[asterisk-users] OT - Incoming fax cuts ADSL line

[Larry Moore]

On 17/05/2012 12:18 AM, James Sharp wrote:
> On 5/16/2012 12:07 PM, Tim Nelson wrote:
>> ----- Original Message -----
>>> Hi,
>>>
>>> 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
>>> equation).
>>>
>>> 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
>>> different
>>> 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.
>>
>> --Tim
>
> 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.

Larry.