by Philo Farnsworth

Created on: January 23, 2009   Last Updated: January 24, 2009

So you want to know what that blinky thing you plug into your computer does? Whether you use dial-up, DSL or cable, you no doubt know it as a modem. All modems contain a tuner, demodulator, MAC (media access control) address, a microprocessor and a transmitter.

There are two basic types of modems; analog and digital. When you are talking about dial-up, you are talking about analog. In analog systems, your computer speaks a different language than the data transportation system. The modem translates (modulates) signal so you computer can use it.

In simple terms, modulation is the process of varying periodic waveforms (tone), in order to convey data. Volume (amplitude), timing (phase) and pitch (frequency) can all be varied to convey a specific meaning. It's easy to think of it in musical terms. Think of a trumpet. There are only three valves on a trumpet. However, a musician can still play an infinite number of melodies by varying the volume, timing and pitch.

A device that performs modulation is known as a modulator and a device that decodes the modulation is known as a demodulator. A device that can do both operations is a modem, an acronym for "modulate-demodulate".

Cable and Digital Subscriber Line (DSL) modems work a bit differently than analog. Your computer "speaks" in digital terms (binary) and the transportation signal is also digital. The term "modem" in this sense is not exactly correct, as digital modems work more like an interface device in a LAN (local area network) than an actual modem.

With DSL, there are two main pieces of equipment that do the work. At the customer's location there is a DSL transceiver (the modem). The device that the DSL transceiver talks to is called a DSL Access Multiplexer (DSLAM.) A DSLAM takes connections from many points and combines them onto a single, high-capacity connection to the internet.

Cable modems work in much the same way. However, cable systems are closed radio (RF) systems. So the signal coming from your computer enters the modem at the modulator, which converts the digital data into an RF signal. The RF signal then enters the transmitter, which sends it out on the network. Cable modems send and receive signals to a CMTS (cable modem termination system.) If the modem wants to send something, it can only send it to the CMTS and only in time slots allocated and controlled by the CMTS.

Congratulations, you've just taken a swim in the geek pool.

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