Why do you need a modem in a computer? Purpose, functionality and parameters of the modem. What types of modems are there and how they are designed

IN modern world Internet users receive information, explore the Internet space, without thinking about how they can do this. Almost always, users confuse a router with what is a modem. Let's look at what it is in this article.

The ancestor of the current data device appeared in 1962. His the creator is AT and T company. At that time, the speed of information exchange was only three hundred bits per second. Then in 1991 this data increased to fourteen kilobits per second.

What is a modem

A modem is a device for receiving and sending information through the telephone system. Streams of information enter it, where they are transformed into the necessary signal that passes through telephone line. It goes to the other end of the wire, where another similar device is already demodulating the signals, turning them into computer signals, and they enter the computer, and then are displayed on the screen user. The word itself comes from an abbreviation of two English words: modulator and demodulator.

What are these devices for?

Modems are in use for connection with the Internet via a telephone line. This device is a kind of bridge between the Internet and home or office equipment. Modern models can be used as a router, sharing the Internet between several devices.

It is worth noting that it will not be able to fully replace the router, since it is not possible to receive the Internet via rj45 from the provider.

Types and types of modems

All such gadgets can be conditionally divide by types and types. Let's look at them more specifically:

• By type of connection modems are made wired and wireless. Wireless well used by laptop owners. Since they connect to the laptop via a USB connector.

Wired connected to the computer using a cable.

• According to the operating principle divided into hardware and software. Hardware differ from software ones in that all signal processing functions are performed by the device itself. Software They give all the work to the computer processor.
• By type of connection devices are divided into telephone, mobile, Dial Up. Analog modems or Dial Up operate over the telephone network. Their speed reaches only 56 kilobits per second. ADSL technology has replaced analog gadgets and is now used everywhere. The transmission speed of information via ADSL reaches 100 MB/s. Mobile phones include those available in the form of a keychain. They work using EDGE, 3G, 4G protocols. Data transfer speed in 3G is up to 3.5 MB/s. While the speed of 4 G is 100 MB/s.
• Broadband. These are ADSL modems. Today, the fastest devices for data transfer.

Popular manufacturers

Modems are produced by many companies. But the most popular of them are Cisco, Zixel, TP LINK, ASUS. These models are famous for being complete. Can work like router.

They are often equipped with DLNA, file and FTP server. In addition, they have an interface to support up to 4 computers. Web interface support.

What does a modem consist of?

Almost the only external hardware components are the input and output ports. This also includes universal, signal and modem processors, read-only storage, RAM, and device status indicators.

The functions that the device can perform are determined mainly by the activities of the universal processor and the program located in the ROM. If update ROM or reprogram it, you can improve the functions of a particular device.

The signal processor converts incoming and outgoing signals into those needed by the device that is connected to it. Buffered in RAM incoming and outgoing data, compression algorithms and other functions occur. Adapters allow you to exchange data on the one hand between the modem and the Internet line, and on the other hand between the computer and the modem.

Operating principle

This device (regardless of USB or landline) turns normal signal to digital. This device has a built-in modulator that converts these signals. The modulator converts signals from the computer, before starting to transmit information, into the signals required by the Internet line. Then the data is transported. And the device at the other end is already demodulating these signals to those that are necessary for the PC with which it is connected.

This is how the information needed by the user is supplied.

What is the difference between a router and a modem?

Many people confuse a router with a modem. This not the same devices. Routers have the following features:

• The modulator-demodulator converts the signal, and the router divides it among network users.
• The first one works with one user, the router with several.
• A router, unlike a signal converter, is a multifunctional device.
• Routers are assigned their own IP address.

Although it is worth noting that for the latest models such the differences are not relevant. Almost all functions of the router and modem are now identical, except that the router cannot transmit data through the telephone line. IN modern devices this can be considered the main and only difference.

Modems (the name comes from the merger of two words - modulator and demodulator)- These are devices that allow you to organize communication between computers located at a distance from each other. If the computers are nearby, then you can organize communication between them using a serial, parallel port, USB, Blutooht. However, such communication is only possible at close distances, determined by the capabilities of the port. Over long distances, the signal weakens and special devices are required that can convert the signal into a form that allows the signal to be transmitted over long distances. For this purpose, a device called a “modem” is used - from the word MOdulator-DEMOdulator. The modulator allows you to convert a digital signal to analog, and the demodulator allows you to do the reverse conversion, that is, convert from analog to digital form(in a more precise sense, modulation is a change in characteristics carrier signal(usually low-frequency periodic oscillations) with a high-frequency control signal that allows the necessary information to be transmitted). Demodulation is the separation of an information signal from a combination of carrier and information signals). A fax works on almost the same principles, which is why modems that are produced with fax transmission capabilities are called a fax modem. Modems can be internal (inserted into expansion slots), external (connected to COM, LPT, USB or network cable to the RJ-45 connector of a computer's network card, usually have an external power supply), built-in like a laptop or in the form of a card for connecting to a PCMCIA connector for laptop computers(the latter is also called an expansion card PC Card and is practically obsolete. The standard currently used ExpressCard with bus connection USB and PCI Express ). Recently, wireless modems (called a module or gateway) using communication lines from cellular operators have become widespread (the most famous are USB modems) . The operating principles of all devices are the same.

Modems can be analog And digital. Analog modems (dial-up) were the first to be used. Due to the fact that the data transfer speed through these modems was not high (up to 56 Kbps), they began to switch to digital modes (with an operating frequency of 4 KHz to 2 MHz and, accordingly, speeds of up to several megabits/sec). In addition, you cannot conduct a conversation while transmitting data via an analog modem.

Most users used the telephone network to transmit data. In order to use digital transmission, it is necessary that both the sender and the recipient have a digital telephone exchange. In addition, there should not be a paired telephone and burglar alarm on the telephone line. Some users still use analog modems.

Main characteristics of modems:

- interior or external. An internal modem is a card that plugs into a slot on the motherboard. This modem is inserted like a regular card, but you need to connect the wires as indicated below. An internal modem is usually cheaper than an external one. But it does not require space on the desk or take up the computer's serial port.

External modems (new) are connected to a USB, PCMCIA or ExpressCard connector and do not require additional power, as they receive it from the connector.

An external modem (old ones) is connected to the serial port and is located in a separate housing. This type requires connection to the electrical network via a transformer. Its advantages include the fact that it does not occupy an expansion slot and makes it easy to transfer it from one computer to another.

Supported standard And transmission speed;

Size of RAM or flash memory.

Additional modem features: voice digitization and translation into analog signal for talking while transferring data; fax; automatic detection caller numbers; answering machine; electronic secretary and other capabilities that telephone sets have.

As a rule, a modern modem has the following phone capabilities, which we will present. These are: negotiations with several subscribers; temporarily muting the microphone; turning on external speakers; memory for subscriber numbers; calling the subscriber again; auto dialer; automatic number identification; remembering called numbers and time of call; detection of the second ring during a conversation; protection from unwanted calls; recording received messages; answering machine; remote control; the phone panel may have buttons with functions: auto-repeat, listening to left messages, turning off the phone, turning off external speakers, etc.; there may be indicators on the telephone panel that determine the operating mode, picking up the handset, etc.; there may be a display with data on incoming and outgoing calls, talk time, etc.; voice dialing, the user calls the subscriber’s last name by voice, and the modem connects with his number; speed dial, dialing a number using one or two keys; auto attendant, answering incoming calls during a conversation with another subscriber; collecting statistics on the number of calls received, their numbers, call time during the day, etc.; other functions, for example, dialing a specific number at a certain time of the day, alarm clock, etc.

If the modem freezes, you can restore its functionality by resetting the power (remove the external one and reinsert it again), but you do not need to turn off the computer. In addition, it has an indication by which you can determine the status of the modem.

Digital modems.

Several are currently in use formats: ADSL, HDSL, IDSL, ISDN, HPNA, SHDSL, SDSL, VDSL, WiMAX and wireless modems using wireless communication (Wi-Fi). They are often called xDSL (Digital Subscriber Line). subscriber line).

ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) appeared in 1987 and is one of the very first and most widespread digital format data transfer. Allows you to send data from the user to the network at speeds from 16 to 640 kbit/s (according to standards 0.5, 0.8, 1.2, 1.3, 3.5 Mbit/s, and receive data at speeds of 1.5, 0.8, 5, 8, 12, 25 Mbit/s sec). Since the user usually receives data rather than sends it, this separation of speeds is not felt by the user, except in cases of video communication. Therefore, over time, other types of formats began to appear using coaxial cable (cable television, speed up to 100 Mbit/s) and an Ethernet connector (local network with speeds up to 1 Gigabit/s). In a number of European countries, the ADSL standard has become the standard by which every resident receives Internet access.

A regular telephone line uses frequencies from 0.3 to 3.4 KHz to pass through; for an ADSL modem, the lower frequency for the outgoing stream is 26 kHz, the upper frequency is 138 KHz, and for the incoming stream it is from 138 kHz to 1.1 MHz. This way, you can talk on the phone and send and receive data at the same time.

However, the first modems did not allow comfortable conversations on the phone, since the high-frequency part of the modem introduced extraneous noise into the telephone conversation(and vice versa, the conversation introduced distortions into the data transmission). To avoid this, they began to use a frequency filter (Splitter), which allowed only low frequencies to pass through to the phone.

HDSL (H igh D ata rate digital Subscriber Line (high-speed digital subscriber line) developed in the late 80s. It uses not one, but two pairs of wires and has a speed of either 1.5 Mbit/s (American standard) or 2.0 Mbit/s (European standard) and allows you to transmit a signal up to 4 kilometers, and in some cases up to 7 kilometers. Mainly used for organizations.

IDSL(ISDN Digital Subscriber Line - IDSN digital subscriber line) allows you to transfer data at a speed of 144 Kbps.

ISDN(Integrated Services Digital Network) appeared in 1981 and has a data transfer rate of 64 Kbps.

HPNA(Home Phoneline Networking Alliance is the name of a joint association of non-profit industrial companies) works with either standard telephone or coaxial cable. The latest standard (3.1) allows you to transfer data at speeds of up to 320 Mbit/s, according to standard 2.0 – 10 Mbit/s.

SHDSL (Symmetric High-speed DSL - symmetric high-speed DSL) allows you to transmit data over one pair of wires at speeds from 192 Kbps to 2.3 Mbps, and over two pairs twice as much over a distance of up to 6 km.

SDSL(Symmetric Digital Subscriber Line - symmetric digital subscriber line) uses one pair of cables with speeds from 128 to 2048 Kbps. Valid at a distance of 3 to 6 km.

VDSL(Very-high data rate Digital Subscriber Line - ultra-high-speed digital subscriber line) has a high data transfer rate from 13 to 56 Mbit/s from the network to the user and 11 Mbit/s in the opposite direction over a distance of up to 1.2-1.4 km.

WiMAX(Worldwide Interoperability for Microwave Access) is a wireless communication in the wave range from 3.5 to 5 GHz according to the 802.16-2004 standard (or fixed WiMAX) and 2.3-2.5, 2.5-2.7, 3.4-3.8 GHz according to the 802.16-2005 standard (or mobile WiMAX). It has many similar parameters to Wi-Fi, but differs in that it can transmit a signal over a long distance and, in addition, is somewhat more expensive.

Bluetooth(translation - blue tooth) was developed in 1998 and is used for wireless communication with a computer in the license-free range of 2.4 - 2.4835 GHz. It does not have a connector and is located inside the computer (device), used to transmit data using radio waves between various types computers, cell phones, printers, cameras, keyboards, mice, joysticks, headphones, MFPs, scanners and others.The essence of the method is that in a certain range the frequency changes abruptly 1600 times per second. This change in frequency occurs simultaneously for the receiver and transmitter, which operate synchronously according to this scheme.The devices can be located at a distance of up to 200 meters from each other, depending on the obstacles between them (walls, furniture, etc.).

The transmit/receive device is located inside the computer and is not visible. If your computer does not have such a device, you can connect an external device via a USB connector that allows you to work with this type of data transfer.

There are standards: 1.0 (1998), 2.0 EDR (2004) with a data transfer rate of 3 Mbit/s, in practice about 2 Mbit/s, 2.1 (2007) using energy-saving technology, simplified communication between devices, has also become more secure, 2.1 EDR required even less power, further simplified the connection of devices and increased reliability, 3.0 HS (2009) with transmission speeds of up to 24 Mbit/s. 4.0 began to be used in the iPhone in 2011, allowing data transfer at a speed of 1 Mbit/sec. in portions from 8 to 27 bytes.

There are profiles for this standard, which are a set of functions. In order for devices to work using a specific profile, both devices must support this profile. For example, A2DP (two-channel stereo audio), AVRCP (standard TV functions), BIP (image forwarding), BPP (text, email to printer forwarding) and so on

WiFi used to create wireless network. Developed in 1991 by NCRCorporation and AT@T, supported by the Wi-Fi Alliance and compliant with the IEEE 802.11 standard. Used to connect computers and cell phones to a network (local and Internet).

The transmitting and receiving device is located inside the computer and is not visible. If your computer does not have such a device, you can connect an external device via a USB connector that allows you to work with this type of data transfer.

The following standards are available: 802.11a uses frequencies of 5 GHz, providing speeds (in theory) of up to 54 Mbit/s; 802.11b uses frequencies of 2.4 GHz, providing speeds (in theory) of up to 11 Mbit/s. (practically not used); 802.11g uses frequencies of 2.4 GHz, providing speeds of up to 54 Mbit/s. (the most common); 802.11n uses frequencies of 2.4 and 5 GHz, providing speeds from 150 to 600 Mbit/s. (newly developed, starting to gain momentum). This standard increases the data transmission range and reduces communication barriers. This standard uses MIMO (Multiple Input Multiple Output) technology, which allows the use of reflected waves from walls. If the device has one antenna, it can operate at a speed of 150 Mbit/s, two antennas - 300 Mbit/s, three - 450 Mbit/s, four (not yet available) - 600 Mbit/s. However, the declared data transfer speed differs from the actual one. So, instead of 300 Mbit/s, it turns out about 100-130 Mbit/s (since half of the transmitted information is service characters), which is also enough for work. And if there are walls, the speed drops further, for example, for three walls it will drop to 50 Mbit/sec.

Since some household appliances operate on the 2.4 GHz frequency (such as a microwave oven), they may cause interference. Therefore, it is advisable to have a device that operates at two frequencies: 2.4 and 5 GHz.

There are also cable modems for connecting to a cable television channel.

Typically, digital modems may contain elements that are used as gateway between the local network and the Internet: router, firewall, etc.

Modem indicators

The following may be available indicators:

A.A.(Auto Answer - auto answer) - auto answer mode, providing a response to the subscriber’s request in automatic mode;

CD(Carrier Detect - carrier detection or DCD) - lights up during a communication session;

CTS or C.S.(Clear To Send) - the modem is ready to receive data from the computer. Goes off while receiving data;

DATA– lights up when data is being transmitted;

DC (Data Compression) - compression data ;

FAX– when the modem operates as a fax;

H.S.(High Speed) – lights up when the modem is operating at maximum speed;

E.C. (Error Control or ARQ) - error correction mode;

M.R.(Modem Ready – modem readiness or DSR) - indicates that the modem is connected to the power supply and is ready for operation;

OH(Off Hook – off hook) - lights up when the hook is hung up;

ON(PWR) - power indicator;

PWR (PoWeR) – power on;

R.D.(Receive Data - receiving data or RX or RXD) - indicates that data is being sent to the computer;

SD(Send Data – sending data or SX or TXT) - indicates that data is being received from the computer;

TEL– lights up when the handset on a parallel connected telephone is lifted;

RTS (Request To Send) - the modem is ready to receive data from the computer. Lights up when waiting for data from the computer, goes off during data transfer;

T.D. (Transmit Data or TXD) – lights up or flashes when data is being transferred from the computer to the modem. May light up when transmitting data at maximum baud rate;

TST (TeST) - flashes during testing;

TR(Terminal Ready – device readiness or DTR) - lights up when receiving a control signal;

USB– lights up when the modem is connected to the computer via USB bus.

The modem body may also have a volume control.

On the back external modem may have connectors with icons:

A.C. IN – connecting the power adapter;

LINE – connection to a telephone line;

ON / OFF – turning on/off the modem;

PHONE – connecting a telephone;

R.S. -232 – connector for connecting to a computer serial port;

USB – connector for connecting to the USB bus.

Analog modem

Data transfer. Telephone lines are adapted to analog signals. Due to the fact that human speech has a range from 30 Hz to 10 KHz (music has a larger range), to save money, the telephone line passes a signal from 100 Hz to 3 KHz. It is this limitation that limits the ability to transmit data at high speeds. Computers can be connected not only through a telephone line, but also using radio waves and infrared radiation. In this case, no wires are needed.

Ultimately, the data sent in the parallel channel is converted into a serial transmission with start-stop bits at the serial port, sent to the modem, where it is simulated, that is, superimposed on the carrier frequency of the signal transmitted along the line, then sent to another modem. Next, they are converted into digital form, sent to the serial port, where they are converted into parallel form, and then sent to the processor for processing.

Digital data is sent bit by bit, and the sending can be of two types: synchronous and asynchronous. During synchronous transmission, a data packet consists of a header, which includes the destination address, the data itself, and a checksum. Asynchronous transmission transmits a start bit, 8 data bits, possibly a parity bit, and a stop bit indicating the end of the transfer. This type is used in a serial channel.

In addition, three modes can be used when transmitting data: duplex, in which data is transmitted in both directions simultaneously, half-duplex, in which data can be transmitted in both directions, but in one direction at a time, and simplex - data transmission only in one direction.

Data transfer from modem to modem and from modem to computer has different speeds, therefore, to prevent data from being lost, the modem has a buffer where the received data is stored.

Some modems compress data before sending it, and when received, another modem decrypts the data. There are files that have already been compressed, so this method may not provide any transfer benefits. To avoid data loss, the data transfer speed from the modem to the computer must be several times higher than between modems, which is what is implemented in practice.

When transmitting data, the unit is often used baud, which is sometimes confused with bits/sec. Actually it's different sizes. 1 baud is one character sent per unit of time, and this can be not only data, but also control signals. A character can represent multiple bits. If the signal consists of two types: 0 and 1, then the symbol indicates 1 bit, if 512, then 9 bits (2 9 = 512). When transmitting data at low speeds, 1 baud is approximately equal to 1 bit/sec. At high speeds, the modem sends data at several frequencies, so at each moment in time, not one, but several bits are transmitted, that is, the speed measured in bits/sec, and not baud/sec, will be several times higher than the speed in bauds . Often the baud rate indicated implies the speed in bits/sec.

When transmitting via a modem, you can approximately determine how long it takes to transfer by dividing the transfer rate by 10, for example, if the transfer occurs at a speed of 28,800 bps, then approximately 2,880 bytes or characters will be transferred per second (28,800/10= 2 800).

The modem connects to the computer's serial port and works with serial data. Typically, a modem is used to work on the Internet, but it can also serve to communicate directly between two arbitrary computers. Modems are also used as fax machines to transmit fax messages. They may have a built-in adapter for creating voice messages in answering machine mode.

When connected, the modem sends signals that are also output to the speakers and can be heard as a continuously changing sound for several seconds. The receiving modem determines the standard by which it can operate, and also makes adjustments to the clock frequency, that is, it performs phase modeling. After this, the speaker turns off, but the signals continue to arrive, in particular, they can be heard through a parallel telephone.

Modems come in two types: internal and external. The internal ones are made in the form of expansion cards and are inserted into the motherboard connector, the external ones have their own housing and are connected to the serial port using a cable. The latest types of modems can be connected via USB (and sometimes receive power from the computer), so they can be used while the computer is running, free up a connector, and have other advantages. When connecting a modem to a serial port, high-speed models require that the port also be fast. So, for modems with a speed of 56 Kbps, a speed on the serial port of 115 Kbps is required. The higher port speed is needed because it also sends control signals between the computer and the modem that are not transmitted over the telephone line. If the port does not support high speeds, data may be lost. External devices can be turned off by turning off the power supply, and the internal one can only be turned off when the computer is turned off, which is inconvenient when the modem freezes.

Modems can be divided into two categories: the first type (Class2) has an internal processor that processes data, in the second the data is processed central processor(Class1), they are also called Windows modems, somewhat cheaper than the first type. Such a modem, if the processor is old, can greatly slow down the computer, but if the user rarely accesses the Internet and sends only a small number of messages from time to time email, then this is acceptable. It is quite advisable to use it even if the computer has a powerful processor.

Often the modem is characterized protocol with whom he works. There are signal modulation protocols, error correction protocols, data compression And work with by fax(fax). The modem has several protocols for each of these types. Error correction protocols include V.42, MNP2-4, MNP10, data compression protocols – V42bis, MNP5.

One of the main characteristics of the modem is the data transfer speed, and the indicated maximum speed can be 33.6 or 56 Kbps for modern devices. If a speed of 33.6 Kbps is specified, then the entire bandwidth is used and data is transmitted in both directions at a speed of 33.6 Kbps. if the line allows it. If the line does not allow this, then a transition to a lower speed occurs. Speed ​​56 Kbps. ensures that data is received at a higher speed than when sending it, since there are more frequencies for reception than for transmission, but transmission from the modem is carried out at a lower speed.

In addition, it is necessary that both modems have the same characteristics, otherwise data transfer will not reach the maximum speed. To do this, before purchasing a modem from your provider, you need to clarify the type of modem with which it works best. Below is a table of correspondence between some protocols and their transmission speed.

The prefix bis indicates that the standard has been revised. Starting from speed 14,400, all protocols are duplex, that is, they transmit messages in both directions simultaneously. The names of not only standards that define a data transfer protocol, but also other types of protocols can begin with the symbol V, for example, V.24 contains a list of specific signals between two modems, V.25bis is a command language for controlling a modem, etc. there are other names, for example, MNP, some begin with the symbol V, but then there are not numbers, but symbols, for example, V.FC.

The following MNP protocols are in effect: MNP1 And MNP2- outdated and not currently used; MNP3– provides synchronous transmission; MNP4- transmits data in synchronous mode in packets from 32 to 256 bytes of data, while the size of the packet depends on the quality of the telephone line. For a lower quality line, a smaller package is used, for a higher quality line, a larger one is used; MNP5- provides synchronous mode, which uses data compression, has two algorithms for compressing repeated messages; MNP6- provides synchronous mode, also uses data compression; MNP7, MNP8, MNP9- provides synchronous mode, while using more advanced compression methods; MNP10- used when the data transmission line is of poor quality. At the moment of starting work, it sets the lowest speed, and if the line is capable of operating with a higher gear, then the speed increases.

The following protocols also exist:

Xmodem- protocol issued in 1977. The transmitting modem sends a special NAK signal, then, upon reception, the receiving modem issues a NAK signal until it receives a data packet, which consists of a start of data character (SOH), block number, 128 byte data and a checksum (CS) . When data is received and checked for correctness using a checksum, a signal is sent that the data has been received (ACK), and if it is received incorrectly, a signal (NAK) is sent. If there are multiple failed data transfers, the communication session is terminated. At the end of the transmission, an EOT character is sent indicating the end of the session.

There are modifications to this protocol, for example in Xmodem CRC the checksum has been increased to 16 bytes, which increases transmission reliability, Xmodem 1k– data block size increased to 1 kilobyte, Xmodem G- transmits data, and the checksum is located at the end not of the data block, but of the file.

Ymodem- based on the Xmodem protocol, with a transmitted data size of 1 kilobyte, transmits the file name and its attributes. In addition, the first block contains information about whether there are further files to transfer.

Kermit- uses data packets up to 94 bytes, mainly used in Unix systems.

Zmodem- transmits data ranging in size from 64 to 1024 bytes with compression. If there is a failure, it sends data from the moment when the failure occurred.

Bimodem– further development of the Zmodem protocol with the ability to send data in two directions simultaneously.

Sometimes may be required modem commands, for example, to test it. Below is a list of some modem commands (note that modifications of modems may have a different set of commands):

ATA- the modem is ready for operation;

ATADP number– pulse dialing of a telephone number;

ATADT number– tone dialing of a telephone number;

ATW– carrier waiting;

ATMx– loudspeaker operation, where 0 is off, 1 is on;

ATLx– loudspeaker volume from 0 to 7;

ATQx– modem messages about command execution: 0-enabled, 1-disabled;

ATHx– 0—disconnect the modem from the line, 1—connect;

ATZ– restoration of the original operating mode;

AT&W– recording current modem parameters into memory;

ATSx=value– determination of modem characteristics;

+++ - switching the modem to command mode;

A\– repeating the last command.

When transmitting data via modem, special protocols are used to compress data, for faster transmission, and error correction methods. Such standards are designated MNP (Microcom Networking Protocol), as well as some of the standards starting with the letter V (V.41, V42 and V42bis).

To transmit data, a special protocol is used, that is, a rule according to which data is transmitted and received. For normal operation, both modems (sending and receiving) must be able to work with these protocols. With data correction methods, in addition to them, a special CRC combination is sent, which is used to identify errors. Upon reception, the data is checked, that is, calculations and comparisons are made of CRC blocks (calculated and verification) and, in the case of normal operation, a signal is sent that the data was received correctly.

Notes. The country code on your computer matches the international phone prefix. The telephone number consists of the following digits: Country code (10 for Russia), + region code (495 or 499 for Moscow) + PBX number (3 digits) + telephone number within the PBX (4 digits)

If you have experimented with the modem and it does not work, then to reset the parameter values, you can restart the computer, while turning the modem off and on, or enter the AT&F command, and enter AT&V to determine the modem parameters.

The transmission of text information over telephone channels is called dayphone communication.

Modems contain contains: I/O port adapter for working with a telephone line; I/O port adapter for working with a computer; a processor that modulates/demodulates the signal and provides a communication protocol; memory where the chip control program is stored, modem parameters are maintained, and RAM; a controller that manages communications with the computer and modem components.

The modem may have some of these components, and the missing part will be modeled by a central processor, for example, a controller. Such modems are called software modems.

The most important characteristic is the data transfer speed. More recently, the standard speed was 14.4 Kbps (of course, there were lower speeds), then devices appeared that allowed information to be transmitted at speeds of 28.8 and 33.6 Kbps. Now the maximum transmission speed has reached 128 Kb/sec and has provided maximum transmission capability over the telephone network.

Of course, devices that operate at 33.6 KB/sec can also operate at slower speeds, namely 28.8 and 14.4 KB/sec, but not vice versa. So, if there is a modem at one end that provides a transfer speed of 28.8 Kbps, and at the other - 14.4, then the transfer will occur at a speed of 14.4 Kbps.

Modem installation

Installing a modem. Installing a modem, as a rule, is not a big problem, since after installation operating system It finds it itself and installs the standard driver. If a driver is supplied with the modem, it is advisable to install it, since compared to the standard driver, it provides additional capabilities.

To install, you need to perform the following sequence of actions:

Turn off the computer (if you are connecting an internal modem or an external one to the serial port);

If it is an internal modem, install it as an expansion card. At the same time, hold the board by the edges without touching the conductors and microcircuits on the boards. If it is an external modem, then connect to a serial port or USB port. If the number of pins on the serial port connector does not match, you will need an adapter, since one of the ports may already be occupied;

If the modem has one output for a telephone, then you need to connect the wire at one end to the modem, and the other end to the telephone socket. In this case, you can use a special type of socket that has two outputs: one for the phone, the other for the modem. The appearance of such a socket is shown in the figure on the right; it has two types of connectors.

One coincides with the standard in force in our country, and the second with that adopted in the West; it is found in many modems sold.

You can use a special splitter, which has one connector at one end and two at the other. One connector is installed in the telephone, the other two connect the wire to the telephone socket and the wire to the modem.

If the modem has two telephone connectors, then you need to connect the wire from telephone socket(inscription near the line connector), the other - to the telephone set (inscription phone). If there is no inscription, then look at the back wall of the modem, where there may be a contact diagram, or refer to the documentation. If the connection is made incorrectly, the modem will not work. In this case, change the contacts. The external modem must also be connected to the network via a power supply. To install an internal modem, use the description of installing boards in the system unit;

After installation, turn on your computer and install software, supplied with the modem.

Laptops have one output for connecting to a telephone line. When working with a modem, it is better not to use a parallel telephone or connect it through the corresponding socket on the modem, otherwise interference from the telephone line may occur and noise may appear.

In Windows, after installing the modem, a message will appear on the screen stating that the system has detected a new device, after which the system itself will try to determine its characteristics. Follow the instructions that came with your modem. It is necessary to make the correct installation so that there are no conflicts due to the use of system resources.

Installation The modem is made in the same way as other devices. If the modem supports the Plug & Play standard, then when you turn on the computer, a “installation wizard” will appear on the screen, which will help you install the modem with the help of questions and answers. If the modem does not support the Plug & Play standard (for very old models), then you need to use the mode: Start → Settings → Control Panel → Modems (2) → Properties (modems) → add → (do not define modem type) Next. If you have a disk for the modem, then you need to use the “Install from Disk” mode or, if it is not available, select the manufacturer (if unknown, then “Standard modem types”) and Model → Next → having selected the appropriate model, click Next → (select required port) Next.

One of the most important parameters that needs to be set is the type of dialing, which should be pulse, since in our country no other type is used. To install it, in the Properties window: Modems: General, click “Communication settings”, where select pulse dialing.

To check, whether the installation was completed correctly, use the mode: Start → Settings → Control Panel → System (2) → Devices, where there is a list of devices. If there is a plus sign next to the name “Modem”, then you need to click on this icon to expand the list of modems. Then you should make sure that there are no question marks or exclamation marks near the installed device.

Modem parameters can be look And change via: Start →Settings →Control Panel →Modems →Properties →General, where you change the port, speaker volume, and indicate the maximum speed. In this case, the maximum speed is meant between the modem and the computer, and not between modems. Usually the maximum speed is set, and in the case bad connection it is reduced.

Other questions

In general, communication channels are divided into:

Analogue (for example, telephone), through which information is transmitted in the form of a continuous signal;

Digital, transmission of digital (discrete or pulse) signals

or

Simplex,

Half duplex,

Duplex

or

Switched networks created for the duration of information transfer are then disconnected;

Non-switched (dedicated), dedicated for a long period of time

or

Low-speed (telegraph) with a speed of 50-200 bytes/sec.;

Medium-speed (telephone) with a speed of 300-56,000 bytes/sec.;

High-speed, over 56,000 bps.

To transfer data from high speed Twisted pair wire (twisted together), coaxial cable (as in a television antenna), fiber optic (made of glass fibers) and radio channel (via radio waves) are used.

Radio waves can be ultra-long (3-30 kHz), long (30-300 kHz), medium (300-3000 kHz), short (3-30 MHz), ultra-short (30 MHz-3 GHz), submillimeter (300-6000 GHz).

When transmitting data, several types of modulation are used: frequency (V21), phase (V22), amplitude and quadrature amplitude modulation, in which the phase and amplitude change, more noise-resistant than the previous ones, so it is used in the V22.bis standard and higher.

The protocol also contains the ability to split messages into blocks, restore communication, correct errors, etc. These include Xmodem, Ymodem, Zmodem, Kermit, etc. The most common is Zmodem.

Network cards serve to connect a computer to a network of computers and act as an intermediary between the computer and the network for data transfer. The network card has its own processor and memory. The main characteristics of a network card are the bus to which it is connected, memory size, card capacity (8, 16, 32 bits), types of connectors for thin and thick cables. Network cards require setting an interrupt line (often 3 or 5), a DMA channel, and a memory address (C800).

Network cable can be of several types:

twisted pair. Consists of several copper conductors twisted together in one cable, which can be unshielded (UTP) or shielded (STR).

Coaxial cable consists of a central and shielding wires, between which there is insulation. There are two varieties of this cable: thin (0.2 inches thick) and thick (0.4 inches thick).

Fiber optic cable consists of two wires consisting of light fibers. It has a large capacity, but is very expensive, so it is rarely used.

When using a cable, pay attention to the characteristic impedance, often 50 ohms. When laying, you need to have cables of the same brand, preferably from the same manufacturer. After laying the thin cable, connectors are installed, for example, Russian production(CP50) or crimp BNC connectors. A plug is installed at the ends and one of them must be grounded.

Thick cables are routed through transceivers, using one transceiver per computer, and the ends of the cables leading to the computer must have 15-pin DIX connectors (or AUI). At the end of the cables are installed: N-terminators, one of which is grounded. To increase the length local network(for a thin cable it cannot be more than 185 meters), repeaters are used (Repeater - repeater).

A twisted pair cable is used in conjunction with a hub or hub, from which a cable no more than 100 meters long is laid to each computer. At the ends is an RJ-45 connector, which looks similar to a telephone connector, but has 8 pins (rather than 4). Hubs can have a different number of ports, for example, 8, 12, 16, corresponding to the maximum number of connected computers.

When the modem is operating as fax, he works according to his own standards. When sending faxes at 14.4 Kbps, the standard is V.17 (14,400), V27 ter (4,800), V29 (9,600) and T.30 for the protocol itself. When transmitting an image of a sheet, the following resolution modes for fax transmission can be used: Standard – 100x200 dpi; high-quality (Fine) – 200x200 dpi; high quality (Superhigh) – 400x200dpi; photo mode (Photo) transmits 64 shades of gray.

A modern modem supports most standards, at least those that operate at less than the maximum speed of the modem.

In addition to ordinary modems, there may be very specific modems, for example, cable modems, when the signal is transmitted through tv cable . In this case, the cable is connected to a special socket, which has a connector for the TV and for the serial channel of the computer. Work on cable networks allows you to transfer data at high speed. However, over time, as the number of users increases, the throughput per user may become low. And now, while there are few users, they give a small number of users great advantages of working on the Internet.

Can be used satellite devices, in which users send a message to the provider via phone about which pages he wants to receive, and receives them via satellite.

Currently, more and more information is used to transmit mobile communications. In this case, the modem is connected to the mobile phone via a special cable.

In our country, the most widespread data transmission is voice and digital, there is a standard GSM- Global System for Mobile Communication, which can be translated as “global system for mobile communications" The essence of this standard is that all transmitted information is divided into so-called frames, divided into eight intervals. Depending on how busy the line is, one interval or another may be used. But this way mobile communications designed primarily for transmitting voice messages that have priority over digital data. Ultimately, the data transfer speed does not exceed 9.6 Kbps.

Other standard GPRS(General Packet Radio Service) allows you to increase this speed to 50 Kbps, and theoretically can reach 100 Kbps. Unlike GSM, here, to send information, it is possible to use other time intervals in the frame, up to all eight, and this circumstance increases the speed of data sending. In addition, this mobile communication option reduces user costs, since the volume of transmitted information is paid for, unlike GSM.

GPRS devices are divided into three classes according to their capabilities:

Class A. Such devices are capable of simultaneously transmitting both types of information – voice and digital – in each unit of time.

Class B. These models allow you to work alternately with either digital data or voice.

Class C. Only digital data is sent here.

Good day, my dear friends. Today I will share some nostalgia with you, remembering what a modem is. Oh, what a time it was... I feel that not everyone understands me now, especially young readers who heard the name of this device for the first time.

Then I explain by turning to history.

Let's say that you have a PC, but there is no Internet, none at all. Do you feel bad without him? Yes, especially considering that you know him limitless possibilities. So here it is. In the 90s, computer owners in Russia for the first time learned about such a Wonder of the World as the global web. And it turns out that you can connect to it via a simple telephone network. Which was at that time the only means of wired communication laid even in remote villages.

But this required that there be a provider in your city that provides Internet access. And the user had a device called a modem that connected between the PC and the telephone line.

Where did it all start?

It just so happened that after the fall of the Iron Curtain, the Internet came to our country with ready-made hardware and software solutions. But in reality, both the network itself and the devices serving it had to go through a thorny path to finding optimal solutions.

The history of modem data processing begins in the post-war mountains in the United States. In 1950, a road was laid between military air defense bases located throughout North America, which connected radars, terminals, and command centers. Signal processing at the ends of the lines was carried out using modulating transformations.

This principle became the basis for the creation of similar devices for network communication between personal computers.

In 1979, Micromodem II appeared, intended exclusively for the Apple II PC. This is how the device got its name “modem”, derived from the name of the two operations it performs: MODULATION and DEMODULATION.

I will now explain the purpose of these functions and how they work. So that digital code pulses, which are a “rectangular” graph, can be transmitted over a channel intended for audio communication, they are pre-modulated, after which the analog signal familiar to a telephone line successfully travels through the network.

In turn, the computer is not able to perceive analog information that is unusual for it. Therefore, the smooth change in line voltage is demodulated back into pulses that can be read by the processor.

But if you study the operation of the modem in more detail, you should pay attention to its interaction with the network and PC, which implies a system of queries, recognition, changing connection parameters and other control operations.

All this is recorded using special program code. In 1981, Hayes introduced the Smartmodem 300. It used a system of such commands that later became a standard in the modem industry.

What types of modems are there and how they are designed

But, since we started to delve into such subtleties. Then I propose to immediately consider the modem device, consisting of:

• power supply unit;
• two connectors for connecting a telephone cable (incoming from the socket and branching to the telephone) and a network LAN or serial LPT port for connecting to a computer;
• controller responsible for exchanging streaming data from a PC;
• a signal processor that directly performs signal conversion operations;
• a set of memory chips, including RAM and two types of non-volatile memory: ROM (with firmware, drivers) and NVRAM with operating settings.

I’ll say right away that this “set” is taken from the design of an external modem, which were the most common during the period of their greatest popularity. Some people probably remember models such as Courier or Zyxel.

Over time, internal modems appeared that were inserted into the motherboard. They no longer required a power supply and an external com-port. But a connector was used that could be useful for other needs. Some manufacturers even made built-in modems integrated into the motherboard.

Connection method that determines Internet speed

A conversation about modems will not be complete if we ignore the methods for connecting them to the Internet. And the first, of course, is Dial-up - regular dialing, dialing a number. Of course, this function was performed by the electronic filling of the device, accompanying its work with characteristic sounds coming from the speaker. They were not specifically disabled (despite the annoyingness) in order to control the process. Dial-up connection had its own characteristics:

• dialing took a long time, and while working on the network, using the phone was impossible;
• Very low speed connections 40 - 45 kbit/s, which, moreover, strongly depended on the quality of the telephone line;
• billing was often per minute, and taking into account slow speed- Expensive…

As an alternative, corporate and wealthy users were offered a dedicated Dial-up communication line. The advantage of which was the absence of dialing and best quality channel.

The practice of using the Internet has shown that the volume of information downloaded by a user is much higher than that transmitted to the network. Therefore, ADSL technology was proposed - asynchronous frequency division of data streams using a multiplexer and a modem capable of working with such a signal. This provided new opportunities and obvious advantages:

• it became possible to simultaneously use the phone while working with the Internet;
• speed increased to 25 Mbit/s;
• Connection time has been significantly reduced;

Transferring information with help allowed us to take a fresh look at the possibilities of high-speed Internet, but, unfortunately, it turned out to be a limiting factor at this stage throughput telephone lines. Providers began to directly lay communication lines ( twisted pair or optical fiber) to the user, bringing the connection speed to 100-300 Mbit/s.

No longer relevant?

What about our modems, you ask, are they missing? Well, not really. Still, laying a new line is a troublesome task, and the telephone cable is already lying there. It would be a sin not to use it. Therefore, providers still offer this service. Another thing is that telephone connection has changed, digitized, become mobile and provided new prospects in the form of 3G/4G USB modems.

Now you can connect to the Internet anywhere within the coverage area of ​​your mobile operator. Moreover, the data transfer speed will allow you to easily watch streaming video. Over time, USB modems have competitors - smartphones. Which, in capable hands, can easily distribute the Internet via wi-fi.

But that's another story. And I think it’s possible to finish this one. See you soon, my dear readers.

They ushered in a new era of communications and played a major role in the development of the Internet. It's about modems.

Modem- (abbreviated from modulator-demodulator) - a device that, through modulation and demodulation of signals, transmits digital data through analog channels- mostly telephone wires.

Thus, modem converts one type of signal to another. Using modulation, one or more characteristics of an analog signal are changed: amplitude, frequency, phase. The demodulator performs the inverse function. Currently modems associated with the Internet. They are used to communicate with the provider through various channels (telephone lines, cable TV lines, base stations mobile operators). Those. The modem acts as a kind of bridge, because Only an analog signal is possible in telephone lines, and a computer only accepts a digital signal.

History of modems.

The first digital modems began to be developed back in the 50s in North America with the purpose of converting signals for air defense. Modems were used to transmit data over regular telephone networks. In 1962 the first commercial modem, was created by AT&T. It was a model Bell Dataphone 103. The data transfer rate over the telephone line was 300 bps.

Subsequently speed modems passed through values ​​such as 1200, 2400, 4800 and 9600 bps. Modem speed increased to 14.4 kb/sec only 1991 year. In 1994 it reached 28.8 kb/sec. The next speed threshold is 33.6 kb/sec, which became the limit for the telephone network. IN 1996 year, a 56K modem appears, invented by the company Dr. Brent Townshend, which gave further development modems. However, let's return to 70s gg. IN 1977 the year the first was invented modem For personal computer - 80-103A. It was a real success. Later there were a number of other models, this was the Hayes Microcomputer Products company.

IN 1981 Hayes released modem which has become legendary - Smartmodem 300 bps. A special command system was developed for it, which is now used. Then a real race for speeds and prices unfolds modems. The company occupies a leading position U.S. Robotics. She releases a whole series Courier modems: starting in 1986 with the Courier HST model - 9600 bps.

Types and types modems.

According to their design, modems are:

• internal modems – located inside the device; they do not have their own power supply.
• external modems – have own building and a power supply, connected to the computer via a cable, have their own indicators;

Internal modems

According to the operating principle:

• hardware - all signal conversion operations are carried out by itself modem;
• software - all signal conversion operations are implemented in software and are performed by the computer’s central processor;

By type of connection:

• analog modems – work through a regular telephone network;
• cable modems - use a regular television cable or coaxial cable to connect to the Internet;

• radio modems allow the user to work with the network via radio air;
• cellular modems- work according to protocols cellular communication- GPRS, EDGE, etc. Often available in the form of a USB keychain;
• ADSL modems– new generation of modems, also work with telephone network, however, unlike analog ones, they use their own range.