Sep 14, 2012

What is a modem?

MoDem(Modulator And Demodulator)
The most common type of Modem in a regular household is either a dsl,cable, or dialup modem, and they are all used to connect with the internet..

The length of a digital signal is very limited based on it's high speed and nature. 

when longer connection distances are needed, special analog types of communication are needed which can reliably transfer the signal to the end point (ISP, phone switching station, Cable Distribution network, etc).
Since data is a set of bits (0 and 1) [look up binary], men have to come up with symbols and tables of different levels within their modulation scheme (Frequency, Amplitudes, Phase angle [to the carrier), etc) to help the modem encode and decode the analog steam it's about to send/receive on the analog medium (line,cable,phone cable, airwave).

If the Signal to Noise Ratio (SNR) is high enough, than many symbols can be created to represent more than one bit at a time [look up bit, byte, nibble). If this is true, then the
over throughput of the data is much higher than when there is a lot of noise on the line/medium/cable and the modulator has to leave bigger gaps between his symbols to it's easier to decode them.

Two interesting MODEM standards are the DOCIS standard for cable modems which use a combination of amplitude and phase shift to send symbols to the other end. And, Wireless Modems (Wireless Cards for WiFI) also use a great range of radio modulation/demodulation tricks to help squeeze as many symbols into the air as possible and reconstruct the original signal (without as much of the noise as possible).

The best modems understand the electrical and physical properties of their medium, whether it's the unreliable land-line phone cord, to the 75Ohm coaxial cable, or the ethernet cord, etc... and carefully tune their symbols and interfaces to have the highest chance of being properly decoded on the other side. Also, often a modem will insert a timing frame for reference, and forward error correction to ensure that tiny misses/mistakes can be easily corrected at the receiving end without having to dump the transmission and restart at a lower speed.

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