Wednesday, June 11, 2014

The Modem (Part 1)

Older readers might associate the word "modem" with the dial-up noise. That noise is the sound of the modem, communicating with another modem. The first tones set a speed they can communicate at. Then the SYN-ACK sets other parameters like the bit number and parity. Then they check the rate, set up duplex for simultaneous communication and viola! throughput. For younger readers it may mean nothing at all. It's just a very annoying noise. A modem is a device that modulates an analog carrier signal to encode digital information and demodulates the signal to decode the transmitted information. The word is a Portmanteau of Modulate and Demodulate.

modem (ˈməʊdɛm) — n computing a device for connecting two computers by a telephone line, consisting of a modulator that converts computer signals into audio signals and a corresponding demodulator
Most sources cite the first modems as being invented in "the 1950s" a reference to the DEWline (Distant Early Waming) or sometimes specifically will refer to the PC modem which was invented in 1977 by Dennis C. Hayes and Dale Heatherington. Neither of these were the first modem. There were modems long before there were computers and even long before there was a cold war.

The modem evolved from news wire services in the 1920s. In order to transmit data over a phone line it had to be converted (modulated) into an analog signal so it could be multiplexed on the telegraph wire. To receive it, the same analog signal had to be converted (demodulated) back into signals the telegraph could receive.  This allowed a much more efficient use of the available spectrum. While the goal as multiplexing, those modulate/demodulate processes are comparable to those of later modems. Note how much that sounds like the A-to-D and D-to-A conversions you do in digital audio.