In the theoretical part, we will not delve into the history of creation cellular communication, about its founders, the chronology of standards, etc. To whom it is interesting - there is plenty of material both in print publications and on the Internet.

Consider what a mobile (cellular) phone is.

The figure shows the principle of operation in a very simplified way:

Fig.1 The principle of operation of a cell phone

A cell phone is a transceiver operating at one of the frequencies in the range of 850 MHz, 900 MHz, 1800 MHz, 1900 MHz. Moreover, the reception and transmission are separated by frequencies.

The GSM system consists of 3 main components such as:

Base station subsystem (BSS - Base Station Subsystem);

Switching/switching subsystem (NSS – NetworkSwitchingSubsystem);

Operation and Maintenance Center (OMC)

In a nutshell, it works like this:

Cellular (mobile) phone interacts with a network of base stations (BS). BS towers are usually installed either on their ground masts, or on the roofs of houses or other structures, or on rented existing towers of all kinds of radio / TV repeaters, etc., as well as on high-rise pipes of boiler houses and other industrial structures.

The phone, after turning on and the rest of the time, monitors (listens, scans) the air for the presence of a GSM signal from its base station. The phone determines the signal of its network by a special identifier. If there is one (the phone is in the network coverage area), then the phone selects the frequency that is best in terms of signal strength and sends a request to the BS to register on the network at this frequency.

The registration process is essentially an authentication (authorization) process. Its essence lies in the fact that each SIM card inserted into the phone has its own unique identifiers IMSI (International Mobile Subscriber Identity) and Ki (Key for Identification). These same IMSI and Ki are entered into the Authentication Center (AuC) database upon receipt of the manufactured SIM cards by the telecom operator. When registering a phone in the network, the identifiers are transmitted by the BS, namely AuC. Then AuC (Identification Center) sends a random number to the phone, which is the key to perform calculations using a special algorithm. This calculation takes place simultaneously in the mobile phone and AuC, after which both results are compared. If they match, then the SIM card is recognized as genuine and the phone is registered on the network.

For a phone, the identifier on the network is its unique IMEI (International Mobile Equipment Identity) number. This number usually consists of 15 digits in decimal notation. For example 35366300/758647/0. The first eight digits describe the model of the phone and its origin. Remaining - serial number phone and check number.

This number is stored in the phone's non-volatile memory. In older models, this number can be changed using a special software(software) and the corresponding programmer (sometimes a data cable), and in modern phones it is duplicated. One copy of the number is stored in the memory area that can be programmed, and the duplicate is stored in the OTP (One Time Programming) memory area, which is programmed by the manufacturer once and cannot be reprogrammed.

So, even if you change the number in the first memory area, then the phone, when turned on, compares the data of both memory areas, and if different IMEI numbers are found, the phone is blocked. Why change all this, you ask? In fact, the laws of most countries prohibit this. Phone by IMEI number is tracked on the network. Accordingly, if the phone is stolen, it can be tracked and seized. And if you have time to change this number to any other (working) number, then the chances of finding a phone are reduced to zero. These issues are dealt with by special services with the appropriate assistance of the network operator, etc. Therefore, I will not delve into this topic. We are interested in the purely technical moment of changing the IMEI number.

The fact is that under certain circumstances this number can be damaged as a result of a software failure or incorrect update, and then the phone is absolutely unusable. This is where all the means come to the rescue to restore the IMEI and the device's performance. This point will be discussed in more detail in the software repair section of the phone.

Now briefly about voice transmission from subscriber to subscriber in the GSM standard. In fact, this is a technically very complex process, which is completely different from the usual voice transmission over analog networks, such as a home wired / radio telephone. Digital DECT radiotelephones are somewhat similar, but the implementation is still different.

The fact is that the subscriber's voice, before it is broadcast, undergoes many transformations. analog signal is divided into segments with a duration of 20ms, after which it is converted to digital, after which it is encoded by using encryption algorithms with the so-called. public key - the EFR system (Enhanced Full Rate - an advanced speech coding system developed by the Finnish company Nokia).

All codec signals are processed by a very useful algorithm based on the principle of DTX (Discontinuous Transmission) - discontinuous speech transmission. Its usefulness lies in the fact that it controls the phone's transmitter, turning it on only at the moment when speech begins and turning it off in the pauses between conversations. All this is achieved with the help of the VAD (Voice Activated Detector) included in the codec - a speech activity detector.

At the received subscriber, all transformations occur in the reverse order.

August 17, 2010

Do you know what happens after you dial a friend's number on your mobile phone? How cellular network finds it in the mountains of Andalusia or on the coast of distant Easter Island? Why does the conversation sometimes suddenly stop? Last week I visited Beeline and tried to figure out how cellular communication works...

A large area of the populated part of our country is covered by Base Stations (BS). In the field, they look like red and white towers, and in the city they are hidden on the roofs of non-residential buildings. Each station picks up a signal from mobile phones at a distance of up to 35 kilometers and communicates with a mobile phone via service or voice channels.

After you dialed a friend's number, your phone contacts the nearest Base Station (BS) via a service channel and asks you to allocate a voice channel. The base station sends the request to the controller (BSC), which forwards it to the switch (MSC). If your friend is on the same cellular network, the switch will check the Home Location Register (HLR) to find out where in this moment the called subscriber is located (at home, in Turkey or in Alaska), and will transfer the call to the appropriate switchboard, from where it will forward it to the controller and then to the Base Station. The Base Station will contact the mobile phone and connect you with a friend. If your friend is a subscriber of another network or you call a landline phone, then your switch will contact the corresponding switch of another network.

Difficult? Let's take a closer look.

The Base Station is a pair of iron cabinets locked in a well-air-conditioned room. Given that in Moscow it was +40 on the street, I wanted to live in this room for a while. Usually, the Base Station is located either in the attic of the building or in a container on the roof:

The Base Station antenna is divided into several sectors, each of which "shines" in its own direction. Vertical antenna communicates with phones, the round one connects the Base Station with the controller:

Each sector can serve up to 72 calls at the same time, depending on the setup and configuration. A Base Station can consist of 6 sectors, so one Base Station can serve up to 432 calls, however, there are usually fewer transmitters and sectors installed in the station. Cellular operators prefer to install more BS to improve the quality of communication.

The Base Station can operate in three bands:

900 MHz - the signal at this frequency spreads further and better penetrates inside buildings
1800 MHz - the signal extends over shorter distances, but allows you to install more transmitters in 1 sector
2100 MHz - 3G network

This is what a cabinet with 3G equipment looks like:

900 MHz transmitters are installed at Base Stations in fields and villages, and in the city, where Base Stations are stuck like needles in a hedgehog, communication is mainly carried out at a frequency of 1800 MHz, although transmitters of all three bands can be present at any Base Station at the same time.

A 900 MHz signal can reach up to 35 kilometers, although the "range" of some Base Stations along the routes can reach up to 70 kilometers, by reducing the number of simultaneously served subscribers at the station by half. Accordingly, our phone, with its small built-in antenna, can also transmit a signal up to 70 kilometers ...

All Base Stations are designed to provide optimum ground level radio coverage. Therefore, despite the range of 35 kilometers, the radio signal is simply not sent to the altitude of the aircraft. However, some airlines have already begun installing low-powered base stations on their aircraft that provide coverage inside the aircraft. Such a BS is connected to the terrestrial cellular network using 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.

The phone can measure signal strength from 32 Base Stations simultaneously. It sends information about the 6 best (by signal level) via the service channel, and the controller (BSC) decides which BS to transmit the current call (Handover) if you are on the move. Sometimes the phone can make a mistake and transfer you to the BS with worst signal, in which case the conversation may be interrupted. It may also turn out that at the Base Station that your phone has selected, all voice lines are busy. In this case, the conversation will also be interrupted.

I was also told about the so-called "top floor problem". If you live in a penthouse, then sometimes, when moving from one room to another, the conversation may be interrupted. This is because in one room the phone can "see" one BS, and in the second - another, if it goes to the other side of the house, and, at the same time, these 2 Base Stations are located on far away from each other and are not registered as "neighboring" mobile operator. In this case, the transfer of a call from one BS to another will not occur:

Communication in the metro is provided in the same way as on the street: Base Station - controller - switch, with the only difference that small Base Stations are used there, and in the tunnel the coverage is provided not by an ordinary antenna, but by a special radiating cable.

As I wrote above, one BS can make up to 432 calls at the same time. Usually this power is enough for the eyes, but, for example, during some holidays, the BS may not be able to cope with the number of people who want to call. This usually happens on New Year when everyone starts to congratulate each other.

SMS are transmitted through service channels. On March 8 and February 23, people prefer to congratulate each other via SMS, sending funny rhymes, and phones often cannot agree with the BS on the allocation of a voice channel.

I was told an interesting story. From one district of Moscow, complaints began to come from subscribers that they could not get through anywhere. Technicians began to understand. Most of the voice channels were free, and all service channels were busy. It turned out that next to this BS there was an institute where exams were taking place and students were constantly exchanging text messages.

Long SMS phone divides into several short ones and sends each one separately. Employees technical service It is advised to send such congratulations via MMS. It will be faster and cheaper.

From the Base Station, the call goes to the controller. It looks as boring as the BS itself - it's just a set of cabinets:

Depending on the equipment, the controller can serve up to 60 Base Stations. Communication between the BS and the controller (BSC) can be carried out via a radio relay channel or via optics. The controller controls the operation of radio channels, incl. controls the movement of the subscriber, signal transmission from one BS to another.

The switch looks much more interesting:

Each switch serves from 2 to 30 controllers. It already occupies a large hall filled with various cabinets with equipment:

10.

11.

12.

The switch performs traffic control. Remember the old movies where people first called the "girl", and then she connected them with another subscriber, rewiring the wires? Modern switches do the same:

13.

To control the network, Beeline has several cars, which they affectionately call "hedgehogs". They move around the city and measure the signal strength own network, as well as the level of the network of colleagues from the "Big Three":

14.

The entire roof of such a car is studded with antennas:

15.

Inside there is equipment that makes hundreds of calls and captures information:

16.

Round-the-clock control over switches and controllers is carried out from the Mission Control Center of the Network Control Center (NCC):

17.

There are 3 main areas for monitoring the cellular network: accident rate, statistics and Feedback from subscribers.

Just like in airplanes, all cellular network equipment has sensors that send a signal to the MCC and output information to the dispatchers' computers. If any equipment is out of order, then the light on the monitor will begin to "blink".

The MSC also keeps track of statistics for all switches and controllers. He analyzes it by comparing it with previous periods (hour, day, week, etc.). If the statistics of some of the nodes began to differ sharply from the previous indicators, then the light on the monitor will begin to "blink" again.

Feedback is received by subscriber service operators. If they cannot solve the problem, then the call is transferred to a technical specialist. If he also turns out to be powerless, then an "incident" is created in the company, which is solved by engineers involved in the operation of the corresponding equipment.

The switches are monitored around the clock by 2 engineers:

18.

The graph shows the activity of Moscow switches. It is clearly seen that almost no one calls at night:

19.

Control over the controllers (sorry for the tautology) is carried out from the second floor of the Network Control Center:

22.

21.

I understand that you still have a lot of questions about how the cellular network works. The topic is complex, and I asked a specialist from Beeline to help me respond to your comments. The only request is to stay on topic. And questions like "Beeline radishes. They stole 3 rubles from my account" - address the subscriber service 0611.

Tomorrow there will be a post about how a whale jumped out in front of me, and I did not have time to photograph it. Stay Tuned!

The communication of mobile, or, as they are also called, cell phones, is carried out not with the help of wires, as in a conventional telephone system, but through radio waves. To make a call on a mobile phone, you need to dial the number as usual. Thus, the radio message arrives at the base station operated by the cellular telephone company.

At the station that handles all calls within a given radius or zone, the controller device determines the call to a free radio channel. In addition, it sends a signal to the automatic telephone exchange of cellular communication. By reading the special codes transmitted by the phone,

ATS monitors the movement of the car in the zone of the first station. If during a call the machine bypasses the zone and enters the next one, the call is automatically transferred to the base station operating in that zone. When calling from a mobile phone, the caller is connected to an automatic telephone exchange for cellular communication, which determines the location mobile phone, requests a free radio channel from the circuit controller and communicates - through the base station - with the desired number. Then the mobile phone rings. When the driver picks up the phone, the circuit is completed.

Base station operation

Each base station receives signals emitted within a radius of three to six miles. To avoid noise, base stations with matching boundaries should operate on different frequency channels. But even within the same city, stations quite remote from each other can easily operate on the same channel.

The local telephone system, which serves both homes and offices, is based on wires that run above and below the ground and are connected to an automatic exchange.

Location and channel

The PBX determines the location of the moving vehicle while the circuit controller routes the call to the communication channel.

Call area

When the car leaves the area of the most remote base station, the driver can no longer use cellular communication. If a call is made on the way to the edge of the zone, the signal gets weaker and weaker and eventually disappears altogether.

On the way from station to station

All over mobile call An automatic telephone exchange for cellular communications fixes the location of a moving car by the strength of the radio signals emanating from it. When the signal becomes too weak, the automatic telephone exchange alerts the base station, which in turn transfers the call to serve the neighboring exchange.

It is hardly possible today to find a person who would never use a cell phone. But does everyone understand how cellular communication works? How is it arranged and how does what we all have long been accustomed to work? Are signals from base stations transmitted over wires, or does it all work in some other way? Or maybe all cellular communication functions only due to radio waves? We will try to answer these and other questions in our article, leaving the description of the GSM standard beyond its scope.

At the moment when a person tries to make a call from his mobile phone, or when they start calling him, the phone connects via radio waves to one of the base stations (the most accessible), to one of its antennas. Base stations can be observed here and there, looking at the houses of our cities, at the roofs and facades of industrial buildings, at skyscrapers, and finally at red-white masts specially erected for stations (especially along highways).

These stations look like rectangular gray boxes, from which various antennas stick out in different directions (usually up to 12 antennas). The antennas here work both for reception and for transmission, and they belong to the mobile operator. Base station antennas are directed in all possible directions (sectors) to provide “network coverage” to subscribers from all sides at a distance of up to 35 kilometers.

An antenna of one sector is able to serve up to 72 calls simultaneously, and if there are 12 antennas, then imagine: 864 calls can, in principle, be served by one large base station at the same time! Although usually limited to 432 channels (72 * 6). Each antenna is connected by cable to the control unit of the base station. And already blocks of several base stations (each station serves its own part of the territory) are attached to the controller. Up to 15 base stations can be connected to one controller.

The base station, in principle, is capable of operating on three bands: the 900 MHz signal penetrates better into buildings and structures, spreads further, so this particular band is often used in villages and fields; the signal at a frequency of 1800 MHz does not spread so far, but more transmitters are installed in one sector, so such stations are more often installed in cities; finally 2100 MHz is a 3G network.

Controllers, of course, in locality or district, there may be several, so the controllers, in turn, are connected by cables to the switch. The task of the switch is to connect the networks of mobile operators with each other and with city lines of the usual telephone communication, long-distance communication and international communication. If the network is small, then one switch is enough; if it is large, two or more switches are used. The switches are interconnected by wires.

In the process of moving a person talking on a mobile phone along the street, for example: he walks, rides in public transport, or moves in a personal car, his phone should not lose the network for a moment, you cannot cut off the conversation.

Communication continuity is obtained due to the ability of the base station network to very quickly switch the subscriber from one antenna to another in the process of moving from the coverage area of one antenna to the coverage area of another (from cell to cell). The subscriber himself does not notice how he ceases to be connected with one base station, and is already connected to another, how he switches from antenna to antenna, from station to station, from controller to controller ...

At the same time, the switch provides optimal load distribution over a multi-layer network scheme in order to reduce the likelihood of equipment failure. A multilevel network is built like this: cellular telephone- base station - controller - switch.

Let's say we make a call, and now the signal has already reached the switch. The switch transfers our call towards the destination subscriber - to the city network, to the international or long-distance communication network, or to the network of another mobile operator. All this happens very quickly using high-speed fiber optic cable channels.

Further, our call arrives at the switchboard, which is located on the side of the receiving call (called by us) subscriber. The "receiving" switch already has data about where the called subscriber is located, in what network coverage area: which controller, which base station. And so, the network polling begins from the base station, the addressee is found, and a call “receives” on his phone.

The entire chain of the described events, from the moment of dialing the number to the moment the call is heard on the receiving side, usually lasts no more than 3 seconds. So we can now call anywhere in the world.

Andrey Povny

The principle of operation of cellular communication

The basic principles of cellular telephony are quite simple. Initially, the FCC established geographic coverage areas for cellular radio systems based on revised 1980 Census data. The idea behind cellular communications is that each area is subdivided into hexagonal cells that, when combined, form a honeycomb-like structure, as shown in the figure. 6.1, a. The hexagonal shape was chosen because it provides the most efficient transmission by approximately matching the circular radiation pattern while eliminating the gaps that always occur between adjacent circles.

A cell is defined by its physical size, population, and traffic pattern. The FCC does not regulate the number of cells in the system and their size, leaving operators to set these parameters in accordance with the expected traffic pattern. Each geographic area is allocated a fixed number of cellular voice channels. The physical dimensions of a cell depend on subscriber density and call structure. For example, large cells (macro cells) typically have a radius of 1.6 to 24 km with a base station transmitter power of 1 W to 6 W. The smallest cells (micro cells) typically have a radius of 460 m or less with a base station transmitter power of 0.1 W to 1 W. Figure 6.1b shows a honeycomb configuration with two cell sizes.

Figure 6.1. – Honeycomb structure of cells a); honeycomb structure with honeycombs of two sizes b) classification of honeycombs c)

Microcells are most commonly used in regions with high population density. Due to their short range, microcells are less susceptible to transmission degradation effects such as reflections and signal delays.

A macro cell may overlap with a group of micro cells, with the micro cells serving slow moving mobile devices and the macro cell serving fast moving devices. The mobile device is able to determine the speed of its movement as fast or slow. This makes it possible to reduce the number of hops from one cell to another and the correction of location data.

The algorithm of transition from one cell to another can be changed at small distances between the mobile device and the base station of the microcell.

Sometimes the radio signals in a cell are too weak to provide reliable indoor communications. This is especially true for well-shielded areas and areas with high level interference. In such cases, very small cells are used - pico cells. Indoor pico cells can use the same frequencies as regular cells in a given region, especially in favorable environments such as in underground tunnels.

When planning systems using hexagonal cells, base station transmitters can be placed in the center of the cell, on the edge of the cell, or at the top of the cell (Figure 6.2 a, b, c, respectively). In cells with a transmitter in the center, omnidirectional antennas are usually used, and in cells with transmitters on the edge or at the top, sector directional antennas are used.

Omnidirectional antennas radiate and receive signals equally in all directions.

Figure 6.2 - Placement of transmitters in cells: in the center a); on edge b); at the top c)

In a cellular communication system, one powerful fixed base station located high above the city center can be replaced by numerous identical low-power stations that are installed in the coverage area at sites located closer to the ground.

Cells using the same radio group can avoid interference if they are properly separated. In this case, frequency reuse is observed. Frequency reuse is the allocation of the same group of frequencies (channels) to several cells, provided that these cells are separated by significant distances. Frequency reuse is facilitated by reducing the coverage area of each cell. The base station of each cell is allocated a group of operating frequencies that are different from the frequencies of neighboring cells, and the antennas of the base station are selected so as to cover the desired coverage area within its cell. Since the service area is limited to the boundaries of one cell, different cells can use the same operating frequency group without mutual interference, provided that two such cells are at a sufficient distance from each other.

Geographic service area cellular system, containing several groups of cells is divided into clusters (Figure 6.3). Each cluster consists of seven cells, which are allocated the same number of full duplex communication channels. Cells with the same letter designations use the same group of operating frequencies. As can be seen from the figure, the same frequency groups are used in all three clusters, which makes it possible to triple the number of available mobile communication channels. Letters A, B, C, D, E, F and G represent seven groups of frequencies.

Figure 6.3 – The principle of frequency reuse in cellular communications

Consider a system with a fixed number of full duplex channels available in some area. Each service area is divided into clusters and receives a group of channels, which are distributed among N cells of the cluster, grouping into non-repeating combinations. All cells have the same number of channels, but they can serve single size areas.

Thus, the total number of cellular communication channels available in the cluster can be represented by the expression:

F=GN (6.1)

where F– number of full-duplex cellular communication channels available in the cluster;

G– number of channels in a cell;

N is the number of cells in the cluster.

If the cluster is "copied" within the given service area m times, then the total number of full-duplex channels will be:

C=mGN=mF (6.2)

where FROM– total number of channels in a given zone;

m is the number of clusters in a given zone.

It can be seen from expressions (6.1) and (6.2) that the total number of channels in a cellular telephone system is directly proportional to the number of cluster "repetitions" in a given service area. If the cluster size decreases while the cell size remains the same, then more clusters will be required to cover a given service area, and the total number of channels in the system will increase.

The number of subscribers who can simultaneously use the same group of frequencies (channels) while not in neighboring cells of a small service area (for example, within a city) depends on the total number of cells in this area. Typically, the number of such subscribers is four, but in densely populated regions it can be much higher. This number is called frequency reuse factor or FRF – frequency reuse factor. Mathematically, it can be expressed as:

(6.3)

where N– total number of full-duplex channels in the service area;

FROM– total number of full duplex channels in the cell.

With the predicted increase in cellular traffic, the increased demand for service is met by reducing the size of the cell, dividing it into several cells, each of which has its own base station. Efficient cell separation allows the system to handle more calls as long as the cells are not too small. If the cell diameter becomes less than 460 m, then the base stations of adjacent cells will influence each other. The ratio between reuse frequencies and cluster size determines how you can change scale cellular system in the event of an increase in subscriber density. The fewer cells in a cluster, the greater the likelihood of crosstalk between channels.

Because cells are hexagonal, each cell always has six equidistant neighboring cells, and the angles between lines connecting the center of any cell to the centers of neighboring cells are multiples of 60°. Therefore, the number of possible cluster sizes and cell layouts is limited. To connect cells to each other without gaps (in a mosaic way), the geometric dimensions of the hexagon must be such that the number of cells in the cluster satisfies the condition:

(6.4)

where N– number of cells in the cluster; i and j are non-negative integers.

Finding a route to the nearest co-channel cells (the so-called first-tier cells) proceeds as follows:

Move on i cells (through the centers of neighboring cells):

Move on j cells forward (through the centers of neighboring cells).

For example, the number of cells in the cluster and the location of the cells of the first tier for the following values: j = 2. i = 3 will be determined from expression 6.4 (Figure 6.4) N = 3 2 + 3 2 + 2 2 = 19.

Figure 6.5 shows the six nearest cells using the same channels as the cell BUT.

The process of handover from one cell to another, ie. when the mobile device moves away from base station 1 to base station 2 (Figure 6.6) includes four main stages:

1) initiation - the mobile device or network detects the need for a handover and initiates the necessary network procedures;

2) resource reservation - with the help of appropriate network procedures, the network resources necessary for handover (voice channel and control channel) are reserved;

3) execution - direct transfer of control from one base station to another;

4) ending - redundant network resources released, becoming available to other mobile devices.

Figure 6.6 – Handover