larson@sri-unix.ARPA (Alan Larson) (11/14/86)
The problem with noise between two modems may not lie in the modems. It may be the telephone switching system introducing noise into the line, resulting in garbage characters. In this area, I have a noise that sounds like a brief buzz in the background that comes every few seconds on the line. It occurs with a frequency similar to the error interval. On the 2400 baud modem, it will cause the 'signal quality' light to go out at the same time as the noise appears on the screen. I suspect that the problem is caused by the switching between different phone switches. I have to call through two telephone company central offices, and the SL/1, so I am going through 3 switches. All are ESS. Most modems use frequency division of the channel to provide full duplex data communication. Any noise that appears only in the frequency range of one direction will not be heard in the other direction. This seems to be the most common form of error I encounter. (Noise that is sent 'to' the computer will frequently appear on the screen, since it is echoed by most of our favorite computers.) Alan