maxim@subch.UUCP (Maxim Samo) (01/15/89)
Most Hayes compatible modems got an option to set a guard tone (none, 550 Hz or 1800 Hz). This mode is only used in CCITT mode (that's what the manual says). I don't have any Idea what a guard tone is and I havn't found any documentation of this feature. Anybody knows what it is? Maxim -- <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><> <> UUCP: <> SMAIL: <> PHONE: <> <> maxim@subch <> Maxim Samo,P.O.Box 188,CH-4125 Riehen <> +41 61 672771 <> <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
Kenneth_R_Jongsma@cup.portal.com (01/21/89)
Guard Tone disables the echo cancellers on some networks. It's supposed to improve the data rate of the circuit....
ch@maths.tcd.ie (Charles Bryant) (01/23/89)
In article <26@subch.UUCP> maxim@subch.UUCP (Maxim Samo) writes: >Most Hayes compatible modems got an option to set a guard tone (none, 550 Hz >or 1800 Hz). This mode is only used in CCITT mode (that's what the manual >says). I don't have any Idea what a guard tone is and I havn't found any >documentation of this feature. Anybody knows what it is? This option put out the tone constantly (along with the normal modem carrier). It prevents problems with in-band signalling. For example, in Great Britain a pure tone of 2280 Hz will clear a call, since the Bell 103 and 212 modems emit 2225 Hz when answering, this would clear a call in Britain (hence a BBS using Bell standards would never let anyone log on!). This can be a problem with answering machines also. If you change the message by dialling up the machine, when you hang up some of this tone gets recorded at the end of your message so every time someone calls you, the machine plays your message, and immediately cuts them off! As the exchanges only respond to pure tones, an extra tone will stop them recognising modem carriers as commands hence the guard tones. The 550 Hz is used in Scandinavian countries, and 1800 Hz elsewhere (though I don't know why there is a difference). Charles Bryant (ch@dce.ie) I work for Datacode Electronics, modem manufacturers and read news on maths.tcd.ie -- Charles Bryant. Working at Datacode Electronics Ltd.