On Aug 15, 1996 at 00:27 hrs, Scott Little of 3:712/848 wrote to All:

Hello Scott,

 SL> What does the guard tone do?

I understand that these are required if the circuit includes overseas
cables and maybe satellites as well.  They are not usually required in Oz.

These circuits have a limited bandwidth and sample the channels to see if
the channel is quiet of not.  If it is quiet another circuit is routed onto
the channel for the period that the original call is quiet.  Since there
often appears gaps in voice conversations it is possible to squeeze up to
15 circuits through a 12 channel system.

The frequency sampled by the equipment is a point where speech energy is
usually found so that it doesn't switch when someone is speaking.  The
equipment will not switch a channel that is in use.  The problem is that
the sample frequency used for voice calls is not a good frequency to sample
for modem to modem calls and these calls are likely to be switched.  To
stop this happening a tone is placed on the channel at the sampled
frequency so that the channel will appear as busy to the equipment and not
be switched.

Regards ... John
@EOT:

--- MsgedSQ/2 3.30