Menu
homeiOS

What is a modem and what do you eat it with? Analog modems

This is what “modem” stands for. In fact, this invention dates back more to the era of the first telephone and telegraph, and not at all to the computer era. The scope of application of devices that can be called modems is extremely wide: in telephone communications, for communication of military command posts.

Some modems have voice functions, that is, they allow voice communication through the built-in speakers, or serve as an answering machine for voice mail. We are only interested in modems used to connect to the World Wide Web.

How the modem works

The main purpose of any modem is to provide physical communication between two objects, one of which transmits data to the other. The modulator transforms the signal before transmitting in accordance with the requirements of the communication channel, and the demodulator at the receiving site performs the reverse operation, providing information to the user in a form convenient for perception.

Modem history

The first company to provide communications between objects using devices operating on the same principle as modern modems was AT&T Dataphone Modems in the United States. It was in the 50s of the last century and this company was part of SAGE - the US air defense system.

AT&T Dataphone Modems' mission was to connect terminals, radar installations and control centers at various military bases to SAGE command centers scattered throughout the United States and Canada.

As for the first modem for a personal computer, it was a device released in 1979. This miracle of technology was called Micromodem II, it was intended for a personal computer under the proud name of Apple II, and the manufacturer was Hayes Microcomputer Products. The modem cost $380 and operated at a speed of 110-300 bps.

In 1981, Hayes released a new modem - Smartmodem. Its operating speed was 300 bits/sec. But what makes this modem interesting is its command system, which is called the Hayes command and, in fact, has become an industry standard.


Types of modems

Modems are different. Well, not green and red, of course (although you can color them). They are divided into the following groups:

By design, modems can be internal or external, by operating principle - hardware and software, by type of network and connection, and by supported data transfer protocols.

So, let's move on to a more detailed description of the various modems.

External modems are connected to the computer via COM, LPT, USB or Ethernet ports. Such modems usually have a separate power supply. Internal modems are additionally installed inside the system unit or laptop. Their location is ISA, PCI, PCI-E, PCMCIA, AMR/CNR slots. Do not confuse internal and built-in modems. The latter are an integral part of the device, but the built-in ones are not.

In hardware modems, all operations to convert the signal and support physical exchange protocols are performed by a computer built into the modem. In addition, the hardware modem has a read-only memory device in which the firmware that controls the modem is stored.


Software modems differ from hardware modems in that all operations for signal encoding, error control and protocol management are implemented in software and are performed by the computer's central processor. The modem contains only input/output analog circuits and converters, as well as an interface controller, such as USB. There are also semi-software modems in which the encoding/decoding functions are divided between the modem and the computer.

Modem device

  1. I/O ports are circuits designed to exchange data between a telephone line and a modem on the one hand, and a modem and a computer on the other. A transformer is often used to interface with an analog telephone line.
  2. Digital Signal Processor (DSP) Typically modulates outgoing signals and demodulates incoming signals at the digital level in accordance with the data protocol used. May also perform other functions.
  3. The controller controls the exchange with the computer.
  4. Memory chips:
    • ROM is a non-volatile memory that stores the modem control firmware - firmware that includes sets of commands and data for controlling the modem, all supported communication protocols and an interface with a computer. Updating the modem firmware is available in most modern models, for which there is a special procedure described in the user manual. To enable flashing, flash memory (EEPROM) is used to store firmware. Flash memory allows you to easily update the modem firmware, correcting developer errors and expanding the capabilities of the device. In some models of external modems, it is also used to record incoming voice and fax messages when the computer is turned off.
    • NVRAM is a non-volatile, electrically reprogrammable memory that stores modem settings (modem profile). The user can change settings, for example, using a set of AT commands.
    • RAM is the modem's RAM, used to buffer received and transmitted data, operate compression algorithms, and so on.

When connecting a laptop or personal computer to the Internet for the first time, poorly versed users usually have the question: “What are modems and why are they needed?” Within the framework of this article, a classification of modems will be given, and an algorithm for their installation and configuration will also be indicated, following which, without much difficulty, a novice computer specialist will be able to select and make such a device work.

What it is?

First, let's figure out what modems are. This is a special component in a computer that is designed to connect it to the The word “modem” was formed by combining two terms. The first of them is a modulator. This is the name in electronics for a special circuit that encodes a signal. And the second is a demodulator. That is, a device that does the opposite of the modulator. One of them encodes and transmits the signal, and the second receives and converts. Thus, until recently, most personal computers were connected to the Internet using telephone wires. Now the situation has changed, and they are slowly being forced out of this market segment by network cards. They have higher speeds, and most motherboards are equipped with them. But there are still wireless modems that do not yet have a real alternative.

When are they needed?

Now let's figure out in what cases they are needed. Essentially, there can be three such moments. The first of them is now slowly becoming a thing of the past. It consists in the fact that a personal computer is connected to the Internet using such a device and a telephone line. Now it has been replaced by network cards. And the cost is lower, and the speed is several times higher. And the reliability of the connection in this case is much better. But for the Client-Bank system such a device is simply mandatory (second case). With its help, the accountant connects to the server of the financial institution. Without leaving the office, he can make a money transfer or check the availability of funds in the account. High speed is not necessary in this case. But connection protection is needed at the proper level. Now many organizations work with banks in exactly this format. The last case when modems are in demand is if a person travels a lot. He needs a wireless internet connection. In this case, the question is: “What are modems, and why are they needed?” - arises by itself. This issue simply cannot be resolved using other technical means.

By method of execution

According to the method of execution, such devices are divided into two types: internal (that is, installed inside the computer system unit) and external (to connect such a device, the expansion slot of a computer, laptop or tablet is used). For the latter, you need to set the hardware toggle switch (if there is one) to the appropriate position. In this case, the following question must arise: “What is modem mode?” They can be digital or analog - determined by the telephone line signal. Only the first of them is available. All cellular networks operate only in this standard. Therefore, such a switch is not provided for wireless devices. One more point needs to be noted. Old motherboards had integrated (that is, soldered) similar devices. But now you can no longer find them on new personal computers.

By connection

The second classification, which has become widespread today, is based on the connection method. In accordance with it, these devices are divided into wired and wireless. In the first case, a special connector is provided into which the telephone wire is installed. In older devices, you could either talk on the phone or surf the Internet. Now there is a special modification of such devices. It allows you to surf the Internet and communicate via telephone at the same time. A special converter that separates the conversation and the transmitted signal into different frequencies. As a result, two data streams are transmitted over the same cable. In the second case, data transmission is provided by electromagnetic radiation without wires.

By type of supported networks

This parameter classifies only wireless devices. In accordance with it, they come in the following types: GSM (they are also sometimes called 2G), 3G and LTE (another name for 4G). All of them are backward compatible with each other. That is, 3G can easily work in the GSM network. Also, users are puzzled by what a USB modem is. It is in this form factor that most of these devices are made. In appearance, it is a flash drive that provides wireless data transfer. It is required to be equipped with a slot for installing a SIM card. It is connected to the rectangular USB connector of a personal computer.

Manufacturers

Conventionally, manufacturers of such equipment can be divided into two classes. The first of them is inexpensive and little-known brands, which include Sierra (their price starts from 180 rubles) and Sprint (the cost of such devices is 120-150 rubles). But the second class is more popular and high-quality devices. They are sold under the Pantech and Huawei brands. The price for them is already 600 rubles or more. But this is true for wireless devices. At the same time, the question often arises about what a 3G modem is. This is a miniature device (very similar to a flash drive in appearance), into which a SIM card of a mobile operator is installed, and with the help of it, data exchange with the Internet is ensured. In turn, among wired devices, the leading positions are occupied by D-Link and A-Corp. It is recommended to pay attention to them when purchasing such a device. The price for some models in this segment starts from 120 rubles. At the same time, their quality is impeccable.

Settings

Let's consider the order. These are all devices of this class, without exception: both wired and wireless. So, the setup order:

  • Connection. For external ones, this means installing them in the expansion slot of a computing device. But when installing such an internal device, you need to remove the side covers of the system unit of the personal computer, install the board in the expansion slot, fix it and put everything back together.
  • Installing drivers. In most cases, it occurs automatically, and user participation in this process is minimized. At the end, a message indicating the successful installation of this software should appear. (If this does not happen, they must be installed manually from a CD or from a website.)
  • Next we connect to the Internet.
  • At the final stage, launch the browser and check the functionality of the connection.

In some cases, you need to adjust the settings of the device (for example, change the analog dialing method to digital). This information is clarified with your provider and telephone operator.

Summary

This article answered the question of what modems are and why they are needed. Possible versions of such devices are given. Their operating modes and other technical characteristics are indicated. A configuration algorithm is also given, following which you can easily and simply configure such a device for connecting to a computer network.

What is a modem and why is it needed?

Its name comes from two words: MODulator and DEmodulator. These two words perfectly reflect the essence of the work performed by the modem. It modulates the signal transmitted to the telephone line with information received from the computer, and vice versa, transmits to the computer what it demodulated from the line. Why is this necessary? - a meticulous reader will immediately ask. But why! As you probably know (and if you don’t know, read more carefully!), all information is represented on the computer in the form of zeros and ones. Zeros and ones, in turn, are encoded by voltage: no voltage - zero, voltage - one. Naturally, computers can only exchange information using zeros and ones. If the distance over which data needs to be transferred is small, such as in a computer - from one microcircuit to another, they are simply connected by wires. What if you want to transfer something to the computer of a friend who is, say, in another area? You will go broke just buying the wire, not to mention having to dig a ditch for this wire, or hang it on poles (otherwise they will get lost!).

Fortunately, the telephone has become widespread in many places - and this is nothing more than a ready-made pair of wires. However, these wires are not as good as we would like, because they are still intended to transmit voice, and not zeros and ones. This is where the modem comes in: converting the zeros and ones into a signal that is more or less similar in its characteristics to a voice, and therefore suitable for transmission over the telephone. At the same time, the modem also performs functions typical of a regular telephone - dialing a number, picking up the handset when they call, etc.

In order to perform all the functions assigned to it, the modem must be very smart, and this is not easy even for people. Essentially, a modem is a small computer. It has a processor, memory, and all sorts of other parts necessary for normal operation. It connects to the telephone line at one end and to the computer at the other. If we have sorted out the telephone line a little, then it’s worth saying a few words about connecting to a computer. Computers - they are also different, big and small, fast and not so fast. In order not to make a different modem for each type of computer, smart people decided to come to an agreement and install the same device in all computers - a communication port (COM port).

If the computer has such a communication port (the standard for it is called RS232C in America, and V24 in Europe), then any standard modem can be connected to it. Naturally, we need to immediately clarify what we mean when we talk about a “standard” modem. Modems, as such, are covered by three types of standards: you already know about one of them - it describes the interaction of the modem with a computer (RS232C/V24), the other determines how data is converted for direct transmission over the phone, and the third describes commands to the modem (by the modem too you can command!).

Let's take a closer look at the standards for data transmission protocols over the telephone network. The speed at which it operates, as well as the very possibility of its operation with any other modem, depends on what protocols your modem supports. Generally speaking, the principle of transmitting information over the telephone is somewhat reminiscent of radio. The modem generates a so-called carrier frequency (“our radio station operates on the frequency...”), and modulates it with information received from the computer according to the rules of a specific protocol. (Very often you will come across the English word CARRIER - don’t be alarmed, it means the carrier frequency). The most common protocols are V21, V22 and V22bis. They determine how signals must be modulated to transmit information over telephone lines at speeds of up to 300, 1200 and 2400 bits per second, respectively. It should be noted here that data over the phone is transmitted sequentially, bit by bit, and in addition to the basic information for which everything is started, service information necessary to “keep the conversation going” is also transmitted. Typically, in addition to the 8 bits of each data byte, 2 bits are added: one at the beginning (start bit) and one at the end (stop bit). Total: a byte will consist of 10 bits, therefore in our case the maximum transfer rates of useful information will be 30, 120 and 240 bytes per second.

Science naturally does not stand still, and recently new protocols have appeared that increase speed and provide additional services. Examples include the MNP and V42/V42bis protocols. Modems that support them can automatically correct errors that occur during transmission and compress the transmitted information, which sometimes increases performance. The V32 and V32bis transmission protocols describe a method for transmitting data at speeds of up to 14,400 bits per second, with the ability to automatically reduce or increase it during transmission depending on the quality of the line. As a rule, modems maintain upward compatibility. That is, modems that support more advanced exchange protocols nevertheless do not stop working with older models. The most important thing is that these old models are standard, which cannot be said about some crafts made by domestic craftsmen. Despite the attractive advertising (“1200!”, “2400!”, “high reliability!”), they are only able to connect to themselves, not to mention the fact that some of them, along with the signal, push a lot of interference into the line than is natural cause the ire of signalmen.

Now let's look at the third type of standards - this is the standard for modem commands. In order to clarify what a “modem command” is, I will make one clarification: for any standard modem, there are two possible modes in which it can be. The first mode is data transfer mode. The modem receives data from the computer, converts it into a signal, and sends it to the telephone line. Similarly, the signal that came from the line is converted into data and transmitted to the computer. The second mode is team mode. In this mode, the modem does not perform any modulation/demodulation and does not send anything to the line. It considers all the data that comes to it from the computer as commands and tries to execute them. This mode is the basic mode for the modem, that is, when you turn on the modem, it starts working in command mode. In this mode, you can send various commands to the modem, force it to pick up or put down the handset, dial a number, turn on or off the speaker, and configure data transfer parameters.

The standard for commands proposed by the American company HAYES (read [hayes]) is currently generally accepted. This is usually written as "HAYES compatible command set", but sometimes it is also called "AT" compatible set - after the first two letters that indicate the command. It is by these first letters that the modem understands that the entered information must be understood as a command to be executed. In your experiments, you should keep in mind that any command to the modem should be no longer than 40 characters, and end with a “carriage return” code (ENTER key), although there are a few exceptions, which we will look at later. If the modem recognizes the command, it tries to execute it and report the result. The simplest command consists of just two letters “AT”; it forces the modem to “give voice” and answer that everything is fine with it. This response looks like an "OK" message from the modem. As a rule, modems are automatically configured for the speed and other parameters of the COM port to which they are connected, so that any working standard modem that is in command mode should respond to this simple command. If the modem cannot understand the nonsense that has fallen on it, it swears “ERROR”, which means an error.

Allowing you to exchange data over regular telephone lines, television cable or channels of mobile operators. Without this device, access to the Internet cannot take place.

In appearance, size and placement, modems can be:
internal (in the form of an expansion card);
external (desktop), made in a separate case;
card (used in laptop computers via USB socket)

What types of modems are there?

From a functional point of view, it is a device with a fairly powerful processor, permanent and random access memory and an analog part consisting of a dialer, an amplifier, an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). Almost all modern modems perform processing information in digital form.

Model types:

Conventional internal modems are connected to the PCI slot on the computer's motherboard, and external modems are connected to the serial port, parallel port or USB port of the computer. The transfer speed is limited to 56 Kbps. Today it is used very rarely.

ISDN modems are in many ways similar to regular modems. They can also be internal or external and connect to a regular telephone line. Data transfer speed is 128 Kbps.

Cable modems-Provide broadband connection through cable television infrastructure. They use a special frequency band that does not interfere with television broadcasts.

Network cards are used to connect external cable modems to computers, and internal modems are inserted into expansion slots on the motherboard. They support asymmetric technology; the maximum possible data reception speed can reach 40 Mbit/s and the data transfer speed is about 10 Mbit/s.

xDSL modems - xDSL modems provide broadband connection through existing telephone lines. They can be internal and external. ADSL (Asymmetrik Digital Subscriber Line) is widely used. Data is transmitted asymmetrically, that is, at different speeds to the user and to the provider (1-20 Mbit/ With).

It is difficult to imagine a personal computer without Internet access. The Internet is an environment where a large amount of information is accumulated, full access to which is only available when using a modem. A modem is a device that is a bridge between the computer and this information. A modem is a device for transmitting data over regular telephone lines, used to connect two computers. The word "modem" itself is an abbreviation for "modulator-demodulator". All telephone lines, as a rule, work with an analog signal, and a computer works with a digital signal. Therefore, the main function of the modem can be considered to be the conversion of a digital computer signal into an analog telephone line and vice versa.

Connecting a modem

Modems can be connected to a computer via a serial RS-232 interface, a parallel interface and a USB interface. Connection to the telephone line is made via an RJ11 cable. In practice, the connection is most often made through the serial interface port COM2, since COM1 is most often occupied by other devices, for example, a mouse.

Port configuration:

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).

Having connected the modem to the COM port and assigned an IRQ, you must check other devices to see if they have the same serial ports and interrupts.

Configuring the modem for a particular port and interrupt (IRQ) is usually done using jumpers, switches or software. General information

Digital data entering the modem from a computer is converted in it by modulation (amplitude, frequency, phase) in accordance with the selected protocol standard and sent to the telephone line. The provider's receiver modem, which understands this protocol, performs reverse conversion (demodulation) and sends the recovered digital data to its computer. Thus, to ensure stable communication, it is necessary that your modem supports a common protocol, is connected directly to the computer, and the communication line, according to its parameters, can transmit modulated signals.

Physically, in modems, all this is implemented quite simply; the signal is a carrier (sinusoid of defined frequency), discretely modeled in phase and amplitude, i.e., fragments of this sinusoid with different amplitudes (possibly several fixed values) and a phase shift relative to the previous fragment (Fig. 1).

Modulation Standards

Modulation is used to transmit data using modems. In order for the transmitting and receiving devices to “understand” each other, they must use the same modulation method. As a rule, different data rates use different modulation methods, but sometimes data transmission at the same speed can also be carried out using different modulation methods.

When transmitting data, the sending modem converts the digital data into an analog signal, which is transmitted over the telephone line. The receiving modem performs the reverse conversion - from analogue to digital form

Types of modulation

Frequency modulation. When zeros are transmitted by a signal of one frequency, and ones by another, we are dealing with frequency modulation (FM). Frequency modulation is the simplest to implement and works very reliably, but it has a natural limit due to the fact that the telephone channel bandwidth is very small. Theoretically, it is only 4 kHz, but due to the high nonlinear distortion at the beginning and end of the passband, a range of 300 Hz to 3400 Hz is actually available. This means that even if the entire signal period is given to one bit, the transmission speed cannot exceed half the bandwidth. Therefore, if the max mod used only frequency modulation, then they would still operate at a speed of 1200-1500 bits per second. But at low speeds, frequency modulation works very reliably. This type of modulation was established by the V.21 standard and was used in early modems, although it is not forgotten today. It is in this mode that modern modems begin their work. When communicating, the modem still “does not know” what properties its partner has, and the two modems need some kind of negotiation process to agree on the parameters of further work. Therefore, at the first moment, modems exchange low-speed, frequency-modulated messages.

Amplitude modulation. If zeros are transmitted by signals of one volume, and ones by another, then this is amplitude modulation (AM). Technically, it is even easier to create amplitude modulation than frequency modulation, but transmission reliability is low, so amplitude modulation is used very limitedly. Modern modems combine it with phase modulation in order to transmit more information (more than one bit of data) in one signal period.

Phase and phase difference modulation The phase modulation (PM) method is based on the fact that if two harmonic (sinusoidal) signals have a phase shift, then it can be detected, measured and used for data transmission (Fig. 2).

Rice. 2. Phase shift of two signals by 90°

Although there are devices in telephone networks that can distort the phase of the signal, nevertheless, this modulation method allows you to more confidently isolate useful data from the background noise than amplitude and frequency modulation. Of course, this conclusion applies only to the audio frequency range that is typical for telephone networks.

Using phase modulation, several bits of information can be encoded in one signal period. For example, an offset of 0° can be assigned the two-bit value 00, an offset of 90° can be assigned the value 01, an offset of 180° can be assigned the value 10, and an offset of 270° can be assigned the value 11.

Please note that a phase shift for one signal does not make sense - you definitely need a pair of signals to have something to compare. In modems, the phase shift of the next signal relative to the previous one is measured. Thus, what plays a role is not what the phase of a given signal is, but what transition occurred in the phase when receiving the next signal. If the previous signal had a phase of 0°, and the subsequent signal had a phase of 90°, then this is the same as a transition from 180° to 270° and, accordingly, the same as a transition from 270° to 0°. Therefore, phase modulation is often called phase-difference modulation. This emphasizes that it is not the phase that is measured, but the phase difference between two successive signals and from it it is determined what data was transmitted.

Main characteristics of the modem

Any computer device has its own characteristics. The main characteristics of the modem (Fig. 3) include:

The maximum data transfer rate, measured in Kbit/sec or baud;

Supported operating protocols;

Possibility of operating the modem as a fax;

Data transfer protocols

The modem's transmission speed also depends on the protocols it can work with. A data transfer protocol is a specific standard by which modems communicate with each other. Each protocol performs a specific action. For example, one is responsible for error correction during data exchange, the other is responsible for the data compression method (allows data to be compressed when transmitting, which reduces transmission time), etc. All protocols can be divided into four groups:

  • interaction and modulation protocols;
  • data compression protocols;
  • error correction protocols.

Interaction protocols describe the order in which modems interact with each other. They indicate what the calling modem should report about itself, and what the called modem should respond. According to the interaction protocol, both modems enter into a dialogue and exchange the parameters necessary to create a reliable and maximum-performing connection.