Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!uakari.primate.wisc.edu!ginosko!usc!bloom-beacon!eru!luth!sunic!mcsun!ukc!tcdcs!tcdmath!ch
From: ch@maths.tcd.ie (Charles Bryant)
Newsgroups: comp.dcom.modems
Subject: Re: Summary: V21(bis) guard tones.
Message-ID: <1217@hamilton.maths.tcd.ie>
Date: 9 Oct 89 12:18:03 GMT
References: <4531@yarra.oz.au>
Reply-To: ch@maths.tcd.ie (Charles Bryant)
Organization: Maths Dept., Trinity College, Dublin, Ireland.
Lines: 42

In article <4531@yarra.oz.au> chris@yarra.oz.au (Chris Jankowski) writes:
>
>The original question was:
>
>> My TrailBlazer manual lists the following options to select guard tones
>> apparently used by V22 and V22bis modems during call establishment:
>> 	S91=0	- no guard tone
>> 	S91=1	- use 1800 Hz guard tone
>> 	S91=2	- use 550 Hz guard tone

Some phone systems use in-band signalling. i.e. A pure tone of the right
freqency triggers some action. To prevent this happening when modems are in use
one of the modems puts out a guard tone (so there can't be a only one pure tone)
It is always the answering modem that puts out this tone. An example of in-band
signalling is given in one of your responses:

>	From: bam@monet.berkeley.edu (Bret Marquis)
>Used to be that I would make a connection from California to London and
>everything would be fine until the connection was established.  The
>line would then immediately disconnect...  Changing guard tones was
>the fix...

I think British Telecom have equipment that clears a call when 2280 Hz is
detected. (Bell 212 answers with 2225 Hz which is close enough). I am told that
this can cause trouble with answering machines - a caller leaves a message and
hangs up, his local exchange emits this tone to clear the call, a bit of the
tone gets recorded at the end of the message, the owner of the answering machine
dials in to hear the messages and is cut off when the tone is played back. [I
have never actually observed this. It would be very annoying if you had given
your answering a command such as "replay and erase all messages"].

Note that the 1800 Hz is *not* the tone that is intended to disable echo
suppressors. That tone is the 3 seconds of 2100 Hz emitted by the answering
modem. (To disable echo cancellers also, phase reversals are put into this tone
every 450ms).

Anyone from B.T. out there who can confim or deny this? I *know* you're there
 - or in eunet.jokes anyway :-)
-- 

		Charles Bryant. (ch@dce.ie)
Working at Datacode Electronics Ltd. (Modem manufacturers)