Subject: Re: tunneling data over fax ?
Date: Sun, 08 Jul 2007 15:24:49 -0400
I've been able to get modems to work over VOIP.
#1. Use the best codec available g711u.
#2. Network usage needs to be low
Many times if sync failed or I got excessive
retries it was because there was too much stuff
going across the network causing a poor voip connection.
I cannot 'use' a specific codec and I cannot make network traffic low.
The voip phones didn't provide any analog connection, so I opted for
keeping the 'analog' line. It just didn't work out as expected.
Modems require precise timing and VOIP is incompatible with modems. If
you have a clean open network and use the g711u codec (64kbps) you
should be able to get that to work. However since you are at
a university resources may be scarce so these ideal conditions
doubtfully exist.
Right. :-(
Post by Gert DoeringHi,
I think this has never been answered...
Post by w***@djo.tudelft.nlcould it be possible to tunnel data over a fax connection?
Has anyone done that yet?
since a major part of the world has switched to VOIP telephony,
our university thought it couldn't stay behind. With as result that
on the now 'analog' phone lines I cannot establish a working data
connection any more. It has even become so bad that the modem drops
the line already at the start of the handshake.
(It sounds awfully 'chunky').
Hooray...
Post by w***@djo.tudelft.nlFax and DTMF applications wouldn't work either.
They played a trick however to ensure that fax will work.
(I've seen outgoing faxes work flawlessly (at 14k4).)
So if the initial fax connection could switch over to data it could
remain unnoticed maybe.
Unfortunately, I don't think this is going to work - Fax is half-duplex
V.29, Modem is full-duplex V.32bis or similar, so it's a different
modulation, and depending on how they do it (uncompressed audio, or
special encodings for Fax), it might or might not work.
Ah. I even expect some more mean tricks like buffering the data stream
a bit, so that it will work for a few minutes to get one or two fax pages
through. That way it will meet the needs for most of the fax users.
Post by Gert DoeringOTOH, if they can make fax work, they should be able to make modem work
Oh, but I wouldn't be happy with just a few seconds for data. It should
better be hours...
Post by Gert Doering- modems do send very characteristic tones at the beginning of the
connections, so the VoIP system should be able to switch to "64 kbit
uncompressed audio". Depending on the network infrastructure in between,
it might still not work (too much jitter) :-(
The whole VoIP stuff is very bad for all sorts of fax/data transmission.
gert
Telex has become extinct, but fax is still in the game.
Thanks to mgetty I've enjoyed modem connections for many years.
Network connections have not been that reliable up to now, so I really
appreciate to have a phone line backdoor at hand.
The current situation is that I managed to get switched back to the old
PXB, but I'm afraid this won't be a long term solution.
I did some experiments worth noting:
Fax connections weren't perfect all the time ( so tunneling wouldn't even
work either).
Data connections up to 2400baud could be maintained for over 20 minutes (so
at least some basic access remains possible, which is much more that nothing
at all :)
Connections of 4800baud and up will get dropped usually within seconds.
Initial handshake must start at low speed. (In that case you'll hear just
one hickup right at the beginning, instead of chunky all the time.)
That leaves a few questions.
How can modems (all, a few, none?) be made to do a retrain and not drop the
connection?
Can modems be made to do a 'fall up' (as the opposite of 'fall back')
because it may be needed to start low again in order to get the codec switch
triggered (again?).
(Various 'Hayes commands' lists on internet aren't that helpful. Various
modem manuals that I have collected are very brief, cryptic or tell nothing
at all. And for most of the modems I found I don't even have a manual.)
Regards,
Wim Osterholt.
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