Modern possibilities of mobile communication in russia. How mobile communication works Mobile communication appeared
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 does something that we are all used to for a long time work and work? Are signals from base stations transmitted via 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 outside 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 is connected 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 high-rise buildings, and finally at the red-and-white masts specially erected for stations (especially along highways).
These stations look like rectangular boxes of gray color, from which various antennas (usually up to 12 antennas) stick out in different directions. The antennas here work both for reception and transmission, and they belong to the cellular operator. The base station antennas are directed to all possible directions (sectors) to provide “network coverage” to subscribers from all directions at a distance of up to 35 kilometers.
An antenna of one sector is able to simultaneously serve up to 72 calls, 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 with a cable to the base station control unit. And already blocks of several base stations (each station serves its part of the territory) are connected to the controller. Up to 15 base stations are connected to one controller.
The base station is, in principle, capable of operating on three bands: the 900 MHz signal penetrates better into buildings and structures, spreads further, therefore, it is this band that is often used in villages and in the fields; the signal at a frequency of 1800 MHz does not travel so far, but more transmitters are installed in one sector, therefore, in cities such stations are more often installed; finally 2100 MHz is a 3G network.
There can be several controllers, of course, in a settlement or area, 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 connection, long distance communication and international communication. If the network is small, then one switch is sufficient, if the network is large, two or more switches are used. Switches are interconnected by wires.
In the process of moving a person talking on a mobile phone along the street, for example: he is walking, riding in public transport, or moving in a private car - his phone should not lose the network for a moment, the conversation should not be interrupted.
Continuity of communication is obtained due to the ability of a network of base stations to very quickly switch a 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 associated 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 to reduce the likelihood of equipment failure. A multi-level network is built as follows: cell phone - base station - controller - switch.
Let's say we make a call, and now the signal has already reached the switchboard. The switch transfers our call to 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.
Next, our call goes to the switchboard, which is located on the side of the subscriber receiving the call (called by us). The "receiving" switch already has data about where the called subscriber is located, in which network coverage area: which controller, which base station. And so, from the base station the polling of the network begins, the addressee is found, and his phone “receives a call”.
The entire chain of described events, from the moment the number is dialed to the moment the call rang out on the receiving side, usually lasts no more than 3 seconds. This is how we can call anywhere in the world today.
Andrey Povny
In this article we will tell you about the history of the emergence of mobile communications
The first radiotelephone communication system appeared in 1946 in the USA - St. Louis. Radiotelephones operated on fixed frequencies and were manually switched. In the Soviet Union, radiotelephone communication appeared in 1959 and was called the Altai system. Naturally, it was not publicly available, but was used as a government connection and special services. In 1990-1994, during the collapse of the USSR, from the Soviet research institutes, a large mass of classified developments, including the development of multi-frequency, multi-base radiotelephone communications, were taken out of the cordon "free of charge". And in 1991 in the USA, and later in Russian Federation appeared new standard radiotelephone - cellular communication NMT-450 ("Sotel"). Used analog signal... Subsequently, digital standards appeared - GSM-900 and GSM-1800.
With the progressive development of cellular communications, mobile telephones have become widely available. As a rule, a mobile telephone (hereinafter MTA) can operate at a distance of up to 1500 m from the base station.
As you know, each cellular device is assigned its own electronic serial number(ESN), which is encoded in the phone's microchip when the phone is manufactured. By activating the SIM-card (Subscriber Identity Module) - a microchip in which the subscriber number is "stitched", the mobile phone receives a mobile an identification number(MIN).
The area covered by the GSM (Global System for Mobile communications) network is divided into separate adjacent cells (cells) - hence the name "cellular communications", in the center of which there are transceiver base stations. Typically, such a station has six transmitters, which are located with a 120 ° radiation pattern and provide an even coverage of the area. One medium modern station can simultaneously serve up to 1000 channels. The area of the "honeycomb" in the city is about 0.5-1 km 2, outside the city, depending on the geographical location, it can reach both 20 and 50 km 2. The telephone exchange in each "cell" is controlled by the base station, which receives and transmits signals in a wide range of radio frequencies (dedicated channel - the step for each cell phone is minimal). The base station is connected to a wired telephone network and equipped with equipment for converting the high-frequency signal of a cell phone into a low-frequency signal wired telephone and vice versa, which ensures the conjugation of these two systems. Technically modern equipment of the base station occupies an area of 1 ... 3 m 2 and is located within one small room, where its work is carried out in automatic mode... For the stable operation of such a station, only a wired connection with a telephone exchange (ATS) and a 220 V mains supply are required.
In cities and towns with a large congestion of houses, base station transmitters are located directly on the roofs of houses. In the suburbs and in open areas, towers are used in several sections (they can often be seen located along the highway).
The coverage area of neighboring stations is contiguous. When a telephone moves between the coverage areas of neighboring stations, it is periodically registered. Periodically, with an interval of 10 ... 60 minutes (depending on the operator), the base station emits a service signal. Having received it, the mobile phone automatically adds its MIN- and ESN-numbers to it and transmits the resulting code combination to the base station. Thus, the identification of a specific mobile cellular telephone set, the account number of its owner and the binding of the device to a certain zone in which it is located in this moment time. This moment is very important - already at this stage it is possible to control the movement of this or that object, and who benefits from it, the question is different - the main thing is there is an opportunity ...
When a user connects with someone on his phone, the base station assigns him one of the free frequencies of the zone in which he is located, makes the appropriate changes to his account (deducts funds) and transfers his call to the destination.
If mobile user during a conversation, it moves from one communication zone to another, the base station of the leaving zone (cell) automatically transfers the communication signal to the free frequency of the adjacent zone (cell).
The analog principle of information transmission is based on the emission of a non-digital radio signal into the air, therefore, having tuned in to the corresponding frequency of such a communication channel, it is theoretically possible to listen to the conversation. However, it is worth "to cool especially hot heads" - to listen to cellular communications of this standard not so easy, since they are encrypted (distorted) and an appropriate decoder is needed for accurate speech recognition. Negotiations of this standard are easier to find than, say, the GSM-digital cellular communication standard, whose mobile phones transmit and receive information in the form of a digital code. Stationary or stationary objects performing cellular communication are easiest to find, while mobile ones are more difficult, since the movement of a subscriber during a conversation is accompanied by a decrease in signal power and a transition to other frequencies (when transmitting a signal from one base station to a neighboring one).
Direction finding methods
The first is a method of targeted direction finding of a cell phone, which determines the direction to a working transmitter from three to six points and gives an intersection of the location of the radio signal source. The peculiarity of this method is that it can be applied at someone's order, for example, bodies authorized by law.
The second method is through a cellular operator, which automatically constantly registers where this or that subscriber is at a given time, even if he does not conduct any conversations. This registration takes place automatically by identifying service signals automatically transmitted by the cell phone to the base station (this was discussed earlier). The accuracy of determining the location of the subscriber depends on a number of factors: topography of the area, the presence of interference and signal reflection from buildings, the position of base stations and their congestion (the number of active mobile phones of the operator in a given cell), the size of the cell. Hence, the accuracy of determining the location of a cellular subscriber in a city is noticeably higher than in an open area, and can reach a spot of several hundred meters. Analysis of data on the subscriber's communication sessions with various base stations (from which and to which station the call was made, call time, etc.) allows you to restore a picture of all the subscriber's movements in the past. The data is automatically registered with the mobile operator (for billing and not only ...), since the payment for such services is based on the duration of the use of the communication system. This data can be stored for several years, and this time is not yet regulated by federal law, only by departmental acts.
You can conclude - confidentiality is provided, but not for everyone. If it is necessary to eavesdrop on your conversations, or determine your location, almost any "equipped" special service, or criminal community, can do this without any effort.
It is more difficult to intercept a conversation if it is being conducted from a moving vehicle. the distance between the cell phone user and the direction finding equipment (when it comes to analog communication) is constantly changing and if these objects move away from each other, especially in rough terrain among houses, the signal weakens. When moving quickly, the signal is transferred from one base station to another, with a simultaneous change of the operating frequency - this makes it difficult to intercept the entire conversation as a whole (if it is not conducted purposefully with the participation of the telecom operator), since it takes time to find a new frequency.
You can draw conclusions from this yourself. Switch off your cell phone if you do not want your location to be known.
Hidden features of cell phones
A modern MTA can be switched on to the dictaphone mode (recording sounds from the built-in microphone) automatically by a signal, or by a given program, without the authorization of its owner. It is not a fact that every MTA records the speech and voice of the owner, and then transmits the information, but such a possibility is technically provided in every modern MTA. It is akin to a gun hanging on the wall. And if the action takes place during a performance in a theater, it is almost obvious that the gun will go off before the end of the performance. So in this case, the MTA has the ability to record and transfer information, and this factor must be taken into account when using your "mobile phone".
The information is received by the station closest to the MTA - cell. How is the transmission of information on the air? The MTA communicates with the station in bursts of digital pulse signals called time slots. The duration of one service communication session can last from fractions of a second to several seconds.
The MTA maintains such intercom sessions with the base station all the time when the cell phone is in the on state. Initially, this happens after turning on the power of the MTA, then the phone, communicating with the nearest communication station of its operator (according to the installed SIM card), positions its position on the ground, broadcasts its data (for example, the identification number of a cell phone in the network, etc.) , that is, it is registered on the network. Based on this registration, during subsequent negotiations, this subscriber is charged for connections, communication services, tariffication of calls and roaming. In addition to the time slots in the communication session when the power is turned on, the MTA periodically, approximately once an hour (and with active movement, constantly) communicates with a nearby base station, positioning its position and, if necessary (going outside the cell), registering in the zone responsibility of another neighboring base station. The duration and frequency of service communication sessions (time slots) for different MTAs is different and is (frequency) from 10 to 35 times a day. In this case, the duration of time slots fluctuates in the range of 2-25 milliseconds.
In many modern MTAs, the functions of various types of service informing the owner are automatically enabled, for example, about the weather forecast or news, so the time slots for such a phone will be more frequent and longer. In this case, it is impossible to determine what kind of signals your "mobile phone" sends to the base station without special equipment. It is only possible to record the very fact of a short communication session that occurred without the participation of the owner of the MTA. In any case, if you received an SMS message, then there was an exchange of time slots.
Every owner of a cell phone should know this feature of "his" MTA, despite the fact that the manufacturing companies are not in a hurry to share this information with the buyers of their goods, or to explain these functions and their purpose. As the saying goes, forewarned is protected ... An indirect sign of the MTA's operation for high-power transmission is a rapidly discharging battery.
How to check a cell phone
At the dawn of the mass popularization of cell phones (and this was not so long ago), mobile telephones (MTA), purchased abroad and requiring Russification, prevailed among the population. In addition, some of the cell phones brought from abroad to the CIS (bought on the secondary market, because it is cheap), when the local operator's SIM card was connected, turned out to be blocked (they did not implement some of the functions declared in the MTA menu and in its operating manual) ... People carried the MTA to the appropriate service (according to the name of the MTA) and sometimes received an answer: your phone will not work in Russia. Since then, MTAs, brought from abroad by private order, began to be secretly divided into "white" and "gray". "White" can be reanimated and used in the CIS "by full program", And the" gray "ones are practically hopeless, or require such investments that outweigh its very cost. Therefore, for some time now, "gray" mobile devices enter Russia only in single copies, or in batches imported by small "shuttle traders", or after Russians have a rest abroad, due to their ignorance. In this regard, a test method for checking MTA was born.
For the test, you must sequentially press the keys on the keyboard: * # 06 #. As a result, the series and model number indicated in the passport data will be displayed. The same data is printed on the MTA case under the battery. How will they help?
The specified data is the IMEI (International Mobile Equipment Identifier) of your MTA. After this procedure of notification to the cellular company, your MTA, together with the SIM-card (or even a newly inserted one), will be under the control of your cellular operator. It is better to find out this number in advance (when buying or using an MTA) and write it down somewhere away from prying eyes. If the device is lost or stolen, this data must be transferred to your cellular operator. This is necessary in order for your MTA to be found exactly, or by at least, would have been blocked from service by the operator you were using before you lost your phone.
Cellular communication has recently become so firmly established in our daily life that it is difficult to imagine modern society without it. Like many other great inventions, the mobile phone has greatly influenced our lives, and many of its areas. It is difficult to say what the future would be like if it were not for this convenient form of communication. Probably the same as in the movie "Back to the Future-2", where there are flying cars, hoverboards, and much more, but no cellular connection!
But today in a special report for there will be a story not about the future, but about how modern cellular communication is arranged and works.
In order to learn about the work of modern cellular communication in the 3G / 4G format, I asked to visit the new federal operator Tele2 and spent a whole day with their engineers, who explained to me all the details of data transmission through our mobile phones.
But first, I'll tell you a little about the history of the emergence of cellular communications.
The principles of wireless communication were tested almost 70 years ago - the first public mobile radiotelephone appeared in 1946 in St. Louis, USA. In the Soviet Union, a prototype of a mobile radiotelephone was created in 1957, then scientists from other countries created similar devices with different characteristics, and only in the 70s of the last century in America the modern principles of cellular communication were determined, after which its development began.
Martin Cooper - the inventor of the prototype of the portable cell phone Motorola DynaTAC weighing 1.15 kg and dimensions 22.5x12.5x3.75 cm
If in Western countries, by the mid-90s of the last century, cellular communication was widespread and used by most of the population, then in Russia it only began to appear, and became available to everyone a little over 10 years ago.
Bulky brick-like mobile phones that worked in the first and second generation formats have gone down in history, giving way to smartphones with 3G and 4G, better voice communication and high Internet speed.
Why is the connection called cellular? Because the territory in which communication is provided is divided into separate cells or cells, in the center of which base stations (BS) are located. In each "cell" the subscriber receives the same set of services within certain territorial boundaries. This means that moving from one "cell" to another, the subscriber does not feel territorial attachment and can freely use communication services.
It is very important that there is continuity of the connection when moving. This is provided thanks to the so-called handover, in which the connection established by the subscriber is, as it were, picked up by neighboring cells on the relay, and the subscriber continues to talk or dig in social networks.
The entire network is divided into two subsystems: a base station subsystem and a switching subsystem. Schematically, it looks like this:
In the middle of a "cell", as mentioned above, there is a base station, which usually serves three "cells". The radio signal from the base station is emitted through 3 sector antennas, each of which is directed to its own "cell". It so happens that several antennas of one base station are directed to one "cell" at once. This is due to the fact that the cellular network operates in several bands (900 and 1800 MHz). In addition, this base station may have equipment of several generations of communication (2G and 3G) at once.
But on the BS Tele2 towers there is only equipment of the third and fourth generation - 3G / 4G, since the company decided to abandon old formats in favor of new ones, which help to avoid interruptions in voice communication and provide a more stable Internet. Regulars of social networks will support me in the fact that nowadays Internet speed is very important, 100-200 kb / s is not enough anymore, as it was a couple of years ago.
The most common location for the BS is a tower or mast built specifically for it. Surely you could see the red and white BS towers somewhere far from residential buildings (in a field, on a hill), or where there are no tall buildings nearby. Like this one that is visible from my window.
However, in urban areas it is difficult to find a place for a massive structure. Therefore, in large cities, base stations are located on buildings. Each station picks up a signal from mobile phones at a distance of up to 35 km.
These are antennas, the BS equipment itself is located in the attic, or in a container on the roof, which is a pair of iron cabinets.
Some base stations are located where you wouldn't even guess. Like on the roof of this parking lot.
The BS antenna consists of several sectors, each of which receives / sends a signal in its direction. If the vertical antenna communicates with telephones, then the round antenna connects the BS to the controller.
Depending on the characteristics, each sector can handle up to 72 calls simultaneously. The BS can consist of 6 sectors, and serve up to 432 calls, but usually fewer transmitters and sectors are installed at the stations. Cellular operators, such as Tele2, prefer to install more base stations to improve the quality of communication. As I was told, the most modern equipment is used here: Ericsson base stations, transport network - Alcatel Lucent.
From the base station subsystem, the signal is transmitted towards the switching subsystem, where the connection is established with the direction desired by the subscriber. The switching subsystem has a number of databases that store information about subscribers. In addition, this subsystem is responsible for security. To put it simply, the switch performs It has the same functions as the female operators who used to connect you with the subscriber by hand, only now all this happens automatically.
The equipment for this base station is hidden in this iron cabinet.
In addition to conventional towers, there are also mobile versions of base stations placed on trucks. They are very convenient to use during natural disasters or in crowded places (football stadiums, central squares) during holidays, concerts and various events. But, unfortunately, due to problems in the legislation, they have not yet found wide application.
To ensure optimal radio coverage at ground level, base stations are designed in a special way, therefore, despite the range of 35 km. the signal does not apply to the flight altitude of the aircraft. However, some airlines have already begun to install small base stations on their aircraft that provide cellular communications inside the aircraft. Such a BS connects to a terrestrial cellular network using 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, for example, turning off the voice on night flights.
I also looked into the Tele2 office to see how specialists control the quality of cellular communication. If a few years ago such a room would have been hung up to the ceiling with monitors showing network data (congestion, network failures, etc.), then over time the need for such a number of monitors has disappeared.
Technologies have developed greatly over time, and such a small room with several specialists is enough to monitor the operation of the entire network in Moscow.
Few views from the Tele2 office.
At a meeting of the company's employees, plans to capture the capital are discussed) From the beginning of construction until today, Tele2 has managed to cover the whole of Moscow with its network, and is gradually conquering the Moscow region, launching more than 100 base stations weekly. Since I now live in the region, it is very important to me. so that this network would come to my town as quickly as possible.
The company plans for 2016 to provide high-speed communication in the metro at all stations, at the beginning of 2016 Tele2 communication is present at 11 stations: 3G / 4G communication at the Borisovo metro, Delovoy Tsentr, Kotelniki, Lermontovsky Prospekt , Troparevo, Shipilovskaya, Zyablikovo, 3G: Belorusskaya (Koltsevaya), Spartak, Pyatnitskoe shosse, Zhulebino.
As I said above, Tele2 abandoned the GSM format in favor of the third and fourth generation standards - 3G / 4G. This allows the installation of 3G / 4G base stations with a higher frequency (for example, inside the Moscow Ring Road, BSs stand at a distance of about 500 meters from each other) in order to provide more stable communication and high speed mobile internet, which was not in the networks of the previous formats.
From the company's office, I, in the company of engineers Nikifor and Vladimir, go to one of the points where they need to measure the communication speed. Nikifor stands opposite one of the masts on which communications equipment is installed. If you look closely, you will notice another such mast a little further on the left, with equipment from other cellular operators.
Oddly enough, but cellular operators often allow their competitors to use their tower structures to accommodate antennas (of course, on mutually beneficial terms). This is because building a tower or mast is expensive and can save you a lot of money!
While we were measuring the speed of communication, Nikifor several times passers-by grandmothers and uncles asked if he was a spy)) "Yes, we are jamming Radio Liberty!).
The equipment actually looks unusual, from its appearance you can assume anything.
The company's specialists have a lot of work, considering that in Moscow and the region the company has more than 7 thousand. base stations: of which about 5 thousand. 3G and about 2 thousand. base stations LTE, and recently the number of BS has increased by about a thousand more.
In just three months, 55% of the total number of new base stations of the operator in the region were put on the air in the Moscow region. At the moment, the company provides high-quality coverage of the territory where more than 90% of the population of Moscow and the Moscow region live.
By the way, in December the 3G Tele2 network was recognized as the best in quality among all 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 Voykovskaya metro station, with the simplest "Very black" tariff for 299 rubles (400 sms / minutes and 4 GB). By the way, I had a similar Beeline tariff, which is 100 rubles more expensive.
I checked the speed on the spot. Reception - 6.13 Mbps, transmission - 2.57 Mbps. Considering that I am standing in the center of a shopping center, this is a good result, Tele2 communication penetrates well through the walls of a large shopping center.
At metro Tretyakovskaya. Signal reception - 5.82 Mbps, transmission - 3.22 Mbps.
And at the Krasnogvardeyskaya metro station. Reception - 6.22 Mbps, transmission - 3.77 Mbps. I measured it at the exit from the subway. If you take into account that this is the outskirts of Moscow, it is very decent. I think that the connection is quite acceptable, we can confidently say that it is stable, considering that Tele2 appeared in Moscow just a couple of months ago.
Tele2 has a stable connection in the capital, which is good. I really hope that they will come to the region as soon as possible and I will be able to take full advantage of their connection.
Now you know how cellular communication works!
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Introduction
Algorithm of functioning of cellular communication systems
Initialization and communication
Authentication and identification
Handover (when routing)
Roaming
Service of calls in the GSM standard
Conclusion
Introduction
The computerization of telecommunication equipment is going on in parallel with the privatization of national communication systems, the emergence of large firms - operators on the market, which leads to increased competition. As a result, prices for telecommunications services are reduced, their range is expanding, and users have a choice.
Most industrialized countries are intensively switching to a digital communication standard, which allows instantaneous transmission of colossal amounts of information with a high degree of protection of its content. In world telecommunications, the trend of development of full-service networks based on the technology of switching packets of services is clearly manifested.
Currently, the top ten countries that have the most developed communication and telecommunication systems that meet world standards include Singapore, Sweden, New Zealand, Finland, Denmark, USA, Hong Kong, Turkey, Norway and Canada. In the ranking of countries in terms of the level of development of telecommunication systems, Kazakhstan is inferior not only to industrially developed, but also to many developing countries.
Demand information Technology, modern computers and office equipment in recent years has had a significant impact on the dynamics and structure of the global economy. A real revolution in the field of information technology was the emergence and rapid development of the cellular communication system, which was formed by the beginning of the third millennium into one of the leading sectors of the world economy.
cellular communication roaming
1. Algorithm of functioning of cellular communication systems
The algorithms for the functioning of cellular communication systems of various standards are basically similar. When a mobile station is in standby mode, its receiver constantly scans either all channels or only control channels (CC). To call a subscriber, a call signal is transmitted by all BSs over the control channels. The called subscriber's PS, upon receipt of this signal, answers one of the free CUs. The BS that received the response signal transmits information about its parameters to the switching center (CC), which switches the conversation to the BS where the maximum signal level of the MS of the called subscriber is fixed.
During dialing, the PS occupies one of the free channels, the signal level of the BS in which is currently the maximum. As the subscriber moves away from the BS or due to the deterioration of the signal propagation conditions, the subscriber automatically switches to another free channel or another BS. A special procedure called relay transmission (Handover) allows you to seamlessly switch the conversation to a free channel of another BS, in the range of which the subscriber is. To control such situations, the BS is equipped with a special receiver that periodically measures the signal level from the SS and compares it with the permissible threshold. (In some PS models, the received signal level is periodically measured and its quality is assessed). If the signal level is less than the threshold, then information about this is automatically transmitted to the switching center via the service communication channel. The switching center issues a command to measure the signal from this subscriber to other BSs (several at once) surrounding the subscriber MS. After receiving a response from these BSs, the switching center selects the most suitable BS.
If all BS channels are busy serving subscribers and at this time a request for service is received from the next subscriber, then, as a temporary measure(before the release of one of the channels), it is possible to use the principle of handover even within one cell. In this case, the call is not blocked, but all the subscribers participating in the communication are alternately switched from channel to channel. In this process, you can alternately give some time from all channels to a new subscriber. A kind of "spare" channel is formed.
One of the most important services of a cellular network is the provision of a set of services for a subscriber from the same mobile station (radiotelephone) in other cities, regions and even other countries, the so-called roaming (Roaming).
To provide such a service between operators of cellular networks, there must be an agreement for the provision of roaming to subscribers who come from areas served by other operators.
2. Initialization and communication
In the operation of a substation in the service area of its "own" network, four modes can be distinguished, which are essentially similar for systems of different standards:
· Standby mode;
· Communication (call) establishment mode;
· Mode of communication (telephone conversation).
If the substation is completely turned off (de-energized), then after turning on the power to the substation, the process is automatically performed initialization -initial launch... During this mode, the PS is configured to work as part of the system - according to signals regularly transmitted by base stations via control channels (CC). Upon completion of initialization, the PS goes into standby mode. The specific content of the initialization operations depends on the standard of the cellular communication system.
In standby, PS monitors:
· Changes in information from the side of the system, associated both with changes in the operation of the system, and in connection with the movements of the PS itself;
· System commands (for example, to confirm operability, measure the level of the received signal, etc .;
· Receiving a call from the system;
· Initiation of a call from own subscriber.
In addition, the MS can periodically, for example, once every 10-15 minutes, confirm its operability by transmitting appropriate signals to the BS or transmit other messages for the system, regardless of the communication session. In the switching center (CC) for each of the switched on MS, a cell is fixed in which it is "registered", which facilitates the organization of the procedure for calling a mobile subscriber. If the PS does not confirm its operability within a certain time, then the CC considers it disabled and the call arriving at this PS is not transmitted. Therefore, the power supply of the MS is usually not turned off and the MS is in the standby mode.
Procedure establishing communication is as follows. If from the side of the system or from the PSTN network a call to the mobile subscriber's number arrives at the CC, then the CC directs this call to the BS of the cell in which the PS was registered, or to several BSs in the vicinity of this cell (taking into account the possible movement of the subscriber). BS transmit the call over the corresponding call channels. If the MS is in the standby mode, then it accepts the call and answers it through its BS, simultaneously transmitting data for the authentication procedure. If the authentication result is positive through the BS, the traffic channel is assigned to the MS and the frequency channel number is reported. The mobile station tunes in to the dedicated channel and, together with the BS, performs the necessary actions to prepare for the communication session. At this stage, the MS, according to the synchronization signals, is tuned to the specified slot number in the frame, specifies the time delay, adjusts the level of the radiated power, etc. The choice of the time delay is made for the purpose of temporal matching of slots in the frame (for reception in the BS) when organizing communication with mobile stations located at different distances from the BS. In this case, the time delay of the burst transmitted by the MS is regulated by the BS commands.
Then the BS issues a call (ring) message, which is confirmed by the mobile station, and caller hears a ringtone. When the called subscriber answers the call (“off-hook”), the MS issues a request to the BS to terminate the connection. With the end of the connection, the actual communication session (conversation) begins.
During the conversation, the PS processes the transmitted and received speech signals, as well as the control signals transmitted simultaneously with the speech. At the end of the conversation, service messages are exchanged between the MS and the BS (request or command to disconnect with confirmation), after which the MS transmitter is turned off and the station goes into standby mode (standby mode).
If the call is initiated by the MS, i.e. the PS subscriber dials the called number, makes sure on the display that the dialing is correct and presses the corresponding call button on the PS panel, then the PS sends through its BS a message indicating the number of the called subscriber and data for station authentication. After successful authentication, the BS assigns a traffic channel. The subsequent steps to prepare a communication session are performed in the same way as when a call comes from the system.
If a connection is established between two mobile subscribers, then the procedure for establishing a connection is practically no different from establishing a connection with PSTN subscribers, since all connections are established through the mobile communications switch CC (MSC). If both mobile subscribers belong to the same cellular system, then communication is established through the CC without going out to the switches of the PSTN network.
3. Authentication and identification
Authentication and identification procedures are performed each time a connection is established. Authentication - the procedure for confirming the authenticity (validity, legality, rights to use the services of the cellular network) of the subscriber. Identification is the procedure for identifying a mobile device (i.e., mobile station). At the same time, the affiliation of the PS to one of the groups with certain characteristics is determined, as well as faulty and stolen devices are identified.
The idea of the authentication procedure in a digital system is to encrypt some passwords-identifiers using quasi-random numbers, periodically transmitted to the PS from the CC, and an encryption algorithm that is individual for each PS. Such encryption, using the same initial data and algorithms, is performed both at the substation and at the CC (or at the authentication center). Authentication is considered successful if both results match.
4. Handover (when routing)
A base station located approximately in the center of a cell serves all MSs within its cell. When the MS moves from one cell to another, its service is accordingly transferred to another BS. The handover process takes place without interruption of communication, i.e. happens relay transmission service. If the MS moves from one cell to another in the standby mode, it simply monitors these movements according to the system information transmitted over the control channels, and at the right moment re-tunes to a stronger signal of the other BS.
The handover decision is made by the switching center. A command is sent from the switching center to the "new" BS for handover so that this BS can allocate the necessary channels, and then the necessary commands are sent to the SS via the "old" BS with an indication of the new frequency channel, working slot number, etc. PS automatically rebuilds to a new channel and tunes to joint work with a new BS. The restructuring process takes a split second and remains invisible to the subscriber.
5. Roaming
Roaming is a function or procedure for providing cellular services to a subscriber of one operator in the system of another operator (of course, in compatible standards). When a subscriber moves to another network and goes online, the central switch of the new network requests (via special communication channels) information about the subscriber from the original network where the user is registered. If the subscriber has a confirmation of authority, the new network registers it with itself. The data on the location of the subscriber is constantly updated in the original network and all incoming calls are automatically forwarded to the network where the subscriber is currently located.
To organize roaming, the networks participating in such a contract must have compatible standards. The switching centers of all networks must be interconnected by special communication channels (wire lines, telephone lines, radio communication, etc.) for the exchange of service data.
There are three types of roaming: manual, semi-automatic and automatic. When roaming is done manually, there may be no service connections between CCs. Just when a subscriber moves to another network, he exchanges his radiotelephone for another connected to the new system. In the semi-automatic version, the subscriber must first inform his operator about the transition to the service system of another network.
Much more complex operations have to be performed with automatic roaming. A subscriber of a cellular network who finds himself on the territory of another network initiates a call in the usual way, as in his own network. The CC of the new network, making sure that this subscriber does not appear in its home register HLR, perceives him as a roamer and enters the guest register VLR. At the same time (or with some delay), he asks the HLR of the “native” roamer system for information related to him necessary for organizing the service (specified types of services, passwords, ciphers), and also informs in which system the roamer is currently. The new location is captured in the native HLR system. After that, the roamer uses cellular communication in the new system, just like at home. Calls outgoing from him are handled in the usual way, with the only difference that the information related to him is not recorded in the HLR. And in the VLR. Calls to the roamer number in its "home" network are forwarded by the "home" network to the system where the roamer is visiting. Upon the return of the roamer home in the HLR of the "native" system, the address of the system where the roamer was located is erased, and in the VLR of that system, the information about the roamer is erased.
In the GSM standard, the roaming procedure is laid down as a mandatory element. In addition, in the GSM standard there is the possibility of roaming with SIM cards with the rearrangement of these cards from one device to another to support various variants of the GSM standard (GSM-900, GSM-1800? GSM-1900), since all three variants of the standard use unified SIM - cards. The roaming procedure in the GSM standard becomes even more convenient with the advent of dual-mode and, in the future, triple-mode subscriber terminals providing operation in all frequency bands of the GSM standard.
6. Service of calls in the GSM standard
When considering cellular telephone networks within the global network, it should be borne in mind that a subscriber is connected not just to a mobile switch, but directly to a network that can unite not only several cellular networks within one country, but also the networks of many countries. In general, the following service areas of the global telephone network can be distinguished:
· Honeycomb (Cell);
· Area of location or search (Location Area);
· The service area of the central mobile switch (MSC Service Area);
· Service area of the cellular telephone network (CTS) of public use with several switching centers (PLMN Service Area);
· The service area of the global system (GSM service Area).
The cell is here understood as the service area of this BS (BTS). A location or search area unites a number of cells controlled by one or more controllers (BSC), but within a single mobile switch (MSC). At the same time, within the limits of the location area, the subscriber can freely move without updating the data in the guest register (VLR). In addition, within this service area, the address is transmitted to search for a specific MS.
The Service Switching Center (MSC) is part of the common system... The subscriber is registered in the VLR of a particular CC and he can freely move within this service area without transferring his subscriber data to another VLR and updating the data in the HLR.
The service area of public cellular communication systems is determined by the service areas of each switching center included in this system and through which access to other telecommunication networks is carried out, including to other service areas of public cellular telephone networks.
The service area of the global cellular telephone network unites all the service areas of the national cellular telephone networks. This means that all national cellular networks must be built in accordance with the GSM standard.
This approach to the functional organization of the global network by zones determines the network numbering system. Taking into account that the GSM cellular telephone network can provide communication between the SS and the subscribers of the fixed PSTN (in the future ISDN), and through it with the subscribers of other telecommunication networks, it should be included in the general numbering plan of the fixed PSTN network in accordance with the recommendations of the CCITT E.164.
In this case, the mobile station number in the general MSISDN numbering plan (Mobile Station ISDN Number) contains: country code, network code, subscriber number. For Russia, such a number will be presented in the form: 7АВСавххххх. However, the GSM STS is dedicated and can unite the STS of different countries. Therefore, in accordance with the recommendations of the GSM standard, a single numbering is adopted within the GSM network, and upon registration, a subscriber is assigned a single international IMSI number, the length of which should not exceed 15 digits. The structure of the IMSI number is similar to the structure of the MSISDN, but the country code in the GSM network has 3 digits; 1-2 digits under the network code; under the subscriber's number maximum 11 digits. In addition, a problem arises when routing incoming calls to the CC from the PSTN network due to the fact that the substation, moving freely, can change the service areas (and, for example, end up in the area of another PBX with a different numbering). As a result, unlike fixed telephone networks, the list number (MSIDN IMSI) cannot contain a logical direction code that uniquely identifies the MSC in the service area of which the called PS is currently located. To provide routing capability, each MSC (VLR) has a plurality of MSRNs at its disposal, which are provided on demand to the head MSC (if the system has multiple MSCs) only for the duration of the call routing to a particular MSC. With this in mind, the MSRN, unlike the MSISDN, does not contain a subscriber number, but a number that identifies the MSC. In the MSC (VLR), the allocated MSRN number is one-to-one with the IMSI number of the called MS. To determine the search area (location) in the GSM network, the LAI number is used, which differs from the IMSI number in that the location area code is indicated here instead of the subscriber number.
Along with the considered numbers used in the call routing process, the GSM standard provides a number for identifying equipment IMEI and a temporary subscriber number TMSI, used to ensure confidentiality. The IMEI number includes codes for the type of equipment and manufacturer, serial number. The TMSI number is determined by the network administration and should be no more than 4 bytes in length.
Subscriber authentication, mobile station equipment identification and information closure
To ensure authentication and closure of information during registration, a subscriber is assigned not only an IMSI number, but also an individual subscriber key Ki, which is stored in the Authentication Center (AUC), as well as in the equipment of the mobile station. The subscriber key Ki in the authentication center is used to form a triplet: the information closing key Kc, the SRES tagged response, and random number RAND (fig. 1). First, a random number RAND is generated. RAND and Ki are the initial data for calculating Kc and SRES. In this case, two different calculation algorithms are used. The generated triplets for each of the subscribers registered in the GSM network are transferred to the HLR register, and, if necessary, are provided to the guest register of the switching center. The algorithm for calculating Kc and SRES is implemented not only in the authentication center, but also in the mobile station.
Rice. 1. Formation of Kc, SRES, RAND
In the GSM standard, the authentication procedure is associated with the use of a Subscriber Identity Module (SIM). A SIM module is a removable plastic card inserted into a subscriber device slot. This card contains an electronic chip in which all the necessary information is "sewn up". The SIM module allows you to conduct a conversation from any device of the same type, including a payphone. The module contains the subscriber's PIN, IMSI, Ki key, individual subscriber authentication algorithm A3, algorithm A8 for calculating the encryption key. Unique identificator The IMSI for the current job is replaced with a temporary TMSI assigned to the machine when it first registers in a particular region, identified by the LAI, and cleared when the machine leaves that region. The PIN identifier is a code known only to the subscriber, which should serve as protection against unauthorized use of the SIM card. For example, if you lose it. After three unsuccessful PIN dialing attempts SIM card blocked. The blocking can be removed either by dialing an additional code (known only to the subscriber) - personal unblocking code (PUK), or by a command from the switching center.
The authentication procedure is as follows. When the MS requests access to the network, the AUC authentication center transmits a random number RAND to the MS via the MSC (Switching Center). The mobile station, having received the RAND number and using the stored subscriber key Ki, uses the A3 algorithm to calculate the tagged SRES response. Having generated the SRES, the mobile station transmits it to the MSC, where the received SRES is compared with the SRES calculated by the network. If they match, access to the network is allowed for the PS. The authentication procedure is carried out when registering a PS, trying to establish a connection, updating data, as well as when activating and deactivating additional types of services. The authentication procedure is shown in Fig. 2.
Rice. 2. Principle of authentication
The identification of the user equipment itself begins with a request from the PS for the IMEI number. The switching center (MSC) transmits the received IMEI number to the Equipment Identity Register (EIR), where there are three lists of PS equipment: permitted for use, prohibited for use in the communication system and faulty. Based on the information in the lists, it is determined to which group the PS with IMEI number). The results are sent to the switching center, where a decision is made about the access of the user equipment to work in the network.
The closure of user information transmitted over the radio channel is carried out in the BS and in the PS. Both use the same encryption algorithms for transmitted messages. To close the user information, the access cycle number and the information close key Kc are used. In the BS, the key Kc from the triplet is used, and in the PS it is calculated based on the received random number RAND and the subscriber key Ki according to the A8 algorithm.
Algorithm A8 is used to compute to compute the message encryption key and is stored in the SIM module. After receiving the RAND, the mobile station calculates, in addition to the SRAS response, also the encryption key Kc using RAND, Ki and the A8 algorithm according to Fig. 2. In addition to RAND, the network sends to the PS the numerical sequence of the encryption key. This number is related to the Kc value and avoids generating an incorrect key. The Kc value is stored in the PS and is contained in every first message transmitted to the network.
Rice. 3. Setting the encryption mode
To set the encryption mode, the network sends the CMC (Ciphering Mode Command) command to the PS to switch to the encryption mode, after which the PS using the Kc key proceeds to encrypt and decrypt messages. The transmitted data stream is encrypted bit by bit or with a stream cipher using the A5 encryption algorithm and the Kc key. The procedure for setting the encryption mode is shown in Fig. 3.
Conclusion
In each country, the management of the telecommunications industry has its own specifics. However, the emergence of digital technologies and the massive introduction of services to provide access to the Internet have led to the fact that today almost any telecom operator operates not only in the local (regional or national), but also in the global telecommunications market.
The advent of digital technology has fueled radical changes in the telecommunications industry. Traditional voice services began to be superseded by interactive services such as the Internet, data transmission, mobile connection.
But, despite the changes, the domestic communication services market remains rather closed. On the one hand, this is due to the huge scale of the country's territory, thanks to which the main incomes of telecom operators are formed. On the other hand, Kazakhstan is still outside the world market of international traffic, which until now has been insufficient high level digitalization of main channels and more Low quality communication compared to world standards. increase.
Despite the high rates of implementation modern technologies, the percentage of coverage of the population of the Republic of Kazakhstan with new types of communication, such as cellular communication, paging, the Internet remains low.
List of sources used
1. Yu.A. Gromakov. TDMA frame structure and signaling in the GSM standard. "Electrosvyaz". N 10. 1993. p. 9-12.
M. Mouly, M.B. Pautet. The GSM System for Mobile Communications. 1992. p.p. 702.
A. Mehrotra. Cellular Radio: Analog and Digital Systems. Artech House, Boston-London. 1994.p.p. 460.
4. Yu.A. Gromakov. TDMA frame structure and signal formation in the GSM standard. "Electrosvyaz" .N10.1993.p.9-12.
5. W. Heger. GSM vs. CDMA. GSM Global System for Mobile Communications. Proceedings of the GSM Promotion Seminar 1994 GSM MoU Group in Cooperation with ETSI GSM Members. 15 December 1994. p.p. 3.1-1 - 3.1-18.
Sukachev E.A. Cellular networks radio communication with mobile objects: Textbook. - Ed. 2nd, rev. and add. - Odessa: UGAS, 2000 .-- 119s
Yu.A. Gromakov. Cellular mobile radio systems. Electronic communication technologies. Volume 48. Eco-Trends. Moscow. 1994.
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