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A huge number of BS - base stations are installed on the territory of Russia. Probably, many of you yourself have seen red and white structures towering in the fields or structures installed on the roofs of non-residential buildings. Each such base station is capable of picking up a signal from a cell phone at a distance of up to 35 km, communicating with it via service or voice channels.

After you have dialed the number of the desired subscriber on your phone, the following happens: the mobile phone finds the nearest BS, contacts it via the service channel and requests a voice channel. After that, the BS sends a request to the controller (BSC), which then goes to the communicator. If the called subscriber is served by the same operator as you, the communicator will check the Home Location Register (HLR) database to find out exactly where the person you are calling is and will redirect the call to the correct switch, which will then transfer the call to the controller and then to the Base Station. Finally, the Base Station will contact the person's mobile phone and connect you to it. And if the person with whom you want to talk is a subscriber of another cellular operator, or you call a city number, the switch will "find" the corresponding switch of the other network and contact him. Sounds confusing enough, right? Let's try to analyze this issue in more detail.

But back to the hardware. As we already said, from the BS, the call is transferred to the controller (BSC). Outwardly, it is not much different from the Base Station:

The number of BSs that the controller is able to service can reach six dozen. The controller and the BS communicate via optical or radio relay channels. The controller manages the operation of radio channels.

Below you can see what the switch is:

The number of controllers supported by the switch varies from two to thirty. Switches are placed in large rooms filled with metal equipment cabinets.

The switch's job is to manage traffic. If earlier, in order to talk to each other, the subscribers had to first contact the telephone operator, who then manually rearranged the necessary wires, now the switch is perfectly coping with her role.

Inside the cars there are devices for reading and processing data:

Controllers and switches are closely monitored 24 hours a day. Tracking is conducted in the so-called CCS (Flight Control Center of the Network Control Center).

Cellular communication has recently become so firmly established in our daily life that it is difficult to imagine modern society without it. Like many other great inventions, the mobile phone has greatly influenced our life, and in many of its areas. It is difficult to say what the future would be like if it were not for this convenient form of communication. Probably the same as in the movie "Back to the Future-2", where there are flying cars, hoverboards, and much more, but no cellular connection!

But today in a special report for there will be a story not about the future, but about how modern cellular communication is arranged and works.

In order to learn about the work of modern cellular communications in the 3G / 4G format, I asked to visit the new federal operator Tele2 and spent a whole day with their engineers, who explained to me all the details of data transmission through our mobile phones.

But first, I'll tell you a little about the history of the emergence of cellular communications.

The principles of wireless communication were tried out almost 70 years ago - the first public mobile radiotelephone appeared in 1946 in St. Louis, USA. In the Soviet Union, a prototype of a mobile radiotelephone was created in 1957, then scientists from other countries created similar devices with different characteristics, and only in the 70s of the last century in America the modern principles of cellular communication were determined, after which its development began.

Martin Cooper - the inventor of the prototype of the portable cell phone Motorola DynaTAC weighing 1.15 kg and dimensions 22.5x12.5x3.75 cm

If in Western countries, by the mid-90s of the last century, cellular communication was widespread and used by most of the population, then in Russia it only began to appear, and became available to everyone a little over 10 years ago.

Bulky brick-like mobile phones that worked in the first and second generation formats have gone down in history, giving way to smartphones with 3G and 4G, better voice communication and high Internet speed.

Why is the connection called cellular? Because the territory in which communication is provided is divided into separate cells or cells, in the center of which base stations (BS) are located. In each "cell" the subscriber receives the same set of services within certain territorial boundaries. This means that moving from one "cell" to another, the subscriber does not feel territorial attachment and can freely use communication services.

It is very important that there is continuity of the connection when moving. This is provided thanks to the so-called handover, in which the connection established by the subscriber is, as it were, picked up by neighboring cells on the relay, and the subscriber continues to talk or dig in social networks.

The entire network is divided into two subsystems: a base station subsystem and a switching subsystem. Schematically, it looks like this:

In the middle of the "cell", as mentioned above, is the base station, which typically serves three "cells". The radio signal from the base station is emitted through 3 sector antennas, each of which is directed to its own "cell". It so happens that several antennas of one base station are directed to one "cell" at once. This is due to the fact that the cellular network operates in several bands (900 and 1800 MHz). In addition, this base station may have equipment of several generations of communication (2G and 3G) at once.

But on the BS Tele2 towers there is only equipment of the third and fourth generation - 3G / 4G, since the company decided to abandon old formats in favor of new ones, which help to avoid interruptions in voice communication and provide a more stable Internet. Regulars of social networks will support me in the fact that nowadays Internet speed is very important, 100-200 kb / s is no longer enough, as it was a couple of years ago.

The most common location for the BS is a tower or mast built specifically for it. Surely you could see the red and white towers of the BS somewhere far from residential buildings (in a field, on a hill), or where there are no tall buildings nearby. Like this one that is visible from my window.

However, in urban areas it is difficult to find a place for a massive structure. Therefore, in large cities, base stations are located on buildings. Each station picks up a signal from mobile phones at a distance of up to 35 km.

These are antennas, the BS equipment itself is located in the attic, or in a container on the roof, which is a pair of iron cabinets.

Some base stations are located where you wouldn't even guess. Like on the roof of this parking lot.

The BS antenna consists of several sectors, each of which receives / sends a signal in its own direction. If the vertical antenna communicates with telephones, then the round antenna connects the BS to the controller.

Depending on the characteristics, each sector can handle up to 72 calls simultaneously. The BS can consist of 6 sectors and serve up to 432 calls, however, usually fewer transmitters and sectors are installed at the stations. Cellular operators, such as Tele2, prefer to install more base stations to improve the quality of communication. As I was told, the most modern equipment is used here: Ericsson base stations, transport network - Alcatel Lucent.

From the base station subsystem, the signal is transmitted towards the switching subsystem, where the connection is established with the direction desired by the subscriber. The switching subsystem has a number of databases that store information about subscribers. In addition, this subsystem is responsible for security. To put it simply, the switch performs It has the same functions as the female operators who used to connect you with the subscriber by hand, only now all this happens automatically.

The equipment for this base station is hidden in this iron cabinet.

In addition to conventional towers, there are also mobile versions of base stations placed on trucks. They are very convenient to use during natural disasters or in crowded places (football stadiums, central squares) during holidays, concerts and various events. But, unfortunately, due to problems in the legislation, they have not yet found wide application.

To ensure optimal radio coverage at ground level, base stations are designed in a special way, therefore, despite the range of 35 km. the signal does not apply to the flight altitude of the aircraft. However, some airlines have already begun to install small base stations on their aircraft that provide cellular communications inside the aircraft. Such a BS connects to a terrestrial cellular network using a satellite channel. The system is complemented by a control panel that allows the crew to turn the system on and off, as well as certain types of services, such as turning off the voice on night flights.

I also looked into the Tele2 office to see how specialists control the quality of cellular communication. If a few years ago such a room would have been hung up to the ceiling with monitors showing network data (congestion, network failures, etc.), then over time the need for such a number of monitors has disappeared.

Technologies have developed greatly over time, and such a small room with several specialists is enough to monitor the operation of the entire network in Moscow.

Few views from the Tele2 office.

At a meeting of the company's employees, plans are being discussed to capture the capital) From the beginning of construction until today, Tele2 has managed to cover the whole of Moscow with its network, and is gradually conquering the Moscow region, launching more than 100 base stations weekly. Since I now live in the region, it is very important to me. so that this network would come to my town as soon as possible.

The company plans for 2016 to provide high-speed communication in the metro at all stations, at the beginning of 2016 Tele2 communication is present at 11 stations: 3G / 4G communication at the Borisovo metro, Delovoy Tsentr, Kotelniki, Lermontovsky Prospekt , Troparevo, Shipilovskaya, Zyablikovo, 3G: Belorusskaya (Koltsevaya), Spartak, Pyatnitskoe shosse, Zhulebino.

As I said above, Tele2 abandoned the GSM format in favor of the third and fourth generation standards - 3G / 4G. This allows the installation of 3G / 4G base stations with a higher frequency (for example, within the Moscow Ring Road, BSs stand at a distance of about 500 meters from each other) in order to provide more stable communication and high speed of mobile Internet, which was not the case in the networks of previous formats.

From the company's office I, in the company of engineers Nikifor and Vladimir, go to one of the points where they need to measure the communication speed. Nikifor stands opposite one of the masts on which communications equipment is installed. If you look closely, you will notice another such mast a little further on the left, with equipment from other cellular operators.

Oddly enough, but cellular operators often allow their competitors to use their tower structures to accommodate antennas (of course, on mutually beneficial terms). This is because building a tower or mast is expensive and can save you a lot of money!

While we were measuring the speed of communication, Nikifor several times passers-by grandmothers and uncles asked if he was a spy)) "Yes, we are jamming Radio Liberty!).

The equipment actually looks unusual, from its appearance you can assume anything.

The company's specialists have a lot of work, considering that in Moscow and the region the company has more than 7 thousand. base stations: of which about 5 thousand. 3G and about 2 thousand. base stations LTE, and recently the number of BS has increased by about a thousand more.
In just three months, 55% of the total number of new base stations of the operator in the region were put on the air in the Moscow region. At the moment, the company provides high-quality coverage of the territory where more than 90% of the population of Moscow and the Moscow region live.
By the way, in December the 3G Tele2 network was recognized as the best in quality among all the capital's operators.

But I decided to personally check how good Tele2's connection is, so I bought a SIM card in the nearest shopping center on Voykovskaya metro station, with the simplest "Very black" tariff for 299 rubles (400 SMS / minutes and 4 GB). By the way, I had a similar Beeline tariff, which is 100 rubles more expensive.

I checked the speed on the spot. Reception - 6.13 Mbps, transmission - 2.57 Mbps. Considering that I am standing in the center of a shopping center, this is a good result, Tele2 communication penetrates well through the walls of a large shopping center.

At metro Tretyakovskaya. Signal reception - 5.82 Mbps, transmission - 3.22 Mbps.

And at the Krasnogvardeyskaya metro station. Reception - 6.22 Mbps, transmission - 3.77 Mbps. I measured it at the exit from the subway. If you take into account that this is the outskirts of Moscow, it is very decent. I believe that the connection is quite acceptable, we can confidently say that it is stable, considering that Tele2 appeared in Moscow just a couple of months ago.

Tele2 has a stable connection in the capital, which is good. I really hope that they will come to the region as soon as possible and I will be able to take full advantage of their connection.

Now you know how cellular communication works!

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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 whole 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 player, PDA devices and GPS navigation system.

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 the Italian Guglielmo Marconi began to talk about wireless communication 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 was powered by a single main antenna, installed 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 (i.e. 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 a 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 an 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 own 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 your 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 connects to 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 field of the base station when 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 allows you 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 don't have to pay "inflated" tariffs later. Therefore, check immediately the cost of this service.

Please note that the phone must operate 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 fully utilize 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 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 vice versa encodes the outgoing audio signal from the analog system to digital and the incoming signal from the digital system to the analog system.

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

The ROM chips and the memory card chip can store the operating system data of the mobile phone and other user data such as phone book data. 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 the mobile phone has more features. 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 wristwatch batteries are used in the internal chip of a 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 fits 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 individual words.

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

FDMA places each call on a separate frequency TDMA allocates a certain time to each call on the specified frequency 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, it is necessary 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 digital information, it is not used, since it is considered less efficient.

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 operates 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) standard - a global digital standard for mobile cellular communications, with channel division based on the TDMA principle and a high degree of security due to public key encryption. 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 are based on Intel Pentium III. GSM systems use a coding method to make phone calls from mobile phones classified. 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 cards store 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 USA, 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 can switch from one standard to another if necessary. 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 will 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 US, 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 e-mails on their 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, the US mobile system 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 a speed of 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, let's take second generation mobile phones - the fastest 2G phone can reach data transfer rates 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 that “serves” 3 different mobile operators. If you look at the base of the base stations, you can see that each mobile operator has installed their own equipment, which nowadays takes up very little space (at the base of older towers for such equipment, small rooms were built).

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 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.

Here's 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.

Almost everyone used a cell phone, but few people thought - how does it all work? In this literary opus, we will try to consider how communication occurs from the point of view of your telecom operator.

When you dial a number and start calling, well, or someone calls you, your device communicates over the radio channel with one of the antennas of the nearest base station.

Each of the base stations contains from one to twelve transmit-receive antennas directed in different directions to provide communication to subscribers from all directions. Antennas are also called "sectors" in professional jargon. You yourself have probably seen them many times - large gray rectangular blocks.

From the antenna, the signal is transmitted via the cable directly to the control unit of the base station. The combination of sectors and a control block is usually called - BS, Base Station, base station... Several base stations, whose antennas serve a specific territory or area of ​​the city, are connected to a special block - the so-called LAC, Local Area Controller, "local area controller" often referred to simply as controller... Up to 15 base stations are usually connected to one controller.

In turn, the controllers, of which there can also be several, are connected to the most central "brain" unit - MSC, Mobile services Switching Center, Mobile Services Control Center, more commonly known as switch... The switch provides an exit (and entrance) to city telephone lines, to other cellular operators, and so on.

That is, in the end, the whole scheme looks something like this:

In small GSM networks, only one switch is used, in larger ones serving more than a million subscribers, two, three or more can be used MSC united with each other.

Why so much complexity? It would seem that you can simply connect the antennas to the switch - and that's it, there would be no problems ... But not everything is so simple. It's about one simple English word - handover... This term refers to handoff in cellular networks. That is, when you walk down the street or drive a car (train, bike, roller skates, asphalt paver ...) and at the same time talk on the phone, then in order for the connection not to be interrupted (and it is not interrupted), it is necessary to switch on time Your phone is from one sector to another, from one BS to another, from one Local Area to another, and so on. Accordingly, if the sectors were directly connected to the switch, then all these switches would have to be managed by the switch, which already has something to do. A multi-level network scheme makes it possible to evenly distribute the load, which reduces the likelihood of equipment failure and, as a result, loss of communication.

Example - if you move with a phone from the coverage area of ​​one sector to the coverage area of ​​another, then the BS control unit is engaged in the transfer of the phone, without affecting the "higher" devices - LAC and MSC... Accordingly, if the transition occurs between different BS, then it is controlled by LAC etc.

The operation of the switch should be considered in more detail. A switch in a cellular network performs almost the same functions as a PBX in a wired telephone network. It is he who determines where you call, who is calling you, is responsible for the operation of additional services, and, in the end - in general, determines whether it is possible to call or not.

Let's dwell on the last point - what happens when you turn on your phone?

Here, you turn on your phone. Your SIM card has a special number, the so-called IMSI - International Subscriber Identification Number, International Subscriber Identification Number... This number is unique for every SIM-card in the world, and it is precisely by this number that operators distinguish one subscriber from another. When the phone is turned on, it sends this code, the base station transmits it to LAC, LAC- to the switch, in turn. This is where two additional modules associated with the switch come into play - HLR, Home Location Register and VLR, Visitor Location Register... Respectively, Home Subscriber Register and Guest Subscriber Register... V HLR are kept IMSI all subscribers who are connected to this operator. V VLR in turn, it contains data on all subscribers who are currently using the network of this operator. IMSI transmitted to HLR(of course, in a highly encrypted form; we will not go into details about encryption, we will only say that another block is responsible for this process - AuC, Authentication Center), HLR, in turn, checks whether he has such a subscriber, and, if so, whether he is blocked, for example, for non-payment. If everything is in order, then this subscriber is registered in VLR and from that moment can make calls. Large operators may have not one, but several parallel operating HLR and VLR... And now let's try to display all of the above in the figure:

Here we briefly reviewed how the cellular network works. In fact, everything is much more complicated there, but if you describe everything as it is thoroughly, then this presentation in terms of volume may well exceed "War and Peace".

Next, we will consider how (and most importantly - for what!) The operator debits money from our account. As you probably already heard, there are three different types of tariff plans - the so-called "credit", "advance" and "prepaid", from English Pre-Paid, that is, prepaid. What is the difference? Let's consider how money can be written off during a conversation:

Let's say you called somewhere. It was registered on the switchboard - the subscriber called so-and-so, talked, say, forty-five seconds.

The first case - you have a credit or advance payment system. In this case, the following happens: data about your and not only your calls are accumulated in the switchboard and then, in the order of the general queue, are transferred to a special block called Billing, from English to bill - pay bills. Billing is responsible for all issues related to subscribers' money - calculates the cost of calls, charges a subscription fee, charges money for services, and so on.

Information transfer rate from MSC v Billing depends on what the computing power is billing, or, in other words, how quickly he manages to translate technical data about the calls made into direct money. Accordingly, the more subscribers talk, or the more "slow" billing, the slower the queue will move, respectively, the greater the delay between the conversation itself and the actual write-off of money for this conversation. This fact is associated with dissatisfaction often expressed by some subscribers - “They say, money is being stolen! I didn’t speak for two days - they wrote off a certain amount ... ”. But at the same time, it is not at all taken into account that for conversations that took place, for example, three days ago, money was not immediately written off ... People try not to notice good things ... And these days, for example, billing could simply not work - because of an accident, or because it was somehow modernized.

In the opposite direction - from billing to MSC- there is another line in which billing informs the switchboard about the status of subscribers' accounts. Again, a fairly common case - the account debt can reach several tens of dollars, and you can still call by phone - this is precisely because the "return" queue has not yet arrived and the switchboard does not yet know that you are a malicious defaulter and You should be blocked for a long time.

Advance tariffs differ from credit tariffs only in the method of settlement with the subscriber - in the first case, a person deposits some amount into the account, and the money for calls is gradually deducted from this amount. This method is convenient in that it allows you to plan and limit your communication costs to some extent. The second option is credit, in which the total cost of all calls for a certain period (" billing cycle»), Usually for a month, is issued in the form of an invoice, which the subscriber must pay. The credit system is convenient because it insures you against those cases when you urgently need to call, and the money on the account suddenly runs out and the phone is blocked.

The pripedes are arranged in a completely different way:

Prepaid billing as such is usually called " Prepaid platform».

Immediately at the beginning of the telephone connection, a direct connection is established between switch and prepaid platform... No queues, data is transmitted in both directions directly during the conversation, in real time. In this regard, the following characteristic features are inherent in the pripades - this is the absence of a monthly fee (since there is no such thing as billing period), a limited set of additional services (it is technically difficult to charge them in “real time”), the impossibility of “going negative” - the conversation will simply be interrupted as soon as the money in the account runs out. A clear dignity prepaids is the ability to accurately control the amount of money in the account, and, as a result, your expenses.

V prepaid still sometimes there is some funny phenomenon - if prepaid platform refuses to work for any reason, for example, due to overload, then, accordingly, for subscribers prepaid tariffs at this time all calls become absolutely free. That, in fact, they - subscribers - can not but rejoice.

And how is our money calculated when we talk while in roaming? And how does the phone work in roaming? Well, let's try to answer these questions:

Number IMSI consists of 15 digits, and the first 5 digits, the so-called CC - Country Code(3 digits) and NC - Network Code(5 digits) - clearly characterize the operator to which this subscriber is connected. By these five numbers VLR the guest operator finds HLR home operator and looks in it - but, in fact, can this subscriber use roaming with this operator? If yes, then IMSI prescribed by VLR guest operator, and in HLR home - link to the same guest VLR to know where to look for the caller.

The situation with writing off money in billing is also not very simple. Due to the fact that the calls are processed by the guest switchboard, but the money is calculated by its own, "home" billing, large delays in debiting funds are quite possible - up to a month. Although there are systems, for example, “ Camel2», Which work according to the prepaid principle in roaming, that is, write off money in real time.

Here another question arises - why is the money written off in roaming? If "at home" everything is clear - there are clearly defined tariff plans, then the situation with roaming is different - they write off a lot of money and it is not clear why. Well, let's try to figure it out:

All phone calls while roaming are divided into 3 main categories:

Incoming calls - in this case, the cost of a call consists of:

Cost of an international call from home to the guest region
+
The cost of an incoming call from a guest operator
+
Some surcharge depending on the specific guest operator

Outgoing call home:

The cost of an international call from the guest region to home
+
The cost of an outgoing call from a guest operator

Outgoing call in the guest region:

The cost of an outgoing call from a guest operator
+
Operator-specific surcharge

As you can see, the cost of calls in roaming depends only on two things - on which operator the subscriber is connected to at home and which operator the subscriber uses when visiting. At the same time, one very important thing is revealed - the cost of a minute in roaming does not depend at all on the tariff plan chosen by the subscriber.

I would like to add one more remark - if two phones of one operator are together in roaming with another operator (well, for example, two friends went to rest), then it will be very costly for them to talk to each other - the caller pays like an outgoing home, and the receiving a call - like an incoming call from home. This is one of the disadvantages of the GSM standard - in this case, communication goes through the house. Although it is technically quite realistic to arrange a connection "directly", but which of the operators will go for it, if you can leave everything as it is and earn money?

Another question that has recently often interested the owners of more than one mobile phone - how much will a forwarded call from one phone to another cost? And the answer to this question is quite realistic:

Let's say that call forwarding is set from phone B to phone C. From phone A they call phone B - accordingly, the call is forwarded to phone C. In this case, they pay:

Phone A - as outgoing to phone B
(in fact, this is logical - after all, he calls him)
Phone B - pays the forwarding price
(usually a few cents per minute)
+
the cost of an international call from the region where B is registered to the region where C is registered
(if the phones are of the same region, then this component is equal to zero).
Phone C - pays as incoming from phone A

At the end of the topics, I would like to mention one more subtle point - how much will the call forwarding cost in roaming? And this is where the fun begins:

For example, the phone has call forwarding on condition of employment to a home number. Then, with an incoming call, the so-called " roaming loop"- the call will go to the home phone through the guest switch, respectively, the cost of such a forwarded call for roamer will be equal to the sum of the cost of incoming and outgoing calls to home, plus the cost of the forwarding itself. And what is funny at the same time - the roamer may not even know that such a call took place, and subsequently be surprised to see the communication bill.

Hence follows a practical advice - when traveling, it is advisable to turn off all types of call forwarding (you can leave only unconditional - in this case, the "roaming loop" does not work), especially forwarding to voice mail - otherwise you can later be surprised for a long time - "Where did this money go? a?"

List of terms used in the text:

AuC- Autentification Center, the Authentication Center, is responsible for the encryption of information when transmitting on the network and receiving from the network
Billing- Billing, operator's cash accounting system
BS- Base Station, base station, multiple transmit and receive antennas belonging to one control device.
Camel2- one of the Prepaid systems, which implements instant debiting of funds in roaming
CC- Country Code, country code in the GSM standard (for Russia - 250)
Gsm- Global System for Mobile Communications, the world's most widespread cellular standard
Handover - transfer control of a handset from one antenna / base station / LAC to another
HLR- Home Location Register, register of home subscribers, contains detailed information about all subscribers connected to a given operator.
IMEI- International Mobile Equipment Identification, the international serial number of equipment in the GSM standard, is unique for each device
IMSI- International Mobile Subscriber Identification, the international serial number of a subscriber for GSM services, is unique for each subscriber
LAC- Local Area Controller, a device that controls the operation of a number of base stations, whose antennas serve a specific area.
Local Area- Local area, territory served by BSs that are part of one LAC
MSC- Mobile services Switching Center, Mobile Services Control Center, switch is the central link of the GSM network.
NC- Network Code, Network Code, the code of a specific operator in a given country in the GSM standard (for MTS - 01, BeeLine - 99).
Prepaid- Prepaid, prepayment - billing system based on instant debiting of funds.
Roaming- Roaming, using the network of another, "guest" operator.
SIM- Subscriber Identification Module, Subscriber Identification Module, SIM card - an electronic unit inserted into the phone on which the subscriber's IMSI is recorded.
VLR- Visitor Location Register, register of active subscribers - contains information about all subscribers who are currently using the services of this operator.