The recent discussions of tapping modem transmissions reminds me of a scrap of data I saw about Caller ID [the service provided by the phone company that displays the caller's phone number whenever your phone rings]. Supposedly, the phone company transmits the caller's number as a burst of 1200 or 2400 baud ASCII between the first and second rings; if your modem is set up 'just right,' you can capture the number with a PC.
The number is transmitted in ascii at 1200 bits per second. The standard used to transmit the data is not the "normal" 1200 bps mode of your modem. Normally 1200 bps is accomplished by sending 300 symbols per second where each symbol conveys 4 bits of information. The caller ID info is transmitted at 1200 bps by sending 1200 symbols per second where one symbol conveys 1 bit of information. The method used is FSK, one frequency is sent for a 1 bit, and another for a zero bit. I dont have the frequencies on me, but if you are really interested I can get them to you later. I have no idea how you would get your modem to go into the right mode (and how it would react to data on the line before it goes off hook).
Philippe D. Nave, Jr. | The person who does not use message encryption pdn@dwroll.dw.att.com | will soon be at the mercy of those who DO... Denver, Colorado USA | PGP public key: by arrangement.