Modem
The telephone line to a house was built to carry voices, not 1s and 0s. To send a computer’s data down it, something has to turn the data into the kind of signal a voice line carries. That something is a modem.
1Two kinds of signal
Has only two levels, high and low, standing for 1 and 0. It jumps straight from one to the other. This is what computers use.
Rises and falls smoothly, like a wave, and can take any value in between. This is what telephone lines were built to carry.
2What a modem is
- Modulation — digital to analog. Done by the modem at the sender’s end.
- Demodulation — analog to digital. Done by the modem at the receiver’s end.
Watch the wave change at each modem. Then switch to receiving. Drag sideways to turn.
The analog wave is drawn the way the first modems made it. Modern modems use cleverer wave patterns to fit far more bits in, but the job is the same.
Notice that the bits never change — only the form of the signal carrying them does. And the two modems swap jobs when the direction changes. Every modem can modulate and demodulate, which is why both halves are in its name.
3Where a modem is used
A modem sits where a home or office network meets the line from its Internet Service Provider (ISP) — the company that connects it to the Internet. Telephone lines, cable TV lines and the lines of broadband companies all need one. Fibre-optic connections use a similar box that works with light, often still called a modem.
4Check yourself
What does MODEM stand for?
Fill in the blank: The modem at the sender's computer end acts as a ________.
Which device converts analog signals to digital signals and vice versa?