chris@yarra.oz.au (Chris Jankowski) (10/03/89)
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 > Questions: > -What are the guard tones for? > -Why you have three options here and are they mutually exclusive? > -Is there any indication what modem may use which option - by country > of manufacturing perhaps? Responses (edited by me _without_ authorisation): From: CHRISTER OLSSON <mcvax!tekno.chalmers.se!CTH_CO@murtoa.cs.mu.oz> Some long distance phone lines have simplex transmission of voice. It is done this way to prevent echos in very long analog phone lines. The guard tone shuts down simplex transmission and lets the phone line to be used in true duplex transmission mode with modem use. Those simplex lines are rather rare. Especially in USA. 1800 Hz guard tone is used in Great Britain and 550Hz in Sweden. The guard tone is the second tone the modem answers with. From: smb@ulysses.att.com They're used to disable echo suppressors on assorted national phone systems. I think that CCITT specifies 1800, the U.S. doesn't use it, and I have no idea what Australia uses. (More precisely, the normal answer tone here (USA) also activates the echo suppressors.) From: jwb@cit5.cit.oz (Jim Breen) The 1800Hz is the "standard" guard tone. The 550Hz was to keep the Yanks happy - something about compatibility with 212A, or the like. V22 and V22bis both say that the suppression of the 1800 and the inclusion of the 550 is a "national option". The guard tones are only used while transmitting - 1800 is used while the upper band is being used, 550 while the lower. I think it is something the receiver can lock into as a reference, but am really not too sure. From: bam@monet.berkeley.edu (Bret Marquis) Can't answer all your questions, but the reason for having different guard tones are because some international phone companies use different tones to deactivate their echo supressors. Also depending on the country contacted, different tones are used to terminate a phone connection. 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... ******************************************** Well, I am still a bit confused. What exactly goes wrong if you use/do not use/use the other guard tone? Why this is only important in V22(bis) mode?
ch@maths.tcd.ie (Charles Bryant) (10/09/89)
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)