6.6. Modems

A modem − is functional unit which provides modulation and demodulation of signals, id est device which converts digital signals into an analog form and vice versa, for transfer of them by analog type lines.

A word "modem" is formed according to the two stories − "MOdulator and DEModulator", that explain basic operation principle of modems.

As computers can interchange only digital signals, and in communication channels analog signals pass better in all, then for this purpose necessary bridge which converts a signal. In addition, a modem executes other functions yet and, in particular correction errors and data compression. The first mode provides additional signals by means of which modems check data on two ends of line and cast aside unmarking information, and the second squeezes information for rapid and clear transfer, and then resumes it. Both these modes increase speed and cleanness of information transfer considerably.

During transfer a modem gets from a computer digital signal, modulates it, id est converts from digital into an analog. In modulation process a digital signal as if it is laid on analog carrier. Thus in an analog signal one of property (amplitude, frequency, phase) changes by law change of digital signal. Depending on that, which characteristic changes, the type of modulation differentiates. On an acceptance a modem distinguishes from got analog signal a carrierу, id est converts an analog signal into digital − demodulation. A modem is also able to execute error correction, which arise up during information transfer at bad quality of connection. For this purpose during transfer excessive information (bits of evenness) is added which is checked up on an acceptance. If control information does not answer proper, then the repeated query is done on transfer of certain information.

During operation a modem engages with other modem by a chart: point − point, id est no other modem can not "interferes in talking". Operation mode, if the transfer of data is carried out only one-way, are called a half duplex, and in both directions − full duplex.

Modems divide on such characteristics: by implementation, support of protocol of data transfer, protocol of error correction, possibility of vocal and facsimile transfer of data.

After implementation (original appearance, placing of modem relatively computer) modems are:

• internal − they are inserted in a computer as cards of expansion;

• desktop (external) − have a separate corps. They are set next to a computer and connect a cable with port of computer;

• a modem as a card − is miniature for connection with a portable computer;

• the portable modem − is similar to the desktop modem, but has less overall sizes and stand-alone feed;

• stanchion modems − if amount of modems over ten, then they are set in the special modem bar which promotes the ease of use.

Modems classify also after types:

• the asynchronous modem − executes transfer on an analog telephone network and operatess only with asynchronous communication ports of terminal devices;

• the fax-modem − it is a modem with the added fax possibility which enables to interchange faxes with faxe-devices and other fax-modems;

• the vocal modem − it is a modem, capable not only to execute the functions of fax-modem but also accept from a telephone network a voice message, writing down them in a file;

modem with subinsurance of the dedicaded line by switched line – they use, if necessary reliability of connection. In these modems there are two independent inputs for a line: so, one unites with the dedicaded line, and second − with switched;

• SVD - modem (Simultaneous Voice and Data) − enables simultaneously with transfer of data to conduct conversation by means of receiver, connected to the modem;

• the synchronous modem − supports synchronous and asynchronous modes of transfer;

• the four-wire modem − operatess on the two dedicaded lines (one is used only for transfer, second − only for receiving) in a duplex mode;

• the cellular modem – it is used for mobile radiotelephony to which a cellular belongs also;

• ISDN-modem − unites an ordinary modem and ISDN-adapter in a single-hull;

• the radiomodem − uses ether as environment of transfer instead of telephone wires;

• the network modem – it is a modem with the built-in network adapter of local network for the compatible use in local network;

• a cable modem − it is a modem which enables to use for information transmit the channels of cable television with speed about 10 Мb/s.

On the type of transmission environment modems divide on:

• modems for double-wire copper lines (ordinary, professional, ADSL -, SR -, ER-modems);

• modems for four-wire copper lines (ordinary, professional, HDSL -, ISDN -, SR -, ER -, MR-modems);

• modems for fiber-ohtic lines (FOM, FOM-Т1/Е1, FOM Т2/Е2, FOM-ТЗ/E3);

• modems for radio channels (radiomodem, cellular modem);

• cable modems in which use a coaxial cable. Modems are also characterized by speed of data transfer.

It is measured in bits for a second (bps) and it is set by the firm-producer − 2400, 9600, 14 400, 16 800, 19 200, 28 800, 33 600, 56 000 bps.

Actual speed of data transfer depends not only on index in bits for a second. Such property influence on it, as ERCC and data compression. However these characteristics are regulated by protocols (by standards).

MNP (Microm Networking Protocol) − standards, developed by a firm Місrocom.

ССІТТ (Comite'Consultatif International de Telegraphique et Tephonique) − the International advisory commission on telegraph and telephone communication, authorized to accept protocols in an international scale. Standards mark "V.х.х"., where V means information transfer in an analog kind. Standards for transfer in a digital kind belong to the Х-series, and for the devices of faxes − to the Т-series. Веll − old and low-speed protocols.

Almost all modern modems have similar functional diagrams which consist of such components. The controller − realizes protocols of data compression and error correction. In addition, it is an interlink between a modem and software of computer (executes a programmatic interface).

The codec − carries out bilateral conversion of analog signal which comes in from a line, into the flow of digital data.

ROM − microcircuit of memory which keeps the program of modem operation, name also "weaving". The last models of modems assume updating and rewriting of modem weaving by means of the special software (except for those cases, if it is not provided by a producer).

RAM − microcircuit of main memory which keeps data up to the first shutdown of feed. Intended for storage and further processing of data flow, for storage of the current tuning for modem operation.

In the modern understanding a modem concept considerably wider than simply aggregate of modulator and demodulator. Presently modems are intellectual designs which enable except the main task – signals conversion, to realize the enormous amount of other functions, giving to the additional comfort to the users.

Intellectual possibilities of modems are realized due to the presence of control chart, executed on the basis of that or other microprocessor. In the control charts of modem they often apply the microprocessors of general purpose. It is however possible to apply and specialized controllers, that unite both in an signalling processor and in a processor which realizes additional service facilities.

For the programmatic controlling of modem operation (its control charts) the set of the special commands is used from the computer side. Controlling commands are perceived by a modem only then, when it is in the command mode.

Every modem can perceive the determined amount of commands, that not coincide with commands, supported by other modems. For comfort of the modems use and compatibility of the communication programs it is needed to have a standard set of corresponding commands.

Programmatic modems make on the same chart, that and hardware. A basic difference of programmatic modem from hardware consists in that part of its functions is realized due to the center processor of computer and software. Often it is remained only a codec from modem (reduction from coder-decoder), and other functions are executed by a driver which uses the resources of personal computer.