Telecommunications in the modern computer world

I. Introduction

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

There are a huge number of personal computers in the world. It is logical to think about a quick and convenient way to connect them. This method was made possible thanks to a modem. It allows information to be transferred from one computer to another in encrypted form via a telephone line. Computers exchange signals of a certain frequency and volume.

What is possible thanks to a modem?

You can contact another modem user and exchange files with him, regardless of his location. Or play with him computer game, which supports the modem.

If your modem is a fax modem, you can exchange fax messages.

You can use the services of BBS - Bulletin Board System (English - Electronic board advertisements), receive and accept files, chat with other users, and play on-line games, i.e. games via modem in real time.

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 in 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.

II. How does a modem work?

Some general information

So when was the first modem board invented?

In the 80s, the American company 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 changing frequency and current strength. This signal is called analog.

The computer, unlike the modem, only understands the digital signal, i.e. current is only two levels. Each of them denotes one of two values ​​understandable to the computer - logical “0” and “1”. To transmit a digital signal over a telephone line, it needs to be given an analogue form acceptable to it.

This is exactly what the modem does. It also performs the reverse procedure, i.e. translates analog signal into a computer-readable digital format. The word “modem” comes from an 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 transmitting 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 useful information from noise on the line. There are special error correction protocols for this. The most advanced of them is MNP10. In addition, there are MNP1, MNP2, MNP3, MNP4, MNP5, MNP7. Currently, MNP5 is the most common because MNP7 and MNP10 are installed on special modems that operate over dedicated lines. For example, on the global Internet. After the modem has separated useful information from noise on the line, it selects the pumped data from service information. And the downloaded file, which has already undergone such multi-stage processing, is written to hard drive computer. This is how data is exchanged when connecting using the Zmodem, Sealink, Ymodem and many other unidirectional protocols.

Of course, both computers can simultaneously receive and send data. Because they use certain conventions about frequencies that are different for input and output signals. There are special bidirectional protocols for this. For example Bimodem, Puma, Janus, Zedzap.

MNP protocols

MNP (Microsoft Network Protocols) is a series of the most common hardware protocols, first implemented on Microsoft modems. These protocols provide automatic error correction and compression of transmitted data. There are currently 10 known protocols:

MNP1. An error correction protocol that uses an asynchronous half-duplex data transfer method. This is the simplest of the MNP protocols.

MNP2. An error correction protocol that uses an asynchronous full-duplex data transfer method.

MNP3. An error correction protocol that uses a synchronous duplex method of data transmission 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 synchronous transmission uses only eight, this makes it possible to speed up data exchange by 20%.

MNP4. The protocol, which uses a synchronous transmission method, provides data phase optimization that somewhat improves on the inefficiencies of the MNP2 and MNP3 protocols. 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 block size decreases, increasing the likelihood of individual blocks passing successfully. The efficiency of this method is about 20% compared to simple data transfer.

MNP5. In addition to the methods of MNP4, MNP5 often uses a simple method of compressing the transmitted information. Characters that occur frequently in a transmitted block are encoded in bit chains of shorter length than rarely encountered characters. Additionally, long chains of identical characters are encoded. Usually at the same time text files shrink to 35% of their original length. Together with 20% MNP4 this gives an increase in efficiency of up to 50%. Note that if you are already passing compressed files, and in most cases this is true; there is no additional increase in efficiency due to data compression by the modem.

MNP6. In addition to the MNP5 protocol methods, the MNP6 protocol 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 more effective method data compression.

MNP9. Uses the 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 communications, intercity lines, rural lines. This is achieved using the following methods:

Repeated attempts to establish connection

Changes in packet size in accordance with changes in the level of interference on the line

Dynamically changes the transmission speed in accordance with the level of line interference

All MNP protocols are compatible with each other from bottom to top. When a connection is established, the highest possible MNP protocol level is set. If one of the communicating modems does not support the MNP protocol, then the MNP modem works without the MNP protocol.

MNP modems modes

The 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 data transmission efficiency, you can set the computer-to-modem speed higher than the modem-to-modem. In standard operating mode, the modem does not perform hardware error correction.

Direct transmission mode. This mode corresponds to a regular modem that does not support the MNP protocol. There is no data buffering and no hardware error correction is performed.

Mode with error correction and buffering. This is the standard operating mode when connecting two MNP modems. If the remote modem does not support the MNP protocol, communication will not be established.

Error correction mode 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 three other modes is set.

Internal and external modems

Modems can be 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 for this reason they are not considered here.). Internal modems are made in the form of an expansion card, inserted into a special expansion slot on motherboard computer. The external modem, unlike the internal one, is designed as 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

Advantages

1. All internal models, without exception (unlike external ones), have a built-in FIFO. (First Input First Output - first to come, first to be accepted). FIFO is a chip that provides data buffering. A regular modem, when a byte of data passes through a port, requests interruptions from the computer every time. The computer, using special IRQ (Interrupt Request) lines, interrupts the operation of the modem for a while, and then resumes it again. This slows down the computer overall. FIFO allows you to use interrupts several times less often. This is of great importance when working in multitasking environments. Such as Windows95, OS/2, Windows NT, UNIX and others.
2. When using an internal modem, the number of wires stretched in the most unexpected places is reduced. Also, the internal modem does not take up precious space on the desktop.
3. Internal modems are a serial port on the computer and do not occupy existing computer ports.
4. Internal modem models are always cheaper than external ones.

Flaws

1. They occupy an expansion slot on the computer motherboard. This is very inconvenient on multimedia machines that have a large number of additional fees, as well as on computers that work as servers in networks.
2. There are no indicator lights that, if you have a certain skill, allow you to monitor the processes occurring in the modem.
3. If the modem freezes, you can restore functionality only by pressing the “RESET” key to restart the computer.

External modems

Advantages

1. They do not occupy an expansion slot, and if necessary, they can be easily disabled and transferred to another computer.
2. There are indicators on the front panel that help you understand what operation the modem is currently performing.
3. If the modem freezes, you do not need to restart the computer; just turn off and turn on the computer's power.

Flaws

1. A multicard with built-in FIFO is required. Without a FIFO, the modem will of course work, but the data transfer speed will drop.
2. An external modem takes up precious space on your desktop and requires additional wires to connect. This also creates some inconvenience.
3. It occupies the computer's serial port.
4. An external modem is always more expensive than a similar internal one, because includes a housing 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 use.

1. TR(Terminal Ready)

This LED is lit when the modem detects a DTR (Data Terminal Ready) sent by the communications program.

1. HS(High Speed)

And this indicator lights up when the modem is operating at its maximum possible speed.

1. CD(Carrier Detect)

Lights up when the modem detects a carrier. It should be on while the modems are connecting and throughout the entire communication session until one of the modems hangs up.

1. AA(Auto Answer)

Indicates that the modem is in auto-answer mode, i.e. will answer all incoming calls himself. If the modem detects Ring, this indicator flickers.

1. OH(Off Hook)

This indicator is equivalent to an off-hook telephone. It lights up when the modem is occupying the line.

1. RD(Receive Data)

Flickers when the computer is receiving data.

1. SD(Send Data)

This light blinks when the computer is sending data.

Modem brands

Today, the de facto standard is a modem with a connection speed of 14400 and data transfer protocols V32 and V32bis (and improved ones, for example, HST and V32terbo). Today it is worth focusing on this standard. But it, like everything in the computer world, is unstable and is gradually dying out. Of course, it is best to take a modem with a connection speed of 28800 and V34 data transfer protocols (and its subsets V.Fast and V.Everything). There is also an improved version of the V34+ protocol. It allows you to receive/transmit at speeds up to 33600. Modems from 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 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 communicate using proprietary protocols only with similar models.).

Now you can consider some brands of modems.

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 almost accurately picks up the BUSY signal. This is a fax modem and it has a Class II fax. It also implements adjustment of the signal level to the line quality. 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 company has greatly lost its position, mainly against the backdrop of the achievements of USRobotics.

All types of modems from the company ZyXEL divided into series.

Series 1496 - in addition to the standard V32 and V32bis protocols, they have their own protocols: Zyx and ZyCell. These models have a voice mode (VOICE) for sending and receiving voice messages. There is also a number identification mode (Caller ID - Automatic Number Identifier).

Models of the 1496 series have adaptive fax, which means that the modem allows you to automatically identify the subscriber and switch accordingly to fax, modem or voice.

Also modems ZyXEL can operate on dedicated four-wire lines, while developing a transmission speed of up to 115,200 baud.

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 are not widely used. The high-performance Courier series, for some reasons stated below, has not become widespread in our country. Only the Sportster series remains. Modems in this series cover the entire range of speeds from 14400 to 33600. They are both internal and external and have many modifications that differ in both software and hardware. It’s quite convenient that the Sportster series modems have the ability to be upgraded by software and hardware to the more expensive and much more functional Courier series. After the upgrade, the regular USR Sportster turns into a Courier. At the same time, it acquires such an important advantage as the built-in HST (High Speed ​​Technology) protocol.

In 1991, four Canadian programmers wondered: why is the Sportster so similar to the old Courier? When they disassembled several modems, they realized that the Sportster and Courier differ only in the firmware, which auto-detects the modem type using tricky jumpers and NVRAM (Non Violatible Random Access Memory - non-volatile memory of the modem) and for the Sportster series simply disables HST and all other Courier features.

III. Conclusion

A real communication explosion is taking place before our eyes, quite comparable in scale and consequences to that caused by the advent of the first personal computers. A simple and effective device - a modem, combining the potential of two of humanity's greatest inventions, the telephone and the personal computer, gives everyone access to incredible amounts of information and empowers the humble personal computer truly fantastic possibilities.

Universal computer literacy, which we dreamed of just recently, is now more or less a fait accompli. There is no doubt that telecommunications literacy is no less needed now: 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 now, the absence of a modem is comparable to the absence of a printer - both of them largely deprive the computer of its meaning. And the time is not far off when a single computer will be no more useful than a microcalculator is now.

Used literature:

1. Berliner E.M. etc.. Microsoft Windows 95. Microsoft Plus! Russian version. Ed. ABF, Moscow, 1996
2. Zeldner G.A. etc. Computer connected! 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

Advantages.

S Doesn't take up extra space

S Does not require a separate connection to the electrical network

S Somewhat cheaper than external ones S Do not occupy standard

computer serial ports

S A modem installed in a computer is more difficult to take away without your knowledge.

Flaws.

Lack of LED indication

allows you to quickly receive

information about the modem status

To install the modem you need

open the system unit

Frequent conflicts with others

equipment, for example with

video adapter or mouse

Purely program management

and if an error occurs for it

elimination may have to

turn off the entire computer

The modem is always on, even if not

computer __________________

External modems.

Advantages.

Extremely simple connection of the modem to

computer

LED indication allows

quickly find out about the modem status

Possibility of quickly transferring the modem from

one computer to another

If there is an error, you can simply turn it off

powering the modem without turning off the computer

The modem can be connected to almost

any computer _____________

Flaws.

Takes up extra desk space

Requires additional connection to the electrical network.

There are two types of telephone lines.

a) Switched lines- these are, in fact, ordinary telephone communication lines. Users make the connection themselves. Communication is established through the GTS. The data transfer rate usually does not exceed 28800 baud.

b) Leased lines- provide a permanent connection to the network, dedicated specifically for this

user.

The use of leased lines is very expensive, since the user is constantly connected to the network, the speed

transmission over such lines is very high, and the occurrence of errors is unlikely.

Telephone lines are of poor quality. This leads to distortion of the transmitted information. Sometimes one corrupted bit can invalidate the entire information. Highlight:

1) Non-intelligent modems- perform only modulation and demodulation functions.

2) Smart modems- built on the basis of microprocessors, they perform more complex functions: monitoring and correction of errors that occur during data transmission, data compression and encryption, etc.

One of the most important characteristics modem is the data transfer speed. It is measured in baud - the number of bits per second. Most modern modems can operate at speeds of 14400 baud or higher.

After speed, you should pay attention to the manufacturer. More than half of modem users in our country use US Robotics modems. The following companies also produce excellent modems: Motorola (however, they are quite expensive), ZyXEL, GVC, MultiTech, INPRO (which produces IDC modems).

4. Bridge divides the network into parts, transferring information from one part to another, only if such transfer is really necessary, that is, if the address of the destination computer belongs to a different subnet. Thus, a bridge isolates traffic on one subnet from traffic on another, which:

1) increases the overall performance of data transmission on the network

2) reduces the possibility of unauthorized access to data, since data does not leave its subnet unless necessary.

An internal modem is a device installed inside a desktop or laptop computer that allows the computer to communicate over a network with other connected computers. There are two types of internal modems: dial-up and WiFi® (wireless). The first operates over a telephone line and requires a telephone number to access the network and login credentials. The latter can be connected via wireless network and in some cases without credentials.
The term "internal modem" usually refers to a dial-up modem, as subsequent technologies are used with classifiers to differentiate them. Current internal modem, also available as external device, uses the v.92 protocol for communication over copper telephone lines. The modem sends and receives data using modulated audio frequencies, which it converts into digital data bits. The word modem combines the words modulator and demodulator, which refer to this function.

External modems preceded internal models and were used for proprietary purposes for decades until 1981, marking the release of the first affordable and practical modem for public use. The Hayes® Smartmodem® was revolutionary because of its built-in controller that allowed the device to accept, store and execute user-generated commands. This modem could not only dial a number independently, but also receive incoming call from another modem. Before the advent of Smartmodem, modems were designed to operate either as a server or a client (sender or receiver), but not both, and telephone number had to be manually dialed into the telephone base and then the receiver placed on the speaker coupler.

The inexpensive 300 baud Smart modem operated at 300 bps and spawned many clones. Free message board services became ubiquitous, quickly joined by private online services that created graphical environments where people could point and click their way through gated content. Over the next decade, as online content moved from text-based bulletin boards to fully graphical environments, the disadvantages of running at slower speeds became apparent.

Not only did modems become faster, they became standard equipment, available as internal models that were soon included in every new computer. Current 9600 baud dial-up modems operating at a maximum speed of 56 kilobits (kbps) used a combination of techniques to maximize the technology's limitations. Server-side compression, which includes an additional protocol known as v.44, can provide theoretical text speeds of up to 320 kbps.

Although constantly improving, the limitations of the internal dial-up modem have given way to digital subscriber line (DSL) modems, cable modems, and fiber optic modems that take advantage of latest technologies to deliver content tens to hundreds of times faster than connecting via a telephone line. However, an internal modem is still standard equipment as a backup device built into every desktop and laptop. Despite its antiquity, dial-up is still considered the most reliable means of online access, as telephones are usually accessible even when high-speed access is not available.

Modems can be 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 for this reason 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. The external modem, unlike the internal one, is made 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

Advantages

All internal models, without exception (unlike external ones), have a built-in FIFO. (First Input First Output - first to come, first to be accepted). FIFO is a chip that provides data buffering. A regular modem, when a byte of data passes through a port, requests interruptions from the computer every time. The computer, using special IRQ (Interrupt Request) lines, interrupts the operation of the modem for a while, and then resumes it again. This slows down the computer overall. FIFO allows you to use interrupts several times less often. This is of great importance when working in multitasking environments. Such as Windows95, OS/2, Windows NT, UNIX and others.

When using an internal modem, the number of wires stretched in the most unexpected places is reduced. Also, the internal modem does not take up precious space on the desktop.

Internal modems are a serial port on the computer and do not occupy existing computer ports.

4. Installation of external and internal modem.

How to install an internal modem?

Before installing the http://www.megabook.ru/pc_2001/Encyclop.asp?Topic=pc_98 modem on your computer, you must select the serial port number (1, 2, 3 and 4) to which the modem will be connected. Since two different devices cannot use the same COM port, you need to know which ports are already occupied. The modem is an interrupt-driven device and requires a separate interrupt request line that is not occupied by any other device.

If your computer already has one or more serial devices installed (printer, mouse, I/O card), then you need to determine which ports they use. For an internal modem, these settings are made in hardware using two jumpers located on the board. One of them determines the COM port number, the other - the corresponding interrupt.

The modem board should be positioned so that the back plate of the modem with sockets for the telephone cord covers the opening where the plug was removed, and the contacts protruding at the bottom of the board are located directly above the expansion connector. Next, move the modem so that its back plate can be pushed into the place where the plug was.

Carefully, with a little force, you need to press the board so that it falls into place. If the modem is difficult to slide in, you should make sure that it is correctly oriented relative to the socket and that no foreign bodies are preventing it from moving when pressed.

Now you should secure the modem by installing the screw in the old place and install the computer casing. The modem is installed, all that remains is to connect it to the telephone line.

How to install an external modem?

To connect a computer and an external http://www.megabook.ru/pc_2001/Encyclop.asp?Topic=pc_98 modem you will need a standard shielded cable. To connect to a modem, the cable must have a DB25-M plug, and to connect to a computer, it must have a connector corresponding to the connector of the selected port.

You need to connect the cable to the modem and the serial port of the computer. To ensure reliable contact, tighten the screws on both connectors.

Now you need to connect the modem's network transformer to the network. Naturally, it is better to do this through a surge protector. Next, you should connect the cord from the transformer to the modem. The modem switch must be set to the ON position and make sure that the corresponding MRDY indicator on the front panel of the modem lights up.

After installing an external or internal modem, you must connect to the telephone line.

Connection to a telephone line.

Any modem has two telephone connectors: one for connecting to a telephone line, the other for connecting an additional telephone. The delivery set includes a standard telephone cord with RJ-11C connectors. If the telephone line ends with a different connector, an adapter must be installed.

Since dangerous voltage sometimes appears in the telephone line, before connecting the modem you should make sure that the computer is turned off, and then insert the plug of the supplied telephone cord into the modem socket, and its other end into the telephone line socket.

If you wish, you can plug a telephone into the second connector of the modem and use it when the line is not occupied by the modem. However, connecting an additional telephone may degrade call quality.

If there are other parallel devices, you should turn them off; if there is a capacitor installed in any telephone socket, remove it.

5. Review of modern modems.

Genius PCI LT-Winmodem

This is one of the cheapest modems. This was achieved by maximizing the simplification of the element base and performing part of the modem functions by a separate program running WINDOWS OS. The modem is built on a Lucent chipset called "Mars" or 1646. Of the semi-software modems existing today, this is one of the best. This modem works tolerably - but nothing more. The results are average, and on a poor quality line the modem dropped the connection twice.

I would especially like to note the following: if in the test errors appeared only on the “bad” line, then in real life there are a considerable number of very good lines with interference that is quite uneven in time - they may not exist for half an hour, but then there are powerful bursts for a couple of minutes . In such cases, most modems will simply pause data transmission, trying to adjust to the line, but the modem described will simply break the connection after the third retransmission attempt. Moreover, an inexperienced user will remain in the dark about the true reason for the connection loss: it is not displayed in the modem statistics. But statistics can easily mislead such a user, showing a surprisingly large number of retrains from the remote side.

A little about the device's requirements for system resources. The manufacturer claims that the Pentium-90 processor and 16 MB of memory are sufficient. Compared to fully software implementations of modems, this is quite a bit. Such low demands are achieved through the use of a fairly powerful DSP on the device board. It should be noted that other running programs do not reduce the performance of the modem, but the execution speed of many of these programs while the modem is running on a computer with a Celeron-400 processor and 64 MB of memory drops significantly. The simplest MP3 encoder slows down by 7% if a Winmodem is used instead of a hardware modem. The most resources are consumed, oddly enough, at the moment of modem initialization - the mouse cursor stops, pauses occur when playing MP3 audio files. Contrary to popular belief, formatting a floppy disk at the time of data transfer does not slow down the process; obviously, the modem driver has higher priority in the system. But the operation of a scanner connected to the LPT port simultaneously with data transmission by the modem will lead (in about a minute) to a loss of communication, unlike the situation with any hardware modem.

In conclusion, I would like to note that the modem has voice functions, which can only be used if the computer has a sound card, and a very good fax part.

“Cheap and cheerful” is the main motto of the Taiwanese company ACORP Electronic Corporation. When reading the technical manual of their modem, one gets the strong impression that ACORP is, in the truest sense of the word, a “defective” modem. There is no adaptive adjustment to the line, there is no manual adjustment of signal levels, there is no frequency response meter at all, and the collection of statistics is short to the point of disgrace; the relay that turns off the parallel telephone was not present, and to top it all off, the unstable signal recognition was busy.

In a word, ACORP 56K is a model for good telephone lines and unpretentious users, but those who like to “shamanize” with settings are unlikely to be satisfied with this modem due to the lack of settings at all - a kind of “Kodak machine” that has only one button – “descent”.

3Com US Robotics Courier V. Everything

3Com's US Robotics Courier V. Everything modem is one of best models presented on our market. A well-made adaptive adjustment system for a specific line, a high-quality echo suppressor, a carrier and phase jitter compensator, coupled with high sensitivity and satisfactory noise resistance, justify the by no means small cost of this modem.

A distinctive feature of Courier V. Everything is the record high speed of data reception on highly noisy channels. In this regard, it outperforms all other modems, including ZyXEL, and even surpasses the latter several times!

However, ZyXEL is capable of running on much noisier lines than Courier V. Everything, albeit at a low speed. So the question is: which modem should I choose, Courier V. Everything or ZyXEL Pro? – is not simple at all.

ZyXEL Omni 56K Pro

“We are not rich enough to buy cheap things” - this famous German proverb suits ZyXEL products perfectly. Prices are high, but so is the quality. The U-1496 modem works confidently on a line on which it is difficult to hear a human voice. This is understandable - the prefix “U” indicates a professional model recommended by the company itself for banks and railways, that is, for use where requirements for communication reliability are paramount.

"Not wanting to give up a market segment that was coming into its hands, the company drew attention to consumers with a “skinny wallet” and released several models at once - ZyXEL OMNI 56K, ZyXEL OMNI 56K Plus and ZyXEL OMNI Pro - the most advanced of all three, and , which is not surprising, dear.

The suffix "Pro" hints at the modem belonging to the professional series, but it is not one at all - the ZyXEL and ZyCELL protocols are not implemented and there is no possibility of working with a leased line. All the “professionalism” lies in the presence of a multifunctional liquid crystal screen, which displays the current operating mode of the modem, line parameters, including frequency response, and other operational information. The inexplicable appeal of the indicator also extends to those who do not understand anything either in modulation or in the theory of information transmission. True, over time you get used to the indicator and the “disease” of looking at it rather than at the loading site goes away.

It is noteworthy that the modem is built on its own chipset - M4, while most of its competitors use cheaper and lower quality Rockwell. Compared to its closest competitor - US Robotics Courier V. Everything - ZyXEL has twice the noise immunity, better EQ and surprisingly detailed information about the state of the line/connection (which is very useful for “fine-tuning” on bad channels), but is somewhat inferior to it in sensitivity and high-speed quality indicator in terms of noise (more about this in the description of 3Com US Robotics Courier V. Everything).

Not directly related to the modem, but still pleasant additions - an autonomous caller ID and an autonomous answering machine, which work even when the computer is turned off, also tempt you to buy this model - of course, you can (and it will be cheaper) buy a separate answering machine and caller ID, but it is much more convenient to combine all these devices in one unit.

It’s also nice that the company takes seriously the issue of adapting its modems to domestic lines, making changes not only at the software level (like most competitors), but also at the hardware level. The overall impression is spoiled by the fragile-looking case of translucent tones, absolutely unaesthetic in the opinion of the author (the case is designed to hide installation details), but you can put up with this trouble or stick the modem between the computer and the monitor, so that only the indicator is visible.

According to unofficial information, almost all ZyXEL ONMI break down on domestic lines within six months - they are sent under warranty to the manufacturer, where repairmen make a little adjustment to the “patient”, after which, according to them, the modem will work forever. According to the seller, this seriously limits the demand for such modems, because customers do not understand why something that costs over two hundred dollars should obviously break, and is it really difficult for the company to fix this problem at the factory?

Now a little about professional modems. Such equipment is much less common in home use.

The T-288C/NC modem is one of the most advanced high-speed modems currently on the market. Support for international recommendations (from V.34+ to V.21), the ability to flexibly adapt to the parameters of a specific communication channel, the presence of a built-in control channel, password protection of connections, work over both dedicated and switched communication channels, support for fax mode make the modem indispensable element in the integration of local networks.

Serves for secure data transmission Figure 2.4 - External ADSL modem 3. Economic calculation The purpose of the economic calculation of the diploma project is to improve the modem by protecting data transmission, determining the magnitude of the economic effect from using the developed data transmission protection program "Severodonetsk Driving School" qualitative and quantitative assessment.. .

A method of operation that ensures compatibility with low-speed modems. The most commonly used class for relatively inexpensive modems today is class 5. MODEM ERROR CORRECTION PROTOCOLS: V.42 VS MNP2-4 Equipping standard medium-speed modems with hardware-implemented error correction and data compression protocols has become a lately de facto standard in...

Analog standard in the "new digital era" mobile communications. In this regard, D-AMPS retained many of the properties of its analog predecessor. All digital cellular network standards provide interoperability with ISDN and PSTN. Next, we will consider the main features of data transmission in cellular networks. From a data transmission point of view, analogue cellular networks fundamentally need little...