[Asterisk-Users] zaptel and low ring voltage

[Jim Van Meggelen]

asterisk-users-bounces at lists.digium.com wrote:
> Inline...
> 
>>> Several months ago we built an * box with a quad-FXO tdm400p (REV
>>> e/f).
>>>> From the get-go, there has been a problem where occasionally
>>> (2-3 times
>>> a week) zaptel/* will not detect the ringing on a line. (The call
>>> will ring through to telco voicemail).
>>> 
>>> The problem is not specific to a single line or FXO port on the
>>> tdm400p. 
>>> 
>>> I have 2 theories:
>>> 
>>> #1 - the ring voltage for some calls is below acceptable levels
>> 
>> Possible, but also possible that there is too much loss on the
>> circuit. 
>> 
>> You can test the ringing voltage with a meter, it needs to be
>> between 90V and 110V. 
>> 
>> Beyond that you may need to use a transmission test set (such as a
>> Wilcom T136B). I got mine for $20 bucks on eBay. Using a butt set and
>> the test set you'll need to call a 1004Hz source from TELUS and then
>> check that you're within the following specs:
>> Loop mA:		23 or better (too hot is no good either, but I
>> doubt that's your problem)
>> Circuit loss:	between 0 and -8dB. 0 is really too hot, -3 to
-6 is
>> nominal, -8.5 is pushing it, but still within spec.
> 
> Not likely the above is the root-cause as that implies the
> pstn line is 'changing' characteristics on almost a daily
> basis (highly unlikely). Also, the spec's for the 3050 chip (on the
> tdm fxo module) indicates it will detect ringing voltage down to 40v
> (or 
> lower on some
> some specific configurations).

Well, I can't tell you how many times I've been involved with PSTN
circuits that have characteristics change on a daily (or even hourly)
basis. A common cause of impairments on a circuit is moisture. A
perfectly acceptable circuit can become next to useless after a rain
storm. Another fun one can be induced AC voltage caused by power lines.

Also, it has been my experience that when several portions of a circuit
are operating at their limits, intermittant problems are not only
possible, but are in fact to be expected.

Finally, if this chip is capable of detecting voltage as low as 40V, it
still begs the question of wheter the minimum ring voltage is something
that can be configured, and how that is set in the zaptel/zapata
drivers.

All that aside, I do not want to give the impression that the items I
discussed ARE the problem. Merely that those are characteristics which
can be checked if one wants to be sure the line is within
specifications.


>>> #2 - the tdm400p card is bad
>>> 
>>> Assuming #1, can the zaptel driver be tweaked to be more sensitive
>>> to ringing? 
>>> 
>>> Any other ideas or experiences?
>>> 
>>> Running asterisk/zaptel v1.0.2
> 
> If you search through the archives you'll see others have
> also had issues with the tdm using fxo modules. Lots of
> references relative to stop/restart asterisk, unload/reload wctdm &
> zaptel, reboots, hear nothing but white noise, can only reload zaptel
> so many times before a machine reboot is necessary, etc. Seems there
> has been (maybe still are) issues that are driver related,
> kernel related, and/or possibly distro related.
> 
> Some folks seem to have the same issues more frequently then
> others, which would suggest there is some sensitivity to call
> volumes or something like that. Several use the word monthly, others
> weekly, etc. 

Sounds like a memory leak. But it could also be power & grounding,
chipset, power supply, circuit conditions, EMI, heat dissipation, sun
spots, humidity.

If EVERYONE had a problem, then we'd be onto something. If only SOME
people have the problem, it is much more difficult (and misleading) to
assume that there is a single problem which applied to all.

> The downside of this is there are very few developers that
> actually use the tdm-fxo card in their systems, and only a subset of
> those seem to have the skills to diagnose wctdm/zaptel driver issues.

This is very definitely an unfortunate reality.