"Nothing can be bad or good in itself"
Frank Herbert "Dune"

What characteristics of the modem should you pay attention to first?

When buying a 21-inch monitor, we are absolutely sure that it will work at twenty-one inches, and not just seventeen or even worse, fourteen! With modems, the situation is completely different: on our telephone lines, almost none of them “accelerate” to 56 kilobits per second - the most popular speed today, proudly displayed on the boxes. The actual performance of modems varies greatly from one product to another, sometimes differing by tens of times. But no matter how high the speed, the modem is useless if it constantly “drops” the handset or - worse - cannot establish a connection at all.
In the example with monitors, everything is clear - all the main characteristics, such as diagonal, resolution, frequency, graininess, are intuitively clear to every buyer and do not require additional explanation. But what should you first pay attention to when buying a modem? The question does not have a clear answer - it all depends on the type and quality of the telephone line that you are going to use.
For local communication channels, the following are relevant, first of all:

Modem resistance to signal attenuation, resistance to constant noise and impulse noise (crackling).

For intercity, in addition to this, -

maximum permissible frequency divergence, resistance to jitter (sudden phase jumps), degree of tolerance of frequency response nonlinearity ("blockages" and "rises" at some frequencies).

“Cheap and cheerful” is the main motto of the Taiwanese company ACORP Electronic Corporation. When reading the technical manual of their modem (purchased on the occasion of a sale from one entrepreneur for only one hundred and fifty rubles), the author got the strong impression that ACORP was, in the natural sense of the word, a “castrated” modem. Remember, like Winnie the Pooh, “these are the wrong bees, and they bring the wrong honey.”
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 did not stay overnight, and to top it all off, the unstable signal recognition is 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”.
In my own way personal experience The author can say that on local lines of average quality, the modem quite stably maintains a connection of 14.400\19.200, working around the clock. But on a long-distance route... even with well-tuned communication compression equipment at a speed of 19.200, the connection is broken no less than after 60-80 minutes, and on average every 15-20 minutes. Modems and ZyXEL OMNI 56K work flawlessly at 28.800 under the same conditions, and ZyXEL occasionally connects at higher speeds.

Possibilities

ACORP 56K
	No
	No
	No
Line frequency response meter	No
Collecting connection statistics	brief
LCD screen	No
	There is
	No
Recognizes busy signal	No
Built-in Caller ID	No
Built-in Autoresponder	No
Chipset	Rockwell

ACORP 56K
Communication protocol	Yes
Reception/transmission speed	Yes
	No
Signal to noise ratio	yes, last + maximum
Level input signal	Yes
Output level	No
Echo return delay	No
Near echo level	No
Far Echo Level	No
Phase jitter	No
Frequency offset	No
Conditional line quality	Yes
Frequency response measurement	No
	No
	No

3Com US Robotics Courier V. Everything

3Com's US Robotics Courier V. Everything modem is one of best models presented on the Russian market. A well-made adaptive adjustment system for a specific line, a high-quality echo suppressor, a carrier shift 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. According to the author, the best way out of the situation is to buy two modems. And what? After all, not a single woman wears the same dress both in summer and winter - why are we, modem workers, worse?!

Possibilities

3Com US Robotics Courier V. Everything
Adjusting the output level	yes, auto
Adjusting the input level	yes, auto
Mechanism of adaptive adjustment to the line	There is
Line frequency response meter	There is
Collecting connection statistics	yes, detailed
LCD screen	No
Connection speed limit	There is
Relay for disconnecting parallel telephone	There is
Recognizes busy signal	Yes
Built-in Caller ID	There is
Built-in Autoresponder	No
Chipset	Rockwell

Line and connection characteristics

3Com US Robotics Courier V. Everything
Communication protocol	Yes
Reception/transmission speed	Yes
Receiver/transmitter carrier frequency	No
Signal to noise ratio	yes, last + maximum
Input level	Yes
Output level	No
Echo return delay	Yes
Near echo level	No
Far Echo Level	No
Phase jitter	No
Frequency offset	No
Conditional line quality	Yes
Frequency response measurement	Yes
Number of blocks transferred	Yes
Number of blocks transmitted with errors	No

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 author still cannot forget his impression of the U1496 modem, which confidently works on a line on which it is difficult to hear the 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.
“Home” users, not burdened with high salaries, looking at such a modem could only lick their lips and... go to bow to their competitors. Not wanting to give up a market segment that was heading 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 on which current mode modem operation, line parameters, including frequency response, and other operational information. The inexplicable attractiveness 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, more high-quality equalizer 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 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 (ay! iMac!), absolutely unaesthetic in the opinion of the author (after all, 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 one indicator was visible.
According to unofficial information received from the seller of the company " Information Technology", 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 is serious limits the demand for such modems, because customers do not understand why something that costs over two hundred dollars should obviously break down, and is it really difficult for a company to fix this problem at the factory?

Possibilities

ZEXEL 56K OMNI Pro
Adjusting the output level	yes, auto\manual
Adjusting the input level	yes, auto\manual
Mechanism of adaptive adjustment to the line	There is
Line frequency response meter	There is
Collecting connection statistics	yes, detailed
LCD screen	yes, multifunctional
Connection speed limit	There is
Relay for disconnecting parallel telephone	There is
Recognizes busy signal	Yes
Built-in Caller ID	There is
Built-in Autoresponder	There is
Chipset	M4

Line and connection characteristics

ZEXEL 56K OMNI Pro
Communication protocol	Yes
Reception/transmission speed	Yes
Receiver/transmitter carrier frequency	yes, instant + maximum
Signal to noise ratio	Yes
Input level	Yes
Output level	Yes
Echo return delay	Yes
Near echo level	Yes
Far Echo Level	Yes
Phase jitter	Yes
Frequency offset	Yes
Conditional line quality	Yes
Frequency response measurement	Yes
Number of blocks transferred	Yes
Number of blocks transmitted with errors	Yes

So which modem should you choose?

Remember the joke about the guy who, having cut his hair, looked thoughtfully in the mirror and ordered another haircut? You should be prepared for the fact that if you suddenly don’t like the modem you bought, not every seller will agree to exchange it for another. And even if he agrees, there are no guarantees that the other one won’t turn out to be even worse? What, play the music back?
An intelligent choice of a modem requires accurate knowledge of the nature of the interference and the quality of your telephone line, and the only household device available for measuring it is the modem itself, and not just any one, but only an expensive model. Cheap ACORP cannot even measure frequency response, let alone measure noise, echo, carrier shift, phase jitter, etc.
It’s great if you can borrow the ZyXEL OMNI Pro for a short time from a friend or rent it - this will allow you to assess the situation and help you purchase exactly what you need. If this is not possible, it is better not to take risks and opt for ZyXEL OMNI Pro. (Excluding cases with very noisy channels, on which V Everything works fastest)

How to choose the right initialization string...

Ideally, each modem should be able to independently adapt to the telephone line, without requiring the user to “shamanically dance with a tambourine” around him. But life is far from ideal - sometimes the modem behaves like a wild animal: it doesn’t want to establish a connection, then it suddenly breaks the connection. Perhaps the best way out of the situation is to buy another modem that is more suitable for such conditions, but often the problem can be solved with proper configuration.
The flexibility of setting up different modems is not the same and varies greatly from model to model. Many cheap modems cannot be adjusted at all, while professional modems (and similar consumer models) allow manual adjustment of most of their components and - most importantly - are able to measure the main characteristics of the line and the amount of distortion. Need I say how much easier this makes the tincture? After all, if you act blindly, you won’t get far...
OS Windows family support manual configuration of the modem, allowing you to pass all the required parameters through the initialization string. The initialization string consists of modem AT commands transmitted to it before each call.
Teams are divided into basic , common to all modems, and extended – specific to each model. The basic set of AT commands is very ascetic and allows you to manipulate only basic settings, and operations such as changing the output signal level or sensitivity correction are not standardized and are implemented by each manufacturer in their own way (if implemented at all). As a result, all modems require an individual approach and it is simply impossible to give general configuration recommendations that apply to all models.
Below is the configuration of three popular models: ACORP 56K, 3Com US Robotics Courier V. Everything and ZyXEL OMNI 56K Pro. The owner of other types of modem will have to refer to their documentation to find out which commands control which options and whether it is provided at all.

Solutions common to all modems

make sure that there is a dial tone in the telephone line: on some telephone exchanges it appears only a few seconds after picking up the handset; if so, increase the contents of the register S6, storing the beep waiting time in seconds (usually 2 by default);
perhaps the modem does not have time to wait for a response from the remote modem - check the contents of the register S7, which stores the carrier waiting time after dialing a number in seconds - by default it should be equal to 30 seconds (this is more than enough), but there are also deviations to the lower side;
many modems cannot establish a connection due to interference on the line, which they mistakenly take for the carrier frequency of the remote modem, with all the ensuing consequences. Try increasing the register contents S9, which stores the time in tenths of a second during which the carrier frequency should be received. By default it is 6 (i.e. 0.6 seconds), in most cases this is completely insufficient. S9=30 gives a good result. Please note: along with increasing the contents of S9, it is necessary to increase the value of register S10, which stores the time in tenths of a second during which the carrier frequency may be absent without breaking the connection. If S9 = S10, then any, even instantaneous, loss of the carrier leads to a breakdown in communication, and if S9 > S10, then communication cannot be established at all.

The modem often hangs up:

try increasing the register contents S10, which stores the time in tenths of a second during which the carrier frequency may be absent, but the connection does not break. By default it is 7 (i.e. 0.7 seconds), which is clearly not enough when operating the modem on low-quality channels. A good result gives values ​​of 80 tenths of a second or more. Please note that on some modems, the maximum carrier dead time is determined by the difference in the values ​​of registers S10 and S9, and not by the contents of S10 itself. If you write the value 255 to the S10 register, then no matter how long the carrier is absent, the connection will not be broken. This can cause the modem to freeze, which can only be overcome by turning off/on the power.

The modem does not detect a busy signal:

Many cheap modems, including ACORP, are susceptible to this “childish” disease. Fortunately, this disease is not fatal - firstly, you can manually break the connection by hearing the hang-up tone in the speaker, and secondly, even if you don’t do this, the connection will be broken on its own after the time contained in the register has expired S7 and measured in seconds. However, you should not get carried away with its excessive reduction - even if the line is free, but within the specified time interval your modem will not have time to connect to the remote modem - the connection will be mercilessly severed. According to the author, the optimal value of the S7 register is 15-20 seconds.

Modem-Specific Solutions

The modem does not establish a connection:

Perhaps the modem dials the number too quickly, and the telephone exchange simply does not have time to “digest” it. Try reducing your typing speed. In modems ZyXEL To do this, you need to adjust the register value S39. The value “0” corresponds to 10 pulses per second, 1 – 16 and 2 – 20. For example, “S39=1” sets the dial speed to 16 pulses per second;
modems Courier V. Everything often cannot connect to each other due to the tones not being sent for long enough. The solution to the problem is to increase the contents of the register S28, which stores the time the tones are sent in tenths of a second (default 8). For example: "S28=20" sets the sending interval to two seconds.

The modem often hangs up:

If the line quality does not remain constant throughout the entire communication session (as is often the case with poor telephone lines under which a tram passes every N minutes), then the channel characteristics measured at the time the connection is established may turn out to be overly optimistic, and the connection speed may be too high . Try setting the connection speed manually, reducing it until the required connection stability is achieved. It should be noted that the V.34 protocol, used by most modems by default, imposes quite stringent requirements on line quality and sometimes it makes sense to switch to the V.32bis protocol, which, although it limits the maximum speed at 14.400, is much better able to withstand all the hardships of telephone life. Please note that limiting the connection speed in Windows settings has the disadvantage that it does not allow you to select the required protocol, and in most cases the connection is established via V.32Bis. Therefore, it makes sense to include the speed selection command in the modem initialization line:
ACORP: for forcing the speed and connection protocol in modems " ACORP"serves the command "+ MS=", the short form of which looks like this: "+MS=mode, automode, minimum speed, maximum speed." A complete list of modes and their corresponding speeds is contained in the documentation included with the modem. Example of use: "+MS 10,1,9600 ,14400" – causes the modem to connect using the V.32bis protocol at a speed from 9.600 to 14.400;
3Com US Robotics Courier V. Everything: modems of this series do not allow you to force the connection protocol, so you can use Windows settings to limit the speed;
ZyXEL OMNI: protocol selection is carried out using the command " &Nn", where n is the number of the required mode (the list of modes is given in the documentation supplied with the modem). For the V.34 and V.90 protocols, it is possible to force any required connection speed, and for all others, several fixed ranges are provided to choose from. For example, " &N17" forces the modem to connect using the V.32bis protocol at the highest possible speed of the following: 14.400 / 12.000 / 9.600 / 7.200 / 4.800.
modems ZyXEL allow you to control your reaction to changes in line quality. This is "managed" by the team "* Qn", where n can take one of the following values: "0" - do not react in any way to changes in quality and continue to exchange data at the same speed; "1" - renegotiate connection parameters with the remote modem if quality deteriorates; "2+" – adaptive setting and “3” – termination of connection when quality deteriorates. Courier V. Everything do not allow you to control your reaction to changes in communication quality and independently adapt to the conditions of the “environment”, working at the highest possible speed;
Often the cause of frequent disconnections is an incorrectly selected output signal level. In general, modems can (and should) automatically adjust the “volume” of their conversation when establishing a connection, but if the channel attenuation does not remain constant, but changes during the communication session, it makes sense to set the level manually, finding a compromise between the lowest and highest attenuation. You should not set the transmission “volume” to maximum - this will increase distortion, especially echo, and the modem, “hoarse” from its own scream, will be forced to reduce the transmission speed. In modems ZyXEL The output signal level is adjusted by the command "* Pn", where n is the conditional volume, taking values ​​from 0 to 15, with a higher value corresponding to a higher volume. Modems Courier V. Everything And ACORP do not allow you to adjust the output signal level;
modems ZyXEL They also allow you to adjust the sensitivity of the receiver, which is especially useful on channels with strong or inconsistent attenuation. Sensitivity is register adjustable S52: “0” corresponds to a receiver sensitivity of –43 dBm, “8” – –33 dBm and “16” – –26 dBm.

Examples of initialization strings

Each initialization line must begin with the prefix "AT", followed by one or more modem commands, separated for readability by a space character or pressed closely together. For example: "AT S52=0 *P2".
AT is often followed by the command " &F", which restores all factory settings of the modem. The fact is that many communication programs, including the operating system, configure the modem at their discretion and save the settings in its non-volatile memory. Of course, if the result of such a setting is unsatisfactory, then it makes sense to return the factory default settings, otherwise it is better to leave everything as is, especially since the initialization line commands overlap the results of all previously entered commands and allow you to change any setting to your liking.
The following few example initialization strings demonstrate how modems can be used effectively on poor channels:

Examples of initialization strings

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

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

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

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

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

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

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

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

Modem selection.

Everything you need to know about how a modem works: a modem is a device that allows you to connect computers together via telephone network. The features available to you through such connection are determined solely by software, which you will use, and the quality of the modem itself determines the connection speed. All the modem specs you should know:
All other characteristics of modems are of interest only to specialists.
External modems, as a rule, work better than internal ones, they are more visual - the lights on the panel blink, and make a stronger impression on your friends (the larger the modem and the more light bulbs on it, the stronger the impression), but internal ones take up less space in your room (since they are located entirely inside the computer).
Having bought a modem and connected it to a computer (or installed it in a computer), you can, for a trial and out of curiosity, call Data Force IP (tel. 755-9363) and get the necessary data for a test connection to the Internet.

External modems

To connect an external modem to a computer, it is necessary (and sufficient) that it have a free serial port (COM port) and a cable to connect the modem to this port. Typically, a computer has two serial ports; a mouse will be connected to one of them.
Serial port connectors come in 9-pin and 25-pin types. Typically, a computer has one 9-pin connector (the mouse is connected to it) and one 2 5-pin connector (if you don’t have a modem, this connector usually remains free), both are of the “male” type, that is, with pins. The modem usually has a 25-pin female connector, that is, with holes. In this case, you need a female-to-male cable that has 25-pin connectors on both sides. If your computer only has a 9-pin connector available, then you need a cable that has a 9-pin female and a 25-pin male. You can almost certainly purchase the cable in the same place where you purchased the modem. If you purchase a high-speed modem, then for you they become

• important characteristics
• serial port of your computer. You need to have a high speed serial port (like the magic words - UART16550A). Usually on an external modem there is a row of lights, under each of which there are two letters labeled. Here are the most common designations:
• HS - high speed
• AA - willingness to answer a call
• CD - carrier frequency detected
• OH - initialization of dialing
• RD - data is being received
• SD - data is being sent
• TR - ready for work
• MR modem enabled

RS - request to send data

CS - readiness to send data.
To work with serial ports, two IRQs are usually allocated - IRQ3 and IRQ4. These IRQs are distributed among the first four serial ports as follows:

• COM1 - IRQ4
• COM2 - IRQ3
• COM3 - IRQ4
• COM4 - IRQ3

The COM1 port is usually used to connect a mouse. Thus, if your modem uses the COM3 port, then the same IRQ is used to work with it as for the mouse. In practice, this means that if you, working in a Windows environment, start using a modem (you launch a program for working with the modem), the mouse temporarily stops working - until you finish using the modem (close the program for working with the modem ). If you want to use both the modem and the mouse at the same time, you need to have them on different IRQs. To do this, either change the serial port number of the internal modem (to COM4 instead of COM3), or move the mouse to another port (from COM1 to COM2).

Modem speeds

By speed, the main modem options (in order of increasing speed): 2400 baud, 9600, 14400, 19200, 21600, 28800 and 33600.
Higher speeds on Russian telephone lines are difficult to achieve. Any modem is capable of operating not only at its maximum speed, but also at all lower speeds. Full range of speeds: 300, 1200, 2400, 4800, 7200, 9600, 12000, 14400, 16800, 19200, 21600, 24000, 26400, 28800, 31200, 33600. That is, the modem is 336 00 baud is capable of operating at all speeds specified here .
A modem speed of 2400 baud means that 300 bytes are sent per second (byte = 8 bits, one character), per minute - 18 kilobytes, per hour - 1 megabyte. A speed of 28800 baud means that 3600 bytes are sent per second (216 kilobytes per minute, 13 megabytes per hour).
In reality, the efficiency of a modem is usually lower than the transfer speed - due to the poor quality of the telephone line, it is necessary to repeat sending pieces of information two or three (or even more) times.

Modem protocols

To combat the poor quality of telephone lines, various protocols for correcting and compressing data during transmission have been invented.

Basic protocols:

• Bell 209A 9600
• V.29 9600
• V.32 9600
• V.32bis 14400
• V.33 14400 V.32terbo 19200
• V.34 28800 and higher
• V.FC simplified version
• V.34 HST 16800 and above
• ZyX 16800 and higher
• other.

Usually modems “know” at least several protocols, and the modem speed indicated on its box or in the price list is the maximum speed at which it can operate. In order for the modems at both ends of the telephone line to “agree” on the acceptable speed and type of protocol (by discussing this issue in the first few seconds of the connection), it is necessary that both of them be able to work with this protocol at a given speed.
If the speed of the established connection does not suit you (all programs that work with modems always inform the user of this information), try calling back again - the connection through the telephone network occurs each time through different wires, and it is likely that another connection will be more good quality.
On Russian telephone lines, the best results are obtained by the HST and ZYX protocols. Please note: modems that only have the V.34 protocol connect to modems that also have only the V.34 protocol at speeds no higher than 14400.

Fax modems

A fax modem is a modem that is capable of receiving (and storing on a hard drive) faxes and sending faxes prepared specifically for it on a computer.
Received faxes can be printed on a printer using a special program for working with a fax modem.
There is nothing complicated in preparing a fax to be sent; on the contrary, you do not need to print beautiful font on the printer is what you are going to put into the fax machine - in many test editors it is possible to turn the document you are working with into a fax (or even immediately send it via fax modem).
But if you work on the Internet, your modem doesn't have to be a fax machine.