regular modem. Types of computer modems. Modems are divided into external and internal
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 computers are nearby, then you can organize communication between them using a serial, parallel port, USB, Blutooht. However, such communication is possible only at close distances, determined by the capabilities of the port. At 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, 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 the characteristics of a carrier signal (usually low-frequency periodic oscillations) by a high-frequency control signal that allows you to transmit the necessary information). Demodulation is the selection of an information signal from a set of carrier and information signals). Fax works on almost the same principles, so modems that are available with facsimile capabilities are called a fax modem. Modems can be internal (inserted into expansion slots), external (connected to COM, LPT, USB ports or a network cable to the RJ-45 connector of a computer network card, usually have an external power supply), built-in like a laptop or in the form of a connection card to laptop PCMCIA connector(the latter is also called an expansion card PC Card and almost obsolete. The standard currently used express card with bus connection USB and PCI Express ). Recently, wireless modems (called a module or gateway) have become widespread, using communication lines of cellular operators (the most famous are USB modems) . The principles of operation of all devices are the same.
Modems can be analog and digital. Analog modems were the first to be used (dial-up). Due to the fact that the data transfer rate through these modems was not high (up to 56 Kbps), they began to switch to digital modes (with an operating frequency from 4 kHz to 2 MHz and, accordingly, a speed of up to several megabits / s). In addition, when transmitting data via an analog modem, you cannot carry on a conversation.
Most users used the telephone network for data transmission. In order to be able to use the digital mode of transmission, it is necessary that both the sender and the recipient have a digital exchange. In addition, there should not be paired telephones and burglar alarms on the telephone line. Until now, some users use analog modems.
Main characteristics of modems:
- interior or external. An internal modem is a card that plugs into a slot on the motherboard. Such a modem is inserted as regular card however, you need to connect the wires as shown below. An internal modem is usually cheaper than an external one. But it does not require space on the table, does not take up the serial port of the computer.
External modems (new) plug into the USB , PCMCIA or ExpressCard slot and do not require additional power as they get it from the slot.
An external modem (older ones) is connected to the serial port and is located in a separate housing. This type requires connection to electrical network through a transformer. Its advantages include the fact that it does not take up an expansion slot, making it easy to transfer it from one computer to another.
Supported standard and transmission speed;
Size of RAM or flash memory.
Additional features of modems: digitization of voice and its conversion into an analog signal for conversation when transmitting data; Fax; automatic detection caller's number; answering machine; electronic secretary and other features that telephone sets have.
As a rule, a modern modem has the following phone capabilities, which we present. These are: negotiations with several subscribers; temporary muting of the microphone; turning on external speakers; memory for subscriber numbers; repeated call of the subscriber; auto redial; automatic identification of the number; remembering the calling numbers and the time of the call; detection of the second call during a conversation; protection from unwanted calls; recording received messages; answering machine; remote control; on the phone panel there may be buttons with functions: auto-repeat, listening to left messages, turning off the phone, turning off external speakers, etc.; on the telephone panel there may be indicators that determine the operating mode, pick 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 name of the subscriber by voice, and the modem connects with his number; speed dialing, dialing a number using one or two keys; auto attendant, answering incoming calls when talking with another subscriber; collection of statistics on the number of calls received, their numbers, conversation time during the day, etc.; other functions, for example, dialing a specific number at a specific time of the day, an alarm clock, etc.
If the modem freezes, you can restore its performance by resetting the power supply (remove the external one and reinsert it), and 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 referred to as xDSL (Digital Subscriber Line).
ADSL(Asymmetric Digital Subscriber Line - asymmetric digital subscriber line) appeared in 1987 and is one of the very first and most common digital data transmission format. Allows you to send data from the user to the network at a speed of 16 to 640 kbps (according to the standards 0.5, 0.8, 1.2, 1.3, 3.5 Mbps, and receive data at a speed of 1.5, 0.8, 5, 8, 12, 25 Mbps sec). Since the user is normally receiving rather than sending data, this split of rates is not perceived by the user, except in video communications. Therefore, over time, other types of formats began to appear using a coaxial cable (cable television, speed up to 100 Mbps) and an Ethernet connector (local network at speeds up to 1 Gigabit / s). In a number European countries the ADSL standard has become the standard by which every resident has access to the Internet.
An ordinary telephone line uses frequencies from 0.3 to 3.4 kHz for transmission, for an ADSL modem the lower frequency for the outgoing stream is 26 kHz, and the upper one is 138 kHz, and for the incoming stream from 138 kHz to 1.1 MHz. Thus, you can talk on the phone and send and receive data at the same time.
Nevertheless, the first modems did not allow comfortable enough to talk 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 - frequency separator), which passed only low frequencies to the phone.
HDSL (High Data rate digital S ubscriber Line) was developed in the late 80s. It uses not one, but two pairs of wires and has a speed of either 1.5 Mbps (American standard) or 2.0 Mbps (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 - a digital network with service integration) appeared in 1981 and has a data transfer rate of 64 Kbps.
HPNA(Home Phoneline Networking Alliance - the name of the unified 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 up to 320 Mbps, according to standard 2.0 - 10 Mbps.
SHDSL (Symmetric High-speed DSL - symmetrical high-speed DSL) allows you to transfer data over one pair of wires at speeds from 192 Kbps to 2.3 Mbps, and two pairs twice as much at a distance of up to 6 km.
SDSL(Symmetric Digital Subscriber Line - symmetrical digital subscriber line) uses one pair of cables with speeds from 128 to 2048 kbps. Operates 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 of 13 to 56 Mbps from the network to the user and 11 Mbps in the opposite direction at a distance of up to 1.2-1.4 km.
WiMAX(Worldwide Interoperability for Microwave Access) is a wireless communication in the wavelength 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 WMAX). It has many similar parameters with Wi-Fi, but differs in that it can transmit a signal over a long distance and, moreover, 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 has no connector and is located inside the computer (device), used to transfer data using radio waves between different kind computers, cell phones, printers, cameras, keyboards, mice, joysticks, headphones, MFPs, scanners and others.The essence of the method lies in the fact that in a certain range, the frequency is pseudo-randomly changing stepwise 1600 times per second. Such a change in frequency occurs simultaneously for the receiver and transmitter, which operate synchronously according to this scheme.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 inside the computer and is not visible. If the computer does not have such a device, then you can connect an external device via the USB connector, which 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 Mbps, in practice about 2 Mbps, 2.1 (2007) with the use of energy-saving technology, simplified communication between devices, has also become more secure, 2.1 EDR requires even less power, further simplifies the connection of devices and increases reliability, 3.0 HS (2009) with transfer rates up to 24 Mbps. 4.0 became used in the iPhone in 2011, allows you to transfer data at a speed of 1 Mbps. in portions from 8 to 27 bytes.
There are profiles for this standard, which are a set of features. In order for devices to work according to a specific profile, both devices must support this profile. For example, A2DP (two-channel stereo audio transmission), AVRCP (standard TV function transmission), BIP (picture forwarding), BPP (text, e-mail forwarding to printer) and so on.
WiFi used to create wireless network. Developed in 1991 by NRC Corporation and [email protected], is supported by the Wi-Fi Alliance and complies with the IEEE 802.11 standard. Used to connect to the network (local and Internet) computers and cell phones.
The transmit/receive device is located inside the computer and is not visible. If the computer does not have such a device, then you can connect an external device via the USB connector, which 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) up to 54 Mbps; 802.11b uses frequencies of 2.4 GHz, providing speeds (in theory) up to 11 Mbps. (practically not used); 802.11g uses frequencies of 2.4 GHz, providing speeds up to 54 Mbps. (the most common); 802.11n uses frequencies of 2.4 and 5 GHz, providing speeds from 150 to 600 Mbps. (newly developed, starting to gain momentum). In this standard, the range of data transmission is increased, communication is less affected by barriers. This standard uses MIMO technology (Multiple Input Multiple Output - multiple input, multiple output), which allows the use of reflected waves from walls. If the device has one antenna, it can operate at a speed of 150 Mbps, two antennas - 300 Mbps, three - 450 - Mbps, four (not yet available) - 600 Mbps. However, the declared data transfer rate differs from the actual one. So instead of 300 Mbps, it turns out about 100 -130 Mbps (since half of the transmitted information is service characters), which is also enough for work. And in the presence of walls, the speed still drops, for example, for three walls it will drop to 50 Mbps.
Since some household appliances operate at 2.4 GHz (such as a microwave oven), they may cause interference. Therefore, it is desirable to have a device that operates at two frequencies: at 2.4 and 5 GHz.
There are also cable modems for connecting to a cable TV channel.
Typically, digital modems may contain elements that are used as Gateway between the local network and the Internet: a router, a firewall, and so on.
Modem indicators
There may be the following indicators:
AA(Auto Answer - auto-answer) - auto-answer mode, providing an answer to the subscriber's request in automatic mode;
CD(Carrier Detect - carrier or DCD detection) - lights up during a communication session;
CTS or CS(Clear To Send) - the modem is ready to receive data from the computer. Turns off while receiving data;
DATA– lights up during data transmission;
DC (Data Compression) data ;
FAX– when the modem is operating as a fax;
HS(High Speed - high speed) - lights up when the modem is operating at maximum speed;
EU (Error Control or ARQ) - error correction mode;
MR(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) - glows when the handset is hung up;
ON(PWR) - power indicator;
PWR (PoWeR) – power on;
RD(Recieve 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 is off-hook on a telephone connected in parallel;
RTS (Request To Send) - the modem is ready to receive data from the computer. Lights up when waiting for data from the computer, goes out during data transfer;
TD (Transmit Data or TXD) – lights up or flashes when data is being transferred from the computer to the modem. May light when data is being transmitted at the maximum transfer 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 the USB bus.
There may also be a volume control on the modem case.
On the back external modem may have connectors with icons:
AC IN – connecting the power adapter;
LINE – connection to a telephone line;
ON / OFF – modem on/off;
PHONE – telephone connection;
RS -232 – connector for connecting to a computer serial port;
USB – socket 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 of 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 binds the possibilities for data transmission at high speeds. Computers can be connected not only through a telephone line, but also using radio waves and infrared radiation. In this case, wires are not needed.
Ultimately, the data sent on the parallel channel is serialized at the serial port with start-stop bits, transferred to the modem, where it is modeled, that is, superimposed on the carrier frequency of the signal transmitted over the line, then sent to another modem. Next, they are digitized, sent to the serial port, where they are converted to 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. In a synchronous transmission, a data packet consists of a header containing the destination address, the data itself, and a checksum. An asynchronous transfer sends a start bit, 8 data bits, possibly an even parity bit, and a stop bit indicating the end of the transfer. This type is used in a serial link.
In addition, three modes can be used for data transmission: full 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, in which data is transmitted only in one direction.
Data transfer from modem to modem and from modem to computer has a different speed, so that the data is not lost, the modem has a buffer where the received data is stored.
Some modems compress data before sending it, and another modem decrypts the data when it receives it. There are files that have already been compressed, so this method may not give you the best transfer experience. To avoid data loss, the data transfer rate from the modem to the computer must be several times higher than between modems, which is implemented in practice.
When transmitting data, the unit is often used baud, which is sometimes confused with bps. In fact, they are different sizes. 1 baud is one character sent per unit of time, and it can be not only data, but also control signals. A character can be multiple bits. If the signal consists of two types: 0 and 1, then the symbol means 1 bit, if 512, then 9 bits (2 9 = 512). When transferring data at a low speed, 1 baud is approximately equal to 1 bit / s. At a high speed, the modem sends data already at several frequencies, so not one, but several bits are transmitted at each moment of time, that is, the speed measured in bits / sec, and not baud / sec, will be several times higher than the baud rate . Often quoted baud rate refers to bits/sec.
When transmitting via a modem, you can roughly determine how long it takes to transmit by dividing the transmission rate by 10, for example, if the transmission is at 28,800 bps, then approximately 2,880 bytes or characters will be transmitted per second (28,800/10= 2800).
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 be used for direct communication between two arbitrary computers. Modems are also used as fax machines to send fax messages. They may have a built-in adapter for creating voice messages in answering machine mode.
When connected, the modem sends out signals that are also output to the speakers and can be heard as a continuous, changing sound for several seconds. The receiving modem determines the standard by which it can operate, and also makes clock frequency settings, that is, it performs phase simulation. After that, the speaker is turned off, but the signals continue to arrive, in particular, they can be heard through the parallel phone.
Modems are of 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 case and are connected to the serial port using a cable. The latter types of modems can be connected via the USB bus (and sometimes receive power from the computer), so they are used while the computer is running, free up a connector, and have other advantages. When connecting a modem to a serial port for high-speed models, the port must also be high-speed. For example, 56Kbps modems require a serial port speed of 115Kbps. The higher port speed is needed because it also sends control signals between the computer and the modem that are not sent over the phone 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 internal devices 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 the data, the second type has the data processed by the 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 it is acceptable. It is quite advisable to use it even if the computer has a powerful processor.
Often the modem is characterized protocol with which he works. Exist signal modulation protocols, error correction protocols, data compression and facsimile operations (fax). The modem has several protocols for each of these types. Error correction protocols include V.42, MNP2-4, MNP10, data compression - V42bis, MNP5.
One of the main characteristics of the modem is the data transfer rate, and the indicated maximum speed can be 33.6 or 56 Kbps for modern devices. If 33.6 Kbps is specified, then the entire bandwidth is used and data is transmitted in both directions at 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. provides receiving data at a higher speed than when sending them, since there are more frequencies for reception than for transmission, however, transmission from the modem is carried out at a lower speed.
In addition, it is necessary that both modems have the same characteristics, otherwise the data transfer will not reach the maximum speed. To do this, before buying a modem from a provider, you need to clarify the type of modem with which it works best. Below is a table of correspondence between some protocols and the transmission speed over it.
The prefix bis means that the standard has been revised. Starting with a speed of 14400, all protocols are duplex, that is, they transmit messages in both directions at the same time. Not only the names of the standards that define the data transfer protocol, but also other types of protocols can begin with the symbol V, for example, V.24 contains a list of certain signals between two modems, V.25bis is a command language for managing a modem, etc., there are other names, for example, MNP, sometimes starting 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 packet size 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; MNP5- provides synchronous mode, while data compression is used, has two algorithms for compressing repetitive messages; MNP6- provides synchronous mode, also uses data compression; MNP7, MNP8, MNP9- provides synchronous mode, while using more advanced compression methods; MNP10- used in case of low-quality data line. At the start of work, it sets the lowest speed, and if the line is able to work with high gear, then the speed increases.
There are also the following protocols:
Xmodem- the protocol was issued in 1977. The transmitting modem sends a special NAK signal, then, after receiving, the receiving modem issues a NAK signal until it receives a data packet that consists of a start of data character (SOH), a block number, 128 bytes of data, and a checksum (CS) . When data is received and checked for correctness, a checksum signal is sent that the data was received (ACK), and if received incorrectly, a signal (NAK) is sent. If there are multiple failed data transfers, the session is terminated. At the end of the transmission, an EOT character is sent, indicating the end of the session.
There are modifications of this protocol, for example, in Xmodem CRC the checksum has been increased to 16 bytes, which increases the reliability of transmission, Xmodem 1k– data block size increased to 1 kilobyte, Xmodem G- transmits data, and the checksum is at the end of the file, not the data block.
Ymodem- based on the Xmodem protocol, with a transfer size of 1 kilobyte, transfers the file name and its attributes. In addition, the first block contains information about whether the following files are available for transfer.
Kermit- uses data packets up to 94 bytes, mainly used in Unix systems.
Zmodem- transfers data from 64 to 1024 bytes in size with their compression. On failure, sends data from the point at which the failure occurred.
Bimodem– further development of the Zmodem protocol with the ability to send data in two directions simultaneously.
Sometimes you may need modem commands, for example, to test it. Below is a list of some modem commands (note that modem modifications may have a different set of commands):
ATA- readiness of the modem to work;
ATADP number– pulse telephone number dialing;
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 mode of operation;
AT&W– writing the current modem parameters to memory;
ATSx=value– determination of modem characteristics;
+++ - switching the modem to command mode;
A\- repeat the last command.
When transmitting data over a modem, special protocols are used for data compression, for faster transmission, and error correction methods. These standards are referred to as MNP (Microcom Networking Protocol), as well as some of the standards beginning with the letter V (V.41, V42, and V42bis).
For data transmission, a special protocol is used, that is, the rule by which data is transmitted and received. For normal operation, it is necessary that both modems (sending and receiving) can work with these protocols. With data correction methods, in addition to them, a special CRC combination is sent, which serves to determine errors. Upon receipt, the data is checked, that is, the CRC blocks (calculated and check) are calculated and compared, and in the case of normal operation, a signal is sent that the data was received correctly.
Remarks. The country code in the computer is the same as the international phone prefix. The phone number consists of the following digits: Country code (10 for Russia), + region code (495 or 499 for Moscow) + PBX number (3 digits) + internal PBX phone number (4 digits)
If you experimented with the modem and it does not work, then to reset the parameters, 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 textual information over telephone channels is called dayphone.
Modems contain in itself: 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 microcircuit control program is stored, modem parameters are supported, 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 the central processor, for example, the controller. Such modems are called software.
The most important characteristic is the data transfer rate. More recently, the standard was 14.4 Kbps (of course, there were lower speeds), then devices appeared that allowed information to be transmitted at a speed of 28.8 and 33.6 Kbps. Now the maximum transfer rate has reached 128 Kb / s and provided the maximum possibility of transmission over the telephone network.
Of course, devices that operate at 33.6 KB can also operate at slower speeds, namely 28.8 and 14.4 KB/s, but not vice versa. So, if at one end there is a modem that provides a transfer rate of 28.8 Kb / s, and at the other - 14.4, then the transfer will occur at a speed of 14.4 Kb / s.
Installing the modem
Modem installation. Installing a modem, as a rule, is not a big problem, because after installation operating system finds it and installs the standard driver. If a driver is attached to the modem, then it is advisable to install it, since compared to the standard driver, it provides additional features.
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 modem 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 this is an external modem, then connect to the serial port or USB port. If the number of pins in the serial port connector does not match, an adapter will be required, as one of the ports may already be occupied;
If the modem has one output for the phone, 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 outlet that has two outputs: one for the phone, the other for the modem. The view of such a socket is shown in the figure on the right, it has two such types of connector.
One coincides with the standard in force in our country, and the second - with the one adopted in the West, it is available 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 set, the wire to the telephone socket and the wire to the modem are connected to the other two.
If the modem has two telephone jacks, then you need to connect the wire from the telephone socket to one (the inscription near the line connector), the other to the telephone set (the inscription phone). If there is no inscription, then look at the back of the modem, where there may be a pin diagram, or refer to the documentation. If the connection is not made correctly, the modem will not work. In this case, change contacts. The external modem must also be connected to the network via the power supply. To install an internal modem, use the description of installing boards in the system unit;
After installation, turn on the computer and install software supplied with the modem.
Notebooks have one output for connecting to a telephone line. While working with the modem, it is better not to use a parallel telephone or connect it through the appropriate socket on the modem, otherwise interference from the telephone line may occur, noise may appear.
In the Windows system, 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 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 “setup wizard” will appear on the screen, which, using questions and answers, will help you install the modem. 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 detect modem type) Next. If there is a disk for the modem, then you need to use the "Install from the Disk" mode or, if it is not available, select the manufacturer (if it is unknown, then "Standard modem types") and Model → Next → selecting the appropriate model, click Next → (select desired port) Next .
One of the most important parameters that needs to be set is the type of dialing, which must be pulsed, since we do not use another type in our country. To install it, in the Properties: Modems: General window, click "Connection settings", where select pulse dialing.
To verify, whether the installation was done 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 and exclamation marks near the installed device.
Modem settings can be look and change through: Start → Settings → Control Panel → Modems → Properties → General, where the port changes, the speaker volume, the maximum speed is indicated. In this case, the maximum speed is meant between the modem and the computer, and not between the modems. Usually, the maximum speed is set, and in case of poor communication, 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, created at the time of information transfer, are then turned off;
Unswitched (dedicated), long-term dedicated
or
Low-speed (telegraphic) 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.
For data transmission at high speed, twisted pair wire (twisted together), coaxial cable (as in a television antenna), fiber optic (from glass fibers) and a radio channel (through 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), ultrashort (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, which is 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, and others. The most common is Zmodem.
network cards serve to connect a computer to a network of computers and are an intermediary between a computer and a 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, the size of the memory, the bit depth of the card (8, 16, 32 bits), and the types of connectors for thin and thick cables. Network cards require the setting of an interrupt line (often 3 or 5), a DMA channel, a memory address (C800).
Network cable may be of several types:
twisted pair. It consists of several twisted copper conductors in one cable, which can be unshielded (UTP) or shielded (STR).
coax cable consists of a central and shielding wires, between which there is insulation. There are two varieties of this cable: thin (0.2 inch thick) and thick (0.4 inch thick).
Fiber optic cable consists of two wires consisting of light fiber fibers. It has a large throughput, 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 one manufacturer. After laying a thin cable, connectors are installed, for example, Russian-made (CP50) or crimp BNC connectors. A plug is installed at the ends and one of them must be grounded.
The thick cable is laid through the transceivers, and one transceiver is used to one computer, and at the ends of the cables leading to the computer, there must be 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), use repeaters (Repeater - repeater).
A twisted pair cable is used together with a hub or hub (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 is similar in appearance to a telephone jack, but has 8 pins (not 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 running fax, he works by his own standards. When sending faxes at 14.4 Kbps, V.17 (14400), V27 ter (4800), V29 (9600) and T.30 are used for the protocol itself. When transmitting a sheet image, the following modes with resolution for fax transmission can be used: standard (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 this modem.
In addition to ordinary modems, there can be very specific modems, for example, cable ones, 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, throughput per user may become low. And now, while there are few users, they give a small number of users the big benefits of working on the Internet.
Can be used satellite devices, while users send a message to the provider via phone, which pages he wants to receive, and receives them via satellite.
Currently, more and more information is being used mobile connection. In this case, the modem is connected to the mobile phone through a special cable.
In our country, the most widespread in 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 such a standard is that all transmitted information is divided into so-called frames, divided into eight intervals. Depending on the busyness of the line, one interval or another can be used. But this method of mobile communication is intended primarily for the transmission of voice messages, which take precedence over digital data. Ultimately, the data transfer rate does not exceed 9.6 Kbps.
Other standard GPRS(General Packet Radio Service - general packet radio service) allows you to increase this speed to 50 Kbps, and theoretically it can reach 100 Kbps. Unlike GSM, here, for sending information, it is possible to use other time intervals in the frame, up to all eight, and this circumstance increases the speed of sending data. In addition, this option of mobile communication provides a reduction in user costs, since the volume of transmitted information is paid, unlike GSM.
GPRS devices are divided into three classes according to their capabilities:
Class A. Such devices in each unit of time are capable of simultaneously transmitting both types of information - voice and digital.
Class B. These models allow you to work alternately, either with digital data or with voice.
Class C. Only digital data is sent here.
There is no strict classification of modems due to the diversity of both the modems themselves and the scope and modes of their operation. Conventionally, modems can be classified according to their design, scope, functionality, type of channel used, transmission method, intellectual capabilities, etc.
By design modems are divided into external, internal, portable, group.
External modems are produced in a separate housing and have a built-in or remote power supply. Their advantages: the presence of a light indication on the front panel, and more recently an LCD, which makes it easy to determine its status; controls are located on the front panel, which allows you to control the modem and change its configuration. On the rear panel of the modem there are connectors for connecting to a power source, a serial port of a computer, a telephone line (LINE), and also for connecting a telephone set (PHONE). Modems are equipped with an internal speaker, sometimes there is a volume control. Disadvantages of external modems: an additional socket for the power supply is required, additional space on the desktop, foreign-made modems are sometimes designed to be connected to an electrical network with a voltage of 110-115 V, which requires the replacement of the power supply.
Interior A modem is an expansion card that is inserted into the corresponding slot of the system bus of the computer motherboard. Such modems do not have their own power supply and receive power from the device in which they are installed. On the rear panel of the modems there are connectors for connecting a communication line and a telephone. Advantages: do not need to be turned on/off, do not occupy standard computer COM ports, are cheaper than an external modem, etc. Disadvantages: the need for a free slot on the motherboard, increased load on the power supply, no external indication, complicated installation, restarting the entire computer when the modem "freezes".
Portable modems are designed to be used with Notebook computers. Such a modem is similar to an external one, only a reduced size. It has a more durable housing and universal power supply (from the mains through an adapter, from car network 12 V and from the internal battery. Modems have appeared that connect to a portable computer through a special connector, through which it also receives power. Disadvantages: such modems are much more expensive than other modems with similar capabilities.
Group (rack) modems are a collection of individual modems arranged in a common unit and having a common power supply, display and control devices. These are professional modems. And they are inserted not into a computer, but into a special modem rack called a modem manager.
By area of application modems can be divided into several groups:
for dial-up telephone channels - such modems must be able to work with automatic telephone exchanges, distinguish their signals and transmit their own dialing signals;
for dedicated (leased) channels;
for physical trunks (bandwidth is not limited to 3400 Hz and depends on the type and length of the cable);
for digital transmission systems - connected to digital channels such as ISDN;
for cellular communication systems - they support special modulation and error correction protocols that allow efficient data transmission in cellular channels with a high level of interference and constantly changing parameters;
for packet radio networks - several radio modems use the same radio channel in the multiple access mode;
for local radio networks - such radio modems provide data transmission over short distances (up to 300 m) at a high speed (2-10 Mbps), comparable to the transmission rate in wired local networks;
cable modems - used to transmit cable television channels; The transmission speed can reach 10Mbps.
Consider the types of modems by function.
Fax modems. To increase the functionality of a classic modem, fax exchange with fax machines and with other fax modems is almost always added. Advantages fax modems:
1. saving thermal paper,
2. a message file is somewhat better read than a fax,
3. sending a fax using a modem is faster,
4. you can program multiple faxes to be sent at a given time,
5. It is more convenient to process incoming documents as files.
Voice modems(VOX) are implemented as an extension of fax modems. These modems receive voice messages from the telephone network, recording them in the form of a computer file, and also play previously generated voice files into the telephone network. The file can be output to the computer speaker, to a separate handset connected to the modem, or to the handset of a telephone set using a special switch.
SVDmodems allow you to transfer data and at the same time have a conversation using a handset connected to the modem, and in full duplex mode.
High speed modems - these are SR modems and cable modems. They use completely different transmission protocols than on telephone lines. SR- modems are used for data transmission over short distances, i.e., when it is possible to connect two terminal devices with a direct cable and there is no need to "squeeze" into the standard telephone channel width. This allows you to increase the data transfer rate to 80 kbps for distances up to 15 km, and for a good twisted pair cable and shorter distances, the speed can reach up to 2 Mbps. Cable modems use cable television channels for transmission (transmission speed reaches 10 Mbps).
Radio modems instead of telephone wires, radio is used as a medium for transmitting information. Instead of a telephone jack, they have an antenna jack where the antenna is inserted. It is the same as a classic modem: it connects to a computer via a standard RS-232 interface. A whip antenna (up to 30 cm) is connected to the antenna connector, or, if a longer range is needed, an antenna cable, an amplifier and a directional antenna. In addition, the radio modem contains a radio transceiver. They are used in cases where the laying of a telephone or cable line is difficult. The main difference between radio modems is that they are designed to work in a single radio channel with many users, and not in a point-to-point channel.
Cellular modems used for mobile cellular radiotelephony. They do not contain transceivers in their composition, but use transceivers that are part of a cell phone, transmitting their signals to them. The speed of data and facsimile information transmission in such a radio channel is up to 14.4 kbps (without compression). These modems are portable and housed in a rugged housing. They are used on noisy and unstable lines: satellite, rural and intercity.
By transmission method modems are divided into asynchronous and synchronous. Transmission over the DTE-DCE interface can be synchronous or asynchronous. The modem can work with a computer in asynchronous mode and simultaneously with a remote modem in synchronous mode and vice versa. Then we can say that the modem operates in a synchronous-asynchronous mode. Most often, synchronization is implemented in one of two ways, related to how the sender and receiver clocks work: independently of each other (asynchronously) or in concert (synchronously).
Asynchronous transfer mode is used when the transmitted data is generated at random times (by the user). In this case, the receiving device must resynchronize at the beginning of each received character. To do this, each transmitted character is provided with an additional start bit and one or more stop bits.
With synchronous transmission method combines a large number of characters or bytes into separate blocks or frames. The entire frame is transmitted as a single bitstring with no delay between the eight-bit elements. In order for the receiving device to provide different levels of synchronization, the following requirements must be met:
the transmitted sequence must not contain long sequences of zeros and ones so that the device can allocate a clock frequency.
each frame must have reserved sequences of bits or characters marking its start and end.
By intellectual capabilities modems there are:
without control system;
supporting a set of AT commands; (allows the user to control the modem's characteristics and communication parameters).
with support for V.25bis commands; (allows you to control the connection establishment and auto call modes).
with proprietary command system; (specialized industrial modems).
supporting network layer protocols; (allow the administrator to manage network elements and modems from a remote terminal).
What is a modem and why is it needed at all?
Its name comes from two words: MODulator and DEModulator. These two words perfectly reflect the essence of the work performed by the modem. It modulates the signal transmitted to the telephone line with information received from the computer, and vice versa, transmits to the computer what it demodulated from the line. Why is this needed? - the meticulous reader will immediately ask. But why! As you must know (and if you don't know, read more carefully!), all information is represented in the computer in the form of zeros and ones. Zeros and ones, in turn, are encoded by voltage: no voltage - zero, there is voltage - one. Naturally, computers can exchange information only with the help of zeros and ones. If the distance over which the data needs to be transferred is small, as for example in a computer - from one microcircuit to another, they are simply connected by wires. And if you want to transfer something to the computer to a friend who is, say, in another area? You will go broke just buying the wire, not to mention ordering the digging of a ditch for this wire, or hanging it on poles (otherwise they will steal it!).
Fortunately, in many places the telephone has become widespread - and this is nothing more than a ready-made pair of wires. However, these wires are not as good as we would like, because they are still intended to transmit voice, and not zeros and ones. This is where the modem's job lies: to convert zeros and ones into a signal that is more or less similar in its characteristics to a voice, and therefore suitable for transmission over a telephone. At the same time, the modem also performs the functions characteristic of a conventional telephone - it dials a number, picks up the phone when they call, etc.
To perform all the functions assigned to it, the modem must be very smart, and this is not easy even for people. Basically, a modem is a small computer. It has a processor, memory, and all sorts of other parts necessary for normal operation. One end is connected to the telephone line, and the other to the computer. If we figured out a little with the telephone line, then a few more words should be said about connecting to a computer. Computers - after all, they are also different, big and small, fast and not very fast. In order not to make their own modem for each type of computer, smart people decided to agree and install the same device in all computers - a communication port (COM port).
If a computer has such a communication port (the standard for it is called RS232C in America, and V24 in Europe), then any standard modem can be connected to it. Naturally, you need to immediately clarify what we mean by talking about a "standard" modem. Modems, as such, are affected by three types of standards: you already know about one of them - it describes the interaction of the modem with a computer (RS232C / V24), the other determines how data is converted for direct transmission over the phone, and the third describes commands to the modem (the modem also you can command!).
Let us consider in more detail the standards for data transmission protocols over the telephone network. The protocols supported by your modem determine the speed at which it operates, as well as the very possibility of its operation with any other modem. Generally speaking, the principle of transmitting information over the telephone is somewhat reminiscent of radio. The modem generates the so-called carrier frequency ("our radio station operates on a frequency..."), and modulates it with information coming from the computer according to the rules of a certain protocol. (Very often you will come across English word CARRIER, - do not be alarmed, it refers to the carrier frequency). The most common protocols are V21, V22 and V22bis. They define how signals must be modulated to transmit information over telephone lines at speeds up to 300, 1200 and 2400 bits per second, respectively. It should be noted here that data is transmitted over the phone sequentially, bit by bit, and in addition to the basic information for which everything is started, the service information necessary to "keep the conversation going" is also transmitted. As a rule, in addition to 8 bits of each data byte, 2 bits are added: one at the beginning (start bit) and one at the end (stop bit). Total: a byte will consist of 10 bits, therefore, in our case, the maximum transfer rates useful information will be 30, 120 and 240 bytes per second.
Science naturally does not stand still, and recently new protocols have appeared that increase speed and provide additional services. Examples include the MNP and V42/V42bis protocols. Modems that support them can automatically correct errors that occur during transmission and compress the transmitted information, which sometimes increases performance. The V32 and V32bis transmission protocols describe a way to transfer data at up to 14400 bits per second, with the ability to automatically decrease or increase it during transmission, depending on the quality of the line. As a rule, modems maintain compatibility from the bottom up. That is, modems that support more advanced exchange protocols do not stop working with older models. The most important thing is that these old models are standard, which cannot be said about some crafts of domestic craftsmen. Despite enticing advertising ("1200!", "2400!", "high reliability!"), they are able to connect only with themselves, not to mention the fact that some of them, along with the signal, push a bunch of interference into the line, which is natural provoke the ire of the messengers.
Now consider the third type of standards - this is the standard for modem commands. In order to clarify what a "command to a modem" is, I will make one explanation: for any standard modem, there are two possible modes in which it can be. The first mode is the data transfer mode. The modem receives data from the computer, converts it into a signal, and sends it to the telephone line. Similarly, the signal that came from the line is converted into data and transmitted to the computer. The second mode is command mode. In this mode, the modem does not do any modulation / demodulation, and does not send anything to the line. All the data that comes to him from the computer, he considers as commands, and tries to execute them. This mode is basic for the modem, that is, when you turn on the modem, it starts its work in command mode. In this mode, you can, by sending various commands to the modem, make it pick up or drop the handset, dial a number, turn the speaker on or off, and configure data transfer settings.
The standard for teams proposed by the American company HAYES (read [hayes]) is currently generally accepted. This is usually written as "HAYES compatible command set", but sometimes it is also called "AT" compatible set - after the first two letters that indicate the command. It is by these first letters that the modem understands that the information being entered must be understood as a command to be executed. In your experiments, you should keep in mind that any command to the modem should be no longer than 40 characters, and end with a "carriage return" code (ENTER key), although there are a few exceptions that we'll look at next. If the modem recognizes the command, it tries to execute it and report the result. The simplest command consists of only two letters "AT", it causes the modem to "voice", to answer that everything is in order with it. This response looks like an "OK" message from the modem. As a rule, modems are automatically configured for the speed and other parameters of the COM port to which they are connected, so that any working standard modem in command mode should respond to this simple command. If the modem cannot understand the nonsense that has fallen on it, it swears "ERROR", which means an error.
Good day to you my dear friends. Today I will nostalgic 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 first heard the name of this device.
Then I explain by referring to history.
Let's say that you have a PC, but there is no Internet, none at all. Are you bad without it? Yes, especially considering that you know its limitless possibilities. So. In the 90s, computer owners in Russia first learned about such a Wonder of the World as the global web. And it turns out that you can connect to it over 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 was a provider in your city that provides access to the Internet. And the user had a device called a modem, and connected between the PC and the telephone line.
How 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 of finding optimal solutions.
The history of the emergence of modem data processing begins in the post-war mountains in the United States. In 1950, between military air defense bases located throughout North America, was laid, which connected radars, terminals, command centers. The 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, the Micromodem II appeared, designed exclusively for the Apple II PC. So the device got its name "modem", formed from the names of the two operations it performs: MODulation and DEMODulation.
I will now explain the purpose of these functions and the principle of operation. So that the 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 passes through the network.
In turn, the computer is not able to perceive analog information unusual for it. Therefore, the smooth voltage change in the line is demodulated back into pulses that are understandable for reading by the processor.
But if you study the operation of the modem in more detail, then you should pay attention to its interaction with the network and PC, which implies a system of requests, recognition, changing connection parameters and other control operations.
All this is recorded by a special program code. In 1981, Hayes introduced the Smartmodem 300. It used a system of such commands, which later became the standard in the modem industry.
What are modems and how are they arranged
But, since we began 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 a branch to the telephone) and a network LAN or serial LPT port for connecting to a computer;
- a controller responsible for exchanging streaming data with a PC;
- a signal processor that directly performs signal conversion operations;
- a memory chipset that includes operational RAM and two types of non-volatile: ROM (with firmware, drivers) and NVRAM with working settings.
I must say right away that this “set” was taken from the design of an external modem, which were the most common during their greatest popularity. Someone probably remembers such models as Courier or Zyxel.
Over time, there were also internal modems 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 the speed of the Internet
The conversation about modems will not be complete if you ignore the ways they connect to the Internet. And the first, of course, is Dial-up - the usual dialing, dialing. 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 turned off (despite the annoying) in order to control the process. Dial-up connection had its own characteristics:
- dialing took a long time, and in the process of working on the network, using the phone was impossible;
- very low connection speed of 40 - 45 kbps, which, moreover, strongly depended on the quality of the telephone line;
- billing was often per minute, and given the slow speed - expensive ...
As an alternative to corporate and wealthy users, a dedicated Dial-up line was offered. The advantage of which was the lack of dial-up and the best quality of the channel.
The practice of using the Internet has shown that the amount of information downloaded by the 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 clear benefits:
- it became possible to use the phone in parallel in the process of working with the Internet;
- speed increased to 25 Mbps;
- significantly reduced connection time;
The transfer of information with the help of a new look at the possibilities of high-speed Internet, but, unfortunately, the deterrent at this stage turned out to be throughput telephone lines. Providers began to directly lay communication lines (twisted pair or fiber) to the user, bringing the connection speed to 100-300 Mbps.
Not relevant now?
And what about our modems, you ask, are gone? Well, not quite. Still, laying a new line is troublesome, and the telephone cable is already lying. It's a sin not to use them. Therefore, providers still have such a service. Another thing is that telephone communication has changed, digitized, become mobile and provided newer perspectives in the face of 3G / 4G USB modems.
Now you can connect to the Internet at any point within the coverage area of your mobile operator. Moreover, the data transfer rate will allow you to safely watch streaming video. Over time, USB modems also had competitors - these are smartphones. Which in skillful hands easily distribute the Internet via wi-fi.
But that's another story. And this one, I think it is possible to finish. See you soon, my dear readers.
A modem is a device designed to modulate a signal, that is, to convert an analog signal into a digital one. It is from the word "modulation" that the name "modem" comes from. Using a modem, the user accesses the Internet. The first similar device appeared in 1979. During this time, of course, a lot has changed. The speed has also changed, which can vary greatly among users, so some want to measure the speed of the Internet.
Types of modems
1) Fiber optic modem. The device connects a computer to global network via fiber optic cable.
2) Cable modem. It allows you to transmit a signal through a standard television cable. At the same time, working on the Internet does not affect the quality of the television signal transmission.
3) ISDN modems. Such modems are used to work in digital networks - with their help it is possible to transmit voice, text information and graphics at the same time at a constant high speed.
4) ADSL modems. They are connected to a telephone line, but they work according to a special technology, due to which the access speed increases significantly. Such modems are not common due to the fact that special complex equipment is required, which does not always justify itself.
By functional feature modems are classified as follows:
1) Analog modems work to transmit information and receive a signal.
2) Fax modems are convenient in that they perform the function of a fax.
Modems are divided into external and internal.
The external modem looks like a small box and is connected to the PC via the main COM port, in some cases via the USB port. The external modem is equipped with indicators by which you can read the necessary information.
Modems tend to freeze, in which case it needs to be turned off and on again. Connecting an external modem is easier than an internal one - you need to connect the cable to the modem at one end and to the computer with the other.
An internal modem is a small board that is installed in a special PCI slot located inside the computer. Internal modems are cheaper and do not require a power supply and a separate outlet for connection.