The word Modem is an acronym for Modulate-Dem
odulate. A modem converts (modulates) digital signals into analog signals that
can be sent over telephone lines. The Modem at the receiving end converts the
analog signals back into digital signals.
Getting pages from this web site to your PC requires a modem of some type (dial-up, cable or DSL), which converts the digital data into analog signals that travel to the phone company and then out over the Internet.
The move to Broadband
Most people still use dial-up modems. If you do, you should consider switching to a cable or DSL (broadband) modem. Both operate at much higher speed than dial-up modems.
We will provide information on this site that can help you decide.
PCs, printers and other digital output devices can not connect directly to telephone lines.
Digital devices produce outputs that can be in one of two distinct states, 0 or 1. Like a stair step, they jump from one level to the other, never stopping in between.
In contrast, analog signals like that which is produced by a human voice, vary continuously in time in a smoother, more hill shaped manner.
Digital signaling produces very high frequencies, that get attenuated (reduced) as the signal goes down the line. Loss of these frequencies at the receiving end can result in a loss of information.
The way around this is to use the digital 1s and 0s to vary an analog carrier, which is sent down the line.
Modulate
Modulate means to encode. A simple modulation type is Frequency Shift Keying. In this modulation scheme when the modem sees a digital 1 it sets the carrier to one tone (A) and sends it down the phone line. When it sees a 0 it changes the carrier to a different tone (B) and sends it down the line.
Demodulate
Demodulate means to decode. The receiving modem upon detecting a tone A converts it to a 1. And when it detects tone B, it converts it to a 0.
Training Tones
Data is not sent until after the modems have connected and
trained. When your computer first connects to a line, the sporadic tones you
hear are your modem and the one at the other end going through a training mode.
They are searching for a rate they can both support using the current phone
line. If your modem is 56K and the other is 14.4k, the fastest they can go is
14.4K. If during training the line won't support 14.4k, due to noise which
causes errors, they have to drop to a lower rate that the line can support,
like 9.6K for instance.
Morse Code
Morse code is a very old code where each character (letter, number or control) is assigned a unique sequence of dots and dashes. A Morse code operator sending dots and dashes down a telegraph line is modulating a carrier tone. A dot is a short burst of an analog tone. A dash is long burst of the same analog tone. Morse code was first used in telegraph communications. Many ham radio operators still use it today.
The Morse code operator receiving the stream knows what the relative length of the tones mean. He knows that a short tone represents a dot and a long tone is a dash. So he demodulates (decodes) the tones he hears on the line back into dots and dashes. Between each coded character sequence of dots and dashes is a short rest period which is required so that the the beginning and end of each charater is known.
In Morse code the sequence 'dot dot dot' represents the letter 'S' and 'dash dash dash' represents the letter 'O.'
So the sequence:
'dot dot dot - rest - dash dash dash - rest - dot dot dot'
coming down the line represents the letters SOS which happens to be the universal distress signal. People who have become trapped will often tap out SOS, hoping that rescuers will hear their call for help.
Summary
coming down the line represents the letters SOS which happens to be the universal distress signal. People who have become trapped will often tap out SOS, hoping that rescuers will hear their call for help.
Summary
In Morse code, dots and dashes are sent down the line at a very slow rate, about 50 characters a minute. 10baseT Ethernet by contrast sends about 1,000,000 characters per second!
In modern digital communications the digital stream of 1s and 0s are modulated onto phone lines by modems using amplitude, frequency and phase modulation or various combinations of them. The receiving modem uses a compatible demodulation scheme to convert the modulated signal back into the original sequence of 1s and 0s.