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Introduction

Very often in the computer literature, we come across such a concept as a modem.

There are a huge number of personal computers in the world. It is also logical to think about a quick and convenient way of connecting between them. This method became possible thanks to the modem. It allows you to transfer information from one computer to another in encrypted form via a telephone line. Computers exchange signals of a certain frequency and volume. What is possible with a modem?

You can contact another user of the modem, and exchange files with him, regardless of his location. Or play with him a computer game that supports a modem. If your modem is a fax modem, you can exchange fax messages. You can use BBS - Bulletin services

Board System, receive and receive files, communicate with other users, and play on-line games, i.e. in real-time modem games. You can connect to global networks.

For example FidoNet, or Internet / Relcom. By connecting to them, you can become a participant in many teleconferences, which makes it possible to exchange information with people of your circle of interests. If you are a businessman, then with the help of a modem you will always be aware of the latest events and news.

How does a modem work

Some background information - So when did you come up with the first modem card?

In the 1980s, the American firm Hayes finally released the first modem for the IBM PC. Of course, telephone lines were designed to transmit only the sounds of the human voice over a distance. Generally speaking, natural sounds are characterized by variable pitch and continuously varying intensity. For transmission by telephone, they are converted into an electrical signal with a continuously and correspondingly varying frequency and current. This signal is called analog. A computer, unlike a modem, understands only a digital signal, i.e. current of only two levels. Each of them denotes one of two computer understandable values, logical "0" and "1". To transmit a digital signal over a telephone line, it needs to be given an analog form acceptable to it. This is what the modem does. He also performs the opposite procedure, i.e. converts an analog signal into a digital one that a computer can understand. The word "modem" comes from the abbreviation of two terms:

Modulator / DEModulator. The modem organizes a bridge between the digital signal issued by the computer and the analog signal, which, as mentioned above, is understood by the telephone line. When transferring data from a computer to a modem, the former outputs a sequence of zeros and ones, and the latter converts them into an analog signal. The data is then sent to the telephone line and received by the modem at the other end of the line. When the modem receives data, it filters out useful information from line noise. For this, there are special error correction protocols. The most advanced of these is the MNP10. In addition, there are MNP1, MNP2, MNP3, MNP4, MNP5, MNP7. Currently, MNP5 is the most common, since MNP7 and MNP10 are installed on special modems that work over leased lines. For example, in the global Internet. After the modem has separated the useful information from the noise in the line, it selects the pumped data from the service information. And the transferred file, which has already passed such multistage processing, is written to the hard disk of the computer. This is how data exchange occurs when connecting using the Zmodem, Sealink, Ymodem and many other unidirectional protocols. Of course, both computers can send and receive data at the same time. Because they use certain frequency conventions that are different for the input and output signals. There are special bi-directional protocols for this. For example, Bimodem, Puma, Janus, Zedzap.

MNP - Protocols MNP (Microsoft Network Protocols) - a series of the most common hardware protocols, first implemented on Microsoft modems. These protocols provide automatic error correction and compression of transmitted data. Currently 10 protocols are known: MNP1. An error correction protocol that uses an asynchronous half-duplex transmission method. This is the simplest of the MNP protocols.

MNP2. An error correction protocol that uses an asynchronous duplex transmission method.

MNP3. An error correction protocol that uses a synchronous duplex method of transferring data between modems (the modem-computer interface remains asynchronous). Since asynchronous transmission uses ten bits per byte - eight data bits, a start bit and a stop bit, and with synchronous only eight, this is the opportunity to speed up data exchange by 20%.

MNP4. A protocol using a synchronous transfer method provides data phase optimization that slightly improves the inefficiencies of the protocols

MNP2 and MNP3. In addition, when the number of errors on the line changes, the size of the transmitted data blocks changes accordingly. As the number of errors increases, the size of the blocks decreases, increasing the likelihood of successful passage of individual blocks. The efficiency of this method is about 20% compared to simple data transfer.

MNP5. In addition to the MNP4 methods, MNP5 often uses a simple method of compressing the transmitted information. Characters that occur frequently in a transmitted block are encoded with bitstrings that are shorter in length than rare characters. Additionally, long strings of identical characters are encoded. Typically, this will compress text files to 35% of their original length. Together with 20% MNP4, this gives an efficiency increase of up to 50%. Note that if you are transferring files that are already compressed, and most of them are, there is no additional efficiency gain due to data compression by the modem.

MNP6. In addition to MNP5 methods, MNP6 automatically switches between full-duplex and half-duplex transmission methods depending on the type of information. The MNP6 protocol also provides compatibility with the V. 29 protocol.

MNP7. Compared to earlier protocols, it uses a more efficient data compression method.

MNP9. Uses V. 32 protocol and associated operating method to ensure compatibility with low speed modems.

MNP10. Designed to provide communication on highly noisy lines, such as cellular lines, long distance lines, rural lines. This is achieved using the following methods: Repeated attempts to establish a link; packet size changes in accordance with the change in the level of interference on the line; Dynamic change in the transmission rate in accordance with the level of line interference. All MNP protocols are upward compatible with each other. When the connection is established, the highest possible level of the MNP protocol is set. If one of the communicating modems does not support the MNP protocol, then the MNP modem works without the MNP protocol. Modes of MNP-modems An MNP-modem provides the following data transmission modes: Standard mode. Provides data buffering, which allows you to work with different data transfer rates between a computer and a modem, and between two modems. As a result, to improve the efficiency of data transfer, you can set the computer-to-modem exchange rate higher than that of the modem-to-modem. In standard mode, the modem does not perform hardware error correction. Direct transfer mode. This mode corresponds to an ordinary modem that does not support the MNP protocol. Data is not buffered and no hardware error correction is performed. Error correction and buffering mode. This is the standard mode of operation when two MNP modems are communicating. If the remote modem does not support MNP, communication will fail. Mode with error correction and automatic tuning. The mode is used when it is not known in advance whether the remote modem supports the MNP protocol. At the beginning of a communication session, after determining the mode of the remote modem, one of the other three modes is set. Internal and external modems Modems are internal and external (There are also special types of modems in the form of PC - cards (PCMCIA), but they are intended for computers such as laptops, and therefore they are not considered here.). Internal modems are made in the form of an expansion card inserted into a special expansion slot on the computer motherboard. An external modem, unlike an internal one, is designed as a separate device, i.e. in a separate case and with its own power supply when the internal modem receives electricity from the computer's power supply. So what are the advantages and disadvantages of external and internal modems? Internal modem Merits. All internal models without exception (as opposed to external ones) have a built-in FIFO. (First Input First Output - first in, first out). FIFO is a data buffering chip. An ordinary modem, when passing a data byte through the port, each time requests interrupts from the computer. The computer on special IRQ (Interrupt Request) lines interrupts the modem for a while, and then resumes it again. This slows down your computer as a whole. FIFO allows you to use interrupts several times less often. This is of great importance when working in multi-tasking environments. Such as Windows95, OS / 2, Windows NT, UNIX and others. Using an internal modem reduces the amount of wires pulled in the most unexpected places. Also, the internal modem does not take up precious desktop space. Internal modems are the serial port of the computer and do not occupy the existing ports on the computer. Internal models of modems are always cheaper than external ones. Disadvantages. They occupy an expansion slot on the computer motherboard. This is very inconvenient on multimedia machines with a large number of add-on cards installed, as well as on computers that run servers on networks. There are no indicator lights, which, with a certain skill, allow you to monitor the processes taking place in the modem.

If the modem hangs, then it can be restored only by pressing the "RESET" button to restart the computer. External modems Advantages. They do not occupy an expansion slot, and if necessary, they can be easily disconnected and transferred to another computer. There are indicators on the front panel that help you understand what operation the modem is currently performing. If the modem hangs, you do not need to restart the computer, just turn off and on the power of the computer.

Disadvantages. Requires a multicard with built-in

FIFO. Without FIFO, the modem, of course, will work, but the data transfer rate will drop. An external modem takes up precious desktop space and requires additional wires to connect. This also creates some inconvenience. It occupies the serial port of the computer. An external modem is always more expensive than a similar internal one, since it includes a case with indicator lights and a power supply.

The role of indicator lights

1. MR (Modem Ready) Indicates that the modem is turned on and ready to work.

2. TR (Terminal Ready) This indicator is on when the modem detects DTR (Data Terminal Ready) transmitted by the communication program.

3. HS (High Speed) And this indicator lights up when the modem is operating at the maximum speed possible for it.

4. CD (Carrier Detect) Lit when the modem detects a carrier. It should be on when the modems are connected and throughout the entire communication session until one of the modems hangs up

5. AA (Auto Answer) Indicates that the modem is turned on in auto answer mode, i.e. will answer all incoming calls himself. If the modem detects Ring, this indicator will blink.

6. OH (Off Hook) This indicator is equivalent to an off-hook telephone. It is on when the modem is on the line.

7. RD (Receive Data) Flickers when the computer is receiving data.

8. SD (Send Data) This indicator flashes when the computer is sending data. Modem Brands The de facto standard today is a 14400 modem with V32 and V32bis data transfer protocols (and enhanced for example HST and V32terbo).

Today it is worth focusing on this standard. But he, like everything in the computer world, is unstable, and gradually dies out. Of course, it is best to take a modem with a connection speed of 28800 and data transfer protocols V34 (and its subsets V. Fast and V. Everything). There is also an improved version of the V34 + protocol. It allows receiving / transmitting at speeds up to 33600. Modems of some companies have specialized protocols for special operating conditions. Usually on very noisy lines. On them, these protocols behave flawlessly. But what then is the conversation about normal "clean" lines? Such protocols are HST, developed by USRobotics®. There are also two protocols developed by Zyxel®. These are Zyx and ZyCell. Zyx is a protocol with the ability to communicate with similar models at speeds of 16800 and 19200. And ZyCell is a special protocol for satellite and cellular communications. The only drawback of such protocols is that they only communicate on proprietary protocols with similar models.). Now you can consider some brands of modems. GVC This company is known primarily for producing inexpensive, but fairly reliable models. For example, the GVC 14440 F1114HV model is a well-proven model in our conditions. It catches the BUSY signal almost flawlessly. It is a fax modem and has a class II fax. It also implements the adjustment of the signal level to the quality of the line. One of its advantages is the silent reed relay. ZyXEL A couple of years ago it was one of the most popular and prestigious models, but today the firm has lost ground, mainly against the backdrop of the achievements of USRobotics. All types of ZyXEL modems are divided into series. Series 1496 - apart from the standard V32 and V32bis protocols, they have their own protocols: Zyx and ZyCell. These models have a VOICE mode for sending and receiving voice messages. There is also a number identification mode (Caller ID Automatic Caller ID). Models of the 1496 series have adaptive fax, which means that the modem can automatically identify the subscriber and switch accordingly to fax, modem or voice. Also, ZyXEL modems can operate on dedicated four-wire lines, while developing a transmission rate of up to 115200 baud. USRobotics® This company produces several series of modems: USR Sportster, USR Courier, USR WorldPort and others. WorldPort models are designed for laptop computers. Because of this, they have not become widespread. The high-performance Courier series, for some of the reasons stated below, has not received wide distribution in our country. Only the Sportster series remains. Modems of this series cover the entire range of speeds from 14400 to 33600. They can be both internal and external and have many modifications that differ both in software and hardware. Conveniently enough, the Sportster series modems are upgradeable to the more expensive and much more functional Courier series. After the upgrade, the regular USR Sportster turns into a Courier. In doing so, it gains such an important advantage as the built-in HST (High Speed Technology) protocol. In 1991, four Canadian programmers wondered: why is this Sportster so similar to the old Courier? When they disassembled several modems, they realized that the Sportster and Courier differ only in firmware, which auto-detects the modem type by tricky jumpers and NVRAM (Non Violatible Random Access Memory) and for the Sportster series it simply disables HST and all other Courier features.

Conclusion

Before our eyes, a real communication explosion is taking place, quite comparable in scale and consequences to the one that was caused by the appearance of the first personal computers. A simple and effective device - a modem that unites the potential of two of the greatest inventions of mankind, the telephone and the personal computer, gives everyone access to incredible amounts of information and endows the humble personal computer with truly fantastic capabilities. Universal computer literacy, which we dreamed of quite recently, is now more or less a fait accompli. There is no doubt that telecommunications literacy is now needed no less: without it, it is simply unthinkable to keep up with the civilized world, either in business, or in science, or in education, or in many other areas. Already, the absence of a modem is comparable to the absence of a printer - both of which largely deprive a computer of its meaning. And the time is not far off when a lonely computer will be no more useful than a calculator is now.

References

modem indicator development

1. Berliner E.M. etc. Microsoft Windows 95. Microsoft Plus! Russian version. Ed. ABF, Moscow, 1996

2. Zeldner G.A. etc. The computer is in touch! Fax modem, modem, global networks, E-Mail, BBS. Ed. ABF, Moscow, 1996

3. Kirsanov D. Fax-modem: from purchase and connection to Internet access. Ed. "Symbol-Plus", St. Petersburg, 1995

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300 bps modem

Let's start considering the principles of modem operation from devices whose data transfer rate was only 300 bits per second. Their example is the easiest way to explain the principle of operation of the modem as such. These modems used a frequency offset key to transmit information over the telephone line; frequency shift keying (FSK). In this "language" certain sounds correspond to certain bits. These sounds of a specific frequency were called modem tones.

The terminal modem (dial-up modem) called the computer's modem and asked for permission to establish a connection. If we talk about the terminal modem, then the 1070-hertz tone corresponded to logical zero, and 1270-hertz tone corresponded to logical one. The computer's modem (the answering modem) transmitted a 2025 hertz tone, which corresponded to zero, or a 2225 hertz tone, which corresponded to one.

Since the dialing and answering modems used different tone frequencies, they could use the telephone line at the same time. This mode of operation was called duplex or simultaneously two-way. Modems that could not simultaneously transmit and receive information were called half-duplex. But such imperfect devices were quite rare even in those years when digital information devices were taking their first timid steps.

So, the connection is established, and the user enters the letter "a" on the keyboard. Its decimal code is 97; binary: 01100001. The terminal had a special device named UART or universal asynchronous receiver-transmitter. It split the bytes into individual bits and sent them through the RS-232 serial port. A modem was connected to this port, which transmitted bits to the remote computer one by one over the telephone line.

Modem with a speed of 56 kilobits per second

To bring the speed of the modems in line with the needs of the users, the designers were already lacking the capabilities of the FSK technology. First, it was replaced by PSK, and then QAM. We will not go into the details of these technologies. We only note that they allowed the transmission of unthinkable volumes of information at that time, squeezing everything possible out of the 3000-hertz bandwidth of a regular telephone line, originally intended for voice conversations.

The modem, whose theoretical maximum speed could reach 56 kilobits per second, actually connected to the remote computer at about 48 kilobits per second. Provided a perfect telephone line. The better the line quality, the higher the connection speed. To determine the achievable speed, such modems used the concept of "gradual degradation". The modem tested the line and disconnected. Then I connected again at a lower speed. And so on until the speed matched the real capabilities of the telephone line.

ADSL modems

The next stage in the evolution of modems was the ADSL devices in this category. ADSL is an acronym for asymmetric digital subscriber line. What is its asymmetry expressed in? These modems are capable of transferring data faster in one direction than the other. A special copper cable ran between the home or office and the telephone company. This cable made it possible to exchange much larger amounts of data with a remote computer than was possible using a regular telephone line. Recall that its bandwidth reached only 3000 hertz.

If ADSL modems were available in both the home and the telephone company office, the cable from one modem to the next acted as a high-speed data backbone. Between the house and the telephone company (that is, in the opposite direction to the main traffic), the maximum speed could reach about one million bits per second, that is, one megabit per second. In the opposite direction (from the telephone company to the house), the speed could theoretically reach 8 megabits per second. But this is under ideal conditions, almost unattainable in the real world. The same line could be used for telephone conversations, and not only for the transmission of digital data streams.

At its core, the principle of ADSL modems is very simple. The bandwidth of the new, more capacious telephone line, in the range from 24 thousand hertz to 1 million 100 thousand hertz, was divided into segments of 4 thousand hertz each. Each such segment was associated with a virtual modem. In total, there were 249 virtual modems, each of which tested its own section of the bandwidth and tried to squeeze the maximum speed out of it. Thus, the ADSL modem baud rate is calculated as the sum of the 249 virtual modem baud rates. If we simplify the description of this technology as much as possible, then we can say that almost two and a half hundred traditional modems are combined in one box and the same number of telephone lines in one cable.

Point to point. PPP protocol

Our story would look incomplete and incomplete if we did not pay attention to the PPP protocol. These days, a user no longer communicates directly with another user's computer. Your computer's modem contacts an Internet service provider (ISP). allows us to connect to any server on the web. Thanks to the global network and providers, there is no longer a need for character-by-character transmission of information. Your modem is responsible for routing TCP / IP packets between your computer and your ISP.

The standard routing technology for these packets for your modem is PPP (Point-to-Point Protocol). Its basic idea is quite simple: your computer's TCP / IP stack forms TCP / IP datagrams, that is, data packets containing the address to which they should be delivered. These packets are sent to the modem for subsequent transmission. The provider's computer receives each datagram and sends it over the Internet to the specified address. The process of transferring data from the provider to you follows a similar scenario.

Yes, in just two decades since computers were just beginning to emerge and until the end of the nineties of the last century, the speed of information transfer has increased tens of thousands of times. Today - already in the millions! The decade and a half that lie between the year 2000 and the present can become the topic of a separate interesting story. This is a story that is being created and written before our eyes.

Based on materials from computer.howstuffworks.com

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 are there and what is its purpose.

What devices are there for connecting to a local and worldwide network?

This word is formed by the merger of 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 transforming it.

ATTENTION. Earlier, computers were connected to the Internet using telephone wires. Network cards are replacing, as they have a higher speed. There are also wireless modems that are not yet so popular.

Why and when are they needed?

There are only two moments when we need a modem. One of them, or rather the first, refers to the recent past. Connection to a computer was then ensured with the help of such equipment, as well as a telephone line. This moment 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 reliability of the connection is much better. And the second point is relevant for people who travel. They need the Internet, which does not need wires and unnecessary devices - wireless Internet.

By way of execution

According to the method of execution, the specified device is divided into two types: internal and external. Internal ones are installed inside the system unit. And to provide an external modem connection, 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 set in the appropriate position. If you have a question "What is a modem mode?", Then we will now answer it. There are two modes in total: digital and analog. It depends on the signal of the telephone line. If you have a wireless device, then only digital mode will be available to you.

By connection type

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

By type of supported networks

This characteristic only applies to wireless devices. There are the following types of networks: GSM or 2G, 3G, LTE or 4G. All of these networks are backward compatible. In simple terms, 3G will work in the GSM network without any problems. If you wondered what a USB modem is, now you will get an answer to it. 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 SIM card slot. It is connected to a computer in a USB slot.

Broadband modem

A broadband modem is a device that will satisfy all the needs of the user. It provides high speed internet as well as data transfer from your computer. A broadband modem is about 40 times more powerful than a regular one. Its main advantage is a reliable connection to the network and fast sending and receiving of documents, etc. In simple words, a modem is a device that allows us to access the Internet.

In the article, we have laid out everything on the shelves with you, learned about its types. We hope that after reading the article, you have no questions left about this wonderful device. Share the article with your friends and like it. You can write all your questions in the comments, and we will answer them as soon as possible.

So, modems and modulation-demodulation ...

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

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

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 the so-called analog (sound) signals. The analog signal identifies information that is transmitted continuously, while the digital signal identifies only the data that is determined at a particular stage of transmission. The advantage of analog information over digital is the ability to fully represent a continuous stream of information.

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

If this whole thing is represented graphically, then analog signals are sine waves, while digital signals are represented as square waves. For example, sound is an analog signal because the 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 to the original digital data.

Interfaces

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

MNP-5 Serial RS-232 interface.

MNP-5 4-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 transferring data, a speed greater than 600 bits per second (bps or bit / sec) 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 tricky scheme). The very process of such a transfer allows the transfer of many bits of data at the same time. It is clear that computers are more sensitive to the 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 apply certain compression algorithms to them. This is how two so-called compression protocols appeared:

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

V.42bis (transmission protocol having 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.

I must say that when transferring files, if the V.42bis protocol is not available at all, then it is best to disable the MNP-5 protocol as well.

Correction of errors

Error correction is a method by which modems test the transmitted information for the presence of certain damages in it that occurred during the transmission. The modem breaks up this information into small packets called frames. The transmitting modem attaches a so-called checksum to each of these frames. The receiving modem checks if the checksum of the sent information matches. If not, then the frame is forwarded again.

Frame is one of the key terms for data transmission. A frame is understood as a basic data block with a header, information and data attached to this header, which terminate the frame itself. The added information includes the frame number, data on the size of the transmitted block, synchronization characters, 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 a whole.

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. Data stop bits are a type of so-called border control bits. The table bit defines the end of the cycle for asynchronous transmission (the time interval between the transmitted characters changes) of data in a short cycle.

MNP2-4 and V.42 protocols

Although error correction can slow down data transmission on noisy lines, this method provides reliable communications. MNP2-4 and V.42 are error correction protocols. These protocols define how modems validate data.

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

Flow Control or 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 this modem at some point in time pauses the reception of data. Flow control can be implemented at both software (XON / XOFF - Start / Stop) and hardware (RTS / CTS) levels. Flow control at the software level is carried out through the transfer of a specific character. After the signal is received, another character is transmitted.

For example, in Windows 98, there is an option in the modem settings Data bits, which allows you to set the 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 pseudo graphics (only works with 128 characters), report it by selecting option 7.

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

which allows you to determine the way to implement data exchange. Here you can correct possible errors that occur when transferring data from the computer to the modem. The default setting XON / XOFF means that data flow control is carried out programmatically through standard ASCII control characters, which send a command to the modem pause / resume transfer.

Software flow control is only possible when using a serial cable. Since the flow control at the software level regulates the transmission process by sending some symbols, 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 delivering information is much faster and safer than through software flow control.

FIFO buffer and UART ICs

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

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

S-registers

The 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. The modem has a lot of registers, 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 to be set, and xx defines the register itself. For example, through the SO register, you can set the number of rings to answer.

IRQ interrupts

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

Communication information port or just COM port

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

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

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

COM 1 is tied 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 the external ports of other I / O 1/0 devices or controllers can use the same IRQ interrupts.

Therefore, by assigning a COM port or IRQ to the modem, you should check other devices to see if they have

the same serial ports and interrupts.

I must say that devices connected to the telephone line parallel to the modem (especially the caller ID) can significantly degrade * the quality of your modem. Therefore, it is recommended to connect telephones through the designated jack on the modem. Only in this case will he disconnect them from the line during operation.

Flash memory of your modem

Flash memory is read-only memory or EPROM (read-only memory) that can be erased and re-programmed.

All modems with the string "V. Everything" in their names are subject to reprogramming. In addition, "Courier V.34 dual standart" modems are subject to software upgrade if the line Options V.FC protocol is present in response to ATI7 command. If the modem does not have this protocol, then the upgrade to "Courier V. Everything" is done 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.

User Programmable NVRAM

All modem settings are reduced to the correct setting of the values of the NVRAM registers. 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 loaded into RAM at startup. NVRAM programming is performed in any terminal program using AT commands. A complete list of commands can be obtained from the documentation for the modem, or obtained in the terminal program by commands AT $ AT & $ ATS $ AT% $. Write the factory settings with hardware data control to NVRAM - command AT & F1, then make adjustments to configure the modem in conjunction with a specific telephone line and write them to NVRAM on command AT&W. Further initialization of the modem must be done through the command ATZ. 4.

Application software for data transmission

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

If you have problems with 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 by 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. Transfer rates over 28,800 bps on Russian telephone lines are usually unattainable. 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 even not always achievable) speed of 9 600 bps, it turns into a continuous wait. Therefore, for stable data transmission in the presence of telephone line interference, a high-end modem is needed, which 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 is 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 there are usually not enough of them), it is difficult to monitor its operation due to the lack of indicators, besides, the described models are fundamentally unsuitable for notebook computers with 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 analogs, does not take up space on the table and does not create a mess of wires. Using an external modem assumes that the computer to which it is connected has the most advanced serial port control (UART) chips. UART microcircuits appeared in the first PCs, because even then it became clear that the exchange of data via a serial port was too slow and complicated operation and it would be better to entrust it to a special controller. Since then, several UART models have been released. In computers such as the IBM PC and XT, as well as those fully compatible with them, the 8250 chip was used, in AT it was replaced by the UART 16450. Until recently, most computers based on the i386 and i486 processors were equipped with a 16550 controller, which had internal hardware buffers like " turn ", and today the standard is UART 16550A - a microcircuit similar to the previous one, but with eliminated flaws. The absence of buffers in all the microcircuits, except for the last one, leads to the fact that data transmission through the serial port at speeds above 9600 bps becomes unstable (using MS Windows reduces this threshold to 2400 bps).

If you need to connect a high-speed external modem to a computer using an outdated UART microcircuit, you must either change the multi-card or add a special expansion card (which will take up one bus slot and deprive the external modem of its most important advantage). Internal modems do not have this problem - they do not use the COM port (more precisely, they contain it). Now, internal modems have another advantage, also related to the speed of operation. According to the V.42bis specification, data in transmission can be compressed by about four times, therefore a modem operating at a speed of 28800 bps must receive data from a computer or send it to it at a speed of 115 600 bps, which is the limit for serial PC port. However, 28,800 bps is not the limit for a telephone line, where the maximum is somewhere in the region of 35,000 bps, and on digital lines (ISDN) the bandwidth exceeds 60,000 bps. Therefore, in this situation, the serial port will become the "bottleneck" of the entire system, and the potential of the external modem will not be realized. Modem makers are now developing models that could connect to the faster parallel port, but obviously devices sold now will not be able to accommodate this.

At the same time, many modems can be upgraded to work at high speeds, up to the ability to work on ISDN. But everything rests against 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 be tomorrow (and 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 counterparts.

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, communication methods and data compression. The most common standard is V.32bis for modems with a baud rate of 14000 bps. For modems with a speed of 28800 bps, the standardized protocol is V.34.

In addition, it should be emphasized that modems with baud rates of 16800, 19200, 21600 or 33600 are not standard.

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

About the exterior and the interior. An external modem is connected via a special cable to your serial port. Such a modem, as a rule, has a volume control, information indicators, a power supply and other sometimes useful devices. If you are a professional, then you shouldn't care which modem to choose - internal or external. Usually, a good internal modem through a special software emulates well all the visibility of an external modem.

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

In Russia, this choice is very limited. This market was hammered by two companies: ZyXEL from sunny Taiwan and the U.S. Robotics from the USA. The modems of the latter company are chosen by professionals (Courier), the first - by all the others, that is, all those users who choose the so-called ultra-reliable ZyCell protocol.

So, choose Courier. And, believe me, this is not an advertisement.

A modem is a device that allows you to exchange data over a telephone line.

A modem is a device that allows you to exchange data over a telephone line.

If the computers are too far away and cannot be connected with a standard network cable, the connection between them is carried out using a modem. In a networked environment, modems are used to connect individual networks to each other or between a LAN and the rest of the world. Computers cannot communicate directly through the telephone line, since they exchange data using digital electronic pulses, and only analog signals (sounds) can be transmitted through the telephone line.

A digital signal can take only two values - 0 or 1. An analog signal is a smooth curve that can have an infinite number of values. The modem on the transmitting side converts digital signals to analog and transmits them over the telephone line. The modem on the receiving end converts the incoming analog signals into digital signals for the receiving computer. In other words, a transmitting modem modulates a digital signal to an analog signal, and a receiving modem demodulates an analogue signal to a digital signal.

Modem hardware

Modems have two standard physical interfaces:

Serial data interface (RS-232)

Telephone line interface RG-11 (four-pin telephone connector)

There are internal and external modems. Internal modems fit into expansion slots on the motherboard like other cards.

An external modem is a box that is connected to a computer using a serial (RS-232) cable. This cable connects the serial port of the computer to the connector on the modem that is intended for communication with the computer. A cable with RG-11 connector is used to connect the modem to the telephone line.

Modem standards

Industry standards exist for virtually every area of networking, and modems are no exception. The standards provide for the interoperability of modems from different manufacturers. The specifications known as V-series include a standard number. Sometimes the word "bis" is included. It indicates that this standard is a revised version of an earlier standard. If the name contains the word "terbo" it means that the second "bis" standard has also been modified.

Modem performance

Originally, the speed of modems was measured in bits per second or in units called "baud". Many people confused them, believing that they mean the same thing. In fact, baud refers to the frequency of the oscillations of the sound wave carrying data bits over the telephone line. In the early 1980s, the baud rate was equal to the transmission rate of modems. Then engineers developed methods for compressing and encoding information. As a result, each modulation of sound could carry more than one bit of information, therefore the transmission rate in bits per second can be higher than the baud rate, so it is necessary to first pay attention to the rate in bits per second, and then in baud. For example, a 28800 baud modem can actually transmit data at 115200 bps. Modern modems have such industry standards for data compression as V.42bis / MNP5, and have a data transfer rate of 57600 bps, and some - 76,800 bps.

Modem types

There are different types of modems as there are different transmission media that require different transmission methods. These types can be roughly divided by taking the link synchronization criterion as a basis. Communication can be asynchronous and synchronous. The type of modem will depend on the environment and the purpose of the network.

Asynchronous communication

Asynchronous communication is the most common form of data transfer. The reason for this popularity is the use of standard telephone lines with this method. In asynchronous transmission, data is transmitted in a serial stream. Each character - letter, number or sign is decomposed into a sequence of bits. Each such sequence is separated from the other by a start and stop bit. The transmitting and receiving devices must agree on the sequence of start and stop bits. Communication of this type is not synchronized, the transmitting computer transmits, and the receiving computer receives without coordinating the interaction of devices. The receiving computer then checks the received data for errors and receives the next block of information. 25% of the traffic goes to the transmission of matching information.

Error control

The possibility of errors is never ruled out, so in asynchronous transmission a special parity bit is used. An error checking and correction scheme that applies it is called parity. With parity, the number of one bits sent and received must be the same.

The V.32 modem standard did not provide error control. To address this problem, Microcom created its own asynchronous data error control standard called Microcom Network Protocol (MNP). This method was so successful that other companies borrowed not only the initial version of it, but also other versions, called classes. Currently, MNP classes 2,3, and 4 are used.

In 1989, the CCITT published an asynchronous error control scheme called V.42. This hardware error correction standard includes two protocols. The basic error control scheme is Link Acces Procedure for Modem (LAPM), however V.42 also uses MNP4. LAPM is used to connect V.42 modems, but if one of the modems only supports MNP4, MNP4 will be used.

Increasing the transmission speed

Correction / Compression Algorithm

When transmitting information using the correction protocol (MNP4, v.42), 10 bits received from the computer are cut off to 8 information bits (start and stop bits are removed) (10 bits = start_bits + 8 informational + stop_bits - see Asynchronous protocol RS232). And vice versa, when 8 information bits are received from the line, the modem converts them into 10 and transmits them to the computer. Thus, less information goes along the line than the modem received from the computer. But that is not all. When using the compression protocol (MNP5, v.42bis), there is also a decrease in the amount of useful information, so only a part of the 10 bits that the modem received from the computer will get into the line (and to the remote modem) ...

The performance of a communication channel is influenced by two factors:
Channel rate - characterizes how quickly the bits are encoded and transmitted over the communication channel
Bandwidth - characterize the share of useful information transmitted over the channel
Transfer rate and bandwidth are not the same thing. By compressing data, you can increase the bandwidth - compression reduces the time required to transfer data (by removing redundant elements and empty areas). One of the common data compression protocols is MNP5 - transfer times can be cut in half

When using the V.42bis standard, you can achieve the best performance, since it describes the hardware implementation of continuous information compression. Throughput at 9600bps can reach 38400bps. Currently, high-speed protocols such as x2 and V.90 are used.

Combining standards

A combination of data transmission and error correction protocols is used to increase performance. For example, with asynchronous transmission, the following combination gives good results:
V.32bis-transmission
V.42 error correction
V.42bis compression

Synchronous communication

Synchronous communication is based on a synchronization scheme agreed between two devices. Its purpose is to separate bits from the group when transmitting them in blocks. These blocks are called frames. To establish synchronization and check the correctness of its work, special characters are used. Since the bits are transmitted synchronously, start and stop bits are unnecessary. The transmission ends at the end of one frame and starts at the beginning of another. This method is more efficient than asynchronous transmission. In the event of an error, the synchronous error recognition and correction circuitry repeats the frame transmission.

Synchronous protocols perform the following actions not covered by asynchronous protocols:

Break data into blocks
Add control information
Checks data for errors

Main protocols of synchronous transmission:

SDLC Synchronous Channel Control Protocol
HDLC high-level channel control protocol
BISYNC Binary Synchronized Communication Protocol

Synchronous communication is used mainly on leased digital lines, and is usually not used at home.