The peripheral intending for information transfer on the telephone line is called as the modem. The word is formed from reduction "modulator demodulator".
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On the execution modems are subdivided on external, internal and built-in. Connection of external modems is carried out through USB, COM or LPT port, or by means of the RJ-45 socket in the network interface card (Ethernet interface). As a rule, have the separate power unit, however there are modems which receive food by means of USB. Internal modems are installed on the computer motherboard for what one of the following interfaces is used: PCI, PCI-E, PCMCIA, ISA, CNR or AMR. The built-in modems represent one of parts of the device which they are built in, an example is the laptop.
By the principle of work modems share on hardware, program and semi-program. In hardware modems all work is carried out at the expense of the calculator who is built in it. In addition, it contains the constant memory (CM), in it there is a microprogram exercising control. In program modems performance of all tasks is carried out by means of the program, and loading of calculations is carried out by means of the central processor. In semi-program modems the central processor of the computer carries out only certain part of tasks.
As connection modems are subdivided on:
- modems of the switched telephone lines;
- ISDN modems (intend for digital telephone lines);
- DSL modems (intend for creation of the allocated lines, the telephone network is used thus, work in the range of frequencies different from what is used by modems of the switched telephone lines);
- cable modems (for their work special cable lines are used);
- radio modems (use a radio channel for the work);
- cellular modems (work at a basis of the GPRS, EDGE, 3G protocols, etc., are often executed in the form of a charm);
- satellite (work with a satellite signal);
- PLC (for work use cables of an electric network).