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. cellular communication consists in the fact that each zone is subdivided into cells of a hexagonal shape, which, combined, form a structure resembling a honeycomb, as shown in Figure 6.1, a. The hexagonal shape was chosen because it provides the most efficient transmission, roughly corresponding to pie chart directivity and at the same time 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 a high level of interference. In such cases, very small cells are used - pico cells. Indoor pico cells can use the same frequencies as conventional cells this region, especially when favorable environment as, for example, 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 chosen 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 fixed amount 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.
In this way, total number cellular communication channels available in a 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 needed 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 relationship between frequency reuse and cluster size determines how one 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) termination - excess network resources are released, becoming available to other mobile devices.
Figure 6.6 – Handover
To do this, we suggest that you go to the Beeline company.
On the territory of Russia established great amount BS - base stations. Probably, many of you have seen red and white structures towering in the fields or structures installed on the roofs of non-residential buildings. Each such base station is able to pick up a signal from cell phone at a distance of up to 35 km, contacting him via official or voice channels.
After you dialed the number of the desired subscriber on your phone, the following happens: the mobile phone finds the nearest BS, contacts it via the service channel and requests a voice channel. After that, the BS sends a request to the controller (BSC), which then goes to the communicator. If the called party is served by the same operator as you, the communicator will check the Home Location Register (HLR) database to find out exactly where the person you are calling is located and redirect the call to the correct switchboard, which will then transfer the call to the controller and then to the Base Station. And finally, the Base Station will contact the mobile phone of the right person and connect you with him. And if the one you want to talk to is a subscriber of another cellular operator, or you call a landline number, then the switch will “find” the corresponding switch of another network and turn to it. Sounds pretty confusing, right? Let's try to analyze this issue in more detail.
But back to the equipment. As we have already said, from the BS the call is transferred to the controller (BSC). Outwardly, it is not much different from the Base Station:
The number of base stations that are able to serve the controller can reach six dozen. The controller and the BS communicate via optical or radio relay channels. The controller controls the operation of the radio channels.
Below you can see what a switch is:
The number of controllers serviced by the switch varies from two to thirty. The switches are placed in large rooms filled with metal cabinets with equipment.
The task of the switch is to manage traffic. If earlier, in order to talk to each other, subscribers had to first contact the telephone operator, who then manually rearranged the necessary wires, now the switch does an excellent job with her role.
Inside the cars there are devices designed for data collection and processing:
Controllers and switches are under vigilant control 24 hours a day. Tracking is carried out in the so-called CKC (Air Control Center of the Network Control Center).
aslan wrote in February 2nd, 2016Cellular communications have recently become so firmly established in our daily lives that it is difficult to imagine modern society without it. Like many other great inventions, the mobile phone has greatly influenced our lives, and many of its areas. It's hard to say what the future would be like if it wasn't for this convenient form of communication. Definitely the same as in the movie "Back to the Future 2", where there are flying cars, hoverboards, and more, but no cellular service!
But today in a special report for will be a story not about the future, but about how modern cellular communications are arranged and work.
In order to learn about the operation of modern cellular communication in the 3G / 4G format, I invited myself to visit the new federal operator Tele2 and spent the whole day with their engineers, who explained to me all the intricacies of data transmission through our mobile phones.
But first, let me tell you a little about the history of the emergence of cellular communications.
The principles of wireless communication were tested almost 70 years ago - the first public mobile radiotelephone appeared in 1946 in St. Louis, USA. In the Soviet Union, a prototype mobile radiotelephone was created in 1957, then scientists from other countries created similar devices with different characteristics, and only in the 70s of the last century in America were determined the modern principles of cellular communication, after which its development began.
Martin Cooper - inventor of the portable cell phone prototype Motorola phone DynaTAC weighing 1.15 kg and measuring 22.5x12.5x3.75 cm
If in Western countries, by the mid-90s of the last century, cellular communications were widespread and used by a large part of the population, then in Russia it only began to appear, and became available to everyone just over 10 years ago.
Bulky brick-shaped mobile phones that worked in the formats of the first and second generations have gone down in history, giving way to smartphones with 3G and 4G, better voice communication and high speed Internet.
Why is it called cellular? Because the territory on which communication is provided is divided into separate cells or cells, in the center of which there are base stations (BS). In each "cell" the subscriber receives the same set of services within certain territorial boundaries. This means that when moving from one "cell" to another, the subscriber does not feel territorial attachment and can freely use communication services.
It is very important that there is continuity of the connection when moving. This is ensured by the so-called handover, in which the connection established by the subscriber is as if picked up by neighboring cells in a relay race, and the subscriber continues to talk or dig in social networks.
The entire network is divided into two subsystems: the base station subsystem and the switching subsystem. Schematically, it looks like this:
In the middle of the "cell", as mentioned above, is the base station, which usually serves three "cells". The radio signal from the base station is radiated through 3 sector antennas, each of which is directed to its own "cell". It happens that several antennas of one base station are directed to one "cell" at once. This is due to the fact that the cellular network operates in several bands (900 and 1800 MHz). In addition, this base station may have equipment of several generations of communication (2G and 3G) at once.
But on the Tele2 BS towers there is only equipment of the third and fourth generation- 3G/4G, as the company decided to abandon the old formats in favor of new ones that help avoid breaks voice communication and provide more stable internet. Regulars of social networks will support me that in our time the speed of the Internet is very important, 100-200 kb / s is no longer enough, as it was a couple of years ago.
The most common location for a BS is a tower or mast built specifically for it. Surely you could see the red and white BS towers somewhere far from residential buildings (in a field, on a hill), or where there are no tall buildings nearby. Like this one, which is visible from my window.
However, in urban areas it is difficult to find a place for a massive structure. Therefore, in large cities, base stations are placed on buildings. Each station picks up a signal from mobile phones at a distance of up to 35 km.
These are antennas, the BS equipment itself is located in the attic, or in a container on the roof, which is a pair of iron cabinets.
Some base stations are located where you would not even guess. Like on the roof of this parking lot.
The BS antenna consists of several sectors, each of which receives / sends a signal in its own direction. If the vertical antenna communicates with phones, then the round one connects the BS to the controller.
Depending on the characteristics, each sector can serve up to 72 calls at the same time. A BS can consist of 6 sectors and serve up to 432 calls, but usually fewer transmitters and sectors are installed at stations. Cellular operators, such as Tele2, prefer to install more BS to improve the quality of communication. As I was told, the most modern equipment is used here: Ericsson base stations, transport network - Alcatel Lucent.
From the subsystem of base stations, the signal is transmitted towards the switching subsystem, where the connection is established with the direction desired by the subscriber. The switching subsystem has a number of databases that store information about subscribers. In addition, this subsystem is responsible for security. To put it simply, the switch is It has the same functions as the female operators, who used to connect you with the subscriber by hand, only now it all happens automatically.
The equipment for this base station is hidden in this iron cabinet.
In addition to conventional towers, there are also mobile variants of base stations placed on trucks. They are very convenient to use during natural Disasters or in crowded places (football stadiums, central squares) during holidays, concerts and various events. But, unfortunately, due to problems in the legislation, they have not yet found wide application.
In order to provide optimal ground level radio coverage, base stations are designed in a special way, therefore, despite a range of 35 km. the signal does not extend to the flight altitude of aircraft. However, some airlines have already begun installing small base stations on their aircraft to provide cellular communications inside the aircraft. Such a BS is connected to the ground cellular network by 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.
I also looked into the Tele2 office to see how specialists control the quality of cellular communications. If a few years ago such a room would have been hung to the ceiling with monitors showing network data (congestion, network failures, etc.), then over time, the need for such a number of monitors has disappeared.
Technology has evolved over time, and such a small room with a few specialists is enough to monitor the operation of the entire network in Moscow.
A few views from the Tele2 office.
At a meeting of company employees, plans are being discussed to capture the capital) From the beginning of construction to the present day, Tele2 has managed to cover all of Moscow with its network, and is gradually conquering the Moscow region, launching more than 100 base stations weekly. Since I now live in the area, it is very important to me. so that this network comes to my town as soon as possible.
The company plans for 2016 to provide high-speed communications in the metro at all stations, at the beginning of 2016 Tele2 communications are present at 11 stations: 3G / 4G communications at the Borisovo, Delovoy Tsentr, Kotelniki, Lermontovsky Prospekt metro stations , Troparevo, Shipilovskaya, Zyablikovo, 3G: Belorusskaya (Koltsevaya), Spartak, Pyatnitskoye Highway, Zhulebino.
As I said above, Tele2 abandoned the GSM format in favor of third and fourth generation standards - 3G / 4G. This allows you to install 3G / 4G base stations with a higher frequency (for example, inside the Moscow Ring Road, BS stand at a distance of about 500 meters from each other) to provide more stable communication and high speed mobile internet, which was not in the networks of previous formats.
From the company's office, I, in the company of engineers Nikifor and Vladimir, go to one of the points where they need to measure the communication speed. Nikifor stands in front of one of the masts, on which the equipment for communication is installed. If you look closely, you will notice a little further to the left one more such mast, with the equipment of other cellular operators.
Strange as it may seem, but cellular operators often allow their competitors to use their tower structures to accommodate antennas (naturally on mutually beneficial terms). This is because building a tower or mast is expensive, and such an exchange saves a lot of money!
While we were measuring the speed of communication, passers-by grandmothers and uncles asked Nikifor several times if he was a spy)) "Yes, we are jamming Radio Liberty!").
The equipment actually looks unusual, you can assume anything from its appearance.
The company's specialists have a lot of work, given that in Moscow and the region the company has more than 7 thousand employees. base stations: about 5 thousand of them. 3G and about 2 thousand. LTE base stations, and recently the number of BS has increased by about a thousand more.
In just three months, 55% of the total number of new base stations of the operator in the region were put on the air in the Moscow region. AT this moment the company provides high-quality coverage of the territory where more than 90% of the population of Moscow and the Moscow region lives.
By the way, in December, the 3G Tele2 network was recognized as the best in quality among all metropolitan operators.
But I decided to personally check how good Tele2's connection is, so I bought a SIM card in the nearest shopping center on Voikovskaya metro station, with the most simple tariff"Very black" for 299 r (400 sms/minutes and 4 GB). By the way, I had a similar Beeline tariff, which is 100 rubles more expensive.
I checked the speed without moving far from the cash register. Reception - 6.13 Mbps, transmission - 2.57 Mbps. Considering that I am standing in the center of a shopping center, this is a good result, Tele2 communication penetrates well through the walls of a large shopping center.
At metro station Tretyakovskaya. Signal reception - 5.82 Mbps, transmission - 3.22 Mbps.
And on m. Krasnogvardeiskaya. Reception - 6.22 Mbps, transmission - 3.77 Mbps. Measured at the exit of the subway. If we take into account that this is the outskirts of Moscow, it's very decent. I think that the connection is quite acceptable, we can confidently say that it is stable, given that Tele2 appeared in Moscow just a couple of months ago.
There is a stable Tele2 connection in the capital, which is good. I really hope that they will quickly come to the region and I will be able to fully use their connection.
Now you know how cellular communication works!
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Telephone communication is the transmission of speech information over long distances. Telephony allows people to communicate in real time.
If at the time of the emergence of technology there was only one method of data transmission - analog, then at the moment the most different systems communications. Telephone, satellite and mobile connection, as well as IP-telephony provide reliable contact between subscribers, even if they are in different parts of the world. How does it work telephone communications when using each of the methods?
Good old wired (analogue) telephony
The term "telephone" communication is most often understood as analog communication, a method of data transmission that has become familiar for nearly a century and a half. When using this, information is transmitted continuously, without intermediate encoding.
The connection of two subscribers is regulated by dialing, and then communication is carried out by transmitting a signal from person to person over wires in the most literal sense of the word. Subscribers are no longer connected by telephone operators, but by robots, which greatly simplified and reduced the cost of the process, but the principle of operation of analog communication networks remained the same.
Mobile (cellular) communication
Subscribers of cellular operators mistakenly believe that they "cut the wire" connecting them to telephone exchanges. In appearance, everything is so - a person can move anywhere (within the signal coverage), without interrupting the conversation and without losing contact with the interlocutor, and<подключить телефонную связь стало легче и проще.
However, if we understand how mobile communications work, we will find not so many differences from the operation of analog networks. The signal actually "hovers in the air", only from the caller's phone it gets to the transceiver, which, in turn, communicates with the similar equipment closest to the called subscriber ... via fiber optic networks.
The data radio stage covers only the signal path from the phone to the nearest base station, which is connected to other communication networks in a completely traditional way. How cellular communication works is clear. What are its pros and cons?
The technology provides greater mobility than analog data transmission, but carries all the same risks of unwanted interference and the possibility of listening to lines.
Cell signal path
Let us consider in more detail exactly how the signal reaches the called subscriber.
- The user dials a number.
- His phone establishes a radio link with the nearest base station. They are located on high-rise buildings, industrial buildings and towers. Each station consists of transmitting and receiving antennas (from 1 to 12) and a control unit. Base stations that serve one area are connected to the controller.
- From the control unit of the base station, the signal is transmitted via cable to the controller, and from there, also via cable, to the switch. This device provides signal input and output to various communication lines: long-distance, urban, international, and other mobile operators. Depending on the size of the network, it may involve one or several switches connected to each other by wires.
- From "its" switchboard, the signal is transmitted via high-speed cables to the switchboard of another operator, and the latter easily determines which controller the subscriber to whom the call is addressed is located in the coverage area of.
- The switch calls the desired controller, which sends a signal to the base station, which "interrogates" the mobile phone.
- The called party receives an incoming call.
Such a multi-layer structure of the network allows you to evenly distribute the load between all its nodes. This reduces the possibility of equipment failure and ensures uninterrupted communication.
How cellular communication works is clear. What are its pros and cons? The technology provides greater mobility than analog data transmission, but carries all the same risks of unwanted interference and the possibility of listening to lines.
Satellite connection
Let's see how satellite communications, the highest level of development of radio relay communications today, work. A repeater placed in orbit is capable of covering a vast area of the planet's surface alone. A network of base stations, as in the case of cellular communications, is no longer needed.
An individual subscriber gets the opportunity to travel with virtually no restrictions, staying connected even in the taiga or the jungle. A legal entity subscriber can bind an entire mini-PBX to one repeater antenna (this is the already familiar “dish”), however, one should take into account the volume of incoming and outgoing, as well as the size of the files that need to be sent.
Technology cons:
- serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time.
- if something is physically broken on a satellite transponder, the period that will pass until the functionality is fully restored will stretch for a very long time.
- the cost of communication services without borders often exceeds the more usual bills. When choosing a communication method, it is important to consider how much you need such a functional connection.
Satellite communications: pros and cons
The main feature of the "satellite" is that it provides subscribers with independence from land lines. The advantages of such an approach are obvious. These include:
- equipment mobility. It can be deployed in a very short time;
- the ability to quickly create extensive networks covering large areas;
- communication with hard-to-reach and remote territories;
- redundancy of channels that can be used in the event of a breakdown of terrestrial communications;
- the flexibility of the technical characteristics of the network, allowing it to be adapted to almost any requirements.
Technology cons:
- serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time;
- if something physically fails on a satellite transponder, the period that will pass until the system functionality is fully restored will stretch for a long time;
- the cost of communication services without borders often exceeds the more usual bills.
When choosing a communication method, it is important to consider how much you need such a functional connection.
"In any field of science, professors prefer their own
theories to the truth, because their theories are their personal property, and the truth is the common property"
Charles Colton
The principle of building a network and the basic elements of a network
The study of any subject begins with the basics, which is the basis on which the hierarchy of the knowledge tree is built. Without this, any, even the most cunning structure will crumble like a house of cards. Only fools begin to build a house from the roof ... Although if we are talking about metro builders or miners, then this rule does not apply. But their work is not limited to the thoughtless transfer of earth's bowels to iron trolleys. One of our acquaintances independently got acquainted with each event or form, starting with the basics. Any conversation with him, on the most trifling topic, could drag on for several hours. He carefully processed his victim, methodically pumping her brain with a maximum of information about the subject of the conversation. In other words, if you asked him about the principle of operation of an emitter follower, then initially you would have to listen to an hour-long lecture on the creation and evolution of semiconductors. Boredom? For most of us, it may seem that way. However, the real fundamental approach to knowledge lies precisely in this. You can talk about complex things for a long time and abstrusely, but if you do not have basic knowledge, then everything said is as beautiful and fleeting as champagne splashes. Today we will build a certain basis of knowledge about cellular communications. We will talk about the basics of building a modern mobile telephone network.Cellular networks
Telephone communication has penetrated so deeply into our environment that we cannot imagine life without it. Pick up the phone, dial a number and hear the voice of a friend or loved one? What could be easier? But behind this is the enormous work of physicists, technologists, electricians and people of other specialties. In 1947, an event occurred that served as the starting point for the creation of cellular communications. An employee of Bell Laboratories, D. Ring, in an internal memorandum put forward the idea of a cellular principle for organizing mobile networks. The engineer proposed the basic ideas that underlie modern cellular networks to this day. On the one hand, cellular communication is simple and clear, like the movement of a wheel, but as soon as we begin to examine it more closely, all sorts of technical subtleties are revealed, supported by dozens of patents and copyright certificates. At a distance, these details are lost and again the view of an indivisible whole opens up - a cellular communication complex. So, let's discuss building a cellular communication system. It is necessary to identify the main problems that we will face when creating it. To create a cellular network, you need to get a set of frequencies or a frequency range. It is in it that the base station will communicate with your mobile terminal. The basic principle of cellular networks is the principle of frequency reuse. It is he who allows you to significantly increase its capacity and cover an almost unlimited space, while using a finite set of frequencies. Let's pay attention to the picture.We have three frequencies at our disposal (f1, f2, f3). In the first cell (cell) we use the frequency f1. In the second cell (cell), we cannot use the same frequency, that is, f1, due to the interference phenomenon. Interference is a physical phenomenon that occurs when two (or more) waves from the same sources are superimposed and leads to an increase or decrease in the wave amplitude. Therefore, the fight against interference is one of the main tasks in frequency planning, that is, the distribution of frequencies over cells (cells). So, since we cannot use the frequency f1 in the second cell (cell), we use the frequency f2. In the third cell we use the frequency f3, and in the fourth cell we can again use the frequency f1. The picture is extremely simple. However, in practice, engineers face serious problems. Indeed, it is possible to draw the boundaries of honeycombs with thin straight lines only on paper. The real landscape, especially urban, imposes serious restrictions on the geometry of the coverage area of each base station. Therefore, the actual coverage can only be verified experimentally. Since the number of points in space is infinite, it is impossible to check them all. Even if we approximate each place of space in the coverage area of the base station to a cubic meter, then the work is impossible. Hence the appearance of white spots on the coverage map and places with active interference, which leads to interference. In accordance with the recommendations of CEPT, the GSM-900 standard provides for the operation of transmitters in two frequency bands. The frequency band (frequencies at which information is transmitted) 890-915 MHz is used to transmit information from a mobile station (mobile phone) to a base station (uplink). Frequency band 935-960 MHz - for transmitting information from the base station to the mobile station (downlink). When switching channels during a communication session, the duplex spacing (the difference between the transmit and receive frequencies) is constant and equal to 45 MHz. The frequency spacing between adjacent communication channels is 200 kHz. Thus, 124 communication channels (124 channels for all GSM operators in the given region) are located in the 25 MHz wide frequency band allocated for reception / transmission. In addition, another popular range is well known in our country - GSM-1800. The bandwidth for transmitting information from a mobile station (phone) to a base station (uplink) is 1710-1785 MHz, and the bandwidth for transmitting information from a base station to a mobile station (downlink) is 1805-1880 MHz. Duplex spacing - 95 MHz. There are 374 communication channels in the 75 MHz bandwidth. The use of GSM-1800 is expedient in urban environments. The density of subscribers is higher here, and therefore the additional channel is very useful. In addition, high-frequency electromagnetic oscillations have a better penetrating ability through all kinds of technical structures, of which there are a great many in cities. What is the beauty of GSM-900? Since this range lives, it has its own advantages. The main asset can be considered its sufficient purity and accessibility due to ancestry. One can argue with this. However, we believe that this is so. Of course, both the military and special services are sitting in it, but everyone knows that GSM is rushing there, like a locomotive. This is a huge machine that has practically grown together with the state and gives it a lot of money. In addition, GSM-900 works better over long distances. We will return to this issue a little later. Discussion of other frequency bands is beyond our interests, since they have not taken root in Russia and Europe. I would like to note only one thing - there are no significant differences. Everything is practically the same. Just a different frequency range. So, we have discussed the basic operating environment of the GSM cellular network. It is time to dissect its contents, which will tell us what, where and for what is responsible.
Basic elements of a GSM network
Structure and nomenclature - two concepts lead us to the understanding of any entity. Imagine that you have in your hands one of the most important ciphers that reveals the death of President John F. Kennedy. The value of this dispatch is directly proportional to whether you own the code from it. Or suppose you are sitting in a restaurant and the waiter who comes up to you speaks only a rare African dialect. In either case, it is important to understand what they are talking about. Therefore, we start talking about the basic elements of a GSM network. The GSM network structure includes:- BSS (Base Station Subsystem)- subsystem of base stations.
- SSS (Switching Subsystem)- switching subsystem
- OSS (Operation Subsystem)- subsystem of operation and maintenance.
So, the scheme is logically divided into three squares. Each of them is a closed system that performs a specific role assigned to it. Experience has shown that such a separation is worthwhile in terms of control, tracking errors and failures, and building a network. We have to analyze all the elements of this scheme. To begin with, let's take into consideration the BSS base station subsystem (Base Station Subsystem). It consists of:
- - base transceiver stations;
- - base station controller;
- - transcoder.
- radio coverage;
- receiving and transmitting data and service information from/to the mobile station;
- mobile station power control;
- quality control of information transfer, etc.
As we wrote above, the basis of the GSM base station is made up of transceivers. They allow the operator to use up to eight channels. The GSM standard says that two channels are needed to control and exchange information. The number of transmitters at each base station can be up to 24 pieces. It depends on the type of base station and its purpose. Note that one base station can configure up to four cells. Experiments on wave interference and the creation of distant cells have completely failed. We will talk about configuring cell stations in the next material, when we consider the interfaces and principles of GSM communication. Installing base stations and calculating the number of transmitters on them is a separate art. First of all, it is necessary to carry out radio reconnaissance of the territory. For example, the case when you raised one of the base stations high and provided good communication from it over long distances, where other cells are already operating, is unacceptable. Mobile phones will be en masse hung on a cell with a good signal and "spoil" its normal operation. It is very important to consider the number of transmitters on one BS. If the BS/transmitter ratio is less than 1:5, then very often the network will generate an “overload” signal. Any base station is equipped with additional radio relay communication. This is done to apply additional communication bridges within the network. The frequency range for this connection is 3-40 GHz. The power of transmitters can be tens of watts and is regulated by special documents. To communicate with a mobile phone, the base station transmitter radiates five to ten watts of power. All of you, probably, paid attention to the antennas of base station transmitters. They are clearly visible on the towers. In our country, we met only two types of antennas:
- weakly directional with a circular radiation pattern (DN) in the horizontal plane ("Omni" type)
- directional (sectoral) with an opening angle (width) of the main lobe of the DN in the horizontal plane, usually 60 or 120 degrees
- – switching center;
- HLR (Home Location Register)– home location register;
- – guest location register;
- AuC (Authentication Center)– Authentication center.
- signal routing (direction), that is, number analysis for outgoing and incoming calls;
- establishment, control and disconnection of connections.
VLR (Visitor Location Register)- guest location register. HLR (Home Location Register)- the home location register is a computer database of home subscribers - mobile phone users, regardless of the state of the mobile phone (on or off). It contains identification numbers and addresses, as well as subscriber authentication parameters, a list of communication services. The recorded data allows the subscriber to use certain basic and additional services provided by the system. The HLR also stores that part of the mobile station location information that allows the switching center (MSC) to deliver the call to that station. The Home Location Register (HLR) contains the International Mobile Subscriber Identity (IMSI-International Mobile Subscriber Identity). It is used to identify the mobile station at the Authentication Center (AuC). All MSCs and VLRs have remote access to the data contained in the HLR. If there are multiple HLRs in the network, then each HLR represents a specific part of the network's overall subscriber database. VLR (Visitor Location Register)- the guest location register contains approximately the same data as the HLR, but only about active subscribers, that is, about those who are currently in the coverage area of the switch (MSC) to which the VLR belongs. The number of guest location registers (VLRs) is equal to the number of switches (MSCs). Each guest location register is assigned to a specific switch. VLR contains a database of roamers (roamers - subscribers of another GSM system, temporarily using the services of this system as part of the "roaming" procedure) located in the VLR zone. So, the communication subsystem takes on a lot of functions. The GSM switching center directly serves a group of cells and provides all types of connections (voice, messaging and data transmission). Theoretically, the MSC mimics the operation of an ISDN exchange. It is the interface between the fixed networks and the mobile network. Of course, you will not be able to work according to the principle “Young lady? Connect ... ". However, technically, this gateway is not much more complicated than modern switches that are installed for fixed networks. It provides call routing and call control features. However, its important difference is that at the same time it has to solve the problems of switching radio channels. Because of this, communication continuity is achieved when the mobile station moves from cell to cell. In addition, the communication center decides on the switching of working channels in the cell when interference or malfunctions occur. Huge piles of service information flow from it in a continuous stream to the control and maintenance center. This is the statistical data needed to monitor and optimize the network. In addition, the MSC maintains security procedures used to control access to radio channels. Have you heard of roaming? We think so. When two operators agree on the roaming of their subscribers, this means that they can use HLR (Home Location Register) and VLR (Visitor Location Register) together. Rather, each of them gets access to each other's guest register. With the home register, things are a little more complicated. We will discuss this in more detail in the following chapters. The Authentication Center (AuC) perched in a small square on the diagram to the home location register. AuC (Authentication Center)- the authentication center generates parameters for the authentication procedure and determines the encryption keys of subscribers' mobile stations. Authentication procedure - a procedure for confirming the authenticity of a subscriber (validity, legality, availability of rights to use cellular services) of the GSM network. Performing this procedure excludes the presence of unauthorized users ("cell twins") of GSM services. At the moment, the operation of this block in GSM networks has been brought to a fantastic level. Of course, this is only a machine controlled by a program that a person wrote. However, the years of work have not gone unnoticed. It is practically impossible to deceive the authentication center from outside the system. Attempts to clone GSM-devices have failed almost everywhere. The theoretical possibility remains. However, economically, such a twin is absolutely not justified. It remains for us to get acquainted with the last subsystem - operation and maintenance (OSS). OSS (Operation Subsystem)- the subsystem of operation and maintenance provides quality control of the network and management of its components. OSS can troubleshoot the network automatically or with the active intervention of personnel; allows you to manage the network load, provide equipment status checks. OSS consists of two components:
- - operation and maintenance center;
- - network control center.