Mobile Internet - devices, technologies, subtleties. How do cellular networks work? How Cellular and Internet Work

A mobile, or cellular, telephone is a miniature combination of a telephone, a radio receiver, and a radio transmitter, made possible only by the advances of modern physics (Figure 18.11).

The main advantage of such a telephone is that it maintains a constant radiotelephone connection when the subscriber moves within the so-called "coverage area".

The entire coverage area is divided into cells, also called "cells" (hence the name of the phone). Each cell has its own receiver-transmitter (their antennas are installed on TV towers, tall buildings and on specially built towers). Included cellular telephone automatically sends signals after a certain period of time, maintaining radio contact with the nearest receiver-transmitter, which provides it with one of the free channels.

When moving mobile phone from one cell to another, it automatically switches to a free channel of the nearest receiver-transmitter.

Electrodynamics. 2014

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Cellular communication is considered one of the most useful inventions of mankind - along with the wheel, electricity, the Internet and the computer. And in just a few decades, this technology has experienced a number of revolutions. How wireless communication began, how cells work and what opportunities the new mobile standard will open 5G?

The first use of mobile telephone radio dates back to 1921 - then in the United States, the Detroit police used one-way dispatch communication in the 2 MHz band to transmit information from a central transmitter to receivers in police cars.

How did the cell phone come about

For the first time, the idea of ​​cellular communication was put forward in 1947 - engineers from Bell Labs Douglas Ring and Ray Young worked on it. However, the real prospects for its implementation began to emerge only by the beginning of the 1970s, when the company's employees developed a working architecture for the cellular communication hardware platform.

So, American engineers suggested placing transmitting stations not in the center, but at the corners of the “cells”, and a little later a technology was invented that allows subscribers to move between these “cells” without interrupting communication. After that, it remains to develop operating equipment for such a technology.

Motorola successfully solved the problem - its engineer Martin Cooper demonstrated the first working prototype of a mobile phone on April 3, 1973. He called the head of the research department of a competitor company right off the street and told him about his own successes.

Motorola's management immediately invested $100 million in the promising project, but the technology entered the commercial market only ten years later. This delay is due to the fact that at first it was required to create a global infrastructure of cellular base stations.

The first cell phone went on sale on March 6, 1983. It weighed almost 800 grams, could work on a single charge for 30 minutes of talk time and charged for about 10 hours. At the same time, the device cost $ 3,995 - a fabulous amount at that time. Despite this, the mobile phone instantly became popular.

Why is it called cellular

The principle of mobile communication is simple - the territory on which the connection of subscribers is provided is divided into separate cells or "cells", each of which is served by a base station. At the same time, in each "cell" the subscriber receives identical services, so he himself does not feel the crossing of these virtual boundaries.

Typically, a base station in the form of a pair of iron cabinets with equipment and antennas is placed on a specially built tower, but in the city they are often placed on the roofs of high-rise buildings. On average, each station catches a signal from mobile phones at a distance of up to 35 kilometers.

To improve the quality of service, operators are also installing femtocells - low-power and miniature cellular communication stations designed to serve a small area. They allow you to dramatically improve coverage in those places where it is needed. Cellular communications in Russia will be combined with space

A mobile phone on the network listens to the air and finds a signal from the base station. In addition to the processor and RAM, a modern SIM card has a unique key sewn into it that allows you to log in to the cellular network. Communication between the phone and the station can be carried out using different protocols - for example, digital DAMPS, CDMA, GSM, UMTS.

Cellular networks of different operators are connected to each other, as well as to the fixed telephone network. If the phone leaves the coverage area of ​​the base station, the device establishes communication with others - the connection established by the subscriber is imperceptibly transmitted to other "cells", which ensures continuous communication when moving.

In Russia, three bands are certified for broadcasting - 800 MHz, 1800 MHz and 2600 MHz. The 1800 MHz band is considered the most popular in the world, as it combines high capacitance, long range and high penetrating power. It is in it that most mobile networks now operate.

What mobile communication standards are

The first mobile phones worked with 1G technologies - this is the very first generation of cellular communications, which relied on analog telecommunication standards, the main of which was NMT - Nordic Mobile Telephone. It was intended exclusively for the transmission of voice traffic.

By 1991, the birth of 2G is attributed - the main standard of the new generation has become GSM (Global System for Mobile Communications). This standard is still supported today. Communication in this standard has become digital, it has become possible to encrypt voice traffic and send SMS.

The data transfer rate within GSM did not exceed 9.6 kbps, which made impossible transfer video or high quality audio. The GPRS standard, known as 2.5G, was intended to solve the problem. For the first time, he allowed mobile phone owners to use the Internet.

Another difference with 3G was the assignment of an IP address to each subscriber, which made it possible to turn mobile phones into small computers connected to the Internet. The first commercial 3G network was launched on October 1, 2001 in Japan. In the future, the throughput of the standard has been repeatedly increased.

The most modern standard is fourth-generation 4G communication, which is intended only for high-speed data transfer services. The bandwidth of the 4G network is capable of reaching 300 Mbps, which gives the user almost unlimited opportunities to surf the Internet.

Cellular communication of the future

The 4G standard is designed for the continuous transmission of gigabytes of information, it does not even have a channel for voice transmission. Due to extremely efficient multiplexing schemes, downloading a high-definition movie on such a network will take the user 10-15 minutes. However, even its capabilities are already considered limited.

In 2020, the official launch of a new generation of 5G communications is expected, which will allow the transfer of large amounts of data at ultra-high speeds up to 10 Gbps. In addition, the standard will allow connecting up to 100 billion devices to high-speed Internet.

It is 5G that will allow the real Internet of Things to appear - billions of devices will exchange information in real time. According to experts, network traffic will soon grow by 400%. For example, cars will be constantly in global network and receive traffic data.

Low latency will ensure real-time communication between vehicles and infrastructure. A reliable and always-on connection is expected to open the way for the first time to launch fully autonomous vehicles on the roads.

Russian operators are already experimenting with new specifications - for example, Rostelecom is working in this direction. The company signed an agreement on the construction of 5G networks in the Skolkovo innovation center. The implementation of the project is state program"Digital Economy", recently approved by the government.

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

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

So, most of the densely populated territory of Russia is covered by the so-called BS, which, without abbreviation, are called Base Stations. Many could turn their attention to them, traveling between cities. In the open field, base stations are more like towers that have red and White color. But in the city, such BS are thoughtfully placed on the roofs of non-residential skyscrapers. These towers are able to pick up a signal from any cell phone located territorially within a radius of no more than 35 kilometers. "Communication" between the BS and the phone occurs 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, 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 we will call BSC for short. This same controller redirects the request to the switch. The smart switch MSC will determine which operator the called subscriber is connected to.

If it turns out that the call is made to a phone within the same network, for example, from a Beeline subscriber to another subscriber of this operator, or within MTS, within Megafon, and so on, the switch will begin to find out the location of the called subscriber. Thanks to the Home Location Register, the switch will find where the right person is located. 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 "tangle" will begin to "unwind". That is, the call from the switch - "answerer" will go to the controller - "responder", then to its Base Station and to the mobile phone, respectively.

If the switch finds out that the called subscriber belongs to another operator, it will send a request to the switch of another network.
Agree, the scheme is quite simple, but it is difficult to imagine. How the "smart" Base Station finds the phone, sends a request, and the switch itself determines the operator and the other switch. What is a base station, really? It turns out that these are several iron cabinets that are located either 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 "catch" the connection. The BS antenna consists of several parts (sectors), each of which is responsible for the territory. The part of the antenna that is located vertically is responsible for communication with mobile phones, and the round one is for communication with the controller.

One sector is able to simultaneously receive calls from seventy telephone sets. If we take into account that one BS can consist of six sectors, then at the same time it will easily serve 6 * 72 = 432 calls.

As a rule, such power of the Base Station is enough "with a head". Of course, there are situations when the entire population of our country begins to call each other at the same time. It is a New Year. Some just need to say the cherished phrase “Happy New Year!” into the phone, while others are ready to pronounce hours with an unlimited tariff from Communications Corporation, 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 from six sectors is quite enough, especially for optimal workload, the operator selects Stations 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 BS can operate and which determine the number of supported devices and the distance covered. In the 900 MHz band, the station is able to cover a large area, but in the 1800 MHz band, the distance will be significantly reduced, but the number of connected transmitters will increase. The third band at 2100 MHz already assumes a new generation connection - 3G.
It is clear that in sparsely populated areas it is more expedient to set the Base Station at 900 MHz, but in the city 1800 MHz is suitable in order to better penetrate through thick concrete walls, and these BSs will need ten times more than in the 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 this moment since it serves few phones, it can "break through" up to 70 km. Naturally, our mobile phones can "find" the 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 by communication precisely on the ground, therefore, if it is 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 "break through".

In order to provide their passengers with cellular communications, some airlines are starting to place small base stations on board aircraft. The connection of the "heavenly" Base Station with the "terrestrial" is carried out using a satellite channel. Since work mobile devices can interfere with the flight process, onboard BS can easily be turned on / off, they have several modes of operation, up to complete shutdown transmission voice messages. During the flight, the phone may accidentally be transferred to a base station with a poorer 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 aircraft, some problems arise for residents of penthouses. Even 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 connection. This may be due to the fact that the phone in one room "sees" one BS, and in another it "discovers" another. Therefore, during a conversation, communication is interrupted, since these BSs are at a relative distance from each other and are not even considered "neighboring" by one operator.

Once a friend, who works as an engineer for a mobile operator, offered to arrange a tour of the skyscrapers of Belgorod and talk about how cellular. I, of course, could not refuse this, and this review has become the most interesting thing I have seen lately, not to mention the fact that incredibly beautiful views of my native city open from these rooftops. Before starting this story, I want to sincerely thank Kirill for organizing this excursion and for the technical advice in the process of writing this article.

The first roof was a skyscraper on Kostyukov Street next to the Vladimirsky residential complex. This 18-storey house stands on a hill, thus being one of the highest points in the city.

In any city, the roofs of high-rise buildings are always chosen by cellular operators - many antennas are installed here.

These antennas are of two types. Panel rectangular, similar in shape to marshmallow - cellular antennas, or "sectors". Through them, your cell phone or modem communicates with the control unit - the base station. Usually, several sector antennas connected to one base station are installed at different angles in order to provide communication to subscribers from all directions. For optimal coverage (both technically and economically), base stations are arranged in such a way that their layout resembles a honeycomb - hence the term "cellular communication". In reality, of course, ideal cells are rarely obtained, since many other factors affect their location, such as terrain, the possibility of installing a BS, and the number of subscribers. Sector antennas are also installed directly in buildings to provide a signal inside them. In particular, indoor-sectors are installed inside various Belgorod shopping and entertainment centers (City Mall, Mega-Grinne, Rio, etc.), in otherwise your mobile phones would not catch the network there.

Round antenna - radio relay (RRL). With the help of such antennas, communication is carried out between base stations, if there is no other connection between them. Such a connection is called a radio relay span. When installing 2 antennas, it is very important that they are clearly pointed at each other (this process is called alignment), because at the slightest deviation, the signal becomes worse, reducing throughput. Antennas can be of different diameters (from 30 cm to two and a half meters) and operate in different frequency ranges (depending on the length of the span, type of terrain and operator requirements).

The base station is the room in which the equipment is located. It can be a container, a partition in a building, occasionally a heating cabinet. The container is supplied with an external 3-phase power supply of 380 volts. Electricity enters the introductory shield, from which consumers of alternating current are powered (air conditioners and fire alarms). The UPS is powered from the input shield (source uninterruptible power supply) transforming alternating current to 48 volts DC, which powers most of the base station equipment. The photo shows the equipment racks and the UPS of the GSM base station.

Rack GSM base station inside.

Uninterruptible power system. At the base station and equipment transport network(internal blocks of RRL antennas, multiplexers, etc.) there must be different power sources, since when the power is turned off, the equipment of the transport network should “hold out” longer.

The base station is the lowest level in the network hierarchy. Several dozens of geographically connected stations are connected via cable to the BSC (Base Station Controller, base station controller), which monitors their performance and many processes, such as handover (switching a subscriber from one station to another without breaking the connection), location updates and signaling, as well as operating parameters. Controller cabinet.

Controller hardware.

Optical trunk multiplexer (a device that combines several different data streams for transmission over a single communication line).

BSC is a serious transport hub with a bunch of radio relay, optical equipment, critical to the quality of the power supply, which is why in case of a power outage it is equipped with serious batteries (pictured) or a diesel generator set (diesel generator).

The core of the network in a single city or region, to which the base station controllers are connected, is the MSC (Mobile Switching Center, mobile switching center) - automatic telephone exchange or switch. It is the MSC that determines where to send calls, and also processes calls from external networks (city, other operators) and sends information about the duration of calls and ordered services to the billing center. A multi-layered network structure is necessary for a more even load - after all, if the MSC, for example, had to perform the handover function and other functions of the base station controller, the load on it would increase significantly. Fiber-optic cables are used for the backbone networks laid between regions, allowing instantaneous transmission of huge data streams.

The word "Simka" familiar to you comes from the English abbreviation SIM (Subscriber Identity Module, subscriber identification module). Each card has an IMSI (International Subscriber Identification Number, international subscriber identification number, the very long number that is written on it in small numbers). When you turn on the phone, it transmits this number to the BS, from where it then goes to the BSC and on to the MSC. MSC queries the operator database (HLR) about the presence of such a subscriber, and whether it is possible to provide communication services to him (whether he paid, whether they are connected), and then registers him in the VLR - a temporary database of subscribers located in the coverage area of ​​this MSC (this includes both subscribers of this network and roamers - subscribers of other networks currently connected to it).

GSM (originally derived from Groupe Spécial Mobile) is a mobile communication standard developed in Europe that has actually become global (although there are other standards, such as CDMA, which is popular in America). This standard was focused primarily on voice data transmission. Third generation networks (UMTS), which are sometimes called 3GSM to indicate continuity and which use the HSDPA protocol, which significantly increases the transmission speed, are already more geared specifically for data, although in the absence of a GSM network, you can still make a call through 3G. As for LTE, the 4th generation network, it is fully optimized for high-speed data transmission (although in Russian realities, much depends on the development of the so-called “transport network” by operators - a set of resources and capabilities for transmitting large amounts of information and distributing these volumes by base stations).

The photo below shows an LTE base station. In the event of a power outage, the batteries will last for another 5 hours of operation.

A big barrier to the development of high-speed mobile internet in Russia is a shortage of radio frequencies. You can't just install the equipment and start service, you must first obtain permission to use the frequency. However, the frequencies required to deploy such networks are often reserved by the Department of Defense in accordance with the standards of the 60-70s of the last century, when equipment needed wide ranges. Allocation (“conversion”) of such frequencies for civilian use is an expensive and complicated procedure, both technically and bureaucratically. In addition, until December 2013 in Russia, by law, frequencies were allocated only for specific technologies, and if a company received a frequency on which it was allowed to provide GSM communication services, it did not have the right to use the same frequency for LTE. Because of this, for example, Tele2, which at one time did not receive a separate frequency for 3G and 4G, could not start providing high-speed mobile Internet services for a long time.

Let's digress a bit from the technical side, and look around. From this house you can see the city at a glance. Let's look at the center. In the middle is the city administration building. The Transfiguration Cathedral peeps out from behind it. Visible diorama "Arc of Fire" and art museum, park named after Lenin.

The monument to Prince Vladimir from here seems quite tiny.

Corps of BelSU.

Train station and city beach.

"Technologist", the areas of the Old City and Kreid are visible in the distance.

The inner courtyard of the residential complex "Vladimirsky".

Gotta get the bow

And of course, panoramas. Belgorod, what a beauty!

With a large approximation (unfortunately the weather was not ideal, although the sun was shining, the air was covered with haze, despite the raised contrast, you can not see very well.

Looking south from the same roof. The southern half of the city is popularly called "Kharkov Mountain".

The next object we visited was the 70-meter cell tower near BSTU.

Antennas are located on the tower, and base station equipment is located in metal booths. The tower itself is surrounded by a barbed wire fence to prevent climbing.

To climb the tower, you need to be in good physical shape. Even though I go to the gym all the time, in the end I started to get tired. And Kirill, who himself is a hefty jock, offered to imagine what it was like to climb on it, also dragging heavy equipment on itself.

But in general, the process of climbing pretty delivered. It's great. There is no insurance - hold on tight.

An unused Soviet radio relay antenna installed back in the 1970s, which is too difficult to dismantle due to its weight and dimensions, so it continues to hang here. This particular tower is quite old and was built back in those days, however, most of the towers that we meet appeared already during the rapid development of cellular communications in Russia.

Installation of such towers is usually carried out using a helicopter. The assembled parts of the structure weighing 2-3 tons each are brought to the place on long tractors, after which the machine lifts them, and the installers fix them. Red and white coloring, which increases the visibility of the structure for aircraft, is a requirement prescribed in the Operation Manual for Civil Airfields of the Russian Federation. Such coloring is applied to all high-rise structures, masts and factory chimneys.

At the top of the tower, as well as on skyscrapers, red lights of the light fence are installed, which are needed for the same purposes as coloring, but at night.

As a rule, the equipment of several companies is located on one tower at once. The tower itself, at the same time, can belong to only one operator (and others pay rent), or to several, or even belong to some other organization that does not deal with communication services, but only leases the object. Recently, in Russia, it has also begun to appear sharing base stations.

The tower sways a little in the wind - as it should be.

I want to become a baser more and more.

The Novy-2 microdistrict under construction and garages, from above, reminiscent of the slums of some Latin American city. A separate garage cooperative next to a residential area is such a purely Soviet invention, which causes bewilderment in the West.

Gubkin street, Khargora.

A little further in the middle, the BSTU buildings are visible. To the right is the city beach. At the top left is the skyscraper we just visited.

General view from the top of the building. In the very left part - the beginning of the Bolkhovets district and the western industrial zone (yellow spot on the horizon - a cement plant quarry). Levoberezhye district, center of Khorkina (blue building), BelSU, Victory Park, Suprunovka district built up with cottages, Railway, Salyut district, two city TV towers, Shchorsa street (it would be more correct to call it avenue), Sfera shopping center, Pervomaisky district.

Victory Park and central part city, Suprunovka.

Shchors Street.

Two city TV towers - old (left) and new. At night they are beautifully illuminated. It is very, very difficult to get to them (unless you are Vadim Makhorov).

Plekhanov and Gorovets streets.

Western industrial zone (cement plant, Energomash, Belatsi).

One of the disadvantages of Belgorod is that there are almost no historical sights here, which makes it of little interest to tourists. Pre-revolutionary buildings are dotted in the center, but they are surrounded on all sides by modern ones. There is not a single old street here, such as cities like Tula, Yaroslavl or Vladimir, except for a couple of streets with Stalinist architecture. Now the entire center is being built up with glass-concrete skyscrapers, and other areas are completely composed of such high-rise buildings. The panel houses in the foreground are typical of the 80s, those that are further away are typical modern architecture of the Belgorod sleeping areas.

Building ceramic cladding. In addition to new houses, it is often used for the restoration of buildings of the Khrushchev and Brezhnev era - square buildings, which already two years after whitewashing begin to look shabby, being lined, they look quite modern and aesthetically pleasing.

Ventilation.

In conclusion, we climbed to the roof of the former Elektrokontakt plant. In Soviet times, the plant produced various electrical equipment, but in the nineties it closed due to unprofitability. Now its buildings house offices and shops, workshops are used as warehouses or are empty.

Radio towers are installed on the roof of one of the buildings.

Nice rooftop.

Nearby is one of the city's attractions, the wooden church of the Holy Martyrs Faith, Hope, Love and their mother Sophia. In 2009 epic burned down but has since been rebuilt.

The roofs of the workshops resemble the streets of an abandoned village. Wondering how to get there?

Gorky Street and Shchors-Korolev-Gorky Ring.

I'm not afraid to say that in terms of education, this was the most interesting excursion since visiting the Exclusion Zone 6 years ago. More than anything, I love industrial insider tours, when you can look at what is close to us, closely related to our lives, but at the same time little known and closed to a person from the street.

I visited Dubovoye (a suburb of Belgorod to the south of Khargora). Hi, Nastya:-)

Such a quiet and pleasant sleeping area. Most of it is occupied by cottages.

This year, the ski complex, which was built for a very long time, finally started working here (although, judging by the reviews, it worked rather conditionally).

If you come to the pedestrian bridge over the private sector that separates the northern and southern parts of the city in the evening, you can meet the Lady of the Goats. Can she be included in the list of urban lunatics?

Pleasure boats on Vezelka.

Why are modern graduates so oversized? At the age of 17, I weighed 55 kilograms, and then you look - just some uncles and aunts. Probably, the hungry nineties, in which we grew up, are to blame :-)

Usually, the sight of the sun setting behind the Belgorod University is associated with J:Morse’s song “Pavetra”, but at that moment it was strange to realize that summer had not even begun yet, although “Spring”, “Voplіv Vіdoplyasova”, which was firmly attached to last April, was already here, too. it didn't fit... It would be interesting to understand why I have such a desire to tie soundtracks to everything.

With you I fly there, de lito, there, de lito,
There, de my soul, flow over the edge.
With you I fly there, de lito, there, de lito,
There, de my soul carry me beyond the sky.

With us, it was summer and laughed,
With us, summer shared its warmth,
And in the evening it crept so unremarkably
The sky is blue with gold and wine ©

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 takes place 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 via radio with one of the antennas of the nearest base station.

Each of the base stations contains from one to twelve receiving and transmitting antennas directed in different directions to provide subscribers with communication from all sides. 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 cable directly to the control unit of the base station. The combination of sectors and the control block is usually called - BS, Base Station. Several base stations, whose antennas serve a certain territory or area of ​​the city, are connected to a special unit - the so-called LAC, Local Area Controller, "local area controller", often referred to simply controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, which can also be several, are connected to the most central "brain" unit - MSC, Mobile services Switching Center, commonly known as switch. The switch provides output (and input) to urban telephone lines, to other mobile operators and so on.

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

Small GSM networks use only one switch, larger networks serving more than a million subscribers can use two, three or more MSc, combined with each other.

Why such 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. The point here is one simple English word - handover. This term refers to handover in cellular networks. That is, when you walk down the street or drive a car (train, bicycle, 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), you need to switch Your phone 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 switching would have to be controlled 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 and your phone move from the coverage area of ​​one sector to the coverage area of ​​another, then the BS control unit is engaged in transferring the phone, without affecting the "superior" devices - LAC and MSc. Accordingly, if the transition occurs between different BS, then it is controlled LAC etc.

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

On the last paragraph stop - what happens when you turn on your phone?

Here, 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 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, Register of Home Subscribers and Register of Guest Subscribers. V HLR stored IMSI all subscribers who are connected to this operator. V VLR in turn, contains data on all subscribers who are currently using the network of this operator. IMSI transferred to HLR(of course, in a highly encrypted form; we will not go into details of encryption in detail, we will only say that another block is responsible for this process - AuC, Authentication Center), HLR, in turn, checks whether it has such a subscriber, and, if so, whether it 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 in parallel 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 may well exceed War and Peace in terms of volume.

Next, we will consider how (and most importantly, why!) The operator writes off money from our account. As you have probably heard, tariff plans there are three different types- the so-called "credit", "advance" and "prepaid", from English Pre-paid i.e. prepaid. What is the difference? Consider how money can be written off during a conversation:

Let's say you made a phone call. It was fixed on the switch - such and such a subscriber called there, 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 switch and then, in the order of the general queue, are transferred to a special block called billing, from English to bill - to pay bills. Billing is responsible for all issues related to subscribers' money - calculates the cost of calls, debits the monthly fee, debits money for services, and so on.

Transfer rate of information from MSc v Billing depends on how much processing power billing, or, in other words, with what speed he manages to translate technical data about calls made into direct money. Accordingly, the more subscribers talk, or the more "brake" billing, the slower the queue will move, respectively, the greater the delay between the conversation itself and the actual debiting of money for this conversation. This fact is connected with the dissatisfaction often expressed by some subscribers - “Supposedly, they steal money! I didn’t talk for two days - they wrote off a certain amount ... ”. But at the same time, it doesn’t take into account at all what conversations that took place, for example, three days ago, the money was not written off right away ... People try not to notice good things ... And these days, for example, billing could simply not work - due to an accident, or due to the fact that it was somehow modernized.

V reverse side– from billing to MSc- there is another queue in which billing informs the switch 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 on the phone - this is just because the “reverse” queue has not yet come up and the switchboard does not yet know that you are a malicious defaulter and You should have been banned for a long time.

Advance tariffs differ from credit tariffs only in the way 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 because it allows you to plan and limit your communication costs to some extent. The second option is a credit one, in which the total cost of all calls for any period (“ billing cycle”), usually for a month, is issued as 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 in the account suddenly runs out and the phone is blocked.

Prepays are arranged in a completely different way:

In prepaid billing as such is commonly referred to as " Prepaid by platform».

Directly at the moment of the beginning of the telephone connection, a direct connection is established between switch and solder platform. No queues, data is transmitted in both directions directly during the conversation, in real time. In connection with this, the following characteristic features are inherent in solders - this is the absence subscription fee(because there is no such thing as billing period), a limited set of additional services (it is technically difficult to charge them in “real time” mode), the impossibility of “going into the red” - the conversation will simply be interrupted as soon as the money in the account runs out. Clear dignity prepaid is the ability to accurately control the amount of money in the account, and, as a result, their expenses.

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

But how is our money calculated when we talk, being 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 SS - Country Code(3 digits) and NC-Network Code(5 digits) - clearly characterize the operator to which this subscriber is connected. These five numbers VLR finds a guest operator 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 HLR home - a link to the same guest VLR to know where to look for the caller.

With the write-off of money in billing, the situation is also not very simple. Due to the fact that the calls are processed by the guest switch, but the money is counted 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 also work in roaming on the principle of prepaid, that is, they write off money in real time.

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

Everything phone calls in roaming are divided into 3 main categories:

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

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

Outgoing call home:

Cost of an international call from the guest region home
+
Cost of an outgoing call from a guest operator

Outgoing call by guest region:

Cost of an outgoing call from a guest operator
+
Some surcharge depending on the specific operator

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 at home. At the same time, one very important thing is revealed - the cost of a minute in roaming absolutely does not depend on the tariff plan chosen by the subscriber.

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

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

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

Phone A - as for outgoing to phone B
(actually, this is logical - after all, he calls him)
Phone B - pays the call 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 in the same region, then this component is equal to zero).
Phone C - pays as for incoming calls from phone A

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

For example, the phone has forwarding on a busy condition to a home number. Then, with an incoming call, the so-called " roaming loop"- the call will go to home phone via guest switch, respectively, the cost of such a forwarded call for roamer will be equal to the sum of the costs of incoming and outgoing home calls, plus the cost of the call 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 when he sees the bill for communication.

This implies 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 call forwarding to voicemail- otherwise, you can later be surprised for a long time - "Where did this money go, huh?"

List of terms used in the text:

AuC– Autentification Center, the Authentication Center, is responsible for encoding information when transmitted over the network and received from the network
billing– Billing, accounting system Money at the operator
BS– Base Station, a base station, several transmitting and receiving antennas belonging to one control device.
camel2– one of the Prepaid systems, which implements instant withdrawal of funds in roaming
CC– Country Code, country code in GSM standard(for Russia - 250)
GSM– Global System for Mobile Communications, the world's most widespread cellular communication standard
Handover - handset control transfer from one antenna/base station/LAC to another
HLR– Home Location Register, a register of home subscribers, contains detailed information about all subscribers connected to this operator.
IMEI– International Mobile Equipment Identification, international serial number equipment in the GSM standard, unique for each device
IMSI– International Mobile Subscriber Identification, the international serial number of a subscriber to GSM services, is unique for each subscriber
LAC– Local Area Controller, Local Area Controller, devices, work manager a certain number of base stations whose antennas serve a certain area.
local area– Local area, territory served by BSs that are part of the same LAC
MSc- Mobile services Switching Center, Mobile Services Control Center, the 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 - a 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 currently use the services of this operator.