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Modem is short for Modulator Demodulator.
What modem are really doing is converting your computer's digital signals ie zeros and ones into analog signals that is sound, this so that we can send computer information from a computer via eg telephone network to another computer with a modem.

How is the modulation and demodulation for?

There are different types of modulation, the one called frequency modulation it enters so as to allow the digital signals of ones and zeros represented by two tones.

A different kind of modulation is amplitude modulation it goes much the same way, though there corresponds a strong tone one and a weak tone, or no tone neutral, then you have not decided which notes are ones and which ones are zeros at frequency modulation.

The most widely used of modulation modes is called phase modulation, which are ones and zeros are not a special tone without wave phase turn 180 ° when the information sent switching from one to zero. This method mostly used at higher speeds.

How modems sync at transfer.

For any of these modulation methods to work, the modems to communicate all the time is "in phase" with each other. This is because the modems need to know where a piece begins and where one piece ends.

To do this, there are two methods that are used, asynchronous and synchronous communication. Asynchronous transfer involves the one to each character, that is, byte add a start bit at the beginning of each character, and one or two stop bits at the end of all the characters. To verified the information you sent arrived at the receiver send you something called parity bits. It has now become standard practice to not use parity bits. An advantage of asynchronous transmission is that the modems do not need to be synchronized, that is "in phase" with each other when they do not send any information. A disadvantage is all unnecessary information that must be sent, it will be about 2-3 additional bits for each character.

The second variant, synchronous transmission works so that in the early modem sends one sync dialed characters at the beginning and then every 100 to 10,000 characters. This sign tells mottagarmodemet that now information. This is great when you want to send large amounts. In order to use the synchronous transmission is required to have special equipment, which can interpret the clock pulses. The advantage of synchronous transmission is that the communication goes up to 20% faster. The downside is that when information flows that way, it is easy for the connection between the modem is broken.

Why not collide information?

To prevent the sending of information from both directions and collide makes use of two channels, an A channel and a B channel. When a modem connects to another channel so decides one modem to be an A-modem, the modem will send the A-channel and receiving the B channel, the other modem B modem do the opposite. A becomes A modem and which one gets B modem does not matter but this is determined at call and can, if want to set. When one modem dials the second also determined the speed of the connection between the modems, one can not work faster than the other. The modems must be aware of how fast the other is because the need to know how long wavelength it is for each piece.

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