What types and types of modems are there? Analog modems

He immediately asks himself the question “What is a modem and what is it for?” After reading the article, we will find out what it is, what types there are and what its purpose is.

What devices exist for connecting to the local and worldwide networks?

This word is formed by merging two terms. One term is modulator. This special circuit is responsible for encoding the signal. And the second term is the word demodulator. It is easy to guess that this component performs a completely opposite function. In general, their functions are as follows: encoding and transmitting a signal, receiving and converting it.

ATTENTION. A little earlier, connecting computers to the Internet was carried out using telephone wires. Network cards are replacing them as they have higher speeds. There are also wireless modems, which are not yet very popular.

Why and when are they needed?

There are only two moments when we need a modem. One of them, or rather, the first, dates back to the recent past. Connection to a computer was then provided using such equipment, as well as a telephone line. This point became almost irrelevant when network cards were born. After all, they are much cheaper in cost, and the speed is several times higher. And also the connection reliability is much better. And the second point is relevant for people who travel. They need the Internet, which does not require wires and unnecessary devices - wireless Internet.

By method of execution

According to the method of execution, the specified device is divided into two types: internal and external. Internal install inside system unit. And to provide connection to an external modem, you need an expansion slot for a PC, laptop or tablet. If you have a laptop or tablet, then you will need a hardware toggle switch, of course, if you have one. And it needs to be installed in the appropriate position. If the question arises, “What is modem mode?”, then we will answer it now. There are two modes in total: digital and analog. It depends on the telephone line signal. If you have a wireless device, then only digital mode will be available to you.

By type of connection

The connection for this equipment can be different - both wired and wireless. Wired ones have a special connector for a telephone cable. In older devices, you could do one thing: talk on the phone or surf the Internet. Nowadays, there is a special type of such devices that allows you to do these things at the same time. This device is called an ADSL modem. It converts the separating conversation and the transmitted signal to different frequencies. This means that not one, but two data streams go along one wire (cable). And wireless transmits data using electromagnetic radiation.

By type of supported networks

This feature only applies to wireless devices. The following types of networks exist: GSM or 2G, 3G, LTE or 4G. All of these networks are backward compatible. In simple terms, 3G will work without problems on the GSM network. If you wondered what a USB modem is, now you will get the answer. This device is most often created in this form. A flash drive is what this equipment looks like. Its main function is to provide wireless data transmission. It must have a slot for a SIM card. It connects to the computer in a USB slot.

Digital modem device

As already noted, digital modems include devices such as CSU/DSU (Channel Service Unit/Data Service Unit), ISDN terminal adapters and short-distance modems (Short Range Modem). In terms of their functions, digital modems are very similar to modems for analog channels communications. With the exception of the simplest ones, digital modems have intelligent functions and support a set of AT-commands. This primarily applies to digital modems operating on dial-up lines, for example, in ISDN networks. As an example of a digital modem, consider a CSU/DSU device.

CSU/DSU devices are used to transmit data over digital channels such as E1/T1, Switched 56 and others. The CSU ensures correct matching to the digital channel being used and line frequency equalization. The CSU also supports loopback testing. Light indicators are often installed on CSUs to indicate a breakage.

Rice. 2. 13. CSU/DSU device diagram

lines, loss of communication with the station, as well as operation in loopback test mode. The CSU can be powered by a separate power supply or via the digital line itself.

Data service modules, or digital service modules DSU, are included in the chain between the CSU and DTE (Fig. 2. 13), which is often not only a computer, but also various network equipment, such as a router, bridge, multiplexer or server. The DSU is usually equipped with an RS-232 or V. 35 interface. The main task of the DSU is to conform the digital data stream coming from the DTE to the standard accepted for that digital line.

An analogy can be drawn with equipment for ISDN networks. In this case, CSUs play roughly the same role as NT1, and DSUs are similar to ISDN terminal adapters. DSUs are often integrated into other devices, such as multiplexers. But more often they are combined with CSU. This produces a single device called CSU/DSU or DSU/CSU. CSU./DSU can have built-in compression schemes for transmitted data, as well as redundant switched ports. Often CSU/DSU devices perform error protection functions by implementing one of the HDLC superset protocols. Unfortunately, in the field of digital modems there is no such strict standardization for data compression protocols, error protection and the type of line coding that exists for analog PSTN modems. For this reason, you should be very careful when choosing digital modems from different manufacturers.

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 located nearby, then you can organize communication between them using 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 to 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 the characteristics of a carrier signal (usually low-frequency periodic oscillations) by a high-frequency control signal, which 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 ports or a network cable to the RJ-45 connector of the computer’s network card, usually have external unit 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 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 desk or take up the computer's serial port.

External modems (new) connect to a USB, PCMCIA or ExpressCard connector and do not require additional food, since 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: digitizing voice and converting it into an analog signal for conversation when transmitting data; fax; automatic caller number identification; answering machine; electronic secretary and other capabilities that telephone sets have.

Typically, 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 dialing, 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, such as dialing 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/sec) and Ethernet connector (local network with speed up to 1 Gigabit/sec). 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, conversely, 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 of 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 protected, 2.1 EDR required even less power, connecting devices was further simplified and reliability increased, 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/s. 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/sec, it turns out about 100-130 Mbit/sec (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 an answer 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 blinks when data is being transferred from the computer to the modem. May light up when data is being transmitted via maximum speed transfers;

TST (TeST) - blinks 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.

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. In 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 actually implemented in practice.

When transmitting data, the unit is often used baud, which is sometimes confused with bits/sec. In fact, these are different quantities. 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 baud speed . 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 listened to 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 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, in the second the data is 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 e-mails from time to time, 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 fax communication (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 defining the data transmission 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 - command language to control the 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 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, while data compression is used, 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 the 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 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 is used to identify errors. Upon reception, the data is checked, that is, calculations and comparisons of CRC blocks (calculated and verification) are performed 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 simulated CPU, 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 run at 33.6KB can also run at higher speeds. slow 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, does not cost big problems, 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 this 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 an outlet 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 the telephone socket to one (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 the software that came with your 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 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 transmit data at 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 the cable, pay attention to characteristic impedance, often 50 ohms. When laying, you need to have cables of the same brand, preferably from the same 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.

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 together 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 may have different number 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. Working over 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.

Nowadays, 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 method of mobile communication is intended primarily for the transmission of voice messages, which take 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 Kbit/s, and theoretically can reach 100 Kbit/s. 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.

The intensive development of communication systems is accompanied by a large number of new terms and concepts, names of devices and technologies. This sea of ​​information is difficult to understand not only for the user who wants to choose the optimal device or solution, but also for the specialist on whose shoulders lies the responsibility for automating the enterprise as a whole, from software to cable systems.

This article touches on a small but most interesting area of ​​telecommunications - we will talk about modern analog voice-frequency modems and corresponding modem technologies, which will give users and specialists the opportunity to better understand the specifics of transmitting information by such devices.

Communication lines

By definition, a communication line is a medium capable of carrying electrical or electromagnetic vibrations in a limited frequency range. Before transmitting information, a digital stream consisting of zeros and ones is converted into a signal that can propagate in a particular medium. However, any medium sets its own limitations on signal propagation, which affect the ability to achieve maximum information transmission speed.

Thus, any communication channel has a theoretical limitation on the speed of information transfer. This limit - the Shannon limit - is determined by two parameters: bandwidth and signal-to-noise ratio. Bandwidth is the difference between the maximum (upper) and minimum (lower) frequencies of a signal capable of propagating in a communication channel. The signal-to-noise ratio is a characteristic not so much of the channel itself, but rather of the “communication channel + modem” system. Therefore, when describing communication lines, such characteristics as bandwidth, signal attenuation coefficient per unit length and level of interference (noise) are often used.

Wider bandwidth allows for higher speeds, and lower attenuation allows for greater range. However, some environments are characterized by a situation where frequencies at the edges of the spectrum decay faster than in the middle. Therefore, for them, increasing the range simultaneously limits the maximum speed of information transmission.

Telephone lines

The existing infrastructure of telephone networks allows them to be widely used for data transmission. However, the channel-forming equipment of telephone exchanges imposes a significant limitation on the signal bandwidth - a signal is transmitted only with frequencies from 300 to 3,400 Hz, that is, a width of 3,100 Hz. Such channels are also called voice-frequency channels.

Modern telephone exchanges use analogue signal conversion to digital form, and the conversation is transmitted at a speed of 64 Kbps, which ensures the specified quality. However, from the point of view of analog signal transmission, a digital stream of 64 Kbps cannot pass through proper quality a tone frequency signal whose spectrum is wider than 3,500 Hz. Thus, telephone communication lines transmit an analog signal with a width of 3,100 to 3,500 Hz, depending on the equipment used at telephone exchanges (analog or digital telephone exchanges).

When transmitting information over telephone networks, the problem of signal attenuation is not so pressing. This is explained by the fact that telephone exchanges, as a rule, take care of themselves to deliver the signal to its destination while maintaining the required power level. Of course, if they are interconnected by analog lines, the connection over long distances, when the signal passes through many intermediate nodes, results in a high level of noise in the output signal.

However, such technologies are gradually being replaced, and even in Belarus systems are increasingly being introduced in which communication between automatic telephone exchanges is carried out digitally. This means that the signal can be delivered to any distance without reducing power and with a low noise level.

Copper physical lines

Copper physical communication lines are leased from telephone companies or installed by the organization itself. Such channels are fundamentally point-to-point.

They differ in that signals of different frequencies in them have different attenuation coefficients. The table shows the most common communication channels and the amount of signal attenuation for different frequencies:

Thus, for the spectrum of a voice frequency signal, the attenuation in a 24 AWG cable is about 2 dB/km, in a 26 AWG cable it is 3 dB/km.

Analog modems

Analog modems are devices for transmitting data over telephone communication channels. The narrow bandwidth of such lines requires analog modems to use modulation methods that can increase the speed of information transfer only by increasing the signal-to-noise ratio. A significant breakthrough in achieving high speeds (up to 28.8 Kbps) was the adoption of the V.34 standard in 1994.

Initially, work was carried out to develop the V.FAST standard, which required modems to operate on dial-up telephone lines at speeds of up to 19,200 bps. The 19,200 bps limit was due to the concept of CCITT (now called ITU-T), which consisted of adopting a new V-series standard only if there was a high degree of connection establishment guarantee on real communication lines.

This concept changed during the development of the V.34 standard for two reasons. First, testing of preliminary protocols has shown that speeds in excess of 19,200 bps can be achieved over a sufficiently large number of communication lines. Secondly, when using high-quality channels, there was a margin of bandwidth, that is, part of it was not used. These two arguments led to the development of a new concept that allows higher speeds to be included in the standard, even if they are not always achievable.

Preliminary testing of the V.34 standard in Europe showed that in some countries only a third of the lines were capable of transmitting data at 28,800 bps, while in others all lines tested were capable of transmitting data at maximum speed.

V.34 is new technologies

Firstly, this is digital telephony. In most countries, analog telephone lines have already been replaced by digital lines using pulse code modulation (PCM). Other countries are also in the process of transitioning from analogue to digital lines communications. PCM channels allow you to get more high quality telephone communications, which is reflected not only in the ability to transmit an analog signal with a wider bandwidth (150 - 3,650 Hz compared to 300 - 3,400 Hz), but also in achieving a lower noise level.

Secondly, the implementation of the standard was facilitated by new digital technology signal processors(DSP), implemented in modems starting with the V.32 standard. It helped speed up the convolution operation, which is most often used when implementing basic modem functions. Solving a significant portion of filtering problems using DSPs has made it possible to simplify analog LSIs (large integrated circuits), in addition, it is easier for manufacturers of semiconductor components to implement digital LSIs. Thus, the digital approach enables rapid transition to high levels of integration at lower cost.

Third, the V.34 standard was the culmination of 30 years of research into modulation, encoding, and algorithms for digital signal processors (DSPs). It should be noted that V.34 is not just another step towards increasing the speed of modem communication, but a huge breakthrough in the desire to select all the reserves of voice frequency channels. This breakthrough, based on a system-wide approach to the problem and supported by a sharp leap in tools, allowed us to get as close as possible to Shannon's theoretical limit.

The main advantage of V.34 technology over previous ones is “adaptive intelligence”. Unlike previous standards, V.34 contains many modulation methods and signal filtering algorithms, which make up a whole set of technologies with which modems interact at their own discretion. Using its "intelligence", V.34 allows modems to automatically select and combine technologies from the available set in order to maximally adapt to the characteristics of the communication line.

That is why this topic deserves separate consideration.

What's new in the V.34 standard

Communication parameters exchange protocol V.8

A new protocol for exchanging communication parameters was specially developed for the V.34 standard. This protocol, called V.8, is backward compatible with all low-speed modems in order to recognize them and work with the "auto mode" procedure (V.25 protocol) defined in the V.32bis standard. However, in the V.25 protocol, the determination of the modulation used by the remote modem was based on sequential detection of tones. This procedure takes a lot of time (actually about 9 seconds), and the emergence of new protocols leads to its increase.

In accordance with V.8 recommendations, information about the modem’s capabilities is exchanged using the V.21 protocol (300 bps, frequency modulation), which is much faster and more reliable than tone detection. Using the V.8 protocol, modems exchange the following information:

V.34 protocol identification,

Data mode or phone call,

Supported modulation modes,

Error correction and data compression protocols V.42 and V.42bis,

Wired or cellular mode.

It is noteworthy that the flexibility built into V.8 and the bits reserved for the future allow the capabilities of the V.34 standard to be expanded without developing new methods for exchanging service information.

Line probing

Communication channel analysis - the most important technology Among the innovations introduced into the V.34 standard. It allows the modem to choose optimal parameters to work on a specific telephone channel.

Communication channel analysis is a bidirectional half-duplex procedure that is performed immediately after the exchange of information according to V.8 and consists of transmitting complex signals that allow the remote receiver to analyze the characteristics of the telephone channel before switching to data transmission mode. The modem uses the analysis results to select several key parameters communications, namely:

Carrier frequency and symbol rate. These parameters determine the occupied bandwidth of the output signal and its location (center frequency) within the spectrum offered by the communication channel. The modem has 11 possible options, combining 6 symbol rates, 5 of which have 2 carrier frequencies;

Correction filter before transmission (pre-emphasis). The modem has the ability to select the most suitable filter from the ten defined in the V.34 standard;

Transmitter power level. The modem can select the optimal signal transmission level from a range of 14 dB in 1 dB steps.

Analysis of the communication channel occurs at the beginning of each new connection, as well as during the process of re-entry into communication, which can occur at any time during the current connection. This allows the V.34 modem to adapt not only to specific communication channels with specific characteristics during communication establishment, but also to time-varying parameters.

Precoding

Precoding is essentially a modification of the adaptive signal correction (equalization, filtering) technology developed in 1970 and known as signal correction with feedback and a decision circuit (Decisions Feedback Equalizations, or DFE).

The problem with DFE was that it was quite difficult to set up to work with Trellis encoding. At the same time, DFE turned out to be the most optimal technology for signal correction in the receiving path of analog modems, which makes it possible to effectively combat intersymbol interference introduced by real communication channels. Combating intersymbol interference is especially important for high-speed modems, which need to use every piece of spectrum available on the line.

This problem was solved by distributing the actions performed by the DFE between the transmitter and receiver. As a result, the V.34 modem receiver calculates the optimal signal correction factors, just like a conventional DFE scheme, but feeds them back to the transmitter for pre-correction. Thus, precoding combines DFE with precorrection and Trellis coding.

Adaptive signal pre-correction (pre-emphasis)

This technology is not completely new, but is based on the use of so-called compromise correctors, but with added “intelligence”. Before the advent of the V.34 standard, modem manufacturers sometimes used fixed-structure correction filters in transmitters. According to V.34, the use of this technology is based on adaptation to the current characteristics of the communication line.

Adaptive signal pre-correction means that before being sent into the line, the signal passes through an equalization filter, which enhances some parts of the spectrum and attenuates others. This technology is very effective against signal-dependent distortion. Its main idea is to pre-compensate for distortions, the existence of which the modem can know in advance during the communication channel analysis phase. If, for example, while analyzing a communication channel, the modem detects that the upper part of the spectrum is attenuated more than the lower part, using an appropriate filter in the transmitter path will compensate for this distortion.

As studies have shown, the use of correction filters in the transmitter path allows one to obtain not only a direct effect - compensation linear distortion, but also to reduce the influence of more severe nonlinear distortions.

The intelligence of adaptive pre-correction, according to V.34, lies in the automatic selection of a compensating filter. This standard defines 10 different filters. The information obtained during the modem's analysis of the communication channel serves as the basis for making a decision on choosing the optimal filter, but the specific method for making such a decision is left to the discretion of modem developers.

Adaptive Transmitter Power Control

Correct selection of transmitter power is very important for high-speed modems operating over a two-wire line using echo cancellation.

Unlike 4-wire or low-speed V.22bis modems, maximum speed V.34 modems claim that more high power the transmitter is always preferable turns out to be incorrect. Echo cancellation algorithms require the selection of the optimal transmitter power, since increasing the power improves the signal-to-noise ratio at the remote receiver, but introduces unnecessary interference in the form of echoes at the local receiver.

Adaptive transmit power control allows you to automatically select the optimal transmission level based on information obtained while the modem studies the characteristics of the communication line. Despite the fairly simple concept, this technology is based on a very cumbersome mathematical model and is very complex from the point of view of implementation.

Multilevel trellis coding

Trellis coding was first introduced in V.32 modems, which provided additional protection against errors and allowed them to be corrected without requiring a retransmission.

The essence of trellis coding is to add an additional bit to each group of information bits (a group of bits that is assigned one baud). This bit is formed by performing a convolution operation (convolutional encoding) on ​​a portion of the bits in a group. The group of bits expanded in this way is subjected to multi-position amplitude-phase modulation and transmitted to the communication channel. Thus, the two-dimensional signal-code structure (SCC) of the V.32bis protocol at a speed of 14.4 Kbit/s was a 32-point quadrature amplitude modulation with a convolutional code for 8 states (two information bits + one additional). During reception, the signals are decoded, which allows, based on the analysis of correlations between groups of bits, to correct a significant part of the errors, due to which the noise immunity of reception increases by 3 - 5 dB.

Today, Trellis encoding technology has advanced significantly beyond that contained in the V.32 recommendation. The V.34 standard recommends three four-dimensional coding schemes - for 16, 32 and 64 states of the matching code. In four-dimensional space, a point has four coordinates. Two symbol intervals are required to transmit each such point. The transition to four-dimensional SCMs made it possible to reduce the number of points in the corresponding two-dimensional projections, which is equivalent to an increase in the distance between neighboring points, and therefore an increase in noise immunity.

Each of the three encoding schemes increases the signal-to-noise ratio of the system by significantly increasing processing power, while also introducing additional delays. In Fig. 1 shows a comparison of the gain provided by each coding scheme and the required computing power (implementation complexity).

Optimal placement of points on the phase plane (shell mapping, shaping)

In high-speed modems, transmitted bits are grouped into symbols, which are then translated into two-dimensional CCMs. The signal point thus created is transformed into an analog equivalent and transmitted to the line.

The goal of optimal point placement on the phase plane is to place points in two-dimensional space in such a way as to improve the signal-to-noise ratio. Theory shows that the optimal shape of a two-dimensional SCS should be spherical. However, this is impossible. Therefore, according to V.34, the modem tries to approximate the square mesh of the 2D SCM to the closest possible shape to a spherical shape.

The result of optimal placement of points on the phase plane is the expansion of the SCM and an improvement in the signal-to-noise ratio by approximately 1 dB. The V.34 specification contains two levels of such conversion: the first level expands the CCM by 12.5%, the second by 25%.

Nonlinear coding (warping)

Nonlinear coding is used to combat signal-dependent distortion, also known as nonlinear distortion, which are present in all telephone channels due to transformers and galvanic isolation circuits and are especially large in PCM channels due to the very nonlinear nature of PCM coding.

Nonlinear encoding results in a deformation of the phase plane in which the inner points, that is, points with small amplitude, are located closer to each other than the outer points (points with large amplitude). This increases noise immunity, and hence performance, in the presence of "digital" noise PCM channels, which are characterized by low level noise when transmitting weak signals (inner points) and high noise levels when transmitting signals with large amplitude (outer points on the phase plane).

Other features of the V.34 standard

A distinctive feature of V.34 is its advanced service, which includes such features as asymmetric transmission and an additional channel.

Asymmetric transmission means that two V.34 modems can have more than just different speeds transmissions, but also different carrier frequencies, use different bandwidths, different SCMs, etc. This feature is very useful when organizing communication over a four-wire line, when the quality of each pair is not the same.

Recommendation V.34 includes the optional possibility of using a low-speed data transmission channel (at a speed of 200 bit/s), which is formed through time multiplexing and is informationally independent from the main one. This channel can be used both for management and for low-speed transmission of asynchronous user data.

V.34 modems

It should be noted that the implementation of the V.34 standard is very difficult. Most of the discussed features of the V.34 standard are optional and are not supported by modems from many manufacturers. To date, only a few modems are known that provide 100% support for all capabilities of the V.34 standard - these are modems from Motorola and the Courier modem from US Robotics.

In general, analog modems can be divided into two classes: personal and professional. Professional modems are used to build corporate networks, which require high reliability, controllability and the ability to work around the clock. They support operation on dedicated 2/4-wire lines, centralized control, can be modular, as well as a range of technical solutions that allow for more reliable and high-speed communications.

Personal modems are designed for home use, and are also intended for automation of small offices in order to organize access to the Internet, use by email etc. These modems usually additionally support voice capabilities(answering machine, voicemail, simultaneous transmission of voice and data, etc.), allow you to send and receive faxes, but they do not support leased lines and some expensive technical solutions, which can improve the quality of communication.

To get a clear picture, let’s compare the most prominent representatives both types of modems, while highlighting their advantages over each other.

Personal modems

These include the Omni288S from ZyXEL, the Sportster 28.8 Voice from US Robotics and the C336Catcher from Tainet. These modems are optimal for creating personal workstations that include the following applications:

Dial-up communication with suppliers information services(Internet, email, BBS, etc.);

Exchange files with other users using standard communication programs such as Zmodem;

Send and receive faxes at speeds up to 14,400 bps;

It should be noted that all operating modes (data transmission, fax and voice mode) are detected automatically. Let us also note the main differences between the listed modems.

The C336Catcher modem additionally provides the ability to simultaneously transmit voice and data. This means that while data is being transferred, the user can pick up the handset (which will cause the remote modem to beep) and talk to the remote user without interrupting the data transfer.

The Omni288S modem, unlike the Sportster and Catcher, provides the ability to transmit data over a dedicated two-wire line. Additionally, the Omni is equipped with Flash memory, so upgrading can be done with a simple software download (Catcher and Sportster software is only upgraded by replacing or flashing the ROM). The Omni has a maximum speed of 28.8 Kbps, while the Sportster and Catcher have a maximum speed of 33.6 Kbps.

The Sportster288 Voice modem has a built-in microphone (Catcher and Omni use external microphones), but its voice capabilities can only be used with the manufacturer's software installed (Catcher and Omni come with standard software from Trio Communications). Additionally, the Sportster does not support the Caller ID features found on the Catcher and Omni.

Among these personal modems, the Omni288S is the most expensive because it supports a dedicated two-wire line, while the Catcher and Sportster cost about the same.

Professional modems

Modems 336S from ZyXEL and T288C from Tainet, operating at a maximum speed of 33.6 Kbps, are usually used to build corporate networks. Let us note the main features of these modems, according to which they belong to the group of professional modems.

Firstly, it is possible to work on a 2- and 4-wire leased line. Using a four-wire leased line eliminates echo, which increases the signal-to-noise ratio, and therefore helps achieve higher speed and reliability of communication.

Secondly, they are available in both desktop and modem rack versions. The ZyXEL 336S modem has its modular analogue ZyXEL 336R, and the T288C has its T288NC. Modem racks RS-1612 for ZyXEL 336R modems and TRS-16 for T288NC modems can accommodate up to 16 devices, have two power supplies and a ventilation system.

Thirdly, they have the ability to centrally manage all modems on the network, collect statistics and information about the status of modems. In networks where a large number of modems are installed, it is quite difficult to keep track of the status of each device. Without the use of a control system, even such a simple task as searching faulty device, becomes extremely difficult, not to mention carrying out configuration work, network planning, etc. Therefore, in large networks it is extremely important to use modems that support centralized management capabilities.

Both models under consideration support the SNMP management protocol, which makes it possible to use not only the management system from the manufacturer, but also any standard one - such as HP OpenView (however, some features become unavailable).

Fourth, synchronous data transfer is supported. Support for synchronous data transfer allows these modems to be used when building X.25, Frame Relay networks, for connecting routers operating using the synchronous PPP protocol, as well as for transmitting any other synchronous traffic.

Fifthly, it is possible to configure modems from the front panel, which allows you to make all settings without using a personal computer.

In addition, modems are capable of automatically reducing and increasing the speed if the quality of the channel changes over time, switching from a leased line to a dial-up line and back if the leased line is broken, and limiting the access of “dial-up” users using a password.

In addition to operating over voice-frequency channels, the modems in question can also operate over copper physical communication lines. And since these communication lines introduce attenuation, it makes sense to calculate the maximum range at which the ZyXEL 336S and T288C modems are capable of operating. Maximum level The signal transmission of the modems under consideration is 0 dB, and the minimum reception level is 43 dB, that is, the operating range is 43 dB. The signal attenuation in a cable with a diameter of 0.4 mm, as already mentioned, is 3 dB/km, so the maximum range will be 14 km (that is, this is the range at which modems can operate at a speed of 33.6 Kbps).

Let us note the main differences between the ZyXEL 336S and T288C modems.

The ZyXEL 336S and 336R modems, like most ZyXEL modems, support voice functions, which allows them to be used as personal modems. The use of flash memory allows for easy upgrades (upgrading T288C modems requires reprogramming the ROM). In addition, the RS-1612 modem rack provides a direct connection to the local network for communication with the control system, in contrast to the TRS-16 modem rack from Tainet, which connects to the local network only through a communication server created on the basis of a personal computer with a special board.

At the same time, despite some inconvenience when connecting a modem rack to a local network, T288C modems have a number of advantages. First of all, it is worth noting the support of the following key technologies V.34 standard, which improves the performance of modems on poor communication lines:

Pre-coding,

Optimal placement of points on the phase plane,

Trellis code for 16, 32 and 64 states,

Nonlinear coding,

Asymmetric data transfer.

In addition, the T288C partially implements a transmitter power control function, the meaning of which is to send a message from the remote modem asking it to reduce, if possible, the transmitter power in accordance with detected distortions. The user can view this message on the modem's LCD display or control system and take necessary action.

The T288C modem has a number of features that allow you to maximally adapt communication parameters to the characteristics of the channel. This is, first of all, the inclusion of an equalizer in the transmitter path, the possibility of a fixed setting of not only the speed on the line, but also the symbol rate, which reflects the frequency range used, adjustment of the transmitter power from 0 to - 31 dB (unlike the ZyXEL 336S, which allows you to adjust the power in range from 0 to - 15 dB).

The cost of ZyXEL 336S modems is slightly lower than T288C.

Thus, ZyXEL 336S modems are best used when building closed corporate networks, when there are no problems with achieving maximum speed. T288C modems are optimal for working on noisy communication lines, as well as for use by companies organizing public access to information resources, for example to the Internet. The last statement is justified by the fact that the T288C modem has a more complete implementation of the V.34 standard, which will allow owners of “good” modems to take advantage of all their capabilities and achieve more reliable and high-speed communication.

Such “good” modems should, of course, include the Courier modem from US Robotics. Therefore, in conclusion, a few words about this modem.

The Courier modem, according to our conditional classification, is neither professional nor personal, since it allows you to work not only over dial-up lines, but also over two-wire leased lines; it does not support voice functions, however, it is one of the few modems that fully implements the V.34 standard.

In this regard, Courier is the most optimal solution for users who need remote access to information resources via dial-up telephone lines, as well as for receiving and sending faxes. Using this modem to access the Internet, use e-mail, download files from BBS allows you to avoid problems with incompatibility and, most importantly, take advantage of all the capabilities of the V.34 standard that modems provide from the providers of the listed services.

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