Chapter 5 - Telephone Modems
It was in the 1970s and later the 80s that a critical peice of technology (for computer networking) was invented and popularized. That technology being the telephone modem. A modem, which is short "Modulator Demodulator" has the purpose of taking an electrical signal and turning it into an audio signal that can survive transmission across whatever systems it might be sent over. The telephone modem was invented to shift the naive electrical signals around until they produced signals in the same frequency ranges as human voice. This is because telephone systems were optimized for those frequencies. With the use of a telephone modem, you can take your fancy personal computer's casette deck connector and instead of plugging it into a casette deck, you can send those signals all the way over a phone connection.
The earliest telephone modems were extremely simple. They had their own microphone and speaker in a "cradle", and you would set the handset from your old timey desktop or wall mounted telephone into that cradle so that the speaker of your phone handset would be next to the microphone of the modem, and the speaker of the modem would be next to the microphone of the handset. The modem would then make its audible sounds to the phone as if it was an actual person talking into the phone. Though, instead of speaking in a human language it mostly just made really awful sounding screeching noises.
The scheme worked like this:
- You dial your friend's phone number
- You tell your friend "I want to send you some data" e.g. a text file with some ascii art nudes
- You and your friend run an application like ZMODEM and configure all the serial parameters (baud rate, data bits, etc)
- You and your friend both place your phone handsets into the cradles of your own telephone modems.
- You let the computers send the data serially, one byte at a time. For "large" files this could take hours.
- You hope the phone call doesn't get disconnected
This setup doesn't use literally the same structure as UART but it follows the same principles. Your computer wiggles the voltage "high" and "low", the modems modulate and demodulate the signal to make sure that it survives the phone line, then the other computer watches it go "high" and "low" so it can decode the binary data.
The usage of telephone modems wasn't limited exclusively to transmitting files. It was also used (and in fact this use case came much earlier) for transmitting live TTY data. You could take your fancy video terminal teletype machine like the DEC VT100, make a phone call to a computer across town, set the phone into your serial modem's cradle, and allow the video terminal direct access to that computer. This was often done to access some type of mainframe system, like a Bulletin Board System. A Bulletin Board System was, more or less, a computer whose basic purpose was to allow people to sign-in and post on the very first message boards. You could send a message to one of these boards, then someone else who calls the BBS later can log in and read the message from that board.