Linda/Joe, To the best of my knowledge, guard tones were used to avoid disconnection by voice detectors, which monitored the mid band energy.
Linda, if you need any specific details about modems let me know. Roger Magnuson Manager, Marketing & Business Development TGC AB - The experts in analog terminal equipment Dalvagen 28, 169 56 SOLNA, Sweden TEL: +46 856250050 (direct) FAX: +46 87050045 (NOTE our new fax number from Sep 10: +46 856250045) mobile: +46 707770594 mailto:[email protected] internet: http://www.tgc.se -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of [email protected] Sent: Thursday, September 09, 1999 6:26 AM To: [email protected] Cc: [email protected] Subject: Re: Stupid Question, I Think In a message dated 9/8/99, Linda Bottero writes: > I have a very basic question. Analog modems:Where can I get information > regarding analog coding patterns 0,1 and 1:1, guard tone and scrambling > method in terms that I can understand after being out of engineering for 15 > years. Linda: The root source for the analog coding patterns and the scrambler polynomial can be found in the specific ITU standard for the modem in question. The modulation technique and symbol constellation tend to be different for each modem. I would be the last to suggest that simply reading the ITU standard will explain these items in terms that can be readily understood, but this is the starting point for narrowing the scope of your investigation. In general, the ITU standards are rather cryptic, and make sense only to modem designers. If you have been away from this for 15 years, you will be astounded at the complexity of the more recent modems such as V.34 and V.90. Things were far simpler back in the days of 1200 bps V.22 modems (I think V.22 was state of the art 15 years ago). Your question about guard tone is a little easier to answer. Guard tone was only used for split-band modems, to comply with the spectral density requirements in certain countries. These modems only place transmit energy in either the upper or lower half of the voice band, so some energy outside of the high band was added to comply with the regulatory requirement. I can no longer recall the technical reason for guard tone, but it had something to do with keeping the network happy. I do not think that guard tone is required anymore on most networks. For instance, an 1800 Hz guard tone was often used in conjuction with V.22 (1200 bps) and V.22bis (2400 bps) modems. The 1800 Hz tone falls in the dead region between the transmit band and the receive band. I seem to recall that the ITU also defined an optional 550 Hz guard tone, which falls below the low band. Guard tone is not used for higher speed echo-canceling modems such as V.32, V.34, etc. Anyway, I think your first step should be to identify which specific modem is of interest and get the relevant ITU standard. If you can use this information to narrow the scope of your questions, I would be happy to take a second stab at providing specific answers. By the way, why in the world are you interseted in this level of detail? (Just curious. If it's top secret, I understand). Joe Randolph Telecom Design Consultant Randolph Telecom, Inc.
