Digital telecommunication system. The main types of telecommunications systems What is telecommunications

Network classification

The classification of fuel assemblies is based on the most characteristic functional, informational and structural features.

According to the degree of territorial dispersal network elements (subscriber systems, communication nodes) distinguish between global (state), regional and local computer networks (WAN, RCS and LAN).

By the nature of the implemented functions networks are divided into computing (the main functions of such networks are information processing), informational (for obtaining reference data at the request of users), information and computing, or mixed, in which computing and information functions are performed in a certain, non-constant ratio.

By way of management TVS are divided into networks with centralized(the network has one or more governing bodies), decentralized(each AS has the means to manage the network) and mixed management, in which, in a certain combination, the principles of centralized and decentralized control are implemented (for example, under centralized control, only tasks with the highest priority associated with the processing of large amounts of information are solved).

On the organization of information transfer networks are divided into networks with information selection and information routing. In networks with a selection of information, built on the basis of a monochannel, the interaction of the AU is carried out by selecting (selection) the data blocks (frames) addressed to them: all the AUs of the network have access to all the frames transmitted in the network, but only the AUs to which they are intended take a copy of the frame. In networks with information routing Multiple routes can be used to transfer frames from a sender to a receiver. Therefore, with the help of communication systems of the network, the problem of choosing the optimal (for example, the shortest time to deliver a frame to the addressee) route is solved.

By type of data transfer organization networks with information routing are divided into networks with circuit (channel) switching, message switching and packet switching. Networks that use mixed data transmission systems are in operation.

According to the topology those. configuration of elements in TVS, networks are divided into two classes: broadcast and serial. Broadcast configurations and a significant part of the serial configurations (ring, star with an intelligent center, hierarchical) are characteristic of the LAN. For global and regional networks, the most common is an arbitrary (mesh) topology. The hierarchical configuration and the “star” have also found application.

V broadcast configurations at any time, only one workstation (subscriber system) can work to transmit a frame. Other PCs in the network can receive this frame, i.e. such configurations are typical for a LAN with information selection. The main types of broadcast configuration are common bus, tree, star with passive center. The main advantages of a LAN with a common bus are the ease of expanding the network, the simplicity of the management methods used, the absence of the need for centralized management, and the minimum cable consumption. A tree topology LAN is a more advanced version of a bus topology network. A tree is formed by connecting several buses with active repeaters or passive multipliers (“hubs”), each branch of the tree is a segment. The failure of one segment does not lead to the failure of the others. In a LAN with a star topology, there is a passive connector or an active repeater in the center - quite simple and reliable devices.

In sequential configurations, typical for networks with information routing, data transmission is carried out sequentially from one PC to a neighboring one, and different types of physical transmission medium can be used in different parts of the network.

The requirements for transmitters and receivers are lower than in broadcast configurations. Sequential configurations include: arbitrary (cellular), hierarchical, ring, chain, star with an intelligent center, snowflake. In the LAN, the ring and the star are most widely used, as well as mixed configurations - star-ring, star-bus.

In a ring topology LAN, signals travel in only one direction, usually counterclockwise. Each PC has a memory of up to a whole frame. When a frame moves around the ring, each PC receives the frame, analyzes its address field, makes a copy of the frame, if it is addressed to this PC, and retransmits the frame. Naturally, all this slows down data transfer in the ring, and the duration of the delay is determined by the number of PCs. Removing a frame from the ring is usually done by the sending station. In this case, the frame makes a full circle around the ring and returns to the sending station, which perceives it as a receipt - confirmation of the receipt of the frame by the addressee. Removing a frame from the ring can also be carried out by the receiving station, then the frame does not complete a full circle, and the sending station does not receive confirmation receipts.

The ring structure provides a fairly wide functionality of the LAN with high efficiency of the use of a monochannel, low cost, simplicity of control methods, and the possibility of monitoring the performance of a monochannel.

In broadcast and most serial configurations (with the exception of the ring), each cable segment must provide signal transmission in both directions, which is achieved: in half-duplex communication networks - using one cable for alternate transmission in two directions; in duplex networks - using two unidirectional cables; in broadband systems - the use of different carrier frequencies for simultaneous transmission of signals in two directions.

Global and regional networks, like local ones, in principle can be homogeneous (homogeneous), in which software-compatible computers are used, and heterogeneous (heterogeneous), including software-incompatible computers. However, given the length of the hot water supply and RVS and the large number of computers used in them, such networks are more often heterogeneous.

The main function of telecommunication systems (TCS), or data transmission systems (DTS) is to organize a prompt and reliable exchange of information between subscribers. The main indicator of the effectiveness of the TCS - the time of information delivery - depends on a number of factors: the structure of the communication network, the bandwidth of the communication lines, the methods of connecting communication channels between the interacting subscribers, the information exchange protocols, the methods of subscribers' access to the transmission medium, the methods of packet routing.

Types of networks, lines and communication channels. TVS uses communication networks - telephone, telegraph, television, satellite. The following are used as communication lines: cable (regular telephone lines, twisted pair, coaxial cable, fiber-optic communication lines (FOCL, or light guides), radio relay, radio lines.

Among cable lines communication, the best indicators have light guides. Their main advantages are: high bandwidth (hundreds of megabits per second), due to the use of electromagnetic waves in the optical range; insensitivity to external electromagnetic fields and the absence of their own electromagnetic radiation, low labor intensity of laying an optical cable; spark, explosion and fire safety; increased resistance to aggressive environments; low specific gravity (ratio of linear mass to bandwidth); wide areas of application (creation of collective access highways, communication systems for computers with peripheral devices of local networks, in microprocessor technology, etc.).

Disadvantages of FOCL: signal transmission is carried out only in one direction; connecting additional computers to the light guide significantly attenuates the signal; the high-speed modems required for fiber optics are still expensive; the light guides connecting the computers must be supplied with converters of electrical signals into light and vice versa.

The following have been used in TVS types of communication channels:

simplex, when the transmitter and receiver are connected by one communication line, through which information is transmitted in only one direction (this is typical for television communication networks);

half duplex, when two communication nodes are also connected by one line, through which information is transmitted alternately in one direction, then in the opposite direction (this is typical for information-reference, request-response systems);

duplex, when two communication nodes are connected by two lines (a forward link and a reverse link), along which information is simultaneously transmitted in opposite directions.

Switched and dedicated communication channels. In the TCS, there are dedicated (non-switched) communication channels and those with switching for the duration of information transmission over these channels.

Using dedicated channels communication, the transceiver equipment of communication centers is constantly connected to each other. This ensures a high degree of readiness of the system for information transfer, higher quality of communication, and support for a large amount of graphics. Due to the relatively high costs of operating networks with dedicated communication channels, their profitability is achieved only if the channels are fully loaded.

For switched channels connections created only for the time of transmission of a fixed amount of information are characterized by high flexibility and relatively low cost (with a small amount of traffic). The disadvantages of such channels are: loss of time for switching (establishing communication between subscribers), the possibility of blocking due to the busyness of individual sections of the communication line, lower communication quality, high cost with a significant amount of traffic.

Analog and digital coding of digital data. Data transfer from one TCS node to another is carried out by serial transmission of all message bits from the source to the destination. Physically, information bits are transmitted in the form of analog or digital electrical signals. analog called signals, which can represent an infinite number of values of some quantity within a limited range. Digital(discrete) signals may have one or a finite set of values. When working with analog signals, an analog carrier signal of a sinusoidal form is used to transmit encoded data, and when working with digital signals, a two-level discrete signal is used. Analog signals are less sensitive to media attenuation distortion, but data encoding and decoding is easier for digital signals.

Analog coding It is used in the transmission of digital data over telephone (analogue) communication lines, which are dominant in regional and global TVS and initially focused on the transmission of acoustic signals (speech). Before transmission, digital data, usually coming from a computer, is converted into an analog form using a modulator-demodulator (modem) that provides a digital-to-analog interface.

There are three ways to convert digital data to analog form or three modulation methods:

amplitude modulation, when only the amplitude of the carrier of sinusoidal oscillations changes in accordance with the sequence of transmitted information bits: for example, when transmitting one, the amplitude of the oscillations is set large, and when transmitting zero, it is small or there is no carrier signal at all;

frequency modulation, when, under the influence of modulating signals (transmitted information bits), only the carrier frequency of sinusoidal oscillations changes: for example, when zero is transmitted, it is low;

phase modulation, when, in accordance with the sequence of transmitted information bits, only the phase of the carrier of sinusoidal oscillations changes: when switching from signal 1 to signal 0 or vice versa, the phase changes by 180 degrees ..

The transmit modem converts (modulates) the sine wave carrier signal (amplitude, frequency, or phase) so that it can carry the modulating signal, i.e. digital data from a computer or terminal. The inverse conversion (demodulation) is performed by the receiving modem. In accordance with the implemented modulation method, modems are distinguished with amplitude, frequency and phase modulation. The most widely used frequency and amplitude modulation.

Digital coding digital data is performed directly by changing the levels of signals that carry information.

For example, if in a computer digital data is represented by signals of levels 5V for code 1 and 0.2V for code 0, then when these data are transmitted to the communication line, the signal levels are converted to +12V and -12V, respectively. Such encoding is carried out, in particular, using asynchronous RS-232-C serial adapters when transferring digital data from one computer to another over small (tens and hundreds of meters) distances.

Synchronization of TCS elements. Synchronization is part of the communication protocol. In the process of communication synchronization, synchronous operation of the receiver and transmitter equipment is ensured, in which the receiver samples incoming information bits (i.e., measuring the signal level in the communication line) strictly at the moments of their arrival. Synchronization signals tune the receiver to the transmitted message even before it arrives and keep the receiver synchronized with the incoming data bits.

Depending on how the synchronization problem is solved, a distinction is made between synchronous transmission, asynchronous transmission, and self-tuning transmission.

Synchronous transmission differs in the presence of an additional communication line (except for the main one, through which data is transmitted) for the transmission of synchronization pulses (SI) of a stable frequency. Each SI adjusts the receiver. The transmission of data bits to the communication line by the transmitter and the sampling of information signals by the receiver are performed at the moments when SI appears. In synchronous transmission, synchronization is very reliable, but this is achieved at a high price - the need for an additional communication line.

Asynchronous transfer does not require an additional communication line. Data transfer is carried out in small blocks of fixed length (usually bytes). Synchronization of the receiver is achieved by sending an additional bit, a start bit, before each transmitted byte, and another additional bit, a stop bit, after the transmitted byte. Startbit is used for synchronization. This synchronization method can only be used in systems with low data rates.

Autotuning transmission, also not requiring an additional communication line, it is used in modern high-speed data transmission systems. Synchronization is achieved by using self-synchronizing codes(SK). The coding of the transmitted data using the SC is to ensure regular and frequent changes (transitions) of the signal levels in the channel. Each signal level transition from high to low or vice versa is used to trim the receiver. The best are those SCs that provide a signal level transition at least once during the time interval required to receive one information bit. The more frequent the signal level transitions, the more reliable the synchronization of the receiver is and the more confident the identification of the received data bits is.

The most common are the following self-synchronizing codes:

NRZ code (code without return to zero);

RZ-code (code with return to zero);

Manchester code;

Bipolar code with alternating level inversion (eg AMI code).

Rice. Message encoding schemes using self-synchronizing codes

On fig. coding schemes for message 0101100 using these SCs are presented.

The following are used to characterize and comparatively evaluate the SC. indicators:

level (quality) of synchronization;

Reliability (confidence) of recognition and selection of received information bits;

The required rate of change of the signal level in the communication line when using the SC, if the line bandwidth is set;

The complexity (and hence the cost) of the equipment that implements the SC.

Digital communication networks (DSN). In recent years, digital communication networks using digital technology have become increasingly widespread in TVS.

Reasons for the spread of digital technology in networks:

The digital devices used in the DSS are based on highly integrated integrated circuits; compared to analog devices, they are distinguished by high reliability and stability in operation, and, in addition, they are usually cheaper in production and operation;

Digital technology can be used to transmit any information over a single channel (acoustic signals, television video data, facsimile data);

Digital methods overcome many of the transmission and storage limitations that are inherent in analog technologies.

In the DSS, when transmitting information, the analog signal is converted into a sequence of digital values, and when receiving, the reverse conversion is performed.

An analog signal appears as a constant change in amplitude over time. For example, when talking on a telephone, which acts as an acoustic-to-electrical converter, mechanical air vibrations (alternating high and low pressure) are converted into an electrical signal with the same amplitude envelope characteristic. However, the direct transmission of an analog electrical signal over a telephone line is associated with a number of disadvantages: signal distortion due to its nonlinearity, which is increased by amplifiers, signal attenuation when transmitted through a medium, exposure to noise in the channel, etc.

In the SSC, these shortcomings can be overcome. Here, the form of the analog signal is represented in the form of digital (binary) images, digital values representing the corresponding values of the amplitude envelope of sinusoidal oscillations at points at discrete levels. Digital signals are also subject to attenuation and noise as they pass through the channel, but at the receiving point it is only necessary to note the presence or absence of a binary digital pulse, and not its absolute value, which is important in the case of an analog signal. Consequently, digital signals are received more reliably and can be completely restored before they fall below the threshold due to attenuation.

The conversion of analog signals to digital is carried out by various methods. One of them - pulse code modulation(IKM), proposed in 1938 by A.Kh. Reeves (USA). When using PCM, the conversion process includes three stages: mapping, quantization and encoding (Fig. 12.2).

Rice. 12.2. Converting an analog signal to an 8-element digital code

First stage (display) based on the Nyquist mapping theory. The main position of this theory is: “If an analog signal is displayed at a regular interval with a frequency of at least twice the maximum frequency of the original signal in the channel, then the display will contain information sufficient to restore the original signal.” During the transmission of acoustic signals (speech), the electrical signals representing them in the telephone channel occupy the frequency band from 300 to 3300 Hz. Therefore, in the DSS, the display frequency is equal to 8000 times per second. The mappings, each of which is called a pulse amplitude modulation (PAM) signal, are stored and then transformed into binary images.

At the quantization stage each IAM signal is given a quantized value corresponding to the nearest quantization level. And DSS the entire range of changes in the amplitude of IAM signals is divided into 128 or 256 quantization levels. The more quantization levels, the more accurately the amplitude of the IAM signal is represented by the quantized level.

At the coding stage each quantized mapping is assigned a 7-bit (if the number of quantization levels is 128) or 8-bit (for 256-step quantization) binary code. On fig. 12.2 shows the signals of the 8-element binary code 00101011, corresponding to a quantum signal with level 43. When encoding with 7-element codes, the data rate over the channel should be 56 Kbps (this is the product of the display frequency and the bit depth of the binary code), and when encoding 8- element codes - 64 Kbps.

Modern DSS also uses another concept of converting analog signals into digital ones, in which not the IAM signals themselves are quantized and then encoded, but only their changes, and the number of quantization levels is assumed to be the same. It is obvious that such a concept allows the conversion of signals with greater accuracy.

Satellite communication networks. The advent of satellite communication networks caused the same revolution in the transmission of information as the invention of the telephone.

The first communications satellite was launched in 1958, and in 1965 the first commercial communications satellite was launched (both in the USA). These satellites were passive; later, amplifiers and transceiver equipment were installed on the satellites.

The following methods are used to manage data transmission between the satellite and ground-based RTSs:

1. Conventional multiplexing - with frequency division and time division. In the first case, the entire frequency spectrum of the radio channel is divided into subchannels, which are distributed among users for the transmission of any schedule.

The costs of this method are: in case of irregular transmission, subchannels are used irrationally; a significant portion of the channel's original bandwidth is used as a separator band to prevent subchannels from interfering with each other. In the second case, the entire time spectrum is divided between users who, at their own discretion, dispose of the provided time slices (slots). It is also possible here that the channel may be idle due to its irregular use.

2. Regular discipline “primary/secondary” with using methods and means of survey/selection. As the primary body that implements such a discipline of satellite communications management, one of the terrestrial RTSs more often acts, and less often - a satellite. The polling and selection cycle takes a significant amount of time, especially when there are a large number of ASs in the network. Therefore, the response time to a user request may be unacceptable for him.

3. Primary/Secondary Management Discipline without polling, with the implementation of the method of multiple access with time slicing (TDMA). Here the slots are assigned to the primary RTS called reference. Receiving requests from other RTSs, the reference station, depending on the nature of the schedule and channel occupancy, satisfies these requests by assigning specific slots to the stations for transmitting frames. This method is widely used in commercial satellite networks.

4. Equal rank management disciplines. They are characterized by the fact that all users have equal access to the channel and there is competition between them for the channel. In the early 70s, N. Abramson from the University of Hawaii proposed a method of effective competition for a channel between uncoordinated users, called the ALOHA system. There are several variants of this system: a system that implements a random access method (random ALOHA); peer-to-peer priority slot system (slot ALOHA), etc.

TO main advantages satellite communication networks include the following:

Large throughput due to the operation of satellites in a wide range of gigahertz frequencies. A satellite can support several thousand voice communication channels. For example, one of the commercial satellites currently in use has 10 transponders, each of which can transmit 48 Mbps;

Ensuring communication between stations located at very large distances, and the possibility of servicing subscribers in the most inaccessible points;

Independence of the cost of information transfer from the distance between the interacting subscribers (the cost depends on the duration of the transfer or the volume of the transmitted schedule);

The possibility of building a network without physically implemented switching devices, due to the broadcasting of satellite communications. This capability is associated with significant economic benefits that can be obtained compared to using a conventional non-satellite network based on multiple physical links and communication devices.

Flaws satellite communication networks:

The need to spend money and time to ensure the confidentiality of data transmission, to prevent the possibility of interception of data by “foreign” stations;

The presence of a delay in the reception of a radio signal by a ground station due to large distances between the satellite and the RTS. This can cause problems related to the implementation of channel protocols, as well as response time;

The possibility of mutual distortion of radio signals from ground stations operating at adjacent frequencies;

The susceptibility of signals in the areas Earth - satellite and satellite - Earth to the influence of various atmospheric phenomena.

To solve the problems with frequency allocation in the 6/4 and 14/12 GHz bands and the placement of satellites in orbit, the active cooperation of many countries using satellite communication technology is necessary.

By purpose, telecommunication systems are grouped as follows:

TV broadcasting systems;

Communication systems (including paging);

Computer networks.

By type of information transfer medium used:

Cable (traditional copper);

fiber optic;

Essential;

Satellite.

By way of information transfer:

Analog;

Digital.

Communication systems are divided by mobility into:

Stationary (traditional subscriber lines);

Movable.

Mobile communication systems are subdivided according to the principle of coverage of the service area:

On microcellular - DECT;

Cellular - NMT-450, D-AMPS, GSM, CDMA;

Trunking (macrocellular, zone) - TETRA, SmarTrunk;

Satellite.

TV broadcast systems

Broadcasting systems (TV) according to the method of signal delivery and coverage area are divided into:

Television reception networks;

- "cable" (systems of collective television reception (SKTP));

Technologies for wireless high-speed distribution of multimedia information MMDS, MVDS and LMDS;

Satellite.

Mobile communication systems

Cellular mobile communication systems (MCS), personal radio call networks (PRC) and satellite communication systems are designed to transmit data and provide mobile and stationary objects with telephone communications. The transfer of data to a mobile subscriber dramatically expands its capabilities, since, in addition to telephone, it can receive telex and facsimile messages, various kinds of graphic information, etc. An increase in the amount of information requires a reduction in the time for its transmission and receipt, as a result of which there is a steady increase in the production of mobile radio communications (pagers, cellular radiotelephones, satellite user terminals).

The main advantage of SPS: mobile communication allows the subscriber to receive communication services at any point within the coverage areas of terrestrial or satellite networks; thanks to advances in communication technology, small-sized universal subscriber terminals (ATs) have been created. SPS provide consumers with the opportunity to access the public telephone network (PSTN), the transmission of computer data.

Mobile communication networks include: cellular mobile communication networks (MCSN); trunking communication networks (STS); personal radio call networks (PRP); networks of personal satellite (mobile) communications.

Cellular mobile networks

Among modern telecommunication facilities, cellular radiotelephone networks are developing most rapidly. Their implementation made it possible to solve the problem of economical use of the allocated radio frequency band by transmitting messages at the same frequencies, but in different zones (cells) and to increase the throughput of telecommunication networks. They got their name in accordance with the cellular principle of organizing communications, according to which the service area is divided into cells (cells).

A cellular communication system is a complex and flexible technical system that allows for a wide variety of configuration options and a set of functions performed. It can provide the transmission of speech and other types of information. For voice transmission, in turn, the usual two-way and multilateral telephone communication (conference - with the participation of more than two subscribers in a conversation at the same time), voice mail can be implemented. When organizing a regular telephone conversation, the modes of auto redial, call waiting, call forwarding (conditional or unconditional), etc. are possible.

Modern technologies make it possible to provide SSRS subscribers with high quality voice messages, reliability and confidentiality of communication, miniaturization of radiotelephones, and protection against unauthorized access.

Trunking networks

Trunking networks are somewhat similar to cellular networks: they are also networks of terrestrial radiotelephone mobile communications that provide subscriber mobility within a fairly large service area. The main difference is that STSs are simpler in terms of construction principles and provide subscribers with a smaller set of services, but due to this they are cheaper than cellular ones. STS have a much lower capacity than cellular, and are fundamentally focused on departmental (corporate) mobile communications. The main application of STS is corporate (official, departmental) communication, for example, operational communication of the fire service with the number of exits (channels) "to the city" much less than the number of system subscribers. The main requirements for STS are: providing communication in a given service area, regardless of the location of mobile subscribers; the possibility of interaction between individual groups of subscribers and the organization of circular communication; efficiency of communication management, including at various levels; providing communication through control centers; possibility of priority establishment of communication channels; low energy costs of the mobile station; confidentiality of conversations.

Name trunking communication comes from the English trunk (trunk) and reflects the fact that the communication trunk in such a system contains several physical (usually frequency) channels, each of which can be provided to any of the subscribers of the system. This feature distinguishes the STS from the two-way radio communication systems that preceded it, in which each subscriber had the opportunity to access only one channel, but the latter had to serve a number of subscribers in turn. STS compared with such systems have a significantly higher capacity (bandwidth) with the same indicators of quality of service.

Paging networks

Paging networks (PRP) or paging networks (paging - call) are one-way mobile communication networks that provide transmission of short messages from the system center (from a paging terminal) to miniature subscriber receivers (pagers).

Paging networks provide a convenient and relatively cheap type of mobile communication, but with significant limitations: communication is one-way, not in real time, and only in the form of short messages. SRLs have become quite widespread in the world - in general, of the same order as cellular networks, although their prevalence varies significantly in different countries.

Mobile satellite networks

Along with the already publicly available SPS (personal radio call and cellular), satellite communication networks are developing more and more actively. The following areas of application of mobile satellite communications are relevant:

Expansion of cellular networks;

The use of satellite communications in areas where the deployment of SPS is impractical, for example, due to low population density;

The use of satellite communications in addition to existing cellular, for example, to provide roaming in case of incompatibility of standards, or in any emergency;

Fixed wireless communication in areas with low population density in the absence of SPS and wired communication;

When transmitting information on a global scale (water areas of the World Ocean, places of breaks in ground infrastructure, etc.).

In particular, when a subscriber is removed from the service area of local cellular networks, satellite communication plays a key role, since it has no restrictions on linking the subscriber to a specific area. In many regions of the world, the demand for mobile services can only be effectively met with the help of satellite systems.

Fiber optic networks

A fiber-optic communication line (FOCL) is a type of transmission system in which information is transmitted through optical dielectric waveguides, known as "optical fiber". A fiber-optic network is an information network, the connecting elements between the nodes of which are fiber-optic communication lines. Fiber optic network technologies, in addition to fiber optics, also cover issues related to electronic transmission equipment, its standardization, transmission protocols, network topology issues and general networking issues.

FOCL advantages: wide bandwidth, low attenuation of the light signal in the fiber, low noise level, high noise immunity, low weight and volume, high security against unauthorized access, galvanic isolation of network elements, explosion and fire safety, cost-effectiveness of fiber optic cables (FOC), long service life operation, remote power supply.

Disadvantages of FOCL: the cost of interface equipment (the price of optical transmitters and receivers is still quite high), the installation and maintenance of optical lines (the cost of installation, testing and support of fiber optic communication lines also remains high), the requirement for special fiber protection.

The advantages of using fiber optic communication lines are so significant that, despite the listed disadvantages of optical fiber, further prospects for the development of fiber optic communication technology in information networks are more than obvious.

Telecommunication networks represent the most complex equipment in the world. One has only to think about the telephone network, which includes more than 2 billion fixed and mobile phones with universal access. When one of these phones makes a call, the telephone network is able to connect to any other phone in the world. In addition, many other networks are connected to the telephone network. This suggests that the complexity of the global telecommunications network exceeds the complexity of any other system in the world.

Telecommunication services have a significant impact on the development of the world community. If we know the country's telephone density, then we can assess the level of its technical and economic development. In underdeveloped countries, the density of fixed (fixed) telephones does not exceed 10 telephones per 1,000 inhabitants; in developed countries, such as North America and Europe, it is approximately 500 - 600 phones per 1000 inhabitants. Economic and cultural development developing countries depends (in addition to many other factors) on the availability of efficient telecommunications services. The local area network (LAN) to which our computer is connected is connected to the LAN of other sites located throughout our university. This is necessary for the effective collaboration of various departments. We communicate daily with people in other organizations through e-mail, telephones, facsimiles and mobile phones. It happens at the scale of organizations, at the scale of the country and at the international scale.

Telecommunications play significant role in many areas of everyday life. Each of us daily uses not only telecommunications services, but also services that rely on telecommunications. Here are some examples of services that depend on telecommunications: banking, automated teller machines, e-commerce; aviation, railway, ticketing; sales, wholesale and order processing; payments with a credit card in stores; ordering hotel rooms by travel agencies; procurement of materials by industry; government operations.

Control questions:

1. The concept of a network. List the possibilities of the network.

2. In what year did the first network appear, what was it called and where?

3. Name the main components of the network.

4. List the indicators of computer networks.

5. Describe the levels of the reference model of open systems interaction.

6. Define the concepts of "protocol", "interface", "transparency", "network operating system".

7. What components does the technical support of computer networks include? Describe them.

8. Name the types of networks.

9. Give the classification of networks.

10. Describe the benefits of local area networks.

11. Describe the main hardware components of the LAN.

12. What is the difference between the "file-server" and "client-server" models?

13. Describe the cables used in most networks.

14. What technologies are used to transmit coded signals over the cable?

15. What is a transceiver? What is it for?

16. What are the advantages and types of wireless networks.

17. Describe LAN access methods

18. Give the concept of a telecommunications system.

19. List the types of telecommunication systems.

20. Describe mobile networks.

Topic 9. Internet

2 Two roots of computer networks Computing and telecommunication technologies Evolution of telecommunications Evolution of computer technology Evolution of computer networks Evolution of computer networks at the intersection of computer technology and telecommunication technologies

3 Telecommunication systems 1. Basic information about telecommunication systems The main function of telecommunication systems (TCS), or territorial communication networks (TCN), is to organize an efficient and reliable exchange of information between subscribers, as well as to reduce data transmission costs. The term "territorial" means that the communication network is distributed over a large area. It is created in the interests of the entire state, institution, enterprise or firm with branches in the district, region or throughout the country. The main indicator of the effectiveness of the functioning of telecommunication systems is the time of information delivery. It depends on a number of factors: the structure of the communication network, the throughput of communication lines, methods of connecting communication channels between interacting subscribers, information exchange protocols, subscriber access methods to the transmission medium, packet routing methods, etc.

4 Telecommunication systems 1. Basic information about telecommunication systems Characteristic features of territorial communication networks: diversity of communication channels from wired tone frequency channels (telephone) to fiber optic and satellite; limited number of communication channels between remote subscribers, through which it is necessary to provide data exchange, telephone, video, fax messages; the availability of such a critically important resource as the bandwidth of communication channels. Therefore, a territorial communication network (TCN) is a geographically distributed network that combines the functions of traditional data transmission networks (DTN), telephone networks and is designed to transmit traffic of various nature, with different probabilistic and temporal characteristics.

5 Telecommunication systems 1. Basic information about telecommunication systems Types of networks, lines and communication channels. TVS uses telephone, telegraph, television, and satellite communication networks. The following communication lines are used: cable (telephone lines, twisted pair, coaxial cable, fiber-optic lines), radio relay and radio lines. Among cable communication lines, light guides (i.e. fiber-optic lines) have the best performance. Their main advantages are: high bandwidth (hundreds of megabits per second); insensitivity to external fields and absence of own radiations; low labor intensity of laying an optical cable; spark, explosion and fire safety; increased resistance to aggressive environments; small specific gravity; various fields of application. Disadvantages: signaling is carried out only in one direction; connecting additional computers significantly weakens the signal; high-speed modems necessary for light guides are expensive; the light guides connecting the computers must be supplied with converters of electrical signals into light and vice versa.

6 Telecommunication systems 1. Basic information about telecommunication systems In telecommunication systems, the following types of communication channels have been used: simplex, when the transmitter and receiver are connected by one communication channel, through which information is transmitted only in one direction (this is typical for TV communication networks); half-duplex, when two communication nodes are also connected by one channel, through which information is transmitted alternately in one direction, then in the opposite direction (this is typical for information-reference, request-response systems); duplex, when two communication nodes are connected by two channels (forward and reverse), through which information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decision and information feedback.

7 Telecommunication systems 1. Basic information about telecommunication systems Switched and dedicated communication channels. In networks (TCS, TSS) there are dedicated (non-switched) communication channels and switched channels for the duration of information transmission over them. When using dedicated communication channels, the transceiver equipment of communication nodes is constantly connected to each other. This ensures a high degree of readiness of the system for information transfer, higher quality of communication, and support for a large amount of traffic. Due to the relatively high costs of operating networks with dedicated communication channels, their profitability is achieved only if the channels are fully loaded. Switched communication channels created only for the period of transmission of a fixed amount of information are characterized by high flexibility and relatively low cost. The disadvantages of such channels are: loss of time for switching (establishing communication between subscribers), the possibility of blocking due to the busyness of individual sections of the communication line, lower communication quality, high cost with a significant amount of traffic.

8 Telecommunication systems 1. Basic information about telecommunication systems Analogue and digital coding of digital data. The transfer of data from one network node to another is carried out by serial transmission of all bits of the message from the source to the destination. Physically, information bits are transmitted in the form of analog or digital electrical signals. Analog signals are signals that can represent an infinite number of values of some quantity within a limited range. Digital (discrete) signals can have a single value or a finite set of values. When working with analog signals, an analog carrier signal of a sinusoidal form is used to transmit encoded data, and when working with digital signals, two and a multi-level discrete signal are used. Analog signals are less sensitive to distortion due to attenuation in the transmission medium, but data encoding and decoding is easier for digital signals.

10 Telecommunication systems 1. Basic information about telecommunication systems Synchronization of network elements is part of the communication protocol. The synchronization process ensures synchronous operation of the receiver and transmitter equipment, in which the receiver samples the incoming information bits strictly at the moments of their arrival. A distinction is made between synchronous transmission, asynchronous transmission, and self-tuning transmission. Synchronous transmission is characterized by the presence of an additional communication line (in addition to the main one) for the transmission of synchronization pulses (SI) of a stable frequency. The issuance of data bits by the transmitter and the sampling of signals by the receiver are performed at the moments of the appearance of SI. This is reliable, but an additional line is needed. Asynchronous transmission does not require an additional line. The transmission is carried out in small fixed blocks, and the start bit is used for synchronization. In lockshift transmission, synchronization is achieved through the use of self-synchronizing codes (SCs). The coding of transmitted data using SC is to ensure regular and frequent changes in signal levels in the channel. Each transition is used to tune the receiver.

11 Satellite communication networks (SCN). Communication spacecraft (SC) are launched to a height of km and are in geostationary orbit, the plane of which is parallel to the plane of the equator. Three such spacecraft provide coverage of almost the entire surface of the Earth. Interaction between CCC subscribers is carried out along the chain: AS-sender of information > transmitting ground station >> satellite > receiving ground station > AS-receiver. One ground station serves a group of nearby speakers. The following methods are used to manage data transmission between the satellite and ground stations. 1. Conventional multiplexing with frequency and time division. 2. Regular primary/secondary discipline with or without the use of survey/selection methods and tools. 3. Equal rank control disciplines with equal access to the channel in the conditions of competition for the channel. Telecommunication systems 1. Basic information about telecommunication systems transmitting ground station >> satellite > receiving ground station > receiving AC. One ground station serves a group of nearby speakers. The following methods are used to manage data transmission between the satellite and ground stations. 1. Conventional multiplexing with frequency and time division. 2. Regular primary/secondary discipline with or without the use of survey/selection methods and tools. 3. Equal rank control disciplines with equal access to the channel in the conditions of competition for the channel. Telecommunication systems 1. Basic information about telecommunication systems ">

12 Telecommunication systems 1. Basic information about telecommunication systems The main advantages of satellite communication networks: high bandwidth due to the operation of satellites in a wide range of gigahertz new frequencies. A satellite can support several thousand voice communication channels; providing communication between stations located at very large distances, and the possibility of servicing subscribers in the most inaccessible points; independence of the cost of information transfer from the distance between subscribers; the possibility of building a network without physically implemented switching devices. Disadvantages of satellite communication networks: the need to spend money and time to ensure the confidentiality of data transmission; the presence of a delay in the reception of a radio signal by a ground station due to large distances between the satellite and the communication station; the possibility of mutual distortion of radio signals from ground stations operating at adjacent frequencies; the susceptibility of signals to the influence of various atmospheric phenomena.

13 Telecommunication systems 2. Switching in networks Switching is a vital element of communication between subscriber systems (AS) and with control centers, processing and storage of information in networks. Network nodes are connected to some switching equipment, thus avoiding the need to create special communication lines. A switched transport network is a network in which communication is established between two (or more) end points on demand. An example of such a network is the switched telephone network. There are the following switching methods: switching circuits (channels); store-and-forward switching, divided into message switching and packet switching.

15 Telecommunication systems 2. Communication in networks Switching channels (circuits). When switching channels (circuits) between connected end points throughout the entire time interval of the connection, real-time exchange is provided, and bits are transmitted at a constant rate over a channel with a constant bandwidth. Advantages of the circuit switching method: the development of circuit switching technology; work in interactive mode and in real time; ensuring transparency regardless of the number of connections between AS; wide scope. Disadvantages of the circuit switching method: a long time to establish an end-to-end communication channel due to the possible waiting for the release of its individual sections; the need to retransmit the call signal due to the employment of the switching device in the signal chain; lack of choice of information transfer rates; the possibility of channel monopolization by one source of information; increasing the functions and capabilities of the network is limited; uniform loading of communication channels is not ensured.

17 Telecommunication systems 2. Communication in networks Message switching is an early method of data transmission (used in e-mail, news). Technology - "remember and send." The entire message retains its integrity as it passes from one node to another up to the destination, and the transit node cannot start further transmission of part of the message if it is still being received. Advantages of the method: no need to establish a channel in advance; formation of a route from sections with different throughput; implementation of systems for servicing requests, taking into account their priorities; the ability to smooth peak loads by storing streams; no loss of service requests. Disadvantages: the need to implement serious requirements for memory capacity in communication nodes to receive large messages; insufficient opportunities to implement an interactive mode and work in real time when transmitting data; channels are used less efficiently than other methods.

18 Telecommunication systems 2. Communication in networks Packet switching combines the advantages of circuit switching and message switching. Its main goals are: ensuring full network availability and acceptable response time to a request for all users, smoothing asymmetric flows between users, providing multiplexing capabilities of communication channels and network computer ports, dispersing critical network components. The data is broken into short packets of a fixed length. Each packet is supplied with protocol information: codes for the beginning and end of the packet, sender and recipient addresses, packet number in the message, information for verifying the reliability of transmitted data. Independent packets of the same message can be transmitted simultaneously along different routes as part of datagrams. The packets are delivered to their destination, where they form the initial message. Unlike message switching, packet switching allows you to: increase the number of connected stations; it is easier to overcome difficulties with connecting additional communication lines; implement alternative routing, which creates increased convenience for users; significantly reduce the time for data transfer, increase the bandwidth and efficiency of the use of network resources. Now packet switching is the main one for data transmission.

20 Telecommunication systems 2. Communication in networks Conclusion of the section The analysis of the considered switching technologies allows us to conclude that it is possible to develop a combined switching method based on the use of message and packet switching principles in a certain combination and providing more efficient management of heterogeneous traffic.

21 Telecommunication systems 3. Packet routing in networks Essence, goals and methods of routing. The task of routing is to choose a route for transmission from the sender to the recipient. First of all, we are talking about networks with an arbitrary (mesh) topology in which packet switching is implemented. However, in modern networks with a mixed topology (star-ring, star-bus, multi-segment), the problem of choosing a route for transmitting frames is really worth and solved, for which appropriate tools are used, for example, routers. In virtual networks, the task of routing when transmitting a message that is divided into packets is solved only once, when a virtual connection is established between the sender and the recipient. In datagram networks, where data is transmitted in the form of datagrams, routing is performed on a per-packet basis. The choice of routes in communication nodes of telecommunication networks is made in accordance with the implemented routing algorithm (method).

24 Telecommunication systems 3. Packet routing in networks The routing algorithm is a rule for assigning an output communication line for packet transmission, based on the information contained in the packet header (sender and recipient addresses), information about the load of this node (packet queue length) and the network as a whole . The main goals of routing are to ensure: the minimum delay of the packet during its transmission from the sender to the recipient; maximum network bandwidth; maximum protection of the package against threats for the information contained in it; reliability of package delivery to the addressee; the minimum cost of sending a packet to the destination. There are the following routing methods: - centralized routing; - distributed (decentralized) routing; - mixed routing

25 Telecommunication systems 3. Packet routing in networks 1. Centralized routing is implemented in networks with centralized control. The choice of the route for each packet is carried out in the network control center, and the nodes of the communication network only perceive and implement the results of solving the routing problem. This routing control is vulnerable to central node failures and is not very flexible. 2. Distributed (decentralized) routing is performed in networks with decentralized control. Routing control functions are distributed among network nodes that have the appropriate means for this. Distributed routing is more complex than centralized routing, but is more flexible. 3. Mixed routing is characterized by the fact that it implements the principles of centralized and distributed routing in a certain ratio. The problem of routing in networks is solved under the condition that the shortest route that ensures the transmission of a packet in the minimum time depends on the network topology, bandwidth and load on the communication line.

26 Telecommunication systems 3. Packet routing in networks Routing methods - simple, fixed and adaptive. The difference between them is in the degree to which topology changes and network load are taken into account when choosing a route. 1. Simple routing differs in that when choosing a route, neither a change in the network topology nor a change in its load is taken into account. It does not provide directional packet transmission and has low efficiency. Its advantages are ease of implementation and ensuring the stable operation of the network in the event of failure of its individual elements. Practical application received: random routing - one random free direction is selected for packet transmission. The packet "wanders" through the network and reaches the destination with a finite probability. flooding involves the transmission of a packet from a node over all free output lines. There is a phenomenon of "propagation" of the package. The main advantage of this method is the guaranteed optimal delivery time of the packet to the addressee. The method can be used in unloaded networks, when the requirements for minimizing the time and reliability of packet delivery are quite high.

27 Telecommunication systems 3. Packet routing in networks 2. Fixed routing - when choosing a route, changes in the network topology are taken into account and changes in its load are not taken into account. For each destination node, the direction of transmission is selected from a table of shortest routes. Lack of adaptation to load changes leads to network packet delays. A distinction is made between single-path and multi-path fixed routing. The first is built on the basis of a single packet transmission path between two subscribers, which is associated with instability to failures and overloads, and the second is based on several possible paths between two subscribers, from which the most preferred path is selected. Fixed routing is used in networks with little changing topology and steady packet flows. 3. Adaptive routing is different in that the decision on the direction of packet transmission is carried out taking into account changes in both the topology and the network load. There are several modifications of adaptive routing, which differ in what information is used when choosing a route. Local, distributed, centralized, and hybrid adaptive routing (the meaning is clear from the name) has become widespread.

28 Telecommunication systems 4. Protection against errors in networks When transmitting data, one error per thousand transmitted signals can seriously affect the quality of information. There are many methods for ensuring the reliability of information transmission (error protection), which differ: in the means used, in the time spent on their application, in the degree of ensuring the reliability of information transmission. The practical implementation of the methods consists of two parts: software and hardware. The ratio between them can be very different, up to the almost complete absence of one of the parts. The main causes of transmission errors in networks are failures in some part of the network equipment or the occurrence of adverse events in the network. The data transmission system is ready for this and eliminates them with the help of the means provided for in the plan; interference caused by external sources and atmospheric phenomena.

29 Telecommunication systems 4. Protection against errors in networks Among the numerous methods of protection against errors, three groups of methods are distinguished: group methods, error-correcting coding and error protection methods in feedback transmission systems. Of the group methods, the majority method and the method of transmitting information blocks with a quantitative characteristic of the block have been widely used. The essence of the majority method is that each message is transmitted several times (usually three times). Messages are remembered and compared, the correct one is chosen by coincidence "2 out of 3". Another group method, which also does not require information recoding, involves data transmission in blocks with a quantitative characteristic of the block (number of ones or zeros, checksum of symbols, etc.). At the receiving point, this characteristic is again calculated and compared with that transmitted over the communication channel. If the characteristics match, it is considered that the block does not contain errors. Otherwise, the transmitting side receives a signal with the requirement to retransmit the block. In modern fuel assemblies, this method is the most widely used.

30 Telecommunication systems 4. Protection against errors in networks Noise-immune (redundant) coding involves the development and use of corrective (noise-immune) codes. Transmission systems with feedback are divided into systems with decision feedback and systems with information feedback. A feature of systems with decisive feedback is that the decision on the need to retransmit information is made by the receiver. Error-correcting coding is used, with the help of which the received information is checked at the receiving station. If an error is detected, a re-request signal is sent to the transmitting side via the feedback channel, through which the information is retransmitted. In systems with information feedback, information is transmitted without error-correcting coding. The receiver, having received the information over the direct channel and storing it, transmits it back, where it is compared. If there is a match, the transmitter sends an acknowledgment signal, otherwise, all information is retransmitted, i.e. the decision to transmit is made by the transmitter.

Telecommunications and network technologies are currently the driving force that ensures the development of world civilization. There is practically no area of industrial and social relations that would not use the possibilities of modern information technologies based on telecommunications.

Telecommunication is the transmission of data over long distances.

Telecommunication facilities - a set of technical, software and organizational tools for data transmission over long distances.

Telecommunication networks are:

1 Telephone networks for the transmission of telephone data (voice);

2 Radio networks for audio data transmission;

3 Television networks for video transmission;

4 digital (computer) networks or data networks (DTN) for the transmission of digital (computer) data.

Data in digital telecommunication networks is formed in the form of messages that have a certain structure and are considered as a whole.

Data (messages) can be:

1 continuous;

2 discrete.

Continuous data can be represented as a continuous function of time, such as speech, sound, video. Discrete data consists of characters (symbols).

Data transmission in a telecommunications network is carried out using their physical representation - signals.

The following types of signals are used in computer networks for data transmission:

1 electric (electric current);

2 optical (light);

3 electromagnetic (electromagnetic radiation field - radio waves.

Cable communication lines are used to transmit electrical and optical signals:

1 electric (ELS)

2 fiber optic (FOCL)

The transmission of electromagnetic signals is carried out through radio lines (RLS) and satellite communication lines (SLS).

Signals, like data, can be:

1 continuous;

2 discrete.

At the same time, continuous and discrete data can be transmitted in a telecommunications network either in the form of continuous or discrete signals.

The process of converting (representing) data into the form required for transmission over a communication line and allowing, in some cases, to detect and correct errors that occur due to interference during their transmission, is called coding. An example of encoding is the representation of data as binary characters. Depending on the parameters of the transmission medium and the requirements for the quality of data transmission, various coding methods can be used.

A communication line is a physical medium through which information signals are transmitted, generated by special technical means related to linear equipment (transmitters, receivers, amplifiers, etc.). A communication line is often considered as a set of physical circuits and technical means that have common linear structures, devices for their maintenance and the same propagation medium. The signal transmitted in the communication line is called linear (from the word line).

Communication lines can be divided into 2 classes:

1. cable (electrical and fiber-optic communication lines):

2. wireless (radio links).

Communication channels are built on the basis of communication lines.

A communication channel is a combination of one or more communication lines and channel-forming equipment that provides data transmission between interacting subscribers in the form of physical signals corresponding to the type of communication line.

The communication channel may consist of several serial communication lines, forming a composite channel. At the same time, several communication channels can be formed in one communication line, providing simultaneous data transmission between several pairs of subscribers.

A telecommunications computer network (TVS) is a network for the exchange and distributed processing of information, formed by a multitude of interconnected subscriber systems and means of communication.

The means of transmitting and processing information are oriented in it to the collective use of public resources of hardware, information, and software.

Telecommunication - remote data transmission based on computer networks and modern technical means of communication.

A subscriber system (AS) is a set of computers, software, peripheral equipment, means of communication with a switching subnetwork of a computer network that perform application processes.

The communication subnet, or telecommunications system (TCS), is a combination of the physical medium for information transmission, hardware and software that ensure the interaction of the AU.

With the advent of TVS, two very important problems were solved:

providing, in principle, unlimited access to computers for users, regardless of their territorial movement of large amounts of information over long distances. In the TVS, all different subscriber computer systems that are part of the computer are automatically connected to each other.

Each network computer is adapted both to work in stand-alone mode under the control of its operating system (OS), and as a component of the network.

TVS allows solving such qualitatively new tasks as, for example:

* providing distributed data processing and parallel processing by many computers;

* the possibility of creating a distributed database (RDB), located in the memory of various computers;

* the possibility of exchanging large amounts of information between computers, remote from each other at considerable distances;

* collective use of expensive resources: application software products (APP), databases (DB), knowledge bases (KB), storage devices (memory), printing devices (PU), network operating systems (OS);

* providing a large list of services, including such as e-mail (EP), teleconferences, electronic bulletin boards (EDB), distance learning, organization of paperless workflow, electronic signature, management decision-making;

* increasing the efficiency of the use of computer technology and informatics (STI) due to their more intensive and uniform loading, as well as the reliability of servicing user requests;

* the possibility of prompt redistribution of computing power between network users depending on changes in their needs, as well as redundancy of these capacities and data transmission facilities in case of failure of individual network elements;

* reducing the cost of purchasing and operating SVTI (due to their collective use);

* providing work to improve technical, software and information facilities.

Telecommunications networks are the highest form of multi-machine associations. The main differences between computer networks and a multi-computer complex are as follows:

* dimension, that is, a large number of computers (from ten to several hundred) located at a distance from each other from tens of meters to several hundred and even thousands of kilometers; separation of the computer function, that is, data processing and system management, analysis and storage of information are distributed between different computers in the network;

* the need to solve the problem of message routing in the network, that is, a message from one computer to another in the network can be transmitted along different routes depending on the priority and state of the communication channels connecting the computers to each other.

On a functional basis, the whole set of computer network systems can be divided into subscriber, switching and main (Host) systems.

The subscriber system is a computer oriented to work as part of a computer network and providing users with access to its computing resources.

Switching systems are switching nodes of the data transmission network and ensure the organization of composite data transmission channels between system subscribers. Teleprocessing processors or special switching (network) processors are used as control elements of switching nodes.

The main (Host) systems or network servers are distinguished by a great variety.

It is customary to call a server a special computer that performs the main service functions: network management, collection, processing, storage and provision of information to computer network subscribers.

Depending on the territorial dispersal of subscriber systems, computer (computer) networks are divided into three main classes:

* global networks (WAN - Wide Area Network);

* regional networks (MAN - Metropolitan Area Network);

* local networks (LAN - Local Area Network).

The main function of telecommunication systems (TCS), or territorial communication networks (TCN), in the conditions of functioning of telecommunication computer networks (TCN) is to organize a prompt and reliable exchange of information between subscribers, as well as to reduce the cost of data transmission.

The main indicator of the effectiveness of the functioning of the TCS is the time of information delivery. It depends on a number of factors: the structure of the communication network, the throughput of communication lines, methods of connecting communication channels between interacting subscribers, information exchange protocols, subscriber access methods to the transmission medium, packet routing methods, etc.

The most common telecommunication systems, or territorial communication networks, are: X.25, Frame Relay (FR), IP, ISDN, SDN, ATM. A particularly important advantage of a particular network technology is its ability to make the most of the available bandwidth of the communication channel and adapt to the quality of the channel. The global Internet technologies include X.25 networks, frame relay, SMDS, ATM. All of these networks, except IP, use packet routing based on virtual circuits between network end nodes.

In modern telecommunication systems, information is transmitted using electrical signals (current or voltage), radio signals or light signals - all these physical processes are electromagnetic field oscillations of various frequencies and nature

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Digital telecommunication system. The main types of telecommunications systems What is telecommunications

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