ergo@ames.arc.nasa.gov (Isaac Rabinovitch) (05/26/90)
claris!netcom!onymouse@ames.arc.nasa.gov (John Debert) writes: (about computer terminals in police cars) >I have heard the transmissions to and from these units and estimate >the rate at about 1200baud. It shouldn't be too hard for someone with >perhaps a TNC to connect their scanner to a terminal and read the >traffic. True. But it ought to be possible to encrypt transmissions, if they haven't already done so. I don't know if it's actually possible to provide an unbreakable encryption method (this was claimed at one time; I haven't followed the issue closely but I understand there are doubts) but at least it can put evesdropping out of the reach of the less resourceful villains. For that matter, they could do the same thing with voice channels, but it's a whole lot easier to do this with the relatively small number of bits in a data channel.
brian@ucsd.edu (Brian Kantor) (05/27/90)
The older data terminals (such as the "MODAT") didn't send ASCII, and it wasn't 10-bit async frames. What was actually being sent was the raster image of the characters to be printed, a scanline at a time, on the assumption that if mobile chop (brief periods of squelch closure due to nulls in coverage in a moving car) or ignition noise pulses were to eat some of the data stream, you'd only lose a few dots and the human eye/mind could easily fill in the missing data, since the image of a character contains a LOT of redundant information (just consider how easily you can read a bad photocopy). Those were the days when microprocessors like the 8080 had only been on the market a year or so, and they cost $125 each, so there weren't many outside the lab. Nowadays more sophisticated error-correction systems are used, although I'm not up on the exact details. I'll try to find out - although I'm not in the two-way business anymore, lots of friends still are. In any case, I've done some snooping, and nothing I have here will decode the radio teleprinter stuff I can hear on my spy radio, which means that it's either encrypted and/or it's not ASCII, Baudot, AX.25, SITOR, nor some other common codes. - Brian