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It’s a little sad that the vast majority of people, when asked: “How does cellular communication work?” answer “over the air” or even “I don’t know.”

Continuing this topic, I had a funny conversation with a friend about work mobile communications. This happened exactly a couple of days before what was celebrated by all signalmen and telecom workers "Radio Day" holiday. It so happened that due to his ardent life position, my friend believed that mobile communication works without wires at all via satellite. Exclusively due to radio waves. At first I couldn't convince him. But after a short conversation everything fell into place.

After this friendly “lecture” the idea came to write in simple language about how cellular communications work. Everything is as it is.

When you dial a number and start calling, or someone calls you, then your mobile phone communicates via radio channel from one of the antennas of the nearest base station. Where are these base stations, you ask?

Please note industrial buildings, city high-rises and special towers. Large gray rectangular blocks with protruding antennas are located on them different forms. But these antennas are not television or satellite, but transceiver operators cellular communications. They are sent to different sides to provide communication to subscribers from all directions. After all, we don’t know where the signal will come from and where the would-be subscriber with the handset will take us? In professional jargon, antennas are also called “sectors”. As a rule, they are set from one to twelve.

From the antenna the signal is transmitted via cable directly to the station control unit. Together they form the base station [antennas and control unit]. Some base stations, whose antennas serve a separate area, for example, a city district or a small locality, connected to a special block - controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, of which there may also be several, are connected by cables to the “think tank” - switch. The switch provides output and input of signals to city telephone lines, to other cellular operators, as well as long-distance and international communications operators.

In small networks, only one switch is used, in larger ones, serving more than a million subscribers at once, two, three or more switches can be used, again interconnected by wires.

Why such complexity? Readers will ask. It would seem you can simply connect the antennas to the switch and everything will work. And here are base stations, switches, a bunch of cables... But it’s not so simple.

When a person moves along the street on foot or by car, train, etc. and at the same time talking on the phone, it is important to ensure continuity of communication. Signalmen call the process of handover of service in mobile networks the term "handover". It is necessary to timely switch the subscriber's phone from one base station to another, from one controller to another, and so on.

If the base stations were directly connected to the switch, then all these switching would have to be managed by the switch. And the “poor” guy already has something to do. The multi-level network design makes it possible to evenly distribute the load across technical means . This reduces the likelihood of equipment failure and resulting loss of communication. After all, we all interested in uninterrupted communication, right?

So, having reached the switch, our call is transferred to then - to the network of another mobile operator, city long-distance and international communications. Of course, this happens over high-speed cable channels communications. The call arrives at the switchboard another operator. At the same time, the latter “knows” in which territory [in the coverage area, which controller] the desired subscriber is currently located. The switch transmits a telephone call to a specific controller, which contains information in the coverage area of ​​which base station the recipient of the call is located. The controller sends a signal to this single base station, and it, in turn, “interrogates”, that is, calls the mobile phone. Tube starts ringing strangely.

This whole long and complex process actually takes 2-3 seconds!

Exactly the same thing happens phone calls to different cities of Russia, Europe and the world. For contact switches of various telecom operators use high-speed fiber optic communication channels. Thanks to them, a telephone signal travels hundreds of thousands of kilometers in a matter of seconds.

Thanks to the great Alexander Popov for giving the world radio! If it weren’t for him, perhaps we would now be deprived of many of the benefits of civilization.

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

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

So, most of the densely populated territory of Russia is covered with so-called BS, which without abbreviation are called Base Stations. Many could pay their attention to them while traveling between cities. IN open field. Base stations are more like towers that have red and white. But in the city, such BS are thoughtfully placed on the roofs of non-residential high-rise buildings. These towers are capable of picking up a signal from any cell phone located 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 for a special service channel and asks it to allocate a voice channel. After receiving a request from the device, the tower 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 MSC smart switch will determine which operator the called subscriber is connected to.

If it turns out that a 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, then 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 person you need is located. He can be anywhere, at home, at work, at the dacha 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, a call from the “responder” switch will go to the “responder” controller, 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 a different network.
Agree, the scheme is quite simple, but difficult to imagine. How a “smart” Base Station finds a phone, sends a request, and the switch itself determines the operator and the other switch. What is a Base Station exactly? It turns out that these are several iron cabinets, which 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 well air-conditioned.

It is logical that the BS has an antenna, which helps it “catch” communications. 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 intended for communication with the controller.

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

As a rule, this power of the Base Station is sufficient. Of course, situations happen when the entire population of our country starts calling each other at the same time. This New Year. For some, it’s enough just to say the cherished phrase “Happy New Year!” into the phone, while others are ready to talk through the clock with an unlimited tariff from the Communications Corporation, discussing guests and plans for the whole night.

However, regardless of the duration of the conversation, the Base Stations cannot cope, and it can be very difficult to reach the subscriber. But on weekdays, most of the year, a BS of six sectors is quite enough, especially since for optimal workload the operator selects Stations in accordance with the population of the territory. Some operators give preference to large BSs in order to improve the quality of the communications 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 range, the station is able to cover a large area, but in the 1800 MHz range the distance will be significantly reduced, but the number of connected transmitters will increase. The third band at 2100 MHz already assumes a new generation of communications - 3G.
It is clear that in sparsely populated areas it is more advisable to install a Base Station at 900 MHz, but in the city 1800 MHz is suitable in order to better penetrate thick concrete walls, and you will need ten times more of these BS 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 in at the moment Since it serves few telephones, it can “break through” up to 70 km. Naturally, our mobile phones can “find” BS even at a distance of 70 km. Base Stations are designed to cover the earth's surface as much as possible and provide large number people communicate precisely on the ground, therefore, while it is possible to catch signals at a distance of at least 35 kilometers, the same distance, but into the sky, the Base Stations do not “pierce”.

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

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

• mobile stations (MS), which move with the subscriber;
• a base station subsystem (BSS), which manages the radio link to the mobile station;
• network subsystem (NSS), the main part of which - the mobile switching center (MSC) - performs switching between mobile stations and between mobile or fixed network users. The MSC also manages activities related to subscriber movement.

The base transceiver station houses a transceiver that implements radio link protocols with the mobile station for one specific cell. A large city usually hosts a large number of BTS. Therefore, the main requirements for BTS are strength, reliability, portability and minimal cost.

The base station controller manages radio resources for one or more BTSs: radio link selection and establishment, frequency hopping, and handover, as discussed below. The BSC is connected between the base transceiver station (BTS) and the mobile switching center (MSC).

1.3.3. Network switching subsystem

Mobile Switching Center (MSC)

The central component of the network subsystem is the mobile switching center (MSC). It operates as a regular switching node of a public switched telephone network (PSTN - Public Switched Telephone Network) or an integrated service digital network (ISDN - Integrated Service Digital Network). Additionally, it provides all the functionality of a mobile subscriber, such as registration, authentication, location update, connection handover and call routing when the subscriber moves. These functions are provided jointly by several functional objects, which together form the network subsystem. The MSC provides connectivity to fixed networks (such as the public switched telephone network (PSTN) or ISDN). The transmission of signals between functional objects in the network subsystem uses SS No. 7 (SS7) - a separate signaling channel, the same as is used for exchange in ISDN and in public networks.

A mobile switching center serves a group of cells and provides all types of connections that a mobile station needs to operate. The MSC is similar to an ISDN switching station and implements the interface between fixed networks (PSTN, PDN, ISDN, etc.) and the mobile network. It provides call routing and call control features. In addition to performing the functions of a conventional ISDN switching station, the MSC is assigned the functions of switching radio channels. These include “handover,” which maintains continuity of communication as a mobile station moves from cell to cell, and switching operating channels within a cell when interference or failure occurs.

Each MSC provides service to mobile subscribers located within a certain geographic area (for example, Moscow and the region). The MSC manages call setup and routing procedures. For the public switched telephone network (PSTN), the MSC provides SS7 protocol signaling functions, call transfer, or support for other types of interfaces in accordance with the requirements of a specific project.

MSC generates the data necessary to issue invoices for communication services provided by the network, accumulates data on completed conversations and transmits them to the billing center. MSC also compiles statistical data necessary for monitoring and optimizing the network. He also supports security procedures, used to control access to radio channels.

The MSC not only participates in call control, but also manages location registration and handoff procedures other than handover in the base station subsystem (BSS). Registering the location of mobile stations is necessary to ensure the delivery of calls to moving mobile subscribers from subscribers of the public telephone network or other mobile subscribers. The call transfer procedure allows connections to be maintained and conversations to be maintained as the mobile station moves from one service area to another. Call transmission in cells managed by a single base station controller (BSC) is handled by that BSC. When call handover occurs between two networks controlled by different BSCs, the primary control occurs in the MSC. The GSM standard also provides procedures for call transfer between networks (controllers) belonging to different MSCs. The switching center continuously monitors mobile stations using a home location register (HLR) and a visiting location register (VLR).

Home Location Register (HLR)

The HLR stores that part of information about the location of a mobile station that allows the switching center to deliver a call to a specific mobile station. In practice, HLR is a reference database of subscribers constantly registered on the network. It contains identification numbers and addresses, as well as subscriber authenticity parameters, the composition of communication services, and special routing information. The subscriber's location changes and roaming data are recorded, including data on the Temporary Mobile Subscriber Identity (TMSI) and the corresponding Visiting Location Register (VLR). The HLR register contains the international mobile subscriber identification number (IMSI - International Mobile Subscriber Identity), the composition of communication services, and special routing information. It is used to identify the mobile station in the authentication center (AUC - Authentication Center).

The Home Location Register (HLR), together with the MSC, provides call routing and location changes (roaming) to the mobile station and contains all the administrative information of each subscriber registered in the associated GSM network, along with the current location of the mobile stations. The location of mobile stations is usually in the form of a given mobile station's address in the VLR. The actual routing procedure will be described later. Logically there is only one