Fax protocol

[Qacer]

Can someone explain to me how fax signalling works?

I'm trying to figure out if I need to press a fax issue with Verizon. I have FIOS Digital Voice. I attempted earlier to send a fax within the US. But whenever I called that number with my Verizon phone, all it did was ring. No answer. I tried calling the same number with my cell phone, and I immediately got the fax tones.

I called Verizon tech support, and they said that the reason why the other number is not picking up is because the other line's fax machine does not support calls from a digital line. I don't quite understand this, so I'm trying to figure out how fax machine negotiate calls with other machines.

All I know is that I was able to send fax before using my Vonage line.

Thanks!

 

[Mark R]

The basic answering is very simple:

Receiving fax machine waits.
Incoming call detected by high-voltage ring signal (same signal as causes normal phone to ring)
Receiving fax machine picks up after a pre-set number of rings (usually 1 or 2 for a dedicated fax line)
Receiving fax machine sends greeting tone
Sending fax waits to hear greeting tone. Once tone is heard, fax negotiation begins.

There basically isn't a way for a fax machine to know where a call is coming from before it picks up and tries to negotiation.

If the fax machine isn't picking up - then it's either not switched on, has errored out (e.g. run out of paper), or has been configured not to auto-answer (or the call is not getting to it).

Theoretically, there is the option of checking caller ID - but that would be a very unusual feature to see on a fax machine.

 

[Qacer]

Thanks!

There basically isn't a way for a fax machine to know where a call is coming from before it picks up and tries to negotiation.

This is what I was trying to ping down. Verizon told me that the reason why the other fax machine is not picking up is because the fax machine on the other line is old and does not support faxing on FIOS Digital Voice.

I can understand faxing not complete going thru, but if a fax machine does not pick up if being called on a Verizon line, but yet picks up when called on a non-Verizon line, then I would think it has something to do with Verizon.

 

[Mark R]

One possible explanation is that the ring signal is different when called from Verizon or non-verizon.

If the fax-machine receives a different (non-standard cadence) ring signal, it may not pick up, because it doesn't recognise it as a ring signal. But that's an issue for the line provider at the remote end.

For phones this doesn't really matter - as a filter circuit just diverts the ringing signal direct to the phone's bell - so the bell will ring with whatever cadence is supplied by the network. Non-standard cadence does occasionally cause problems with modems in auto-answer mode (hence why some brands of modem had localised firmware - to ensure that ring cadence was correctly interpreted in the country the modem was to be used in).

 

[Qacer]

Ahh.. I see. I decided to read up on ring cadence, and it seems that the ringing pattern is mostly the same in a region (e.g. US ringing pattern is mostly the same in the country). Is this true?

 

[Mark R]

Ahh.. I see. I decided to read up on ring cadence, and it seems that the ringing pattern is mostly the same in a region (e.g. US ringing pattern is mostly the same in the country). Is this true?

Yes. It's generally the same in a country.

However, some providers can give additional services. E.g. About 10 years ago, when digital exchanges started being introduced here - the phone provider started offering custom rings. They would send a different cadence based upon caller ID.

So if you here 'ring pause ring pause' you know that it's some random caller. 'ring ring pause ring ring pause' you know it's work. 'ring ring ring pause ring ring ring pause' you know it's the in=laws, etc.

Similar features are used on PBXs to discriminate external and internal calls.