From: Timothy Newsham <newsham@wiliki.eng.hawaii.edu>
Date: Mon, 8 Nov 93 15:13:05 PST
To: pdn@dwroll.dw.att.com (Philippe Nave)
Subject: Re: Modem taps/Caller ID
In-Reply-To: <9311082230.AA29752@toad.com>
Message-ID: <9311082312.AA00818@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


> 
> 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.