suggs wrote:

> Just FYI, I'm about 97% positive that the ability to send faxes over GSM
> is dependent upon the carrier.  In the case of T-Mobile (USA) you have
> to call customer support and have them enable it.  I believe that there
> is also a monthly charge as well (maybe ~$5/month).

It certainly is carrier-dependent. In the UK, O2 consumer tariffs do not
support it at all; their business tariffs do, but you have to ask them to
enable it specifically (and then they give you separate fax & data
numbers), though there is no extra charge for this. Not sure about the
other UK operators.

> Other carriers might be different, and it might "just work" but at least
> here stateside then are going to nickel and dime you as much as they
> can.  Besides, doesn't that require some out of band signalling (or
> something similar that isn't in the base GSM spec)?

AFAIK in GSM, fax calls are similar to data calls, where the network does
the work of decoding the modem tones, so you get the full 9600bps of data
(rather than having to try to decode the tones from GSM-encoded sounds!).  
The network knows to do this based on either signalling from the calling
end (eg if the call originates from a GSM phone), or based on the called
number (eg if it is from a landline). This is why O2 give you the separate 
numbers for fax/data - without them, you can make data calls but not 
receive them, and you couldn't receive faxes (in fact you can't send them 
either, but I don't know of a reason for this).

> OT, does anybody actually still use fax machines?

I occasionally get asked how to use the one in the office, so I assume 
peope are still using it :-)

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