What is a modem?

What is a modem? Modem (an acronym formed from the words of the modulator and demodulator) - the device used in communication systems for the physical interface data signal with a medium of its distribution, where it can not exist without the adaptation (ie, transferring it to the carrier with the modulation), and performs the function of modulation and demodulation of the signal (most often in the speech range).
  Modulator in the modem performs the modulation of the carrier signal, ie change its characteristics according to changes in the input data signal, a demodulator - performs the reverse process. The modem performs the function of the terminal equipment line. The very creation of data for transmission and processing of incoming data provides so-called.terminal equipment (in its role can act and a personal computer). Modems are widely used to link PCs (one of them periferiyyh devices), allowing one of them to communicate with others (also equipped with a modem) via the telephone network (telephone modem) or cable connection (cable modem). Modems also previously been used in cellular phones (until they were driven digital data transmission method). Types of computer modems To execute: External - connected via a COM or LPT [1], USB port or a standard plug in the NIC RJ-45, usually have a separate power supply (there are USB-modems bus-powered USB). inside - additionally installed inside the unit (in the slot, PCI, PCI-E, PCMCIA, AMR, CNR, ExpressCard) built - are part of the apparatus, which are built (eg, laptop or docking station). According to the principle of work: hardware - all operations of signal conversion, support for the physical exchange protocols, produced a built-in modem computer (eg using DSP, the controller). Just a hardware modem is ROM, which contains the firmware that controls the modem. software (soft-modems, Host based soft-modem) - all transactions on signal encoding, error checking and management protocols are implemented in software and carried out by the CPU computer. In the modem is only input analog circuitry and converters (ADCs and DACs), and the controller interface (eg USB). poluprogrammnye (Controller based soft-modem) - Modem in which some functions of the modem performs a computer that is connected to the modem. By type of connection: Modems for dial-up telephone lines - the most common type of modem ISDN - digital modems for dial-up telephone lines DSL - are used to organize dedicated (non-switched) lines using a conventional telephone network. Differ from dial-up modems that use a different frequency range, as well as the fact that over the phone line signal is only transmitted to the PBX. Usually allow for simultaneous data exchange to the telephone line as usual. Cable - used to exchange data on specialized cable - for example, via cable television in the collective protocol DOCSIS. Radio - work in radio, have their own sets of frequencies and protocols. Cellular - are working on protocols Cellular - GPRS, EDGE, etc. Most have performance as USB-stick. As such modems also frequently use mobile communication terminals. Satellite PLC - use technology to transfer data over the wire household electrical outlet. Currently, the most common are: Back-End software modem an external hardware modem built-in notebook modems. Device I / O ports - schemes designed to exchange data between the phone line and modem on one side, and the modem and the computer - on the other. To interact with an analog phone line is often used in a transformer. Signal processor (Digital Signal Processor, DSP) typically modulates outgoing signals and demodulates the incoming on a digital level, according to the used communication protocol. Can also perform other functions. The controller manages the exchange with a computer. Memory chips: ROM - non-volatile memory that stores firmware modem control - firmware, which includes sets of commands and data to control the modem, all supported communication protocols and interface with the computer. Modem firmware update is available in most modern models, which is a special procedure described in the user manual. To be able to rewrite the firmware is used for storage of flash memory (EEPROM).Flash memory allows you to easily update the firmware modem, correcting the mistakes of developers and expanding the capabilities of the device. In some models, external modems, it is also used to record incoming voice and fax messages when your computer is off. NVRAM - nonvolatile electrically reprogrammable memory, which stores the settings in the modem (modem profile). The user can change the settings, for example, using a set of AT-commands. RAM - RAM is a modem used for buffering received and transmitted data, the work of compression algorithms and other things.