Mobile communication definition. How does mobile communication work?

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

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

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

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

pay attention to industrial buildings, urban high-rises and special towers. On them are located large gray rectangular blocks with protruding antennas of various shapes. But these antennas are not television or satellite, but transceiver cellular operators. They are directed in different directions to provide communication to subscribers from all directions. After all, we don’t know where the signal will come from and where the unfortunate subscriber with the handset will take us? In professional jargon, antennas are also called “sectors”. As a rule, they are set from one to twelve.

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

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

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

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

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

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

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

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

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

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

Communication is called mobile if the source of information or its recipient (or both) move in space. Radio communication has been mobile since its inception. Above, in the third chapter, it is shown that the first radio stations were intended for communication with moving objects—ships. After all, one of the first radio communication devices A.S. Popov was installed on the battleship Admiral Apraksin. And it was thanks to radio communication with him that in the winter of 1899–1900 it was possible to save this ship, lost in the ice of the Baltic Sea. However, in those years, this “mobile communication” required bulky radio transceiver devices, which did not contribute to the development of much-needed individual radio communications even in the Armed Forces, not to mention private clients.

On June 17, 1946, in St. Louis, USA, telephone business leader AT&T and Southwestern Bell launched the first radiotelephone network for private customers. The elemental base of the equipment was tube electronic devices, so the equipment was very bulky and was intended only for installation in cars. The weight of the equipment without power sources was 40 kg. Despite this, the popularity of mobile communications began to grow rapidly. This created a new problem, more serious than weight and size indicators. An increase in the number of radios, with a limited frequency resource, led to strong mutual interference for radio stations operating on channels close in frequency, which significantly deteriorated the quality of communication. To eliminate mutual interference at repeating frequencies, it was necessary to ensure a minimum one-hundred-kilometer separation in space between two groups of radio systems. That is why mobile communications were mainly used for the needs of special services. For mass implementation, it was necessary to change not only the weight and size indicators, but also the very principle of organizing communication.

As noted above, in 1947 a transistor was invented that performs the functions of vacuum tubes, but has a significantly smaller size. It was the advent of transistors that was of great importance for the further development of radiotelephone communications. The replacement of vacuum tubes with transistors created the preconditions for the widespread introduction of mobile phones. The main limiting factor was the principle of communication organization, which would eliminate or at least reduce the influence of mutual interference.

Studies of the ultrashort wave range, carried out in the 40s of the last century, revealed its main advantage over short waves - wide range, i.e. large frequency capacity and the main disadvantage - strong absorption of radio waves by the propagation medium. Radio waves in this range are not capable of bending around the earth's surface, so the communication range was provided only on the line of sight, and depending on the power of the transmitter, a maximum of 40 km was provided. This disadvantage soon turned into an advantage, which gave impetus to the active mass introduction of cellular telephone communications.

In 1947, an employee of the American company Bell Laboratories D. Ring proposed a new idea for organizing communications. It consisted of dividing space (territory) into small areas - cells (or cells) with a radius of 1–5 kilometers and separating radio communications within one cell (by rationally repeating the communication frequencies used) from communications between cells. Frequency repetition has significantly reduced the problems of using frequency resources. This made it possible to use the same frequencies in different cells distributed in space. In the center of each cell it was proposed to locate a basic receiving and transmitting radio station, which would provide radio communication within the cell with all subscribers. The cell size was determined by the maximum communication range of the radiotelephone device with the base station. This maximum range is called the cell radius. During a conversation, the cellular radiotelephone is connected to the base station by a radio channel through which the telephone conversation is transmitted. Each subscriber must have his own microradio station - a “mobile phone” - a combination of a telephone, a transceiver and a mini-computer. Subscribers communicate with each other through base stations, which are connected to each other and to the public telephone network.

To ensure uninterrupted communication when a subscriber moves from one zone to another, it was necessary to use computer control over the telephone signal emitted by the subscriber. It was computer control that made it possible to switch a mobile phone from one intermediate transmitter to another within just a thousandth of a second. Everything happens so quickly that the subscriber simply does not notice it. Thus, the central part of the mobile communication system is computers. They find a subscriber located in any of the cells and connect him to the telephone network. When a subscriber moves from one cell (cell) to another, computers seem to transfer the subscriber from one base station to another and connect the subscriber of the “foreign” cellular network to “their” network. This happens at the moment when the “foreign” subscriber finds itself in the coverage area of the new base station. Thus, roaming is carried out (which in English means “wandering” or “wandering”).

As noted above, the principles of modern mobile communications were an achievement already at the end of the 40s. However, in those days, computer technology was still at such a level that its commercial use in telephone communication systems was difficult. Therefore, the practical use of cellular communications became possible only after the invention of microprocessors and integrated semiconductor chips.

The first cellular telephone, a prototype of a modern device, was designed by Martin Cooper (Motorola, USA).

In 1973, in New York, on top of a 50-story building, Motorola installed the world's first cellular communications base station under his leadership. It could serve no more than 30 subscribers and connect them to landline lines.

On April 3, 1973, Martin Cooper dialed his boss and said the following words: “Imagine, Joel, that I am calling you from the world's first cell phone. He’s in my hands, and I’m walking down a New York street.”

The phone Martin called from was called Dyna-Tac. Its dimensions were 225x125x375 mm, and its weight was no less than 1.15 kg, which, however, is much less than the 30 kilogram devices of the late forties. Using the device it was possible to make calls and receive signals, and negotiate with the subscriber. This telephone had 12 keys, of which 10 were digital for dialing the subscriber's number, and the other two ensured the start of a conversation and interrupted the call. Dyna-Tac batteries allowed talk time for about half an hour, and required 10 hours to charge.

Although much of the development took place in the United States, the first commercial cellular network was launched in May 1978 in Bahrain. Two cells with 20 channels in the 400 MHz band served 250 subscribers.

A little later, cellular communications began their triumphal march throughout the world. More and more countries realized the benefits and convenience it could bring. However, the lack of a unified international standard for the use of the frequency range eventually led to the fact that the owner of a cell phone, moving from one state to another, could not use the mobile phone.

In order to eliminate this main shortcoming, since the late seventies, Sweden, Finland, Iceland, Denmark and Norway began joint research to develop a single standard. The result of the research was the communication standard NMT-450 (Nordic Mobile Telephone), which was intended to operate in the 450 MHz range. This standard first began to be used in 1981 in Saudi Arabia, and only a month later in Europe. Various variants of the NMT-450 have been adopted in Austria, Switzerland, Holland, Belgium, Southeast Asia and the Middle East.

In 1983, a network of the AMPS (Advanced Mobile Phone Service) standard, which was developed by Bell Laboratories, was launched in Chicago. In 1985, in England, the TACS (Total Access Communications System) standard was adopted, which was a variation of the American AMPS. Two years later, due to the sharply increased number of subscribers, the HTACS (Enhanced TACS) standard was adopted, adding new frequencies and partially correcting the shortcomings of its predecessor. France stood apart from everyone else and began using its own Radiocom-2000 standard in 1985.

The next standard was NMT-900, using frequencies of the 900 MHz range. The new version came into use in 1986. It allowed to increase the number of subscribers and improve the stability of the system.

However, all of these standards are analog and belong to the first generation of cellular communication systems. They use an analog method of transmitting information using frequency (FM) or phase (FM) modulation - as in conventional radio stations. This method has a number of significant disadvantages, the main ones being the ability to listen to conversations of other subscribers and the inability to combat signal fading when the subscriber moves, as well as under the influence of the terrain and buildings. Congestion in frequency ranges caused interference during conversations. Therefore, by the end of the 1980s, the creation of the second generation of cellular communication systems began, based on digital signal processing methods.

Previously, in 1982, the European Conference of Postal and Telecommunications Administrations (CEPT), uniting 26 countries, decided to create a special group Groupe Special Mobile. Its goal was to develop a single European standard for digital cellular communications. The new communication standard was developed over the course of eight years, and was first announced only in 1990 - then the standard specifications were proposed. The special group initially decided to use the 900 MHz band as a single standard, and then, taking into account the prospects for the development of cellular communications in Europe and throughout the world, it was decided to allocate the 1800 MHz band for the new standard.

The new standard is called GSM – Global System for Mobile Communications. GSM 1800 MHz is also called DCS-1800 (Digital Cellular System 1800). The GSM standard is a digital cellular communication standard. It implements time division of channels (TDMA - time division multiple access, message encryption, block coding, as well as GMSK modulation) (Gaussian Minimum Shift Keying).

The first country to launch the GSM network is Finland, which launched this standard into commercial operation in 1992. The following year, the first DCS-1800 One-2-One network went live in the UK. From this moment on, the global spread of the GSM standard throughout the world begins.

The next step after GSM is the CDMA standard, which provides faster and more reliable communications through the use of code division channels. This standard began to emerge in the United States in 1990. In 1993, CDMA (or IS-95) began to be used in the 800 MHz frequency range in the United States. At the same time, the DCS-1800 One-2-One network began operating in England.

In general, there were many communication standards, and by the mid-nineties, most civilized countries were smoothly switching to digital specifications. If the first generation networks allowed the transmission of only voice, then the second generation of cellular communication systems, which is GSM, allows the provision of other non-voice services. In addition to the SMS service, the first GSM phones made it possible to transmit other non-voice data. For this purpose, a data transfer protocol was developed, called CSD (Circuit Switched Data - data transfer over switched lines). However, this standard had very modest characteristics - the maximum data transfer rate was only 9600 bits per second, and then only under the condition of stable communication. However, such speeds were quite enough for transmitting a fax message.

The rapid development of the Internet in the late 90s led to the fact that many cellular users wanted to use their handsets as modems, and the existing speeds were clearly not enough for this.
In order to somehow satisfy the needs of their customers for access to the Internet, engineers invent the WAP protocol. WAP is an abbreviation for Wireless Application Protocol, which translates to Wireless Application Protocol. In principle, WAP can be called a simplified version of the standard Internet protocol HTTP, only adapted to the limited resources of mobile phones, such as small display sizes, low performance of telephone processors and low data transfer rates in mobile networks. However, this protocol did not allow viewing standard Internet pages; they had to be written in WML, which was adapted for cell phones. As a result, although subscribers of cellular networks received access to the Internet, it turned out to be very “stripped down” and uninteresting. Plus, to access WAP sites, the same communication channel was used as for voice transmission, that is, while you are loading or viewing a page, the communication channel is busy, and the same money is debited from your personal account as during the conversation. As a result, a rather interesting technology was practically buried for some time and was used very rarely by subscribers of cellular networks of various operators.
Cellular equipment manufacturers urgently had to look for ways to increase data transfer speeds, and as a result, HSCSD (High-Speed Circuit Switched Data) technology was born, which provided quite acceptable speeds of up to 43 kilobits per second. This technology was popular among a certain circle of users. But still, this technology did not lose the main drawback of its predecessor - the data was still transmitted over the voice channel. The developers again had to engage in painstaking research. The efforts of the engineers were not in vain, and quite recently a technology came into being called GPRS (General Packed Radio Services) - this name can be translated as a packet radio data transmission system. This technology uses the principle of channel separation for voice and data transmission. As a result, the subscriber does not pay for the duration of the connection, but only for the amount of data transmitted and received. In addition, GPRS has another advantage over earlier mobile data technologies - during a GPRS connection, the phone is still able to receive calls and SMS messages. At the moment, modern phone models on the market pause the GPRS connection when making a conversation, which automatically resumes when the conversation ends. Such devices are classified as class B GPRS terminals. It is planned to produce class A terminals that will allow you to simultaneously download data and conduct a conversation with the interlocutor. There are also special devices that are designed only for data transmission, and they are called GPRS modems or class C terminals. Theoretically, GPRS is capable of transmitting data at a speed of 115 kilobits per second, but at the moment most telecom operators provide a communication channel that allows you to reach this speed up to 48 kilobits per second. This is primarily due to the equipment of the operators themselves and, as a consequence, the lack of cell phones on the market that support higher speeds.

With the advent of GPRS, the WAP protocol was again remembered, since now, through the new technology, access to small-volume WAP pages becomes many times cheaper than in the days of CSD and HSCSD. Moreover, many telecom operators provide unlimited access to WAP network resources for a small monthly subscription fee.
With the advent of GPRS, cellular networks ceased to be called second generation networks - 2G. We are currently in the 2.5G era. Non-voice services are becoming increasingly popular as the cell phone, computer and Internet are merging. Developers and operators are offering us more and more different additional services.
Thus, using the capabilities of GPRS, a new message transmission format was created, which was called MMS (Multimedia Messaging Service), which, unlike SMS, allows you to send not only text, but also various multimedia information from a cell phone, for example, sound recordings, photographs and even video clips. Moreover, an MMS message can be transferred either to another phone that supports this format or to an email account.
The increasing power of phone processors now allows you to download and run various programs on it. The Java2ME language is most often used to write them. Owners of most modern phones now have no difficulty connecting to the website of Java2ME application developers and downloading, for example, a new game or other necessary program to their phone. Also, no one will be surprised by the ability to connect the phone to a personal computer in order to, using special software, most often supplied with the handset, save or edit an address book or organizer on a PC; while on the road, using a mobile phone + laptop combination, access the full Internet and view your email. However, our needs are constantly growing, the volume of transmitted information is growing almost daily. And more and more demands are being placed on cell phones, as a result of which the resources of current technologies are becoming insufficient to satisfy our increasing demands.

It is precisely to solve these requests that the fairly recently created third generation 3G networks are designed, in which data transmission dominates over voice services. 3G is not a communication standard, but a general name for all high-speed cellular networks that will grow and are already growing beyond the existing ones. Huge data transfer speeds allow you to transfer high-quality video directly to your phone and maintain a constant connection to the Internet and local networks. The use of new, improved security systems makes it possible today to use a telephone for various financial transactions - a mobile phone is quite capable of replacing a credit card.

It is quite natural that third generation networks will not become the final stage in the development of cellular communications - as they say, progress is inexorable. The ongoing integration of various types of communications (cellular, satellite, television, etc.), the emergence of hybrid devices that include a cell phone, PDA, and video camera will certainly lead to the emergence of 4G and 5G networks. And even science fiction writers today are unlikely to be able to tell how this evolutionary development will end.

Globally, there are currently about 2 billion mobile phones in use, of which more than two-thirds are connected to the GSM standard. The second most popular is CDMA, while the rest represent specific standards used mainly in Asia. Now in developed countries there is a situation of “saturation”, when demand stops growing.

It is hardly possible today to find a person who has never used a cell phone. But does everyone understand how cellular communications work? How does what we have all become accustomed to work and work? Are signals from base stations transmitted through wires or does it all work somehow differently? Or maybe all cellular communications function only through 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 seen 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 gray boxes, from which various antennas stick out in different directions (usually up to 12 antennas). The antennas here work for both reception and transmission, and they belong to the cellular operator. The base station antennas are directed in all possible directions (sectors) to provide “network coverage” to subscribers from all directions at a distance of up to 35 kilometers.

The antenna of one sector is able to service up to 72 calls simultaneously, and if there are 12 antennas, then imagine: 864 calls can, in principle, be serviced by one large base station at the same time! Although they are usually limited to 432 channels (72*6). Each antenna is connected by cable to the control unit of the base station. And blocks of several base stations (each station serves its own 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 inside buildings and structures and spreads further, so this band is often used in villages and fields; a signal at a frequency of 1800 MHz does not travel that far, but more transmitters are installed in one sector, so such stations are installed more often in cities; finally 2100 MHz is a 3G network.

Of course, there may be several controllers in a populated area or region, so the controllers, in turn, are connected by cables to the switch. The purpose of the switch is to connect the networks of mobile operators with each other and with city lines of regular telephone communication, long-distance communication and international communication. If the network is small, then one switch is enough; if it is large, two or more switches are used. The switches are connected to each other 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 driving a personal car, his phone should not lose the network for a moment, and the conversation cannot 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 as he moves from the coverage area of one antenna to the coverage area of another (from cell to cell). The subscriber himself does not notice how he ceases to be connected to 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 across a multi-level network design to reduce the likelihood of equipment failure. A multi-level network is built like this: cell phone - base station - controller - switch.

Let's say we make a call, and the signal has already reached the switchboard. The switch transmits 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 switch, which is located on the side of the recipient of the call (the one we called). The “receiving” switch already has data about where the called subscriber is located, in what network coverage area: which controller, which base station. And so, a network survey begins from the base station, the recipient is located, and a call is received on his phone.

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

Andrey Povny

mobile connection- this is radio communication between subscribers, the location of one or more of which changes. One type of mobile communication is cellular communication.

cellular- one of the types of radio communications, which is based on a cellular network. Key Feature: The total coverage area is divided into cells determined by coverage areas base stations. The cells overlap and together form a network. On an ideal surface, the coverage area of one base station is a circle, so the network made up of them looks like cells with hexagonal cells.

Operating principle of cellular communication

So, first, let's look at how a call is made on a mobile phone. As soon as the user dials a number, the handset (HS - Hand Set) begins searching for the nearest base station (BS - Base Station) - the transceiver, control and communication equipment that makes up the network. It consists of a base station controller (BSC - Base Station Controller) and several repeaters (BTS - Base Transceiver Station). Base stations are controlled by a mobile switching center (MSC - Mobile Service Center). Thanks to the cellular structure, repeaters cover the area with a reliable reception area in one or more radio channels with an additional service channel through which synchronization occurs. More precisely, the exchange protocol between the device and the base station is agreed upon by analogy with the modem synchronization procedure (handshacking), during which the devices agree on the transmission speed, channel, etc. When the mobile device finds a base station and synchronization occurs, the base station controller forms a full-duplex link to the mobile switching center through the fixed network. The center transmits information about the mobile terminal to four registers: the Visitor Layer Register (VLR), the Home Register Layer (HRL), and the Subscriber or Authentication Register (AUC). and equipment identification register (EIR - Equipment Identification Register). This information is unique and is located in the plastic subscription box. microelectronic telecard or module (SIM - Subscriber Identity Module), which is used to check the subscriber’s eligibility and tariffication. Unlike landline phones, for the use of which you are charged depending on the load (the number of busy channels) coming through a fixed subscriber line, the fee for using mobile communications is not charged from the telephone you use, but from the SIM card, which can be inserted into any apparatus.

The card is nothing more than a regular flash chip, made using smart technology (SmartVoltage) and having the necessary external interface. It can be used in any device, and the main thing is that the operating voltage matches: early versions used a 5.5V interface, while modern cards usually have 3.3V. The information is stored in the standard of a unique international subscriber identifier (IMSI - International Mobile Subscriber Identification), which eliminates the possibility of "doubles" - even if the card code is accidentally selected, the system will automatically exclude the fake SIM, and you will not have to subsequently pay for other people's calls. When developing the cellular communication protocol standard, this point was initially taken into account, and now each subscriber has its own unique and only identification number in the world, encoded during transmission with a 64-bit key. In addition, by analogy with scramblers designed to encrypt/decrypt conversations in analogue telephony, 56-bit coding is used in cellular communications.

Based on this data, the system’s idea of the mobile user is formed (his location, status on the network, etc.) and the connection occurs. If during a conversation a mobile user moves from the coverage area of one repeater to the coverage area of another, or even between the coverage areas of different controllers, the connection is not interrupted or deteriorated, since the system automatically selects the base station with which the connection is better. Depending on the channel load, the phone selects between a 900 and 1800 MHz network, and switching is possible even during a conversation, completely unnoticed by the speaker.

A call from a regular telephone network to a mobile user is made in the reverse order: first, the location and status of the subscriber are determined based on constantly updated data in the registers, and then the connection and communication are maintained.

Mobile radio communication systems are built according to a point-multipoint scheme, since the subscriber can be located at any point in the cell controlled by the base station. In the simplest case of circular transmission, the power of a radio signal in free space theoretically decreases in inverse proportion to the square of the distance. However, in practice, the signal attenuates much faster - in the best case, proportional to the cube of the distance, since the signal energy can be absorbed or reduced by various physical obstacles, and the nature of such processes strongly depends on the transmission frequency. When the power decreases by an order of magnitude, the covered area of the cell decreases by two orders of magnitude.

"PHYSIOLOGY"

The most important reasons for increased signal attenuation are shadow areas created by buildings or natural elevations in the area. Studies of the conditions for the use of mobile radio communications in cities have shown that even at very close distances, shadow zones provide attenuation of up to 20 dB. Another important cause of attenuation is tree foliage. For example, at a frequency of 836 MHz in the summer, when the trees are covered with leaves, the received signal level is approximately 10 dB lower than at the same place in the winter, when there are no leaves. The fading of signals from shadow zones is sometimes called slow in terms of the conditions for their reception in motion when crossing such a zone.

An important phenomenon that has to be taken into account when creating cellular mobile radio communication systems is the reflection of radio waves, and, as a consequence, their multipath propagation. On the one hand, this phenomenon is useful, since it allows radio waves to bend around obstacles and propagate behind buildings, in underground garages and tunnels. But on the other hand, multipath propagation gives rise to such difficult problems for radio communications as extended signal delay, Rayleigh fading and worsening of the Doppler effect.

Signal delay stretching occurs due to the fact that a signal passing along several independent paths of different lengths is received several times. Therefore, a repeated pulse can go beyond the time interval allotted for it and distort the next character. Distortion caused by extended delay is called intersymbol interference. At short distances, the extended delay is not dangerous, but if the cell is surrounded by mountains, the delay can extend for many microseconds (sometimes 50-100 μs).

Rayleigh fading is caused by the random phases with which the reflected signals arrive. If, for example, the direct and reflected signals are received in antiphase (with a phase shift of 180°), then the total signal can be attenuated almost to zero. Rayleigh fading for a given transmitter and a given frequency is something like amplitude “dips” that have different depths and are distributed randomly. In this case, with a stationary receiver, fading can be avoided simply by moving the antenna. When a vehicle is moving, thousands of such “dips” occur every second, which is why the resulting fading is called fast.

The Doppler effect manifests itself when the receiver moves relative to the transmitter and consists of a change in the frequency of the received oscillation. Just as the pitch of a moving train or car appears slightly higher to a stationary observer as the vehicle approaches and slightly lower as it moves away, the frequency of a radio transmission shifts as the transceiver moves. Moreover, with multipath signal propagation, individual rays can produce a frequency shift in one direction or another at the same time. As a result, due to the Doppler effect, random frequency modulation of the transmitted signal is obtained, just as random amplitude modulation occurs due to Rayleigh fading. Thus, in general, multipath propagation creates great difficulties in organizing cellular communications, especially for mobile subscribers, which is associated with slow and fast fading of the signal amplitude in a moving receiver. These difficulties were overcome with the help of digital technology, which made it possible to create new methods of coding, modulation and equalization of channel characteristics.

"ANATOMY"

Data transmission is carried out via radio channels. The GSM network operates in the 900 or 1800 MHz frequency bands. More specifically, for example, in the case of considering the 900 MHz band, the mobile subscriber unit transmits on one of the frequencies lying in the range 890-915 MHz, and receives on a frequency lying in the range 935-960 MHz. For other frequencies the principle is the same, only the numerical characteristics change.

By analogy with satellite channels, the direction of transmission from the subscriber device to the base station is called upward (Rise), and the direction from the base station to the subscriber device is called downward (Fall). In a duplex channel consisting of upstream and downstream transmission directions, frequencies differing by exactly 45 MHz are used for each of these directions. In each of the above frequency ranges, 124 radio channels are created (124 for receiving and 124 for transmitting data, spaced at 45 MHz) with a width of 200 kHz each. These channels are assigned numbers (N) from 0 to 123. Then the frequencies of the upstream (F R) and downstream (F F) directions of each channel can be calculated using the formulas: F R (N) = 890+0.2N (MHz), F F (N) = F R (N) + 45 (MHz).

Each base station can be provided with from one to 16 frequencies, and the number of frequencies and transmission power are determined depending on local conditions and load.

In each of the frequency channels, which is assigned a number (N) and which occupies a 200 kHz band, eight time division channels (time channels with numbers from 0 to 7), or eight channel intervals, are organized.

The frequency division system (FDMA) allows you to get 8 channels of 25 kHz, which, in turn, are divided according to the principle of the time division system (TDMA) into another 8 channels. GSM uses GMSK modulation and the carrier frequency changes 217 times per second to compensate for possible quality degradation.

When a subscriber receives a channel, he is allocated not only a frequency channel, but also one of the specific channel slots, and he must transmit in a strictly allotted time interval, without going beyond it - otherwise interference will be created in other channels. In accordance with the above, the transmitter operates in the form of individual pulses, which occur in a strictly designated channel interval: the duration of the channel interval is 577 μs, and the duration of the entire cycle is 4616 μs. Allocation to the subscriber of only one of the eight channel intervals allows the process of transmission and reception to be divided in time by shifting the channel intervals allocated to the transmitters of the mobile device and the base station. The base station (BS) always transmits three timeslots before the mobile unit (HS).

The requirements for the characteristics of a standard pulse are described in the form of a normative pattern of changes in radiation power over time. The processes of turning the pulse on and off, which are accompanied by a change in power by 70 dB, must fit into a time period of only 28 μs, and the working time during which 147 binary bits are transmitted is 542.8 μs. The transmission power values indicated in the table earlier refer specifically to the pulse power. The average power of the transmitter turns out to be eight times less, since the transmitter does not radiate 7/8 of the time.

Let's consider the format of a normal standard pulse. It shows that not all bits carry useful information: here in the middle of the pulse there is a training sequence of 26 binary bits to protect the signal from multipath interference. This is one of eight special, easily recognizable sequences in which the received bits are correctly positioned in time. Such a sequence is fenced with single-bit pointers (PB - Point Bit), and on both sides of this training sequence there is useful encoded information in the form of two blocks of 57 binary bits, fenced, in turn, with boundary bits (BB - Border Bit) - 3 bits each each side. Thus, a pulse carries 148 bits of data, which takes up a 546.12 µs time interval. To this time is added a period equal to 30.44 μs of protective time (ST - Shield Time), during which the transmitter is “silent”. In terms of duration, this period corresponds to the time of transmission of 8.25 bits, but no transmission occurs at this time.

The sequence of pulses forms a physical transmission channel, which is characterized by a frequency number and a time channel slot number. Based on this sequence of pulses, a whole series of logical channels are organized, which differ in their functions. In addition to channels transmitting useful information, there are also a number of channels transmitting control signals. The implementation of such channels and their operation require precise management, which is implemented by software.

Telephone communication is the transmission of voice information over long distances. With the help of telephony, people have the opportunity 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 a variety of communication systems are successfully used. Telephone, satellite and mobile communications, as well as IP telephony, provide reliable contact between subscribers, even if they are in different parts of the world. How does telephone communication work using each method?

Good old wired (analog) telephony

The term “telephone” communication most often refers to analog communication, a method of data transmission that has become commonplace over almost a century and a half. When using this, information is transmitted continuously, without intermediate encoding.

The connection between two subscribers is regulated by dialing a number, and then communication is carried out by transmitting a signal from person to person through wires in the most literal sense of the word. Subscribers are no longer connected by telephone operators, but by robots, which has greatly simplified and reduced the cost of the process, but the operating principle of analog communication networks remains the same.

Mobile (cellular) communications

Subscribers of cellular operators mistakenly believe that they have “cut the wire” connecting them to telephone exchanges. In appearance, everything is so - a person can move anywhere (within 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 many differences from the operation of analogue networks. The signal actually “floats in the air”, only from the caller’s phone it goes to the transceiver, which, in turn, communicates with similar equipment closest to the called subscriber... through fiber optic networks.

The radio data transmission stage only covers the signal path from the phone to the nearest base station, which is connected to other communication networks in a completely traditional way. It's clear how cellular communications work. What are its pros and cons?

The technology provides greater mobility compared to analog data transmission, but carries the same risks of unwanted interference and the possibility of wiretapping.

Cell Signal Path

Let's take a closer look at exactly how the signal reaches the called subscriber.

The user dials a number.
His phone establishes radio contact with a nearby base station. They are located on high-rise buildings, industrial buildings and towers. Each station consists of transceiver antennas (from 1 to 12) and a control unit. Base stations that serve one territory are connected to the controller.
From the base station control unit, 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: intercity, city, international, and other mobile operators. Depending on the size of the network, it may involve either one or several switches connected to each other using wires.
From “your” switch, the signal is transmitted via high-speed cables to the switch of another operator, and the latter easily determines in the coverage area of which controller the subscriber to whom the call is addressed is located.
The switch calls the desired controller, which sends the signal to the base station, which “interrogates” the mobile phone.
The called party receives an incoming call.

This multi-layer network structure allows the load to be evenly distributed between all its nodes. This reduces the likelihood of equipment failure and ensures uninterrupted communication.

It's clear how cellular communications work. What are its pros and cons? The technology provides greater mobility compared to analog data transmission, but carries the same risks of unwanted interference and the possibility of wiretapping.

Satellite connection

Let's see how satellite communications, the highest level of development of radio relay communications today, works. A repeater placed in orbit is capable of covering a huge area of the planet's surface on its own. A network of base stations, as is the case with 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 subscriber who is a legal entity can attach an entire mini-PBX to one repeater antenna (this is the now familiar “dish”), but one must take into account the volume of incoming and outgoing messages, as well as the size of the files that need to be sent.

Disadvantages of technology:

serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time.
If something physically breaks down on a satellite repeater, the time it takes for functionality to be fully restored will take a very long time.
the cost of borderless communication services often exceeds more conventional 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 terrestrial communication lines. The advantages of this approach are obvious. These include:

mobility of equipment. It can be deployed in a very short time;
the ability to quickly create extensive networks covering large territories;
communication with hard-to-reach and remote areas;
reservation of channels that can be used in the event of a breakdown of terrestrial communications;
flexibility of network technical characteristics, allowing it to be adapted to almost any requirements.

Disadvantages of technology:

serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time;
if something physically fails on the satellite repeater, the period until the system’s functionality is fully restored will take a long time;
the cost of borderless communication services often exceeds more conventional bills.

When choosing a communication method, it is important to consider how much you need such a functional connection.