Millions of people around the world use mobile phones because mobile phones have made it much easier to communicate with people around the world.

Mobile phones present a range of functions these days, and there are more and more of them every day. Depending on your mobile phone model, you can do the following:

Save important information
Take notes or make a list of tasks
Record important appointments and turn on an alarm for reminders
use a calculator for calculations
send or receive mail
search for information (news, statements, anecdotes and much more) on the Internet
play games
watch TV
send messages
use other devices such as MP3 players, PDAs and navigation system GPS.

But haven't you ever wondered how a mobile phone works? And what makes it different from a simple landline phone? What do all these terms PCS, GSM, CDMA and TDMA mean? This article will focus on the new capabilities of mobile phones.

To begin with, a mobile phone is, in fact, a radio - a more advanced form, but a radio nonetheless. The telephone itself was created by Alexander Graham Bell in 1876, and wireless communication a little later by Nikolai Tesla in the 1880s (for the first time about wireless communication Italian Guglielmo Marconi began to speak in 1894). It was destined for these two great technologies to come together.

In ancient times, when there were no mobile phones yet, people installed radio phones in their cars to communicate. This radiotelephone system operated from a single main antenna mounted on a tower in the city border and supported about 25 channels. To connect to the main antenna, the phone had to have a powerful transmitter - with a radius of about 70 km.

But not many could use such radio phones due to the limited number of channels.

The genius of the mobile system lies in the division of the city into several elements ("honeycomb"). This promotes frequency reuse throughout the city, so millions of people can use their mobile phones at the same time. "Honeycomb" was not chosen by chance, since it is precisely honeycombs (in the form of a hexagon) that can best cover the area.

In order to better understand the operation of a mobile phone, it is necessary to compare CB radio (ie conventional radio) and a radiotelephone ..

Full duplex handheld versus half duplex - a radiotelephone, like a simple radio, is a half duplex device. This means that two people are using the same frequency, so they can only speak in turn. A mobile phone is a full duplex device, which means that a person uses two frequencies: one frequency is for hearing the person on the other side, the other for speaking. Therefore, you can talk on mobile phones at the same time.

Channels - the radiotelephone uses only one channel, in the radio there are about 40 channels. A simple mobile phone can have 1,664 channels or more.

In half-duplex devices, both radio transmitters use the same frequency, so only one person can speak. In full duplex devices, the 2 transmitters use different frequencies, so people can talk at the same time. Mobile phones are considered full duplex devices.

In a typical US mobile system, a mobile phone user uses about 800 frequencies to talk around the city. A mobile phone divides the city into several hundred. Each cell is of a certain size and covers an area of ​​26 km2. Honeycombs are like hexagons enclosed in a lattice.

Since mobile phones and stations use low-power transmitters, non-adjacent cells can use the same frequencies. Two cells can use the same frequencies. The cellular network is powerful high-speed computers, base stations (multi-frequency VHF transceivers) distributed throughout the entire working area of ​​the cellular network, mobile phones and other high-tech equipment. We'll talk about base stations later, but now let's look at the "cells" that make up the cellular system.

One cell in analog cellular system uses 1/7 of the available two-way communication channels. This means that each cell (out of 7 cells in the grid) uses 1/7 of the available channels, which have their own set of frequencies and, due to this, do not overlap:

The mobile phone user usually receives 832 radio frequencies for city calls.
Each mobile phone uses 2 frequencies per call - the so-called. two-way channel - therefore, there are 395 communication channels for each mobile phone user (the remaining 42 frequencies are used by the main channel - we will talk about it later).

Thus, each cell has up to 56 available communication channels. This means that 56 people will be able to talk on their mobile phones at the same time. The first mobile technology 1G is considered an analogue of the cellular network. Since the introduction of digital transmission of information (2G), the number of channels has increased significantly.

Mobile phones have built-in low-power transmitters, so they operate at 2 signal levels: 0.6 watts and 3 watts (for comparison, we present a simple radio that operates at 4 watts). Base stations also use low-power transmitters, but they have their advantages:

The transmission of the signal of the base station and the mobile phone within each cell does not allow you to go far from the cell. In this way, both cells can reuse the same 56 frequencies. The same frequencies can be used throughout the city.
The charge consumption of a mobile phone, which usually runs on battery power, is not significantly high. Low-power transmitters mean a small battery, which makes mobile phones more compact.

The cellular network needs a number of base stations, regardless of the size of the city. A small town should have several hundred towers. All mobile phone users in any city are managed by one main office, which is called the Mobile Phone Switching Center. This center monitors all telephone calls and base stations in the area.

Mobile phone codes

The Electronic Serial Number of the Device (ESN) is a unique 32-bit number programmed into the mobile phone by the manufacturer.
Mobile Identification Number (MIN) is a 10-digit code derived from a mobile phone number.
The System Identification Code (SID) is a unique 5-digit code assigned to each FCC company. The last two codes, MIN and SID, are programmed into your mobile phone when you buy a card and turn on the phone.

Each mobile phone has its own code. Codes are needed to recognize phones, mobile phone owners and mobile operators. For example, you have a mobile phone, you turn it on and try to call. Here's what happens at this time:

When you just turn on the phone, it looks for an identification code on the main control channel. A channel is a special frequency that mobile phones and base stations use to transmit signals. If the phone cannot find the control channel, then it is out of reach and the message "no network" is displayed on the screen.
When the phone receives an identification code, it verifies it against its own code. If matched, the mobile phone is allowed to connect to the network.
Together with the code, the phone requests access to the network and the Mobile Phone Switching Center records the position of the phone in the database, so the Switching Center knows which phone you are using when it wants to send you a service message.
The switching center receives calls and can figure out your number. At any time, he can view your phone number in his database.
The switching center communicates with your mobile phone to tell you which frequency to use, and after the mobile phone communicates with the antenna, the phone gains access to the network.

The cell phone and base station maintain constant radio contact. The cell phone periodically switches from one base station to another with a stronger signal. If the cell phone leaves the base station field while moving, then it establishes communication with another, nearest base station, even during a conversation. The two base stations "communicate" through the Switching Center, which sends a signal to your mobile phone to change the frequency.

There are times when, while moving, the signal moves from one cell to another belonging to another mobile operator. In this case, the signal does not disappear, but is transmitted to another mobile operator.

Most modern cell phones can operate in several standards, which makes it possible to use roaming services on different cellular networks. The wiring center you now use the cell phone connects to your wiring center and asks for a code confirmation. Your system transfers all data about your phone to another system and the Switching Center connects you to the cells of the new mobile operator. And the most amazing thing is that all this is done within a few seconds.

The most unpleasant thing about all this is that you can pay a pretty big sum for roaming calls. On most phones, when you just cross the border, the roaming service is displayed. Otherwise, you'd better check the mobile coverage map so that you do not have to pay "inflated" tariffs later. Therefore, check immediately the cost of this service.

Please note that the phone must work in multiple bands if you want to use the roaming service, because different countries use different bands.

In 1983, the first analog mobile standard, AMPS (Advanced Mobile Telephone Service), was developed. This analog mobile communication standard operates in the frequency range from 825 to 890 MHz. In order to maintain competition and keep prices in the marketplace, the US federal government required that there be at least two companies in the market, engaged in the same activity. One such company in the United States was the Local Telephone Company (LEC).

Each company had its own 832 frequencies: 790 for calls and 42 for data. To create one channel, two frequencies were used at once. The frequency range for an analog channel was typically 30 kHz. The range of transmission and reception of the voice channel is divided by 45 MHz, so that one channel does not overlap with another.

A version of the AMPS standard called NAMPS (Narrowband Advanced Communication System) uses new digital technologies to enable the system to triple its capabilities. But even despite the fact that it uses new digital technologies, this version remains just an analogue. Analog standards AMPS and NAMPS only operate on 800 MHz and cannot yet offer a wide variety of functions, such as Internet connection and mail handling.

Digital mobile phones are second generation (2G) mobile technologies. They use the same radio technology as analog phones, albeit in a slightly different way. Analog systems do not make full use of the signal between the phone and the mobile network — analog signals cannot be suppressed or manipulated as easily as digital signals can. This is one of the reasons why many cable companies are switching to digital so they can use more channels in a given range. It's amazing how efficient a digital system can be.

Many digital mobile systems use frequency modulation (FSK) to transmit and receive data through the analog AMPS portal. Frequency modulation uses 2 frequencies, one for logic one and one for logic zero, choosing between the two, when transmitting digital information between the tower and the mobile phone. In order to convert analog information into digital and vice versa, modulation and a coding scheme are needed. This suggests that digital mobile phones need to be able to process data quickly.

In terms of "complexity per cubic inch," mobile phones are among the most sophisticated devices available today. Digital mobile phones can perform millions of calculations per second in order to encode or decode a voice stream.

Any ordinary phone consists of several parts:

Microcircuit (board), which is the brain for the phone
Antenna
Liquid crystal display (LCD)
Keyboard
Microphone
Speaker
Battery

The microcircuit is the center of the entire system. Next, we will consider what kind of chips there are and how each of them works. A chip for converting analog information to digital and back encodes the outgoing audio signal from the analog system to digital and the incoming signal from digital system to analog.

A microprocessor is a central processing unit responsible for the bulk of information processing. It manages the keyboard and display, and many other processes.

ROM chips and memory card chip can store mobile phone operating system data and other user data such as data phone book... Radio frequency manages power and charge, and also works with hundreds of FM waves. The RF amplifier controls the signals that enter or reflect the antenna. The screen size has increased significantly since there were more functions in the mobile phone. Many phones have notebooks, calculators and games. And now many more phones connect to a PDA or Web browser.

Some phones store certain information, such as SID and MIN codes, in the built-in flash memory, while others use external cards such as SmartMedia cards.

Many phones have speakers and microphones so tiny that it's hard to imagine how they make any sound at all. As you can see, the speakers are the same size as a small coin, and the microphone is no bigger than a watch battery. By the way, such batteries for wrist watch are used in the internal chip of the mobile phone to operate the watch.

The most amazing thing is that 30 years ago, many such details occupied an entire floor of a building, and now all this is placed in the palm of a person.

There are three most common ways 2G mobile phones use radio frequencies to transmit information:

FDMA (English Frequency Division Multiple Access) TDMA (English Time Division Multiple Access) CDMA (English Code Division Multiple Access) - Code Division Multiple Access.

Although the names of these methods seem so confusing, you can easily guess how they work by simply breaking the name into separate words.

The first word, frequency, time, code, indicates the accessor method. The second word, division, says that it separates calls based on the access method.

FDMA allocates each phone call on a separate frequency TDMA allocates each call certain time on the frequency indicated to him, CDMA assigns a unique code to each call and then transmits it to a free frequency.

The last word of each method “multiple” means that several people can use each cell.

FDMA

FDMA (Frequency Division Multiple Access) is a method of using radio frequencies, when there is only one subscriber in the same frequency range, different subscribers use different frequencies within a cell. It is an application of frequency division multiplexing (FDM) in radio communications. In order to better understand how FDMA works, you need to consider how radios work. Each radio station sends its signal to free frequency bands. The FDMA method is used primarily for the transmission of analog signals. And although this method can undoubtedly transmit and digital information, it is not used because it is considered less effective.

TDMA

TDMA (Time Division Multiple Access) is a method of using radio frequencies, when there are several subscribers in one frequency slot, different subscribers use different time slots (slots) for transmission. It is an application of time division multiplexing (TDM) to radio communications. With TDMA, the narrow bandwidth (30 kHz wide and 6.7 milliseconds long) is split into three time slots.

Narrow bandwidth is commonly referred to as “channels”. Voice data converted into digital information is compressed so that it takes up less space. Therefore, TDMA is three times faster than an analog system using the same number of channels. TDMA systems operate on the 800 MHz (IS-54) or 1900 MHz (IS-136) frequency bands.

GSM

TDMA is currently the dominant technology for mobile cellular networks and is used in the GSM (Global System for Mobile Communications) (Russian SPS-900) - global digital standard for mobile cellular communications, with channel division according to the TDMA principle and a high degree of security thanks to encryption with public key... However, GSM uses TDMA and IS-136 access differently. Let's imagine that GSM and IS-136 are different Operating Systems that run on the same processor, for example, both Windows and Linux operating systems run on Intel Pentium III. GSM systems use a coding method to encrypt phone calls from mobile phones. The GSM network in Europe and Asia operates at 900 MHz and 1800 MHz, while in the USA it operates at 850 MHz and 1900 MHz and is used for mobile communications.

Blocking your GSM phone

GSM is an international standard in Europe, Australia, most of Asia and Africa. Mobile phone users can buy one phone that will work wherever this standard is supported. In order to connect to a specific mobile operator in different countries, GSM users simply change their SIM card. SIM card save all information and identification numbers that are required to connect to a mobile operator.

Unfortunately, the 850MHz / 1900-MHz GSM frequencies used in the United States do not match those of the international system. Therefore, if you live in the United States, but you really need a mobile phone abroad, you can buy a three- or four-band GSM phone and use it at home and abroad, or just buy a mobile phone with GSM 900MHz / 1800MHz standard to travel abroad. ...

CDMA

CDMA (Code Division Multiple Access). Traffic channels with this method of dividing the medium are created by assigning each user a separate numerical code that spreads across the entire bandwidth. There is no time division, all subscribers constantly use the entire channel bandwidth. The frequency band of one channel is very wide, the broadcasting of subscribers is superimposed on each other, but since their codes are different, they can be differentiated. CDMA is the basis for IS-95 and operates in the 800 MHz and 1900 MHz bands.

Dual band and dual standard mobile phone

When you travel to travel, you undoubtedly want to find a phone that will work on multiple lanes, in several standards, or will combine both. Let's take a closer look at each of these possibilities:

A multiband phone can switch from one frequency to another. For example, a dual band TDMA telephone can use TDMA services in an 800 MHz or 1900 MHz system. A dual band GSM phone can use the GSM service in three bands - 850 MHz, 900 MHz, 1800 MHz or 1900 MHz.
Multi-standard telephone. "Standard" in mobile phones means the type of signal transmission. Therefore, a phone with AMPS and TDMA standards, if necessary, can switch from one standard to another. For example, the AMPS standard allows you to use the analogue network in areas where the digital network is not supported.
A multiband / multistandard phone allows you to change the frequency band and transmission standard.

Phones that support this feature automatically change bands or standards. For example, if the phone supports two bands, then it connects to the 800 MHz network, if it cannot connect to the 1900 MHz band. When a phone has several standards, it first uses a digital standard, and if it is not available, it switches to an analog one.

Mobile phones come in two and three bands. However, the word "three-lane" can be deceiving. It can mean that the phone supports CDMA and TDMA standards, and an analog standard. And at the same time, it can mean that the phone supports one digital standard in two bands and an analog standard. For those traveling abroad, it is better to get a phone that works on the 900 MHz GSM band for Europe and Asia and 1900 MHz for the USA, and in addition supports the analog standard. In essence, it is a dual band phone, in which one of these modes (GSM) supports 2 bands.

Cellular and Personal Communications Service

Personal Communications Service (PCS) is essentially a mobile phone service that emphasizes personal communications and mobility. The main feature of PCS is that the user's telephone number becomes his Personal Communication Number (PCN), which is "tied" to the user himself, and not to his phone or radio modem. A user traveling around the world with the PCS can freely receive phone calls and email on your PCN.

Cellular communications were originally created for use in automobiles, while personal communications meant great possibilities. Compared to traditional cellular communications, PCS has several advantages. First, it is completely digital, which allows for faster data transfer rates and facilitates the use of data compression technologies. Secondly, the frequency range used for the PCS (1850-2200 MHz) reduces the cost of the communications infrastructure. (Since the overall dimensions of the PCS base station antennas are smaller than the dimensions of the cellular base station antennas, they are cheaper to manufacture and install).

In theory, mobile system in the USA it operates in two frequency bands - 824 and 894 MHz; PCS operates at 1850 and 1990 MHz. And since this service is based on the TDMA standard, the PCS has 8 timeslots and the channel spacing is 200KHz, as opposed to the usual three timeslots and 30KHz between channels.

3G is the latest technology in mobile communications. 3G means the phone belongs to the third generation - the first generation is analog mobile phones, the second is digital. 3G technology is used in multimedia mobile phones, commonly referred to as smartphones. These phones have multiple bands and high-speed data transmission.

3G uses several mobile standards. The most common are three of them:

CDMA2000 is a further development of the 2nd generation CDMA One standard.
WCDMA (Wideband Code Division Multiple Access) is the radio interface technology chosen by most cellular operators to provide broadband radio access to support 3G services.
TD-SCDMA (English Time Division - Synchronous Code Division Multiple Access) is a Chinese standard for third-generation mobile networks.

The 3G network can transfer data at speeds up to 3 Mb / s (therefore, in order to download an MP3 song of 3 minutes duration, it takes only about 15 seconds). For comparison, here are the second generation mobile phones - the fastest 2G phone can reach data transfer rates of up to 144 Kb / s (it takes about 8 hours to download a 3-minute song). High-speed 3G data transfer is ideal for downloading information from the Internet, sending and receiving large multimedia files. 3G phones are a kind of mini-notebooks that can handle large applications such as receiving streaming video from the Internet, sending and receiving faxes, and downloading e-mail messages with applications.

Of course, this requires base stations that transmit radio signals from phone to phone.

Mobile phone base stations are cast metal or lattice structures that rise hundreds of feet. This picture shows a modern tower serving 3 different mobile operators. If you look at the base of the base stations, you can see that each mobile operator installed his equipment, which nowadays takes up very little space (at the base of older towers, small rooms were built for such equipment).

Base station. photo from the site http://www.prattfamily.demon.co.uk

A radio transmitter and receiver are placed inside such a block, thanks to which the tower communicates with mobile phones. The radios are connected to the antenna on the tower with several thick cables. If you look closely, you will notice that the tower itself, all the cables and equipment of the companies at the base of the base stations are well grounded. For example, a plate with green wires attached to it is a copper ground plate.

In a mobile phone, as in any other electronic device, malfunctions may occur:

Most often, these include corrosion of parts caused by moisture entering the device. If the phone gets wet, make sure the phone is completely dry before turning it on.
Excessive temperatures (eg in a car) can damage the battery or the electronic board of the phone. If the temperature is too low, the screen may turn off.
Analog mobile phones often face the problem of "cloning". A phone is considered "cloned" when someone intercepts its identification number and can call other numbers for free.

This is how cloning works: Before you call someone, your phone sends its ESN and MIN codes to the network. These codes are unique and it is thanks to them that the company knows who to send invoices for calls. When your phone transmits MIN / ESN codes, someone can hear (using a special device) and intercept them. If you use these codes in another mobile phone, then you can call from it completely free of charge, since the owner of these codes will pay the bill.

How many of us wonder what happens after we press the call button on our mobile phone? How do cellular networks work?

Most probably not. Most often, we dial the federal number of the interlocutor on the machine, as a rule, on the case, so what is there and how it works does not interest us at a particular moment in time. But these are amazing things. How can you call a person in the mountains or in the middle of the ocean? Why during a conversation we can hardly hear each other, or even completely interrupt. Our article will try to shed light on how cellular communication works.

So, most of the densely populated territory of Russia is covered by the so-called BS, which are called Base Stations without abbreviation. Many could turn their attention to them when traveling between cities. In an open field, Base Stations are more like towers that have red and White color... But in the city, such base stations are thoughtfully placed on the roofs of non-residential high-rises. These towers are capable of picking up a signal from any cell phone located geographically within a radius of no more than 35 kilometers. "Communication" between the BS and the telephone takes place through a special service or voice channel.

As soon as a person dials the number he needs on a mobile device, the device finds the Base Station closest to him, therefore, to a special service channel and asks her to allocate a voice channel. The tower, after receiving a request from the device, sends a request to the so-called controller, which will be abbreviated as BSC. This very controller redirects the request to the switch. The "smart" switch MSC will determine to which operator the called subscriber is connected.

If it turns out that a call is made to a phone within one network, for example, from a Beeline subscriber to another subscriber of this operator, or inside MTS, inside Megafon, and so on, then the switchboard will begin to find out the location of the called subscriber. Thanks to the Home Location Register, the switch will find where the person is. It can be anywhere, at home, at work, in the country, or even in another country. This will not prevent the switch from transferring the call to the appropriate switch. And then the "ball" will begin to "unwind". That is, the call from the switch - "responder" will go to the controller - "responder", then to its Base Station and to the mobile phone, respectively.

If the switchboard finds out that the called subscriber belongs to another operator, it will send a request to the switchboard of a different network.
Agree, the scheme is quite simple, but difficult to imagine. How a "smart" Base Station finds a phone, sends a request, and the switch itself determines the operator and another switch. What is a Base Station really? It turns out that these are several iron cabinets that are located under the very roof of the building, in the attic or in a special container. The main condition is that the room must be perfectly air-conditioned.

It is logical that the BS has an antenna, which helps it to "catch" the connection. The antenna at the BS consists of several parts (sectors), each of which is responsible for the territory. The part of the antenna, which is located vertically, is responsible for communication with mobile phones, and the round part is for communication with the controller.

One sector is capable of simultaneously receiving calls from seventy telephones. Considering that one BS can consist of six sectors, then at the same time it will quietly serve 6 * 72 = 432 calls.

As a rule, such power of the Base Station is enough "headlong". Of course, there are situations when the entire population of our country starts calling each other at the same time. It's the new Year. Some people just need to say the coveted phrase "Happy New Year!" Into the phone, while others are ready to speak hours with an unlimited tariff from "Corporation of Communications", discussing guests and plans for the whole night.

However, regardless of the duration of the call, the Base Stations cannot cope, and it can be very difficult to get through to the subscriber. But on weekdays for most of the year, BS of six sectors is quite enough, especially for optimal workload, the Stations are selected for the operator in accordance with the population of the territory. Some operators give their preference to large BS in order to improve the quality of the communication provided.

There are three ranges in which the base station can operate and which determine the number of devices supported and the distance covered. In the 900 MHz range, the station is capable of covering a large area, but in the 1800 MHz range, the distance will be significantly reduced, but the number of connected transmitters will increase. The third band at 2100 MHz suggests a new generation of communications - 3G.
It is clear that in sparsely populated areas it is more expedient to install a Base Station at 900 MHz, but in a city, 1800 MHz is suitable in order to better penetrate thick concrete walls, and these BSs will be needed ten times more than in a village. Note that one BS can support three bands at once.

Stations in the 900 MHz mode cover an area with a radius of 35 km, but if at the moment it serves few telephones, then it can "penetrate" up to 70 km. Naturally, our mobile phones can "find" BS even at a distance of 70 km. Base Stations are designed to cover the earth's surface as much as possible and provide a large number of people are connected precisely on the ground, therefore, if possible, to catch signals at a distance of at least 35 kilometers, at the same distance, but into the sky, the Base Stations do not "penetrate".

In order to provide their passengers with cellular communications, some airlines are starting to place small BSs on board aircraft. Communication of the "heavenly" Base Station with the "earth" is carried out using a satellite channel. Since work mobile devices can interfere with the flight process, onboard BS can be easily turned on / off, have several modes of operation, up to a complete shutdown of transmission voice messages... During the flight, the phone may accidentally be transferred to a base station with a worse signal or no free channels. In this case, the call will be terminated. All these are the subtleties of cellular communication in the sky in motion.

In addition to airplanes, penthouse residents also have some problems. Even an unlimited tariff and VIP - the conditions of the mobile operator will not help in the case of different BS. A resident of an apartment on a high floor, moving from one room to another, will lose contact. This can happen due to the fact that the phone in one room "sees" one base station, and in another it "discovers" another. Therefore, during a conversation, the connection is interrupted, since these BSs are at a relative distance from each other and are not even considered "neighboring" for the same operator.

The mobile phone is an integral part of a modern, technologically advanced society. Despite the routine and outward simplicity of this device, very few people know how a mobile phone works.

Mobile phone device

Modern technologies and constantly moving forward progress make it possible to create phones with a huge number of functions and capabilities. With each new model, phones are becoming thinner, more beautiful and more affordable in terms of finance. Despite the huge variety of models and manufacturers, all these devices are arranged according to the same principle.

In fact, a mobile phone is a receiving and transmitting device that has a receiver, transmitter and radio antenna in its body. The receiver receives a radio signal, converts it into electrical impulses and sends it to the speaker of your phone in the form of electrical waves. The speaker converts these electrical impulses into sound that we hear when we talk to the other person.

The microphone picks up your speech, converts it into electrical signals and sends it to the built-in transmitter. The task of the transmitter is to convert electrical impulses into radio waves and transmit them to the nearest station via an antenna. The antenna serves to enhance the reception and transmission of radio waves from the phone to the nearest cellular station.

How a landline phone works

The device of a landline phone is not very different from a mobile one. IN landline phone there is no need to convert electrical impulses into radio waves, since the contact with the subscriber occurs via a telephone cable through an Automatic Telephone Exchange (PBX). The station does not need to search for a phone by its range and when dialing a number, it automatically connects you to the phone to which this number is registered.

How does mobile communication work?

Each of us has the opportunity to visually observe a large number of radio towers located in different parts of the city. These towers, as a rule, are installed at the most elevated places, on the roofs of high-rise buildings, on the structures of other communications or on their own stationary towers. These radio towers are called base stations (BS). You may notice that such stations are installed much more often in cities than in intercity space. This is due to the fact that in urban environments there is a lot of natural interference in the form of concrete buildings and various metal structures, which significantly degrade the signal quality. At the same time, a larger number of subscribers are concentrated in cities, which create a heavy load on the cellular network and to maintain a good quality of communication, an increase in the coverage area is required.

Your phone has its own identification in the form mobile number your SIM card. When turned on, the mobile phone constantly scans the space in search of a network and automatically selects the Base Station that provides the best signal quality. At the same time, it informs the station about its location and status, thus, the central computer of the cellular operator always knows which base station the phone is in the coverage area and whether it is ready to receive a call signal. As soon as the other person calls your number, the computer detects your location and sends a call signal to your phone. If the phone is turned off or is not within the coverage area of ​​the nearest Base Station, the computer informs you that the subscriber is out of the coverage area and cannot receive the call.

We all use mobile phones, but at the same time hardly anyone thinks - how do they work? In this article, we will try to figure out how, in fact, communication is implemented with respect to your mobile operator.

When you make a call to your interlocutor, or someone calls you, your phone connects over the radio channel to one of the antennas of the neighboring base station (BS, BS, Base Station).Each base station of cellular communication (in the common people - cell towers) includes from one to twelve transceiver antennas with directions in different directions in order to provide high-quality communication to subscribers within the radius of their operation. Such antennas are called by experts in their own jargon "Sectors", which are gray rectangular structures that you can see almost every day on the roofs of buildings or special masts.

The signal from such an antenna goes through a cable directly to the control unit of the base station. The base station is a collection of sectors and a control block. At the same time, a certain part of a settlement or territory is served by several base stations at once connected to a special block - local zone controller(abbreviated LAC, Local Area Controller or just "controller"). As a rule, one controller combines up to 15 base stations of a certain area.

For their part, controllers (there can also be several of them) are connected to the most important block - Mobile services Switching Center (MSC), which for simplicity of perception is usually called simply "Switch"... The switch, in turn, provides input and output to any communication lines - both cellular and wired.

If you display what is written in the form of a diagram, you get the following:
Small-scale GSM networks (usually regional) can use only one switch. Large ones, such as our operators of the "Big Three" MTS, Beeline or MegaFon, which simultaneously serve millions of subscribers, use several MSC devices connected to each other at once.

Let's see why we need so much a complex system and why can't the base station antennas be directly connected to the switch? To do this, you need to talk about another term, called in technical language handover... It characterizes the handover of service in mobile networks on a relay basis. In other words, when you are moving along the street on foot or in a vehicle and are talking on the phone, so that your conversation is not interrupted, you should promptly switch your device from one BS sector to another, from the coverage area of ​​one base station or controller. local zone to another, etc. Therefore, if the sectors of the base stations were directly connected to the switch, it would have to carry out this handover procedure for all its subscribers, and the switch already has enough tasks. Therefore, in order to reduce the likelihood of equipment failures associated with its overloads, the scheme for constructing GSM cellular networks is implemented according to a multi-level principle.

As a result, if you and your phone move from the coverage area of ​​one BS sector to the coverage area of ​​another, then this movement is carried out by the control unit of this base station, without touching more "high-end" devices - LAC and MSC. If the handover occurs between different BSs, then the LAC is taken over, and so on.

The switch is nothing more than the main "brain" of GSM networks, so its operation should be considered in more detail. A cellular network switch undertakes approximately the same tasks as a PBX in the networks of wire operators. It is he who understands where you are making a call or who is calling you, regulates the work of additional services and, in fact, decides whether you can currently make your call or not.

Now let's see what happens when you turn on your phone or smartphone?

So, you pressed the "magic button" and your phone turned on. There is a special number on the SIM card of your mobile operator, which is called IMSI - International Subscriber Identification Number... He is unique number for each SIM-card not only for your operator MTS, Beeline, MegaFon, etc., but a unique number for all mobile networks in the world! It is on it that operators distinguish subscribers from each other.

When the phone is turned on, your device sends this IMSI code to the base station, which transmits it further to the LAC, which, in turn, sends it to the switch. In this case, two additional devices come into play, connected directly to the switch - HLR (Home Location Register) and VLR (Visitor Location Register)... Translated into Russian, this, respectively, Home subscribers register and Guest subscriber register... HLR stores the IMSI of all subscribers on its network. The VLR contains information about those subscribers who currently use the network of this operator.

The IMSI number is transmitted to the HLR using an encryption system (another device is responsible for this process AuC - Authentication Center)... At the same time, HLR checks whether a subscriber with a given number exists in its database, and if the fact of its presence is confirmed, the system checks whether he can currently use communication services or, say, has a financial block. If everything is normal, then this subscriber goes to VLR and after that gets the opportunity to call and use other communication services.

For clarity, we will display this procedure using the schema:

Thus, we have briefly described how GSM cellular networks work. In fact, this description is rather superficial, since if we delve into the technical details in more detail, then the material would have turned out to be many times more voluminous and much less understandable for most readers.

In the second part, we will continue our acquaintance with the work of GSM networks and consider how and for what the operator debits funds from our account with you.

How radio communication works

Radio (lat.radio- radiate, emit rays, radius- ray) is a type of wireless communication, in which radio waves are used as a signal carrier, freely propagating in space.

Principle of operation
Transmission occurs as follows: on the transmitting side, a signal with the required characteristics (frequency and amplitude of the signal) is generated. The transmitted signal then modulates the higher frequency oscillation (carrier). The received modulated signal is emitted by the antenna into space. On the receiving side, the radio waves induce a modulated signal in the antenna, after which it is demodulated (detected) and filtered by a low-pass filter (thereby getting rid of the high-frequency component - the carrier). The received modulated signal is radiated by the antenna into space.
On the receiving side, the radio waves induce a modulated signal in the antenna, after which it is demodulated (detected) and filtered by a low-pass filter (thereby getting rid of the high-frequency component - the carrier).). Thus, a useful signal is extracted. The received signal may differ slightly from that transmitted by the transmitter (distortion due to interference and interference).

Frequency ranges
The frequency grid used in radio communications is conventionally divided into ranges:

• Long waves (LW) - f = 150-450 kHz (l = 2000-670 m)
• Medium waves (MW) - f = 500-1600 kHz (l = 600-190 m)
• Short waves (HF) - f = 3-30 MHz (l = 100-10 m)
• Ultrashort waves (VHF) - f = 30 MHz- 300 MHz (l = 10-1 m)
• High frequencies (HF-centimeter range) - f = 300 MHz - 3 GHz (l = 1-0.1 m)
• Extremely high frequencies (EHF-millimeter range) - f = 3 GHz - 30 GHz (l = 0.1-0.01 m)
• Hyperhigh frequencies (HHF - micrometer range) - f = 30 GHz - 300 GHz (l = 0.01-0.001 m)

Depending on the range, radio waves have their own characteristics and propagation laws:

• LWs are strongly absorbed by the ionosphere; surface waves, which propagate around the earth, are of primary importance. Their intensity decreases relatively quickly with distance from the transmitter.
• SW are strongly absorbed by the ionosphere during the day, and the area of ​​action is determined by the surface wave, in the evening they are well reflected from the ionosphere and the area of ​​action is determined by the reflected wave.
• HF propagates exclusively through reflection by the ionosphere, so there is a so-called radio silence zone around the transmitter. During the day, shorter waves (30 MHz) propagate better, at night, longer ones (3 MHz). Short waves can travel long distances with low transmitter power.
• VHF propagates in a straight line and, as a rule, is not reflected by the ionosphere. They easily bend around obstacles and have a high penetrating power.
• HF does not go around obstacles, spreads within the line of sight. Used in WiFi, cellular, etc.
• EHF does not bend around obstacles, is reflected by most of the obstacles, and spreads within the line of sight. Used for satellite communications.
• Hyper-high frequencies do not bend around obstacles, are reflected like light, and propagate within the line of sight. Limited use.

Propagation of radio waves
Radio waves propagate in void and in the atmosphere; earthly solid and water are opaque for them. However, due to the effects of diffraction and reflection, communication is possible between points on the earth's surface that do not have a line of sight (in particular, those located at a great distance).
The propagation of radio waves from a source to a receiver can occur in several ways simultaneously. This spread is called multipath. Due to the multipath and changes in the parameters of the environment, fading occurs - a change in the level of the received signal over time. With multipath, the change in the signal level occurs due to interference, that is, at the point of reception, the electromagnetic field is the sum of time-shifted radio waves of the range.

Radar

Radar- the field of science and technology, combining methods and means of detection, measurement of coordinates, as well as the definition of properties and characteristics various objects based on the use of radio waves. A close and somewhat overlapping term is radio navigation, however, in radio navigation, the object whose coordinates are being measured plays a more active role, most often this is the determination of its own coordinates. The main technical device for radar is a radar station.

Distinguish between active, semi-active, active with a passive response and passive RL. They are subdivided according to the used range of radio waves, by the type of the probing signal, the number of channels used, the number and type of measured coordinates, the location of the radar.

Operating principle

Radar is based on the following physical phenomena:

• Radio waves are scattered by electrical inhomogeneities (objects with other electrical properties different from the properties of the distribution medium). In this case, the reflected wave, as well as the actual radiation of the target, allows you to detect the target.
• At large distances from the radiation source, it can be assumed that radio waves propagate in a straight line and at a constant speed, due to which it is possible to measure the range and angular coordinates of the target (Deviations from these rules, which are valid only in the first approximation, are studied by a special branch of radio engineering - Radio wave propagation. these deviations lead to measurement errors).
• The frequency of the received signal differs from the frequency of the emitted oscillations with the mutual movement of the points of reception and emission (Doppler effect), which allows you to measure the radial speeds of the target relative to the radar.
• Passive radar uses the radiation of electromagnetic waves by the observed objects, it can be thermal radiation inherent in all objects, active radiation created by the technical means of the object, or spurious radiation created by any objects with working electrical devices.

cellular

cellular, mobile network- one of the types of mobile radio communication, which is based on cellular network . Key feature lies in the fact that the total coverage area is divided into cells (cells), which are determined by the coverage areas of individual base stations (BS). The honeycombs partially overlap and together form a network. On an ideal (even and without building) surface, the coverage area of ​​one BS is a circle, therefore, the network composed of them looks like honeycombs with hexagonal cells (honeycombs).

The network consists of spaced-apart transceivers operating in the same frequency range, and switching equipment that allows determining the current location of mobile subscribers and ensuring continuity of communication when a subscriber moves from the coverage area of ​​one transceiver to the coverage area of ​​another.

The principle of cellular communication

The main components of a cellular network are cell phones and base stations, which are usually located on rooftops and towers. When turned on, the cell phone listens to the air, finding a signal from the base station. The telephone then sends its unique identification code to the station. The telephone and the station maintain constant radio contact, periodically exchanging packets. The phone can communicate with the station using an analog protocol (AMPS, NAMPS, NMT-450) or digital (DAMPS, CDMA, GSM, UMTS). If the phone leaves the range of the base station (or the quality of the radio signal of the service cell deteriorates), it establishes communication with another (eng. handover).

Cellular networks can consist of base stations of different standards, which allows to optimize network performance and improve its coverage.

Cellular networks different operators connected to each other, as well as to the fixed telephone network. This allows subscribers of one operator to make calls to subscribers of another operator, from mobile phones to landlines and from landlines to mobiles.

Operators can conclude roaming agreements with each other. Thanks to such agreements, the subscriber, being outside the coverage area of ​​his network, can make and receive calls through the network of another operator. As a rule, this is done at higher rates. The possibility of roaming appeared only in 2G standards and is one of the main differences from 1G networks.

Operators can share network infrastructure, reducing network deployment and operational costs.

Cellular services

Cellular operators provide the following services:

• Voice call;
• Answering machine in cellular communication (service);
• Roaming;
• Caller ID (Automatic Caller ID) and AntiAON;
• Reception and transmission of short text messages (SMS);
• Reception and transmission of multimedia messages - images, melodies, video (MMS service);
• Mobile bank (service);
• Access to the Internet;
• Video call and video conferencing

TV

TV(Greek τήλε - far away and lat. video- I see; from Novolatinsky televisio- far-sightedness) - a set of devices for transmitting a moving image and sound over a distance. In everyday life, it is also used to designate organizations involved in the production and distribution of television programs.

Basic principles

Television is based on the principle of sequential transmission of picture elements by radio signal or by wire. The decomposition of the image into elements occurs using a Nipkov disk, a cathode-ray tube or a semiconductor matrix. The number of image elements is selected in accordance with the radio channel bandwidth and physiological criteria. To narrow the bandwidth of transmitted frequencies and reduce the visibility of flickering on the TV screen, interlaced scanning is used. It also allows you to increase the smoothness of the transmission of motion.

The television path in general includes the following devices:

1. TV transmission camera. Serves for converting an image obtained with a lens on a target of a transmitting tube or semiconductor matrix into a television video signal.
2. Video recorder. Records and plays back the video signal at the right time.
3. Video mixer. Allows you to switch between multiple image sources: camcorders, VCRs and others.
4. Transmitter. The RF signal is modulated by a television video signal and transmitted by radio or wire.
5. Receiver - TV. With the help of sync pulses contained in the video signal, the television image is reproduced on the receiver screen (kinescope, LCD display, plasma panel).

In addition, an audio path similar to a radio transmission path is used to create a television transmission. Sound is transmitted on a separate frequency, usually using frequency modulation, a technique similar to FM radio stations. IN digital television soundtrack, often multichannel, is transmitted in a common data stream with the image.

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Date the page was created: 2016-04-11