Re: [hylafax-users] T.30 T2 Timeout, Digital Phone Lines and Modem Speed

[Lee Howard <faxguy@xxxxxxxxxxxxxxxx>]

Brad Stevens wrote:
> I'll have to differ to all the others who answered this problem in the
> past, as they found the problem I just passed it along.  The good part
> is Hylafax works great with the Multitech. The bad part is that older
> versions of Hylafax used to work great with the old USR modems.
> Something changed that made the USR compatibility go downhill fast.

While older versions of HylaFAX worked better with USR modems by default 
there were still many problems when using USR modems that could not be 
overcome.  One major issue that we were dealing with in the Class 1 
protocol was the issue of how to guarantee the amount of silence on the 
line (audio) before responding to a signal by the remote system.  In old 
versions of HylaFAX this was accomplished with a software pause.  
However, the software doesn't know what's going on with the audio itself 
- that's the modem's job.  And, we discovered that in many situations 
the modem would respond with an "OK" after receiving a signal before the 
line actually went silent.  Consequently, the concept of a software 
pause guaranteeing any particular period of silence on the line was 
regularly flawed.

We tried playing with the length of the pause... increasing it, 
generally.  But then we found that as we increased it we would break 
protocol in situations where "OK" did occur close to the point in time 
when silence began.  So we had to turn to the modems and rely on them to 
tell us when a particular amount of silence had passed.  Many Class 1 
modems have a command for this, AT+FRS (to receive silence) and AT+FTS 
(to send silence), some however do not.  USR modems claim to have this 
support, and it works much of the time, however, it tends to cause the 
USR firmware to crash over time, and it causes more problems for USR 
modems than it resolves.  So USR modem users have to return to rely on 
the old software pauses.

So there's a period of time when the "default" HylaFAX installation and 
configuration for USR modems had them relying on AT+FRS/AT+FTS and so if 
you happened to develop your modem config file based on those prototypes 
you would need to manually alter them to get back to the software 
pauses... or you'd need to re-run faxaddmodem after upgrading HylaFAX to 
a recent version where the default config has them using software pauses.

You can configure a USR modem "correctly" now and you can get back to a 
reliability level that even exceeds that of the "old" versions of 
HylaFAX to which you refer.  For many people this will be more than good 
enough.  However, due to the inability to guarantee periods of silence 
on the line you'll never be able to match the reliability level that you 
can get with now with a modem that properly implements AT+FRS/AT+FTS.

Thanks,

Lee.


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