Re: OT - Incoming fax cuts ADSL line

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On 17/05/2012 12:18 AM, James Sharp wrote:
On 5/16/2012 12:07 PM, Tim Nelson wrote:
----- Original Message -----

I'm facing a strange situation.
Though it's not directly related to Asterisk, I do think it is
interesting to this mailing list.

The setup is a single line which is split between an ADSL
modem/routeur and a fax machine (Asterisk was removed from the

Any time the fax machine rings (incoming fax), the ADSL service is
troubled to the VPN users are disconnected.
It can be reproduced at will.

I've changed the ADSL filter twice (a different unit, then a
model) without any visible change.
What could explain this ?

I've experienced this quite a few times, and after working with a local telco, it has become policy to not place ADSL on lines where fax is going to be used. I'm unsure of the exact technical reasons behind this other than 'the fax signals/frequencies interfere with the ADSL signalling/frequencies used on the circuit'. It sounds like you might want to separate your fax/ADSL lines.


You might also be able to limit the Fax machines maximum transmission rate so the modem's transmission spectrum doesn't inch up into where the ADSL service is.

I have clients with their ADSL2+ service attached to their fax lines with no problems observed.

Perhaps the issue is the fax machines attenuators are not set correctly are are to _loud_ on the PSTN.

In Australia Telstra advised the signal level received at the exchange should be between -15dB and -17dB. They have a Fax On Line Diagnostic System (FOLDS) which you can send a transmission to, a report is returned advising of the quality of your transmission including measured signal and noise levels.

In old days the fax machine might have a wired jumper block to set the attenuation, more modern devices would be configured from the front panel, typically in a maintenance mode. Your good old dial-up modems with fax capabilities would have an S-Register or two to set the attenuation.


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