[Asterisk-Users] Which is the best fax-modem for testing ?

[Rich Adamson]

>      > So I was telling myself : "what if I could buy the most inclusive
>      > fax-modem, connect it to a PC, and run a bunch of test scripts to
>     gather
>      > useful information on both production and preparation systems ?".
> 
>     Total waste of money as the problem isn't the fax modem as noted above.
> 
> Hi,
> 
> In this case, the fax machine doesn't hangup (*) when connected to 
> TDM400P FXS port.
> It seems related to electrical incompatibilities we couldn't remove with 
> the help of Digium support though I can't personnally tell how far we 
> really went into studying this case with them.

Are you in the US?

You might try using a plain old voltmeter on tip & ring to "see" if 
there is any form of disconnect signal. In the US, you should see the 
voltmeter going to zero volts for at least a 1/4 second or so. If you 
don't see the disconnect, then the problem is asterisk/tdm oriented. If 
you do see the disconnect, the problem is in the fax machine.

> You're certainly right in that electrical incompatibilities involve 
> TDM400P capacities and Sangoma's A200D behaves differently.
> 
> Reading past requests on this list, I saw people had fax machines 
> working with TDM400P.

Some folks have been able to make it work, but its a very small 
percentage of implementations. The general consensus is that if zttest 
reports anything less then about 99%, faxes will not work properly.

The problem seems to be oriented around missed/lost data frames (across 
the pci bus) every xx number of seconds using the TDM card. If the 
missed/lost data occurs when the fax modem is actually sending data, the 
reproduced analog signal will be distorted. If it occurs between bursts 
of fax data, its less impacting. Its similar to clock slippage where 
clocking regains sync after xx seconds.

The same missed/lost data frames occur with the A200D, negatively 
impacting fax modem usage "if" the fax call crosses the pci bus. 
However, if the fax call stays on the A200D (as in fxs -> fxo on the 
exact same card), faxes function very reliably.

> So there must be something somewhere explaining why it doesn't work in 
> my case.
> 
> I thought a super fax-modem could be used as a reference case : you send 
> faxes with as many different settings as possible (speeds, protocols, 
> flash signals levels, ...) and then analyse performances.

That might provide some insight into the issue, but I don't believe its 
going to provide much in terms of root cause.

> Regards
> 
> (*) By "fax doesn't hangup", I mean though Asterisk server forward an 
> incoming fax call to the right extension, it keeps on ringing the fax 
> machine which never hangup. Maybe the flash signal is too weak

I'm very confused by the above statement.

What do you mean by "it keeps on ringing" and "machine never hangup" in 
the same sentence?  (No such thing as it keeps ringing and never hangup. 
Hangup occurs after answering, so if its ringing, it can't hangup.)

What do you mean by "flash signal is too weak"? (There's no such thing 
as a weak flash. Sort of equivalent to saying a weak binary "1".)