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Device that modulates an analog carrier signal to encode digital information


A rare serious business person, professional programmer or system operator can imagine a full-fledged work without using such a powerful, efficient and convenient combination as a regular telephone line, modem and computer network.

Without a modem, an electronic communications system is unthinkable. This device allows you to enter the fascinating world of information flows, electronic databases, e-mail, electronic directories, electronic bulletin boards and much more.

How a modem works

When a computer is used to exchange information over the telephone network, a device is needed that can receive the signal from the telephone network and convert it into digital information. At the output of this device, information is modulated, and at the input it is demodulated, hence the name MODEM. The purpose of the modem is to replace the signal coming from the computer (a combination of zeros and ones) with an electrical signal with a frequency corresponding to the operating range of the telephone line.

The modem divides the acoustic channel of this line into low and high frequency bands. The low frequency band is used for data transmission and the high frequency band for reception.

There are many ways to encode information, the most famous of which are the FSK (Frequency Shift Keying) method for transmission rates up to 300 baud (baud is a unit of information transfer rate equal to 1 bit / s) and the PSK (Phase Shift Keying) method for faster modems , transmission speed up to 2400 baud. FSK uses four dedicated frequencies.

When transmitting information, a signal with a frequency of 1070 Hz is interpreted as a logical zero, and a signal with a frequency of 1270 Hz is interpreted as a logical unit. When receiving, zero corresponds to a signal of 2025 Hz, and one corresponds to 2225 Hz. PSK uses two frequencies: for data transmission - 2400 Hz, for reception - 1200 Hz. Data is transmitted in two bits, while encoding is carried out by shifting the phase of the signal. The following phase shifts are used for encoding: 0 degrees for bit combination 00, 90 degrees for 01, 180 degrees for 10, 270 degrees for 11. There are also other types of modulation (DPSK, QAM, TCM).

The modem is presented either in the form of an external device that is connected to a telephone line with one output and a standard computer COM port with another output (RS232 connector according to CCITT V.24 recommendations) or as an ordinary printed circuit board that is installed on a common computer bus.

Internal modem options can be adapted to both regular ISA and PCI buses. The modem controller is a specialized microcomputer of the SC1107 or SC1108 type, containing an eight-bit ALU, 8 KB ROM, 128 bytes RAM, a timer, a command register, an interrupt controller, a stack, an I / O port. If the modem board is connected to the PC system bus, then the "parallel" controller SC1107 is used. If the board works with a computer via RS232, then the "serial" controller C1108 is used. In some designs, the role of the controller is performed by the 8031 ​​processor with external ROM (i2732, 2764) and the 74LS373 chip.

Types of modems

HAYES-compatible modems, named after the manufacturer of one of the first modems, have become widespread. Such modems use AT commands (from the English word ATtention) compatible with Hayes Smartmodem, in addition to the standard command set for all Hayes-compatible modems, each manufacturer individually offers the user a wide range of specific commands that are valid only in models of this company (for example, USRobotics, Rockwell, ZyXEL). In addition to compatibility with the command set, the modem must comply with some standard for transmitting information over telephone lines.

Such standards are the recommendations of the CCITT (international advisory committee on telegraphy and telephony. In the USA and Canada there is a similar Bell standard, the only difference from CCITT is lexical.

Modems that comply with standards for speeds up to 2400 baud can exchange information freely. It should be noted that the CCITT V.32 recommendation is not a standard in the full sense of the word, as virtually every major manufacturer of modems above 2400 baud has a habit of additionally applying one or more specific data transfer protocols. Their use is possible only when connecting similar modems, and in this case, as a rule, a higher transmission rate, noise immunity and connection speed are achieved. The most common and cheapest (which is why it is popular among users) is the HST (High Speed ​​Transfer) protocol, developed by USRobotics back in the late 80s.

Varieties of this protocol: H96, H14, H16, H19, H21, H28, the difference is in the information transfer rate, which is 9600, 14400, 16800, 19200, 21600 and 28800 baud. Due to the low price, wide upgradeability and high noise immunity and high-speed data of the HST protocol, users buy famous USRobotics models such as Sportster, Worldport, Courier. ZyXEL modems are widely used, which have a specific ZYX protocol, which makes it possible to transfer data at a speed of 19200 baud in full duplex.

Expensive, but with a strong and stable signal, capable of ignoring even the protective filters installed on the PBX in order to avoid using modems for free. These are Telebit modems, TrailBlazer brands and the famous PEP protocol (Packet Ensemble Protocol). Almost all high speed modems are compatible with slower standards.


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