Upgrading & Fixing Laptops DUMmIES

When Scotch-born Bostonian Alexander Graham Bell patented his telephone

in 1876 — beating by just a few hours a similar claim by Chicago inventor

Elisha Gray, which is why we old fogies grew up complaining about Ma Bell

instead of Ma Gray — he created a system that was based on analog signaling.

Think of an analogsignal as an analogyrather than a discrete code. The pitch

and volume of the human voice were represented by the peaks and valleys

and frequency of an electrical wave. That system works very well from point

to point — if you can string a continuous pair of wires from one house or

office to another a reasonable distance away. But because of electrical resis-

tance in the wires, once the signal travels more than a few hundred feet it

needs to be boosted back to original levels by an amplifier. And then there is

the issue of being able to connect any one telephone to another on an ad hoc

basis: That requires switching of the signal from one circuit to another.

Bell’s design was intended to be analog all the way, including amplifiers and

switches. It is for that reason that telephone services generally cannot carry

pure digital 0s and 1s any distance. (You might be able to send signals within

a home or office, but once you hit a switch or a non-digital amplifier, you’ve

got a problem.)

So, a standard telephone modem converts the digital bits to an analog

warble. When a modem first connects, you can usually hear the two devices

whistling at each other to agree on a speed and communication method; if

you were to lift a telephone handset on the circuit you would hear the audible

conversation of computers. When a computer communicates over a telephone,

the 0s and 1s are modulated into a high or low warble of tones. At the receiving

end, the warble is demodulated and the highs converted to a 1 and the lows

to a 0. Therein comes the name of the device: Modemcomes as a concatenation

(combining) of the phrase modulator-demodulator.

226 Part IV: Failing to Communicate

AM/FM

If the computer were instead using radio waves

for communication, it could employ a number of

encoding schemes with the most common

being AM(amplitude modulation) or FM(fre-

quency modulation). You’ll recognize those

terms from conventional radio stations.

An AM signal conveys information by the volume

or intensity of its signal. If a computer sends data

by amplitude modulation, the radio wave is made

up of peaks and valleys; think of a high spike as

a 1 and low spike as a 0 and you can understand

the basics. FM signals convey information by

sending out a wave with uniform peaks and val-

leys but varying time intervals between them,

sort of a Morse code of dashes and dots (or in

computer terms, 1s and 0s). The same sort of

method is used when data is sent by infrared or

by pulses of light over a fiber optic cable.