Modems can also be classified according to the protocols they implement.

Protocol is a set of rules that govern the information exchange of interacting devices.

All protocols that regulate certain aspects of the operation of modems can be classified into two large groups: international and proprietary.

International level protocols are developed under the auspices of the Standardization Sector of the International Telecommunication Union (ITU-T - International Telecommunications Union - Telecommunications) and are accepted by it as recommendations. All ITU-T recommendations regarding modems are in the V series. Proprietary protocols are developed by individual modem manufacturing companies in order to outperform the competition. Often proprietary protocols become de facto standard protocols and are adopted in part or in full as ITU-T recommendations, as happened with a number of Microcom protocols. Such well-known companies as AT&T, Motorolla, U.S. Robotics, ZyXEL and others are most actively developing new protocols and standards.

Modem (MOdulator-DEModulator) is a device for converting serial digital signals into analogue ones and vice versa. Standards organizations use the common abbreviations DCE to refer to a modem and DTE to refer to a computer, terminal, or any other device connected to a modem. The modem has two interfaces (Fig. 2.31): interface between DCE and analog line; multi-wire digital interface between DCE and DTE.

Point-to-point channel. The simplest network using modems, is a point-to-point channel in which two modems are connected (“point-to-point”) by one communication line (Fig. 2.32). A discrete channel connects DTE to DTE. The line connects DCE to DCE. A discrete channel consists of a line and two modems (DCE). For transmission speeds up to 20 kbit/s, the V.24/V.28 (RS-232C) interface is used, via a 25- or 9-pin female connector. At transmission speeds from 48 to 168 kbit/s, broadband modems operating with the V.35 interface are required. At speeds up to 20 kbit/s, any of the following analog telephone lines can be used:

4-wire 2-point leased line; 4-wire multipoint leased line; 2-wire 2-point leased line; 2-wire 2-point dial-up (PSTN dial-up); A 4-wire, 2-point switched line created by switching two separate two-wire connections over the PSTN. Telephone channel standards as derivatives of the standard PSTN voice frequency (TV) channel are presented in Table. 2.10.

Modems operating modes. Asynchronous. This mode is implemented by asynchronous modems; such modems are low-speed and operate in the mode of asynchronous start-stop character-by-bit transmission. Asynchronous modems do not generate synchronization signals and can operate at any transmission speed within the speed range set for them. Synchronous. In this mode, data is transmitted in blocks, and the modem generates synchronization signals. Modems that implement only synchronous mode are called synchronous modems. Asynchronous-synchronous. This mode is implemented by asynchronous-synchronous modems, which can carry out both synchronous and asynchronous transmission. The modem removes the start-stop bits before transmitting and restores them after receiving. Modems of this type generate synchronization signals and have a built-in asynchronous-synchronous converter. Asynchronous-synchronous and synchronous modems operate only at fixed transmission rates. When choosing a modem, the type of communication provided by the modem-line combination is important.

Any modem operating on a 4-wire, 2-point line uses one pair to transmit and the other to receive, and therefore can operate in full-duplex mode. Modems operating with a 4-wire multidrop line operate in half-duplex mode only. Synchronous-only modems operate on a 4-wire, 2-point non-switched line or over the PSTN, with a single dial-up connection providing half-duplex mode and a dual-switching connection providing full-duplex mode. Asynchronous-synchronous modems operate on 2-wire lines (either leased or switched), and all of them can operate in full-duplex mode. Modem compatibility. The transmission of data over telephone networks is described by the recommendations of the V series of the International Telecommunications Union (Technical Standards Sector) - ITU-T. The compatibility check is to check the V series number specified by the manufacturer in the modem specifications. The classification of the V series recommendations is shown in Fig. 2.33.

The modem can operate in two modes: command and data transmission. The modem command mode is usually set: when turning on the power; during the initial initialization of the modem; after an unsuccessful attempt to connect to a remote modem; when interrupted from the keyboard by pressing the “hang up” key combination (most often); when exiting the data transfer mode via the ESCAPE sequence. In command mode, the entire data stream entering the modem through the V.24/V.28 interface is perceived by it as a command. The data transfer mode (on-line) is established after the modem sends a CONNECT message in the following cases: when an attempt to establish a connection with a remote modem is successful; when the modem performs self-test. In data transfer mode, the data stream entering the modem from DTE is translated with conversion to the line, and the data stream from the line is translated with inverse conversion to the interface with DTE. Functional modes of the modem. The modem is always in one of two functional modes (except for periods when it switches from one mode to another): command (local) and asynchronous connection mode (ON LINE). The modem transition diagram is shown in Fig. 2.34. When the power is turned on, the modem initializes its parameters in accordance with the configuration stored in non-volatile memory, and goes into asynchronous command mode. Only in this mode does the modem accept AT commands. Using the Z command, the modem restores its working configuration

from non-volatile memory and returns to command mode, the “^-command restores the configuration according to the manufacturer’s profile (default setting) and returns to command mode. The modem “picks up the phone” in auto-answer mode: a) upon receipt of an A-command; b) automatically when S1 = SO, when the counter of incoming calls (calls) becomes equal to the number set for answering; c) upon receipt of a dialing command, when the call line ends with R. Functions of exchange circuits 103, 104, 109 V.24. Let's consider the functions of exchange circuits associated with the transmission and reception of data: 103 (2) TxD (transmitted data) to DCE; 104 (3) RxD (receive data) to DTE; 109 (8) CD (received line signal detector) to DTE. The input stream of serial data entering the modem through circuit 103 is converted by the modulator into a modulated analog signal for output to the line (Fig. 2.35). At the other end of the line, the remote modem demodulator receives the modulated line signal and converts it into a serial data stream for output through the data receive circuit 104.

When a modulated carrier frequency is detected by the demodulator, circuit 109 transitions from the OFF state to the ON state. In this case, a delay is introduced between the moment the carrier is detected and the moment the state of the exchange circuit 109 changes, known as the carrier detection “on” delay. There is also a carrier sensing "off" delay that occurs when the carrier on the other end of the line turns off. Circuit 109 in the internal circuit of the modem is necessary to fix the data exchange circuit 104 (data is received only when circuit 109 is turned on). The CD turn-on delay and data receive circuit latching provide protection against transient bursts of line noise that simulate spurious signals in the data receive circuit 104.

So, modems and modulation-demodulation...

The term "modem" is short for the well-known computer term modulator-demodulator. A modem is a device that converts digital data coming from a computer into analog signals that can be sent over a telephone line. This whole thing is called modulation. The analog signals are then converted back into digital data. This thing is called demodulation.

The scheme is very simple. The modem receives digital information in the form of zeros and ones from the computer's central processor. The modem analyzes this information and converts it into analog signals, which are transmitted through the telephone line. Another modem receives these signals, converts them back into digital data, and sends this data back to the remote computer's central processing unit.

Modulation type which allows you to select frequency or pulse modulation. Pulse modulation is used throughout Russia.

Analog and digital signals

Telephone communication is carried out through so-called analog (sound) signals. An analog signal identifies information that is transmitted continuously, while a digital signal identifies only that data that is defined at a specific stage of transmission. The advantage of analog information over digital is the ability to fully represent a continuous flow of information.

On the other hand, digital data is less affected by various types of noise and grinding noises. In computers, data is stored in individual bits, the essence of which is 1 (start) or O (end).

If we represent this whole thing graphically, then analog signals are sine waves, while digital signals are represented as square waves. For example, sound is an analog signal because sound is always changing. Thus, in the process of sending information over the telephone line, the modem receives digital data from the computer and converts it into an analog signal. A second modem at the other end of the line converts these analog signals into raw digital data.

Interfaces

You can use a modem in your computer using one of two interfaces. They are:

MNP-5 Serial interface RS-232.

MNP-5 Four-pin RJ-11 telephone cable.

For example, an external modem is connected to a computer using an RS-232 cable, and to a telephone line using an RJ11 cable.

Data compression

In the process of data transmission, a speed greater than 600 bits per second (bps or bits per second) is required. This is due to the fact that modems must collect bits of information and transmit them further through a more complex analog signal (a very sophisticated circuit). The process of such transmission itself allows for the transmission of many bits of data at the same time. It is clear that computers are more sensitive to transmitted information and therefore perceive it much faster than a modem. This circumstance generates additional modem time, corresponding to those data bits that need to be grouped somehow and certain compression algorithms applied to them. This is how two so-called compression protocols emerged:

MNP-5 (transmission protocol with a compression ratio of 2:1).

V.42bis (transmission protocol with a compression ratio of 4:1).

The MNP-5 protocol is usually used when transferring certain already compressed files, while the V.42bis protocol is applied even to uncompressed files, since it can speed up the transfer of just such data.

It must be said that when transferring files, if the V.42bis protocol is not available at all, then it is best to disable the MNP-5 protocol.

Error Correction

Error correction is a method by which modems test the transmitted information to determine whether it contains any damage that occurred during transmission. The modem breaks this information into small packets called frames. The sending modem attaches a so-called checksum to each of these frames. The receiving modem checks whether the checksum matches the information sent. If not, then the frame is sent again.

Frame is one of the key terms for data transmission. A frame is a basic block of data with a header, information and data attached to this header that complete the frame itself. The added information includes frame number, transmission block size data, synchronizing symbols, station address, error correction code, variable size data and so-called indicators Start of transmission (start bit)/End of transmission (stop bit). This means that a frame is a packet of information that is transmitted as one unit.

For example, in Windows 98 in the modem settings there is an option Stop bits which allows you to set the number of stop bits. Stop data bits are one of the varieties of the so-called boundary service bits. The table bit determines the end of the cycle during asynchronous transmission (the time interval between transmitted characters varies) of data in a short-term cycle.

MNP2-4 and V.42 protocols

Although error correction may slow down data transmission on noisy lines, this method provides reliable communication. The MNP2-4 and V.42 protocols are error correction protocols. These protocols determine how modems verify data.

Like data compression protocols, error correction protocols must be supported by both the sending and receiving modems.

Flow Control

During transmission, one modem can send data much faster than another modem can receive the data. The so-called flow control method allows you to inform the receiving modem that the modem will stop receiving data at some point in time. Flow control can be implemented both in software (XON/XOFF - Start signal/Stop signal) and in hardware (RTS/CTS) levels. Flow control at the software level is carried out through the transfer of a specific sign. After the signal is received, another character is transmitted.

For example, in Windows 98 in the modem settings there is an option Data bits which allows you to set the information data bits used by the system for the selected serial port. The standard computer character set consists of 256 elements (8 bits). Therefore, the default option is 8. If your modem does not support pseudographics (works only with 128 characters), please indicate this by selecting option 7.

In Windows 98, in the modem settings, there is also an option Use flow control

which allows you to determine how to implement data exchange. Here you can correct possible mistakes problems that occur when transferring data from a computer to a modem. Default setting XON/XOFF means that the data flow is controlled by software using standard ASCII control characters, which send the command to the modem pause/resume transfer.

Software flow control is only possible if a serial cable is used. Since flow control at the software level regulates the transmission process by sending certain characters, a failure or even termination of the communication session may occur. This is explained by the fact that this or that noise in the line can generate a completely similar signal.

For example, with software flow control, binary files cannot be transferred because such files may contain control characters.

Through hardware flow control, RTS/CTS transfers information much faster and more securely than through software flow control.

FIFO buffer and UART universal asynchronous interface chips

The FIFO buffer is somewhat similar to a transshipment base: while data arrives at the modem, part of it is sent to the buffer capacity, which gives some gain when switching from one task to another.

For example, operating room Windows system 98 only supports the 16550 series Universal Asynchronous Receiver Transmitter (UART) chips and allows control of the FIFO buffer itself. Using a checkbox Use FIFO buffers requres 16550 compatible UART (Use FIFO buffers) you can lock (prevent the system from accumulating data into the buffer capacity) or unlock (allow the system to accumulate data into the buffer capacity) the FIFO buffer. By pressing the button Advanced, you turn to dialogue Advanced Connection Settings whose options allow you to configure the connection of your modem.

S-registers

S-registers are located somewhere inside the modem itself. It is in these very registers that settings are stored that in one way or another can affect the behavior of the modem. There are a lot of registers in the modem, but only the first 12 of them are considered standard registers. S-registers are set in such a way that they send a command to the modem ATSN=xx, where N corresponds to the number of the register being set, and xx defines the register itself. For example, through the SO register you can set the number of rings to answer.

Interrupts IRQ

Peripheral devices communicate with the computer processor through so-called IRQ interrupts. Interrupts are signals that force the processor to suspend a particular operation and transfer its execution to the so-called interrupt handler. When the CPU receives an interrupt, it simply suspends the process and delegates the interrupted task to an intermediary program called Interrupt Handler. This whole thing works regardless of whether an error was detected in the operation of a particular process or not.

Information communication port or simply COM port

The serial port is very easy to find out. You can do this by simply looking at the connector. The COM port uses a 25-pin connector with two rows of pins, one of which is longer than the others. At the same time, almost all serial cables have 25-pin connectors on both sides (in other cases, a special adapter is required).

A COM port (serial port) is a port through which computers communicate with devices such as a modem and mouse. Standard personal computers have four serial port.

COM 1 and COM 2 ports are usually used by the computer as external ports. By default, all four serial ports have two IRQs:

COM 1 is bound to IRQ 4 (3F8-3FF).

COM 2 is tied to IRQ 3 (2F8-2FF).

COM 3 is tied to IRQ 4 (3E8-3FF).

COM 4 is tied to IRQ 3 (2E8-2EF).

This is where conflicts can arise, since external ports of other I/O devices 1/0 or controllers can use the same IRQs.

Therefore, having assigned a COM port or IRQ to the modem, you must check other devices to see if they have

the same serial ports and interrupts.

It must be said that devices connected to the telephone line in parallel to the modem (especially Caller ID) can very significantly degrade* the quality of operation of your modem. Therefore, it is recommended to connect phones through the dedicated socket in the modem. Only in this case will he disconnect them from the line during operation.

Your modem's flash memory

Flash memory is a read-only memory or PROM (read-only reprogrammable memory) that can be erased and reprogrammed.

All modems whose names contain the line “V. Everything” are subject to reprogramming. In addition, "Courier V.34 dual standart" modems are subject to software upgrade if the line Options the response to the ATI7 command contains the V.FC protocol. If the modem does not have this protocol, then the upgrade to "Courier V. Everything" is carried out by replacing the daughter board.

There are two modifications of Courier V. Everything modems - with the so-called supervisor frequency of 20.16 MHz and 25 MHz. Each of them has its own firmware versions, and they are not interchangeable, i.e. The firmware from the 20.16 MHz model will not work for the 25 MHz model, and vice versa.

Field programmable NVRAM

All modem settings are reduced to correct installation NVRAM register values. NVRAM is a user-programmable memory that retains data when the power is turned off. NVRAM is used in modems to store the default configuration that is loaded into RAM when turned on. NVRAM programming is done in any terminal program using AT commands. A complete list of commands can be obtained from the documentation for the modem, or obtained in a terminal program using commands AT$ AT&$ ATS$ AT%$. Write factory settings with hardware data control to NVRAM - AT&F1 command, then make adjustments to the modem settings in conjunction with a specific telephone line and write them to NVRAM using the command AT&W. Further initialization of the modem must be done using the command ATZ.4.

Application software for data transfer

Data transfer programs allow you to connect to other computers, BBS, Internet, Intranet and other information services. You may have a very extensive range of such programs at your disposal. For example, in Windows 98 you have at your disposal a very good terminal client, Hyper Terminal.

If you have problems establishing communication with other modems

First you need to assess the nature of the communication line. To do this, after a successful session, before re-initializing the modem, enter the commands ATI6- communication diagnostics, ATI11- connection statistics, ATY16- amplitude-frequency characteristic. The received data must be written to a file. After analyzing the received data, it is necessary to make changes to the current configuration and then write them to NVRAM using the command AT&W5.

Russian telephone lines and imported modems

The choice of modems today is quite large, and the difference in their cost is quite significant. Transmission speeds of more than 28,800 bps are usually unattainable on Russian telephone lines. Above 16,900 bps can only be obtained if the Internet service provider has lines on the PBX to which your phone is connected. In other cases, working on the Internet is too tedious, because at a typical (and not always achievable) speed of 9,600 bps, it becomes a complete wait. Therefore, for stable data transmission in the event of interference in the telephone line, you need a high-quality modem that costs at least $400.

Which modem is better - internal or external?

The internal modem is installed in a free expansion slot on the computer's motherboard and connected to the built-in power supply, while the external modem is a stand-alone device connected to the computer via a standard serial port.

Each of the designs has its own advantages and disadvantages. The internal modem occupies a system bus slot (and, as a rule, there are not enough of them), it is difficult to monitor its operation due to the lack of indicators, and besides, the described models are fundamentally not suitable for notebook-type portable computers that have a narrow-profile case and, in most cases, are not with expansion connectors. At the same time, the internal modem is several tens of dollars cheaper than external analogues, does not take up space on the table and does not create a tangle of wires. Using an external modem means that the computer to which it is connected has the most modern serial port control chips (UART). UART chips appeared in the first PCs, since even then it became clear that data exchange via a serial port was too slow and complex an operation and it was better to entrust it to a special controller. Since then, several UART models have been released. Computers such as IBM PC and XT, as well as those fully compatible with them, used the 8250 chip; in AT it was replaced by UART 16450. Until recently, most computers based on i386 and i486 processors were equipped with a 16550 controller, which included internal hardware buffers of the " queue", and today the UART 16550A is becoming the standard - a chip similar to the previous one, but with the defects eliminated. The lack of buffers in all chips except the last one causes data transfer through the serial port at speeds above 9600 bps to become unstable (using MS Windows reduces this threshold to 2400 bps).

If you need to connect a high-speed external modem to a computer that uses an older UART chip, you must either change the multicard or add a special expansion card (which will take up one bus slot and deprive the external modem of a critical advantage). Internal modems do not have this problem - they do not use a COM port (more precisely, they contain one). Now internal modems have another advantage, also related to speed. According to the V.42bis specification, data can be compressed by approximately four times during transmission, therefore a modem operating at 28800 bps must receive data from or send data to the computer at a speed of 115600 bps, which is the limit for serial PC port. However, 28,800 bps is not the limit for a telephone line, where the maximum lies somewhere in the region of 35,000 bps, and on digital lines (ISDN) the throughput exceeds 60,000 bps. Consequently, in this situation, the serial port will become a bottleneck of the entire system, and the potential capabilities of the external modem will not be realized. Modem manufacturers are currently developing models that can connect to a faster parallel port, but it is obvious that devices sold now will not be able to accommodate this.

At the same time, many modems can be upgraded to operate at high speeds, even being able to operate on ISDN. But everything depends on the restrictive barrier on the computer side, which for the internal modem is significantly higher than 4 MB/s (ISA bus bandwidth). By the way, all ISDN modems are internal. True, all this will happen tomorrow (or maybe the day after tomorrow), but today we can say one thing: choose a device of the type that you like - there are no functional differences between internal modems and their external analogues.

Which modem to choose and how to choose it

The modem cannot be unique. Your modem must be understood by other modems. This means that the modem must support the maximum number of standards, that is, error correction, data exchange methods and data compression. The most common standard is V.32bis for modems with an exchange rate of 14000 bps. For modems with a speed of 28800 bps, the standardized protocol is V.34.

In addition, it must be emphasized that modems with a data exchange rate of 16800, 19200, 21600 or 33600 are not standard.

No error correction should be done in software. Everything must be built into the modem by its manufacturer.

About the outside and the inside. An external modem is connected to your serial port via a special cord. Such a modem, as a rule, has a volume control, information indicators, a power supply and other, sometimes useful accessories. If you are a professional, then you should not care which modem you choose - internal or external. Usually, a good internal modem, through special software, does a good job of emulating all the clarity of an external modem.

Do not buy purely imported modems. These pieces of iron do not get along on our ancient lines. Buy only certified modems, that is, hardware specially tailored for our dirty telephone exchanges.

In Russia, such a choice is very small. This market is dominated by two companies: ZyXEL from sunny Taiwan and U.S. Robotics from the USA. Modems from the latter company are chosen by professionals (Courier), while the former is chosen by everyone else, that is, all those users who choose the so-called ultra-reliable ZyCell protocol.

So, choose Courier. And, believe me, this is not advertising.

The word “modem” comes from the combination “modulator/demodulator” and is used to refer to a wide range of devices for transmitting digital information using analog signals by modulating them - changing over time one or more characteristics of the analog signal: frequency, amplitude and phase. In this case, the modulated analog signal is called a carrier and is usually a signal of constant frequency and amplitude (carrier frequency).

The number of modulations per second is called the modulation rate and is measured in baud (Baud); the amount of information transmitted is measured in bits per second (bits per second or BPS Bits Per Second). One modulation can transmit one bit, or more or less of them. In new modem protocols, the unit of information transmitted per modulation is called a character. A "modem" symbol can generally be of any size.

The original digital signal is fed to a modulator, which converts it into a series of changes to the analog carrier signal, which is transmitted via a communication line to a demodulator, which, based on these changes, recreates the original digital signal. To obtain a symmetrical bidirectional communication line, the modulator and demodulator are combined in one device - a modem.

Although modulators/demodulators are used in many devices network adapters, disk drives, CD recorders, etc., the term “modem” has been fixed to refer mainly to intelligent modems for telephone lines. Such a modem is a complex device, in which the modulator and demodulator themselves are included only as the main functional units.

Modems are used where the communication line does not allow reliable transmission of a digital signal by simply changing the amplitude. Frequency changes are most reliably transmitted - frequency modulation, however, fixing such a change at the receiving end requires several signal periods, which requires the use of carrier frequencies significantly higher than the frequency digital signal. To increase the amount of information transmitted per modulation, parallel phase and amplitude modulations are used.

A typical scheme for organizing communication between two digital devices using modems looks like this:

DTE1 DCE1 Link DCE2 DTE2

The abbreviation DTE (Data Terminal Equipment) in communication system terminology refers to digital terminal devices that generate or receive data. The abbreviation DCE (Data Communication Equipment) refers to modems. The communication line between DCE is analogue, between DCE and DTE is digital.

If a unified digital interface is used to communicate between DTE and DCE, this often makes it possible to connect two adjacent DTEs with a straight digital line, called a null modem cable. In case of DTE diversity long distance Instead of a null modem cable, a pair of modems and an analog communication line are connected into the gap, providing a transparent connection and data transfer.

Modems various types used in many areas of communication; This FAQ covers only smart telephone line modems designed for communication between computers and alphanumeric terminals.

How does a modern modem work and work?

Almost all modern modems have similar functional circuits, consisting of a main processor, a signal processor, random access memory (RAM), read-only memory (ROM), reprogrammable memory (Non-Volatile RAM, NVRAM non-volatile memory with direct access), the modulator/demodulator itself, the line matching circuit and the speaker.

The main processor is actually a built-in microcomputer responsible for receiving and executing commands, buffering and processing data - encoding, decoding, compression/decompression, etc., as well as controlling the signal processor. Most modems use specialized processors based on standard chipsets, and some (US Robotics, ZyXEL) use processors general purpose(Intel, Zilog, Motorola).

The signal processor (DSP, Digital Signal Processor digital signal processor) and modulator/demodulator directly deal with signal operations modulation/demodulation, frequency band division, echo suppression, etc. Such processors are also used either specialized, focused on a specific set of modulation methods and protocols (AT&T, Rockwell, Exar), or universal ones with removable firmware (for example, TMS), which make it possible to subsequently refine and change the operating algorithms.

Depending on the type and complexity of the modem, the main intellectual load shifts towards the DSP or modulator/demodulator. In low-speed (300..2400 bps) modems, the main work is performed by the modulator/demodulator, in high-speed (4800 bps and above) DSP.

ROM stores programs for the main and signal processors (firmware). ROM can be one-time programmable (PROM), ultraviolet erasable reprogrammable (EPROM), or electrically reprogrammable (EEPROM, Flash ROM). The latter type of ROM allows you to quickly change firmware as errors are corrected or new features become available.

RAM is used as temporary memory when operating the main and signal processors; it can be either separate or general. The current set of modem parameters (active profile) is also stored in RAM.

NVRAM stores stored sets of modem parameters (stored profiles), one of which is loaded into the current set every time it is turned on or reset. Typically there are two saved sets: a primary (profile 0) and a secondary (profile 1). By default, the main set is used for initialization, but it is possible to switch to the additional one. A number of modems have more than two saved sets.

Line matching circuits include an isolation transformer for signal transmission, an optocoupler for identifying the ringing signal (Ring), a relay for connecting to the line (“off-hook”, off-hook) and dialing, as well as elements for creating a load in the line and overvoltage protection. Instead of relays, silent ones can be used electronic keys. Some modems use additional optocouplers to control line voltage. Connecting to the line and dialing a number can be done using one or separate keys.

Output to speaker amplified signal from the line for auditory monitoring of her condition. The speaker can be turned on for the duration of dialing and connecting, during the entire connection, or turned off completely.

External modems additionally contain a circuit for generating supply voltages (usually +5, +12 and -12 V) from one alternating (less often direct) voltage of the power source. In addition, external modems contain interface circuits for communication with the DTE.

What is the difference between internal and external modems?

The internal modem is made in the form of an expansion card placed in the computer case, connected directly to the system bus and using the computer's common power supply. An external modem is made as a separate device, connected to one of the serial or parallel ports, and powered from its own network source. The external modem also has operating mode indicators in the form of a set of LEDs or a liquid crystal display.

Advantages of an internal modem:

Disadvantages of an internal modem:

Advantages of an external modem:

Disadvantages of an external modem:

How is data transmission organized via modems?

Data transfer is organized based on a set of protocols, each of which establishes rules for the interaction of communicating devices. The protocols used in modems are divided into four main groups:

The first three groups apply only to DCE-DCE communication, the last only to DCE-DTE communication.

The first group of protocols establishes the rules for modems to enter into communication, its maintenance and termination, the parameters of analog signals, the rules of encoding and modulation. These protocols directly relate to signals transmitted over an inter-modem analog communication line. Connecting two modems is only possible if they support any common or compatible protocols of this group. In the seven-level hierarchy of OSI communication protocols, this group of protocols has level 1 (physical) and forms a digital communication channel in real time, but is not protected from transmission errors.

Protocols physical connection can be simplex (simultaneous transmission in one direction at a time), and duplex (simultaneous bidirectional transmission). The most commonly used protocols are duplex protocols, which can be symmetrical, when the transmission speeds in both directions are equal, and asymmetrical, when the speeds are different. Asymmetrical duplex is used to increase the transmission speed in one direction by reducing it in the opposite direction when the flow of transmitted data has a pronounced asymmetry.

To determine the direction of transmission in a physical channel, the concepts of calling (initiating the connection) and responding modems are used; The direction of transmission is determined by the calling modem.

The second group establishes rules for detecting and correcting errors that occur during the transmission stage using protocols of the first group. These protocols deal only with digital information; To check the integrity of information, it is divided into blocks (packets) equipped with check redundancy codes (CRC Cyclic Redundancy Check). If the control code at the receiving end does not match, the transmitted packet is considered erroneous and its retransmission is requested. This group of protocols transforms an unreliable physical channel into a reliable (error-proof) channel at a higher level, but this leads to loss of real-time communication and comes at the cost of certain overhead costs. In the OSI model, this group corresponds to layer 2 (link).

The third group establishes rules for compressing transmitted data by reducing its redundancy. At the same time, at the transmitting end they are analyzed and packaged, and at the receiving end they are unpacked into their original form. Compression allows you to increase the transmission speed beyond the physical bandwidth of the channel by reducing the amount of data actually transmitted. Implementing compression also requires some overhead for parsing information and generating packets; If compression is ineffective, the transmission speed may be lower than the speed of the physical channel.

The last group of protocols sets the rules for interaction between DCE and DTE. They are divided into physical, relating to cables, connectors and interaction signals, and informational, relating to the format and meaning of transmitted messages. Through these protocols, communication between DTE and DCE is realized during preparation for entering into communication, organizing a call and response, as well as during the data exchange itself.

What modulation protocols are used in modem communications?

Most of the protocols used are standardized by the International Telecommunications Union ITU, formerly called the International Consultative Committee on Telegraphy and Telephony, CCITT (Comite Consultatif Internationale de Telegraphie et Telephonie CCITT). ITU department related to telephone communication, denoted by ITU-T.

Of the physical communication protocols, the most widely used are the following:

56k protocols are aimed primarily at centralized communication systems Internet providers (ISP Internet Service Provider), banking and information networks etc., where the transmission of information from the center to the subscriber (download) predominates, and the transmission from the subscriber to the center (upload) is much less common.

What is CPS?

This is a historically rooted unit of measurement for the transfer rate of data between programs (Characters Per Second characters per second), which denotes the rate at which “computer” (eight-bit) characters (bytes) are transferred between terminal programs. The “modem” speed in BPS is not suitable for this, since it denotes the data transfer speed between modems in a physical channel, and the actual transfer speed over the full channel (between programs) is affected by error correction, data compression, subtleties of hardware and system protocols, and port settings and so on.

CPS is a purely "computer" unit, unrelated to the "modem" modulation symbols introduced in V.FC, V.34 and later protocols.

How do error correction protocols work?

Almost all error correction protocols are based on repeating the transmission of an erroneous block (frame) upon request from the receiving modem. Each block is supplied with a checksum, which is checked at the receiving end, and the block is not given to the consumer until it is received in the correct form. This creates possible transmission delays, but practically guarantees error-free data transmission without additional higher-level control.

To increase transmission efficiency, correction protocols establish a connection in a synchronous mode, in which the bits transmitted over the physical channel are no longer divided into bytes, but are packaged into larger packets. Due to this, the same pair of modems over a clean high-quality channel using protocols with correction most often transmits data faster than using low-level asynchronous protocols without correction.

The most common correction protocols are MNP (Microcom Networking Protocol) Layer 4 (MNP4), introduced by Microcom and becoming the de facto standard, and including its later V.42, also called LAP-M (Link Access Procedure Modems), introduced ITU-T. The latter is more efficient, so when establishing a connection, modems first try to use V.42, and if unsuccessful, try MNP4.

In both MNP4 and V.42, the rejection of an erroneous frame by the receiving modem can be either individual or include all subsequent frames that the remote modem has managed to transmit by that moment. Most often, the second scheme is implemented as a simpler one, but a number of models use selective frame repetition Selective Reject (SREJ), which significantly increases the transmission speed on channels with frequent communication errors.

An even more recent Layer 10 MNP extension targets rapidly changing channels (RF, cellular) and is optimized to reduce losses from such changes.

In addition to error correction, correction protocols can transmit a number of service messages between modems. Basically, two types of such messages are used: a temporary break in transmission signal (Break), transmitted between the computer and the modem in the form of a long series without a stop bit at the end, and a connection break signal (Link Disconnect), transmitted by one modem to another when communication is terminated (multiple block reception failure, DTR drop, ATH command and the like). The first message allows you to transmit a “non-character” signal between computers, which is often called an “attention” signal, and the second one makes it easier and faster to disconnect the connection so that the remote modem does not try to restore it.

How do data compression protocols work?

Data compression is performed by detecting and partially eliminating redundant information in the input stream of the transmitting modem, after which the encoded data blocks of reduced size are sent to the receiving modem, which restores them to their original form. The principle of operation of compression algorithms is in many ways similar to the work of archivers.

The most common compression protocols are MNP5, introduced by Microcom, and V.42bis, introduced by ITU-T. The MNP5 algorithm is based on relatively simple methods compression, its efficiency in the best cases rarely exceeds 2. V.42bis is based on the popular LZW compression method used in most archivers, and in successful cases provides up to four times compression. In modems that implement both protocols, the default connection preference is V.42bis.

In the MNP5 protocol, the compression algorithm is not disabled, and the protocol always attempts to encode incoming data. This often results in uncompressible data becoming larger in size due to encoding, and the effective transfer rate decreases. The V.42bis protocol monitors the effectiveness of stream compression, and temporarily stops working if compression does not achieve its goals. If the modem implements only the MNP5 protocol, it is recommended to disable it for sessions in which data with low redundancy predominates (archives, distributions, images, sound, video, etc.), and enable it for sessions of transferring texts, HTML pages, unpacked databases, etc.

The compression algorithm in a modem always deals with a continuous data stream, which is why only individual, relatively small and independent fragments of the stream are compressed, and this does not allow achieving the same high degree of compression as in archivers. For example, text in Russian is compressed by most archivers by 4-5 times, while the real efficiency of the best modem compression protocols does not exceed 2-3, and a higher degree is achieved only when transmitting repeating series (tables, unpacked databases with high redundancy and so on.).

How does DTE communicate with the modem?

Almost all general-purpose telephone modems have a unified set of commands, proposed and established by Hayes, after which the set itself is named. Another name for the set is AT-set, since most commands begin with the AT prefix (ATtention). A number of specialized modems have their own command sets that are incompatible with Hayes and with each other.

There are two main operating modes of the modem: command mode and data mode. In the first mode, the DTE sends commands to the modem and receives messages; in the second, the modem transparently transmits data between the DTE and the remote modem.

In command mode, the Hayes modem processor constantly monitors the bit stream from the DTE and tries to detect the combination "AT" or "at" transmitted at one of the allowed rates. As soon as such a combination is detected, the processor records given speed and enters command line input mode, writing the received characters into an internal buffer, the volume of which is usually 40 characters. Spaces in commands are ignored unless otherwise specified for individual commands. Incorrectly typed characters can be erased with a backspace character (default BS, code 08 hex), but the AT prefix is ​​not stored in the buffer, so it cannot be erased or the command line input mode can be canceled.

The modem command mode was originally aimed at manually entering commands from a simple terminal, so the input method and command structure were designed in a “human” form. For the same reason, a modem in command mode by default returns (echo mode) each character received from the DTE, allowing you to visually verify the correctness of the command set. In data mode, received characters are not returned by default.

Most Hayes modem commands are designated by the letter "A", "P", or the symbol with the letter &C, %T. The command may have a parameter (usually numeric) X1, &D2. If a numeric parameter is omitted, it is assumed to be zero. A number of commands have syntax that does not follow these rules.

One or several commands can be written on one command line; The exception is cases when the next command leads to a change of modes, making subsequent commands meaningless. Each command is executed after it has been extracted from the command line and parsed. If the command line is executed successfully, an OK message is displayed; lines may be given before it additional information, requested by the entered commands. If an error is detected, an ERROR message is issued and line processing stops, but all previous correct commands will be executed at this point.

Example command lines:

Each line of AT commands ends with a CR character (default code 0D hex, Enter key). After receiving a CR, the modem processor analyzes the command line and, if possible, executes every command in it, after which it issues a confirmation message, an error message, or the information requested by the commands. Hayes modem diagnostic messages are issued in text form by default, but can also be issued in the form of three-digit decimal codes.

AT commands are used to obtain information about the state of the modem, change its operating modes, dial a number, establish/end a connection, and test the modem and line. There are separate commands to change the main parameters; other parameters are stored in the so-called S-registers, which take values ​​from 0 to 255. The values ​​of the S-registers can be used either completely or separately by fields and individual bits. In fact, all or most of the parameters are stored in S-registers, and individual commands to control them are introduced solely for convenience.

With rare exceptions, state change commands only affect the current set of parameters, which lose their values ​​when the modem is turned off or reset. The contents of the current set can be written to one of the saved sets in NVRAM; In addition, a number of commands can directly change the contents of NVRAM.

In addition to command lines starting with AT, Hayes modems also support the “A/” command. It repeats the last command line entered; execution begins immediately upon receiving the "/" character, no CR code is required.

When executing connection commands (call, answer, test), the modems connect and switch to data mode, accompanied by the issuance of the CONNECT message. In data mode, all incoming characters are forwarded transparently by the modem. The exception is the so-called Escape sequence of three identical characters (by default “+”), before and after which guard intervals must be maintained (by default 1 sec). When receiving such a sequence, the modem goes into command mode without breaking the connection; subsequently you can either return to data mode or terminate the connection using any of the appropriate commands.

What are the main commands used in Hayes modems?

Modems that support error correction and data compression almost always have a group of commands "\" and "%":

What is the structure of the dial command?

The dial command D has a parameter in the form of a string of sequentially interpreted characters that control the dialing process:

What is the structure of the S-register command?

The command for working with S-registers S has two forms:

What responses can the modem give to command lines?

A basic set of responses defined for all Hayes modems:

Additional answers introduced in some extensions:

A CONNECT message without parameters is issued either when extended messages (X0) are disabled or a connection is established at 300 bps.

The RING message is issued by the modem after the completion of each ringing signal (interval about 5 seconds). RINGING/RINGBACK messages are not issued by all types of modems.

The VOICE message is supported only by some modems and is issued when a signal is detected on the line that does not belong to any known class of line or modem signals. In this case, it is considered that the subscriber responded by voice, and after issuing a message, the modem is disconnected from the line.

What is a fax modem?

This is a modem with built-in fax protocols for communication, modulation and image transmission. Such a modem can work both with conventional modems via data transfer protocols and with fax machines via image transfer protocols.

The functionality of a fax modem is determined by its class: 1, 2 or 2.0. Class 1 only supports protocols physical level, all other procedures are performed control program computer. Class 2 brings most of the intelligence into the modem itself, but is the de facto “intermediate” standard. Class 2.0 adds image encoding and decoding functionality, contains a number of changes, and is approved as an official standard.

Classes of fax modems are not compatible from the bottom up (functions of lower classes are not supported in higher classes), and higher class modems most often do not support lower classes of fax commands.

Programs designed to work with fax modems (BitFax, BGFax, WinFax, etc.) allow you to send and receive images in various graphic formats (BMP, GIF, TIFF, JPG, etc.). In addition, most programs, as well as the built-in fax services of modern operating systems, allow you to transfer documents of any type, for which a fictitious device of the “printer” class is installed in the system, when “printing” documents to which they are converted into a clear image and sent by fax modem .

What is a voice modem?

This is a modem with the possibility of voice contact between subscribers. The first modems with voice support had only a microphone and telephone amplifier with the ability to connect headphones with a microphone, which added to the modem the functions of a regular telephone set. Modern modems, in addition, are capable of simultaneously transmitting data and voice over a channel, which is why this group of modems has the general designation SVD (Simultaneous Voice and Data), and often allows this to be done using a telephone connected to the modem.

There are two main technologies for transmitting voice and data:

What is Soft-modem?

This is the name of a class of modems, part of the “intelligence” of which is transferred from the modem itself to the main computer. Increased performance central processing units and the emergence of specialized commands for signal processing (MMX) make it possible to transfer some of the functions of modem equipment operating system main computer.

There are also three most common types of soft modems:

How to initially configure a new modem?

For an internal modem, first of all you need to set the number of the COM port and IRq line that it will use. The vast majority of internal modems are visible to the computer as an additional COM port, with the exception of Soft modems with completely program controlled, which can have an arbitrary interface.

When setting the port number, you need to keep in mind that on all modern motherboards There is a built-in I/O controller that supports two serial ports, usually operating as COM1 and COM2 by default. In BIOS Setup, each of these ports can also have an Auto mode, in which the port is turned on only if there are free standard addresses and IRq lines. For example, if the second system port is set to Auto and the board has an internal modem configured as COM2, the BIOS, depending on the type and version, can either move the second system port to COM4 or disable it completely.

If two ports are configured for one IRq line (IRq sharing), then it is possible to work with only one of them at any given time. If you try to activate both ports, neither will be able to work, unless both ports are served by a specialized program that is able to figure out which port generates which interrupt. If two ports are configured to the same address, both will fail.

Internal modems with a Plug & Play interface do not require special configuration; It may only be necessary to set the PnP mode by jumpers if the modem also allows direct configuration of the address and IRq.

On an external modem, you may need to set operating modes using switches, if any.

You can check the correct operation of the modem port using any terminal program (Telix, Terminate, Telemate for DOS, or the standard Hyper Terminal (Communication Program) for Windows 95). When entering the AT&F line, the modem must respond OK. You can also use the ATZ line, however, if the default parameters are set to Q1 mode, the modem will not respond OK to this line.

After making sure that the modem is working, you need to create a set of default parameters. To do this, enter the &Fn command with the required configuration number described in the modem manual; A configuration with hardware (hardware, RTS/CTS) data flow control is highly desirable.

If it is desirable to have some parameters different from the factory configuration, their required values ​​are set after the &Fn command. After setting all the parameters, enter the &W command, which records the generated set as the default set with number 0. Subsequently, each time the modem is turned on or after executing the Z command, this set of parameters will be installed.

To ensure that programs display speed correctly established connection, you need to set the modem to display the real speed in the CONNECT line instead of the modem-DTE speed. The Wn command is used for this; Other commands may also be required (for example, \Vn), which should be found in the description. You can check the format of the CONNECT line on most modems with the &T1 command, which establishes a test connection using the Local Analog Loopback type.

What is an initialization string and why is it needed?

The initialization string is a sequence of commands that brings the modem to a previously known state. Typically, such a line begins with one of the &Fn commands, which sets the factory settings, followed by commands for setting the desired modes.

If the terminal program supports several initialization lines that are output sequentially to the modem, it is convenient to begin the sequence with the Z command. In this case, the active default set of parameters stores the most general settings for all modem applications on a given station.

In the event that one set of parameters is sufficient for all uses of the modem, it will be most convenient to store it in NVRAM. The initialization line in this case is reduced to a single Z command.

How can you optimize the settings of the modem and control program?

In general optimal setting modem and program is very complex and ambiguous, however, in most cases, several most typical points can be identified:

What is the difference between asynchronous and synchronous modes?

In asynchronous mode, data is transferred byte by byte, each byte preceded by a start bit and terminated by one or two stop bits. Thus, the minimum unit of transmission is a byte, and the start/stop bits between the bytes ensure that the start and end of each byte are correctly identified. This mode is convenient from the point of view of reliability of isolating signals from the line; however, it requires packing/unpacking of bit data into bytes, and also reduces the transmission speed in the channel due to redundant start and stop bits (by at least 25% 2/8).

In synchronous mode, data is transmitted bit by bit, without grouping into bytes. In this case, there is no overhead for grouping bits, and the unit of transmission is a single bit. However, to allow the receiver to resynchronize if part of the stream is lost, the bits are often packaged in packets of varying lengths, complete with a header and a checksum. The minimum information unit in this case is a packet. Since the length of the packet is much greater than the length of its overhead, the overhead is much less.

All error correction and data compression protocols establish a synchronous transmission mode with packet exchange between modems. At the same time, the exchange between the modem and the DTE most often occurs in asynchronous mode, which, coupled with the overhead costs of processing and processing packets, creates a difference in speeds in the channel and with the DTE. To compensate for this difference, the modem has a buffer and also uses flow control methods.

Specialized devices (pager stations, industrial information collection systems, etc.) often use synchronous transmission between themselves and the modem, forming packets themselves and monitoring their correctness. In such cases, due to the inability of a regular computer port to operate in synchronous mode, it may not be possible for the computer to communicate with such devices through a pair of modems.

Why doesn't the modem recognize the busy signal?

The vast majority of modems are configured to recognize US/Canadian telephone signals. The “busy” signal in this standard is a combination of two frequencies 480 and 620 Hz, the duration of the tone and pause is 0.5 s, and the volume of the signal is significantly (12 dB) lower than the volume of a continuous beep. In the Russian telephone system, busy signals are transmitted in bursts of frequency 425 Hz, the duration of the tone and pause is 0.35 s, the level of all signals is the same. As a result, if the modem analyzer does not have sufficient signal duration/intensity margin, correct identification of them occurs rarely or does not occur at all.

If the modem has the ability to adjust the sensitivity to station signals and the range of their parameters, you can try to select the appropriate values. Modems aimed at the Russian telephone network (IDC, Russian ZyXEL, Russian Courier) are initially configured to the parameters of domestic signals.

For modems that do not have such adjustments, in the case when the difficulty in recognizing the “busy” signal is caused by its too loud level, you can try to attenuate the input signal by connecting a resistor with a resistance of 50..500 Ohms in series with the line, but this most often has a negative effect on the quality of communication.

What is the difference between working on a dial-up and leased line?

A standard dial-up line is distinguished by the presence of a supply voltage (about 60 volts in Russian telephone networks) and the ability to issue and receive line status and dialing signals. Accordingly, when operating over a dial-up line, the calling modem generally waits for a continuous dial tone, then dials the number, and only then waits for a response from the remote modem. The answering modem, in turn, receives a calling signal (ring), after which it connects to the line (“picks up”) and goes into answering mode.

A leased line is a permanent point-to-point connection between two subscribers. Typically this is a two- or four-wire communication line that directly connects two modems and is not connected in any way to the station equipment. In the simplest case, this can be a regular telephone cable included with the modem; in the most complex case, it can be a section of a multi-channel wire, fiber-optic or radio path, which, using channel equipment, simulates a simple wire connection.

Modems that support operation over a leased line (command &L1) in this mode automatically disable checking for a continuous tone, and also automatically try to restore the connection if it is broken. For initial installation connection, one modem must be activated as calling (command D) and the other as answering (command A). After this, the modems themselves restore the connection in the event of a break in the same roles.

In addition, modems that support leased lines have memorized modes in which communication in the selected role is established automatically when the power is turned on (or after the DTR signal appears). Thus, a pair of such modems, immediately after power-up or the appearance of DTR, creates an automatically maintained connection without the intervention of control programs, which in this case can only monitor the DCD signal and/or CONNECT/NO CARRIER messages. Ideally, such a pair of modems allows you to organize a completely transparent connection, similar to a null modem cable, in which programs are completely unaware of the existence of any additional devices in the tract.

The modem does not dial the number. Why?

If an attempt to connect ends with the message “No dialtone” (No Dialtone), and at the same time you hear a long beep through the modem speaker (if there is one), then most likely your PBX is producing a non-standard dial tone. In this case, the X3 command will help (the modem ignores the dialing signal). If this command doesn't help, try replacing it with X0.

If you do not hear a long beep, then either you have a problem with the line (check by connecting regular phone instead of the modem), or you plugged the telephone cord into the wrong modem connector. A modem usually has two connectors (the exception is inexpensive modems of unknown manufacture, which are better not to purchase) called PHONE and LINE (sometimes WALL). The telephone line cord must be plugged into the LINE (WALL) jack. The second connector is connected to a telephone set (when the modem is working, the telephone set is turned off).

If the X3 (or X0) command did not help, and you are sure that the telephone line is working and connected correctly, then the problem should be looked for in the modem. In this case, you should contact service center manufacturer or to the organization specified in the warranty card.

The remote modem has picked up the phone and is answering, but my modem can't hear it. What to do?

If the modem is working properly and the answer signal has sufficient power, then the reason is most likely that it was not able to recognize the long beep from the telephone exchange before the exchange began (your modem may not be able to simultaneously recognize the beep and the answer signal). This could happen if the beep was very quiet or very short (occurs on some PBXs and multi-line phones). Universal tool command X2.

If this does not help, then most likely your modem does not have the required sensitivity (it simply does not hear the remote modem) or is faulty.

The modems started communicating, the username and password were verified successfully, but the connection was lost when entering the network. Why?

Go to “My Computer” -> “Remote Access”, then right-click on the connection you are setting up and select “Properties” from the menu that appears. Next, go to the “Server Type” tab and uncheck the box next to “Login to the network.”

The modems started communicating, but the connection was lost before the username and password were verified. How to fix it?

Most likely, the connection settings have set the connection wait time to be too short. To change this interval, go to “My Computer” -> “Remote Access”, then right-click on the connection you are setting up and select “Properties” in the menu that appears. Next, click the “Settings” button, select the “Connection” tab. Here, either change the number in the “Cancel call if there is no connection” item (we recommend setting at least 120 seconds), or uncheck the box altogether. Also pay attention to the item “Shutdown when idle for more than...”.

If this does not help, then see the answer to the next question.

How to overcome frequent disconnections?

Reason: poor line quality (high attenuation, impulse noise, periodic signal fading, etc.). First, try adding the following commands to the initialization line: S7=200S10=200. If this does not help, then you can try to select the signal level, reception sensitivity, communication protocol (prohibit V.90), set the connection mode with error correction, or select the speed limit. This process is quite lengthy and tedious, because... The optimal parameters will have to be selected by trial and error. You can find the corresponding commands in the manual for your modem or at the end of this FAQ.

How to overcome low connection speed or short-term interruptions in data transfer?

You should try to adjust the signal level, reception sensitivity, communication protocol (disable V.90) or speed. In some cases, oddly enough, reducing the connection speed or choosing a slower protocol improves overall performance, because the number of long overtrainings decreases. You can find the corresponding commands in the manual for your modem or in the following sections of this FAQ.

Recommended modem settings depending on line quality.

Copyrights

This FAQ draws heavily on the Frequently Asked Questions on dial-up modems compiled by Eugene Muzychenko (2:5000/14@FidoNet, [email protected]). Copyright (C) 1998-99, Eugene V. Muzychenko. All rights reserved.