· Lecture 27. Principles of building telecommunication computing systems.

Introduction

Telecommunicationscan be defined as a technology that connects information arrays, often located at some distance from each other. Telecommunications is currently undergoing a two-fold revolution: rapid changes in communications technologies and equally important changes in the ownership, control, and delivery of communications services. Today's managers need to understand the possibilities and benefits of various communication technologies, as well as be able to balance the costs and benefits of using telecommunications properly.

Telecommunication systemis a set of hardware and software compatible equipment connected into a single system for the purpose of transferring data from one place to another. The telecommunication system is capable of transmitting textual, graphic, voice or video information. This chapter describes the main components of telecommunications systems. The following sections explain how these components work together to form different kinds of networks.

A typical communication system includes servers, user computers, communication channels (in the figure they are indicated by red lines), as well as active equipment - modems, hubs, and so on.

2.Components of the telecommunication system

The main components of a telecommunications system are listed below:

1. Servers that store and process information.

2. Workstations and user PCs used to enter queries to databases, receive and process the results of queries, and perform other tasks of end users of information systems.

3. Communication channels are communication lines through which data is transmitted between the sender and recipient of information. Communication channels use various types of data transmission media: telephone lines, fiber optic cable, coaxial cable, wireless and other communication channels.

4. Active equipment - modems, network adapters, hubs, switches, routers, etc. These devices are necessary for transmitting and receiving data.

5. Network software that controls the process of transmitting and receiving data and controls the operation of individual parts of the communication system.

Functions of the telecommunication system

In order to transmit information from one location and receive it at another, a telecommunications system needs to perform some operations that are mostly hidden from users. Before a telecommunications system can transmit information, it needs to establish a connection between the transmitting (sender) and receiving (receiver) parties. Then calculate the optimal data transfer route, perform the initial processing of the transmitted information (for example, you need to check that your message is transmitted exactly to the person to whom you sent it) and convert the computer's transfer rate to the speed supported by the communication line. Finally, the telecommunications system controls the flow of transmitted information.

Network devices and means of communication.

The most commonly used means of communication are twisted pair, coaxial cable, and fiber optic lines. When choosing the type of cable, the following indicators are taken into account:

· installation and maintenance cost,

· information transfer rate,

· restrictions on the value of the information transmission distance without additional repeater amplifiers (repeaters),

· data transmission security.

The main problem is to achieve these indicators at the same time, for example, the highest data transfer rate is limited by the maximum possible data transfer distance, which still provides the required level of data protection. Easy scalability and ease of expansion of the cable system affect its cost.

3. Types of telecommunication networks.

There are various ways in which active and passive network equipment can work together, and therefore, there are many ways to classify networks. Networks can be classified by configuration, or network topology. According to their geographic size, networks are divided into global and local. Global networks, as a rule, cover fairly large areas - from 1-2 to hundreds of thousands of kilometers. Local area networks unite the computer resources of one or more buildings. In this part, you will get acquainted with different types of computer networks.

Local networks

The local network , LAN (sometimes called a local area network, LAN) - Local Area Network, LAN - covers small spaces, usually one building or several closely standing buildings. Most LANs link computers within 600 meters of each other. LANs need their own telecommunication channels (twisted pair or coaxial cable is most commonly used). Local area networks are widely used in business. They enable organizations to implement applications that can significantly improve productivity and management efficiency. These applications include, first of all, all types of e-mail (plain, text, voice and video mail), tele and video conferencing, Internet technologies. Today it is difficult to imagine an office that is not equipped with a local area network. LANs allow organizations to share software and expensive hardware. For example, users of multiple computers connected by a local area network can share a single laser or inkjet printer connected to the network. Networks are used to work with applications of collective planning, as well as to organize distributed computing.

Without networks, it would be impossible for organizations to share access to the Internet. Typically, in organizations, only one computer is directly connected to an Internet Service Provider (ISP). In order for users of other computers to be able to work with the World Wide Web, special software is installed on the computer that acts as a gateway, which performs requests to the Internet on behalf of users. The staff of the Michelin Corporation in Milan use the local area network mainly for e-mail exchange, as well as for the joint processing of text and graphic information. The cabling system, built on the basis of a UTP5 cable, connects several hubs to which more than 200 computers are connected. The network uses Compaq ProLiant servers with powerful processors and large hard drives, as well as Olivetti workstations and personal computers. Each office has a network laser printer. At night, when there are no employees in the building, all critical information is backed up by a backup system that is equipped with one of the servers - this reduces the risk of losing vital data. The entire Milan branch of Michelin Corporation is connected to the Internet through one of the computers, which acts as a gateway between the company's local network and a fiber optic link to the Internet provider. With a permanent connection to the Internet, Michelin Milano can connect to the mainframe at Michelin Corporation's headquarters in Turin at any time.

4. Topologies of a computer network.

Star topology.

The concept of a star network topology comes from the field of mainframe computers, in which the host machine receives and processes all data from peripheral devices as an active data processing node. This principle is applied in data communication systems such as RELCOM e-mail. All information between two peripheral workstations passes through the central node of the computer network.

Network throughput is determined by the computing power of the node and is guaranteed for each workstation. Collisions (collisions) of data do not occur.

The star topology is the most reliable of all computer network topologies, since data transmission between workstations passes through the central node (if it has good performance) on separate lines used only by these workstations.

Ring topology.

With a ring network topology, workstations are connected to each other in a circle, i.e. workstation 1 with workstation 2, workstation 3 with workstation

4 etc. The last workstation is linked to the first. The communication link is closed in a ring.

Laying cables from one workstation to another can be quite complex and expensive, especially if the workstations are geographically located far from the ring (for example, in a line).

The main problem with a ring topology is that each workstation must actively participate in the transfer of information, and if at least one of them fails, the entire network is paralyzed.

A special form of ring topology is the logical ring network. Physically, it is mounted as a connection of star topologies.

Bus topology.

With a bus topology, the information transmission medium is presented in the form of a communication path, accessible to all workstations, to which they must all be connected. All workstations can communicate directly with any workstation on the network.

Workstations at any time, without interrupting the operation of the entire computer network, can be connected to it or disconnected. The functioning of a computer network does not depend on the state of a separate workstation.

In the standard situation for an Ethernet bus network, a thin cable or a Cheapernet cable with a tee connector is often used. Turning off and especially connecting to such a network requires a bus break, which causes a disruption in the circulating flow of information and a system hang.

The tree structure of the LAN.

Along with the well-known topologies of computer networks ring, star and bus, a combined one is also used in practice, for example, a tree structure. It is formed mainly in the form of combinations of the above topologies of computer networks. The base of the computer network tree is located at the point (root) where information communication lines (tree branches) are collected.

Computing networks with a tree structure are used where it is impossible to directly apply the basic network structures in their pure form. To connect a large number of workstations, according to adapter cards, network amplifiers or switches are used. A switch that simultaneously has the functions of an amplifier is called an active hub.

5. Modem

To connect remote computers with each other, mainly conventional telephone networks are used, which cover more or less vast territories of most states - PSTN (Public Switchable Tele-phone

network). The only problem in this case is the conversion of digital (discrete) signals that the computer operates into analog (continuous) ones.

Devices called modems are designed to solve this problem.

A modem is a peripheral device designed to exchange information with other computers through the telephone network. According to GOST terminology, they are called UPS (signal conversion devices). In fact, the modem is formed by two nodes - a modulator and a demodulator; it performs modulation and demodulation of information signals. Actually the word "modem" is an abbreviation for the other two:

Modulator / Demodulator.

In other words, the modem modulator converts the bit stream from the computer into analog signals suitable for transmission over the telephone channel; The modem's demodulator performs the inverse task - it converts the audio frequency signals into digital form so that they can be perceived by the computer. Thus, the data to be transmitted is converted into an analog signal by the modem modulator<передающего>computer. The receiving modem, located at the opposite end of the line,<слушает>transmitted signal and converts it back to digital using a demodulator.

Therefore, a modem is a device capable of both transmitting and receiving data.

Due to the fact that telephone lines are used as a data transmission medium, it is possible to communicate with any part of the globe.

Modern modems are made on the basis of specialized LSI (large-scale integrated circuits) that perform almost all the functions of a modem. This provides small dimensions, high reliability and ease of use of modems.

In recent years, modems at a transmission rate of 2400, 9600 and 14400 bps are most widely used, while these types of modems allow transmission at lower speeds (1200, 4800, 7200, 12000 bps), as well as interaction with the bulk of modems of earlier years of release.

At present, the functions of protection against transmission errors and the function of data compression have been introduced into the tasks performed by the modem, which has made it possible to radically increase the reliability and speed of information transmission. Thanks to data compression, the actual transmission rate of digital information using modems can be increased to 40-60 Kbps.

Recently, modems have become an integral part of the computer.

By installing a modem on your computer, you actually open up a new world for yourself. Your computer is transformed from a stand-alone computer into a global network.

List of used literature.

1. Sukhman S.M., Bernov A.V., Shevkoplyas B.V. Components of telecommunication systems. Analysis of engineering solutions. - M.: MIET, 2002. - 220 p.

2. Computer Press. – 1998 - No. 8

3. Computer Press. – 1999 - No. 1

4. Website on the Internet: www.iXBT.ru. The link is "communications".

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 (SS) 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: packet start and end codes, sender and recipient addresses, packet number in the message, information to control 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.

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BRANCH OF THE FEDERAL STATE BUDGET EDUCATIONAL INSTITUTION OF HIGHER PROFESSIONAL EDUCATION

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IN TOBOLSK

Tobolsk Pedagogical Institute. DI. Mendeleev

Department of Physics, Mathematics, Informatics and Teaching Methods

Course work

Telecommunication systems

student of the 5th year of correspondence education

Faculty of Natural Sciences,

direction "Vocational training

(electronics, radio engineering and communications)"

Sorochenko Alexander Nikolaevich

Lecturer: candidate of pedagogical sciences,

Associate Professor Kutumova A. A.

Tobolsk 2016

Introduction

1. Characteristics and classification of information networks

2. Multilevel architecture of information networks

3. Varieties of communication channels

4. Organization of access to information networks

4.1 Structure of territorial networks

4.2 Basic types of access

4.2.1 Telecommunication technology service

4.2.2 Email

4.2.3 File sharing

4.2.4 Newsgroups and bulletin boards

4.2.5 Access to distributed databases

4.2.6 WWW information system

Conclusion

Bibliography

Introduction

XXI century without exaggeration can be called the century of information technology. The concept of information technology includes many aspects. One of the most important parts of this direction is the direct transmission of information through information networks.

Telecommunication technologies are the principles of organizing modern analog and digital systems and communication networks, including computer and INTERNET networks.

Telecommunication means is a set of technical devices, algorithms and software that allow transmitting and receiving speech, information data, multimedia information using electrical and electromagnetic oscillations via cable, fiber-optic and radio channels in various wave bands. These are information conversion devices, its encoding and decoding, modulation and demodulation, these are modern computer processing technologies.

1. Characteristics and classification of information networks

Modern telecommunication technologies are based on the use of information networks.

Communication network - a system consisting of objects that perform the functions of generating, converting, storing and consuming a product, called points (nodes) of the network and transmission lines (connections, communications, connections) that transfer the product between points.

A distinctive feature of the communication network is the large distances between the points compared to the geometric dimensions of the space occupied by the points.

Information network - a communication network in which the product of generation, processing, storage and use is information.

Computing network - an information network, which includes computing equipment. The components of a computer network can be computers and peripheral devices that are sources and receivers of data transmitted over the network. These components make up the data terminal equipment (DTE or DTE - Data Terminal Equipment). Computers, printers, plotters and other computing, measuring and executive equipment of automatic and automated systems can act as OOD. The actual transfer of data occurs with the help of media and tools that are combined under the name data transmission medium.

The preparation of data transmitted or received by the DTE from the data transmission medium is carried out by a functional unit called data circuit termination equipment (DCE or DCE - Data Circuit-Terminating Equipment). The DCE may be structurally separate or integrated into the DTE. The DTE and DCE together form a data station, often referred to as a network node. An example of a DCE is a modem.

Computing networks are classified according to a number of features.

Depending on the distances between connected nodes, computer networks are distinguished:

Territorial, covering a significant geographic area; among territorial networks, one can single out regional and global networks, having, respectively, regional or global scales; regional networks are sometimes called MAN (Metropolitan Area Network) networks, and the common English name for territorial networks is WAN (Wide Area Network);

Local (LAN) ? covering a limited area (usually within the remoteness of stations no more than a few tens or hundreds of meters from each other, less often 1 ... 2 km); local networks stand for LAN (Local Area Network);

Corporate (enterprise scale) ? a set of interconnected LANs covering an area where one enterprise or institution is located in one or more closely spaced buildings. Local and corporate computer networks are the main type of computer networks used in computer-aided design (CAD) systems.

They especially single out the unique global network Internet (the World Wide Web (WWW) information service implemented in it is translated into Russian as the World Wide Web); it is a network of networks with its own technology. In the Internet, there is the concept of intranets (Intranet) - corporate networks within the Internet.

There are integrated networks, non-integrated networks and subnets. An integrated computer network (Internet) is an interconnected collection of many computer networks, which are called subnets in the Internet.

In automated systems of large enterprises, subnets include computing facilities of individual design departments. Internets are needed to combine such subnets, as well as to combine the technical means of automated design and production systems into a single integrated automation system (CIM - Computer Integrated Manufacturing).

Internets are usually adapted for various types of communication: telephony, e-mail, video information, digital data, etc., in which case they are called integrated service networks. The development of internets consists in the development of means for interfacing heterogeneous subnets and standards for building subnets that are initially adapted to interfacing. Subnets in internets are combined in accordance with the selected topology using interaction blocks.

2. Multilevel architecture of information networks

In general, for the functioning of computer networks, two problems must be solved:

Transfer data to the destination in the correct form and in a timely manner;

The data received by the intended user must be recognizable and in the proper form for their correct use.

The first problem is related to routing tasks and is provided by network protocols (low level protocols).

The second problem is caused by the use of different types of computers in networks, with different codes and language syntax. This part of the problem is solved by introducing high level protocols.

Thus, a complete end-user-oriented architecture includes both protocols.

The developed Open Systems Interconnection (OSI) reference model supports the concept that each layer provides services to the higher layer and builds upon and uses the services of the lower layer. Each layer performs a specific function for data transmission. Although they must work in a strict order, but each of the levels allows for several options. Consider the reference model. It consists of 7 layers and is a multi-layer architecture, which is described by standard protocols and procedures.

The three lower layers provide network services. Protocols that implement these layers must be provided in each network node.

The top four layers provide services to the end users themselves and thus are associated with them and not with the network.

Physical level. In this part of the model, the physical, mechanical and electrical characteristics of the communication lines that make up the LAN (cables, connectors, fiber optic lines, etc.) are determined.

We can assume that this layer is responsible for the hardware. Although the functions of other levels can be implemented in the corresponding microcircuits, they still belong to the software. The functions of the physical layer are to ensure that characters entering the physical medium at one end of the link reach the other end. When using this downstream symbol transport service, the goal of the channel protocol is to ensure reliable (error-free) transmission of data blocks over the channel. Such blocks are often called cycles or frames. The procedure usually requires: synchronization on the first character in the frame, end-of-frame detection, detection of erroneous characters, if any, and correction of such characters in some way (usually done by requesting a retransmission of a frame in which one or more erroneous characters are found ).

Level channel. The data link layer and the underlying physical layer provide an error-free transmission path between two nodes in the network. This layer defines the rules for using the physical layer by network nodes. The electrical representation of data in the LAN (data bits, data encoding methods, and markers) is recognized at this and only at this level. Here, errors are detected (recognized) and corrected by requiring data retransmission.

network level. The function of the network layer is to establish a route for data transmission over a network or, if necessary, through several networks from a transmission node to a destination node. This layer also provides flow control or congestion control to prevent overflowing of network resources (storage in nodes and transmission channels) that can lead to a shutdown. When performing these functions at the network layer, a lower-level service is used - a data transmission channel that ensures error-free receipt of a data block entered into the channel at the opposite end along the network route.

The main task of the lower levels is to transfer blocks of data along the route from the source to the recipient, delivering them in a timely manner to the desired end.

Then the task of the upper layers is the actual delivery of data in the correct form and recognizable form. These upper layers are unaware of the network's existence. They provide only the service required of them.

Transport level. Provides reliable, consistent communication between two end users. For this purpose, a network layer service is used at the transport layer. It also manages the flow to ensure that blocks of data are received correctly. Due to the differences in end devices, data in a system can be transferred at different rates, so if flow control is not in place, slower systems can become overwhelmed by faster ones. When more than one packet is in processing, the transport layer controls the order in which the message components pass through. If a duplicate of a previously received message arrives, then this layer recognizes this and ignores the message.

Level session. The functions of this layer are to coordinate communication between two application programs running on different workstations. It also provides services to the higher presentation layer. It comes in the form of a well-structured dialogue. These functions include creating a session, managing the transmission and reception of message packets during a session, and terminating a session. This layer also manages the negotiation, if necessary, to ensure that data is exchanged correctly. The dialogue between the user of the session service (ie, the presentation layer parties and the higher layer) may consist of normal or accelerated data exchange. It can be duplex, i.e. simultaneous two-way transmission, when each side has the ability to independently transmit, or half-duplex, i.e. with simultaneous transmission in one direction only. In the latter case, special labels are used to transfer control from one side to the other. The session layer provides a synchronization service to overcome any detected errors. With this service, synchronization marks must be inserted into the data stream by users of the session service. If an error is detected, then the session connection should be returned to a certain state, users should return to the established point of the dialog flow, reset part of the transferred data, and then restore the transfer from this point. computer teleconferencing communication network

Level representation. Manages and transforms the syntax of blocks of data exchanged between end users. This situation can occur in different types of PCs (IBM PC, Macintosh, DEC, Next, Burrogh) that need to exchange data. Purpose - transformation of syntactic data blocks.

Applied level. Application layer protocols give the appropriate semantics or meaning to the exchanged information. This layer is the boundary between the PP and the processes of the OSI model. A message destined for transmission over a computer network enters the OSI model at this point, passes through layer 1 (physical), is forwarded to another PC, and travels from layer 1 in reverse order until it reaches the PP on the other PC through its application layer. Thus, the application layer provides mutual understanding of two application programs on different computers.

3. Varieties of communication channels

Data transmission medium - a set of data transmission lines and interaction units (i.e., network equipment not included in data stations) intended for data transmission between data stations. Communication media can be shared or dedicated to a specific user.

Data line - the means that are used in information networks to propagate signals in the right direction.

Channel (communication channel) - means of one-way data transmission. An example of a channel could be a frequency band allocated to one transmitter in radio communication.

Data transmission channel - means of two-way data exchange, including equipment for terminating a data channel and a data transmission line. By the nature of the physical data transmission medium (PD), there are data transmission channels on optical communication lines, wired (copper) communication lines and wireless.

Wired communication lines: Wired telecommunication lines are divided into cable, air and fiber optic.

Fax: Fax (or phototelegraph) communication is an electrical method of transmitting graphic information - a still image of text or tables, drawings, diagrams, graphs, photographs, etc. It is carried out using facsimile machines: telefaxes and telecommunication channels (mainly telephone).

Fiber optic communication lines: As wired communication lines, telephone lines and television cables are mainly used. The most developed is telephone wire communication. But it has serious drawbacks: susceptibility to interference, attenuation of signals when they are transmitted over long distances, and low bandwidth. All these shortcomings are deprived of fiber optic lines - a type of communication in which information is transmitted through optical dielectric waveguides ("optical fiber").

Optical fiber is considered the most perfect medium for transmitting large amounts of information over long distances. It is made of quartz, which is based on silicon dioxide, a widely used and inexpensive material, unlike copper. Optical fiber is very compact and lightweight, with a diameter of only about 100 microns.

Optical fiber lines differ from traditional wire lines:

Very high speed of information transfer (for a distance of more than 100 km without repeaters);

Security of transmitted information from unauthorized access;

High resistance to electromagnetic interference;

Resistance to aggressive environments;

Ability to transmit up to 10 million telephone conversations and one million video signals simultaneously on one fiber;

Flexibility of fibers;

Small size and weight;

Spark, explosion and fire safety;

Ease of installation and installation;

Low cost;

High durability of optical fibers - up to 25 years.

Currently, the exchange of information between continents is carried out mainly through submarine fiber optic cables, and not through satellite communications. At the same time, the main driving force behind the development of submarine fiber optic communication lines is the Internet.

Wireless communication systems: Wireless communication systems are carried out via radio channels.

In the 1930s meter-long, and in the 40s - decimeter and centimeter waves were mastered, propagating in a straight line, without bending around the earth's surface (i.e., within line of sight), which limits direct communication on these waves to a distance of 40-50 km in flat terrain, and in mountainous areas - several hundred kilometers. Since the width of the frequency ranges corresponding to these wavelengths - from 30 MHz to 30 GHz - is 1000 times the width of all frequency ranges below 30 MHz (waves longer than 10 m), they can transmit huge information flows and carry out multi-channel communication. At the same time, the limited range of propagation and the possibility of obtaining a sharp directivity with an antenna of a simple design make it possible to use the same wavelengths at many points without mutual interference. Transmission over considerable distances is achieved by using multiple retransmissions in radio relay lines or with the help of communication satellites located at a high altitude (about 40 thousand km) above the Earth (see "Space communications"). Allowing tens of thousands of telephone conversations to be conducted simultaneously over long distances and dozens of television programs to be transmitted, radio relay and satellite communications are much more efficient in their capabilities than conventional long-distance radio communications on meter waves.

Radio relay communication lines: Radio relay communication was originally used to organize multichannel telephone communication lines in which messages were transmitted using an analog electrical signal. The first such line with a length of 200 km with 5 telephone channels appeared in the USA in 1935. It connected New York and Philadelphia.

Over the past decades, the need to transmit data - information presented in digital form - has led to the creation of digital transmission systems. Digital radio-relay data transmission systems have appeared that are capable of exchanging digital information.

Satellite communications and navigation: Space or satellite communication is essentially a kind of radio relay communication and differs in that its repeaters are not on the surface of the Earth, but on satellites in outer space.

In the 1980s, the development of personal satellite communications began. At the beginning of the 21st century, the number of its subscribers is several million people, and in another 10 years - much more. Satellite and terrestrial communication systems will be combined into a single global personal communication system. The reach of any subscriber will be ensured by dialing his telephone number, regardless of his location. This is an advantage of satellite over cellular (discussed later in this chapter) because it is not tied to a specific location. After all, at the beginning of the 21st century, the coverage area of ​​cellular communications is only 15% of the earth's surface. Therefore, the demand for personal mobile communication in many regions of the world can only be met with the help of satellite communication systems. In addition to voice (radiotelephone) communication, they allow you to determine the location (coordinates) of consumers.

A satellite phone connects directly to a satellite in Earth orbit. From the satellite, the signal arrives at the ground station, from where it is transmitted to the conventional telephone network. The number of satellites required for stable communication anywhere in the world depends on the radius of the orbit of a particular satellite system.

At present, the first global communication system "Iridium" is in operation. It allows the client to stay in touch, wherever he is, and use the same phone number at the same time.

The system consists of 66 low-orbit satellites located at a distance of 780 km from the Earth's surface. It provides reception and transmission of a signal from a mobile phone located anywhere in the world. The signal received by the satellite is transmitted along the chain to the next satellite until it reaches the ground station of the system closest to the called subscriber. This ensures high signal quality.

The main disadvantage of personal satellite communications is its relative high cost compared to cellular communications. In addition, high power transmitters are built into satellite phones. Therefore, they are considered unsafe for the health of users.

The most reliable satellite phones operate on the Inmarsat network, established over 20 years ago. Inmarsat satellite phones are a flip-top case the size of early laptop computers. The cover of the satellite phone is also an antenna, which must be turned towards the satellite (the signal level is displayed on the phone display). These phones are mainly used on ships, trains or heavy vehicles. Every time you need to make or answer someone's call, you will need to install the satellite phone on some flat surface, open the lid and twist it, determining the direction of the maximum signal. These satellite phones cost more than $2500 and weigh from 2.2 kg. A minute of conversation on such a satellite phone costs 2.5 US dollars and more.

Paging: Paging communication is radiotelephone communication, sending messages dictated by the sender subscriber by telephone and receiving them over the radio channel by the recipient subscriber using a pager - a radio receiver with a liquid crystal display on which the received alphanumeric texts are displayed. A pager is a one-way communication device: you can only receive messages on it, but you cannot send messages from it.

The history of paging as a means of personal radio calling began in the mid-1950s in England. The first such device was developed in 1956. The number of subscribers could be no more than 57. When the subscriber received a tone signal, he had to bring the device to his ear and listen to the message transmitted by the dispatcher in speech form. Doctors became users of the first network in England. The networks that existed at that time were local in nature and served the needs of specific services. The largest of these were airport services. Some of these networks still exist today. Widespread paging began in the late 1970s in the United States.

Since then, paging systems have become quite widespread in the cities of Europe and the USA. At the same time, paging came to Russia.

The first pagers were simple frequency-modulated signal receivers. They contained several tuned circuits that traced the characteristic sequence of low-frequency signals (tones). When these tones were received, the device beeped. Therefore, such pagers are called tonal.

The transition to digital systems was inevitable. Tone coding was not suitable for transmitting alphanumeric messages.

Mobile cellular: 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. 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 they managed to save this ship, iced up in the Baltic Sea.

For many years, individual radio communication between two subscribers required its own separate radio communication channel operating at the same frequency. Simultaneous radio communication over many channels could be achieved by allocating a certain frequency band to each channel. But frequencies are also needed for radio broadcasting, television, radar, radio navigation, and military needs. Therefore, the number of radio channels was very limited. It was used for military purposes, government communications. So, in cars used by members of the Politburo of the Central Committee of the CPSU, mobile phones were installed. They were installed in police cars and radio taxis. In order for mobile communication to become mass, a new idea of ​​its organization was needed. This idea was expressed in 1947 by D. Ring, an employee of the American company Bell Laboratories. It consisted in dividing space into small sections - cells (or cells) with a radius of 1-5 kilometers and in the radio communication department within one cell from communication between cells. This allowed the same frequencies to be used in different cells. In the center of each cell, it was proposed to place a base - transmitting and receiving - radio station to ensure radio communication within the cell with all subscribers. Each subscriber has his own micro-radio station - "mobile phone" - a combination of a telephone, a transceiver and a mini-computer. Subscribers communicate with each other through base stations connected to each other and to the city telephone network.

Each cell must be served by a base radio transmitter with a limited range and a fixed frequency. This makes it possible to reuse the same frequency in other cells. During a conversation, the cellular radiotelephone is connected to the base station by a radio channel, through which the telephone conversation is transmitted. Cell dimensions are determined by the maximum communication range of the radiotelephone with the base station. This maximum range is the cell radius.

The idea of ​​mobile cellular communication is that, without leaving the coverage area of ​​one base station, the mobile phone falls into the coverage area of ​​any neighboring station up to the outer border of the entire network area.

For this, systems of antenna-repeaters have been created that cover their "cell" - the area of ​​the Earth's surface. For communication to be reliable, the distance between two adjacent antennas must be less than their radius of action. In cities, it is about 500 meters, and in rural areas - 2-3 km. A mobile phone can receive signals from several repeater antennas at once, but it always tunes in to the strongest signal.

The idea of ​​mobile cellular communications was also to use computer control over the telephone signal from the subscriber when he moves from one cell to another. 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 fast that the subscriber simply does not notice it.

Computers are the central part of the mobile communication system. They look for a subscriber located in any of the cells and connect him to the telephone network. When the subscriber moves from one cell to another, they transfer the subscriber from one base station to another, and also connect the subscriber from the "foreign" cellular network to "their own" when he is in its coverage area - they perform roaming (which in English means "wandering" or "wandering").

The principles of modern mobile communication 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 systems was difficult. Therefore, the practical use of cellular communication became possible only after the invention of microprocessors and integrated semiconductor circuits.

An important advantage of mobile cellular communications is the ability to use it outside the common area of ​​your operator - roaming. To do this, various operators agree among themselves on the mutual possibility of using their zones for users. The subscriber, leaving the common area of ​​his operator, automatically switches to the zones of other operators even when moving from one country to another, for example, from Russia to Germany or France. Or, while in Russia, the user can make cellular calls to any country. Thus, cellular communication provides the user with the ability to communicate by phone with any country, wherever he is.

Leading cell phone manufacturers are guided by a single European standard - GSM. That is why their equipment is technically perfect, but relatively inexpensive. After all, they can afford to produce huge batches of phones that find sales.

A convenient addition to the cell phone was the system of short messages SMS (Short Message Service). It is used to send short messages directly to the phone of a modern digital GSM system without the use of additional equipment, only with the help of a numeric keypad and a cell phone display screen. SMS messages are also received on a digital display, which is equipped with any cell phone. SMS can be used in cases where a regular telephone conversation is not the most convenient mode of communication (for example, in a noisy crowded train). You can send your phone number to a friend via SMS. Due to the low cost, SMS is an alternative to telephone conversation. The maximum size of an SMS message is 160 characters. You can send it in several ways: by calling a special service, as well as using your GSM phone with the sending function, using the Internet. The SMS system can provide additional services: send exchange rates, weather forecasts, etc. to your GSM phone. Essentially, a GSM phone with an SMS system is an alternative to a pager.

But the SMS system is not the last word in cellular communications. In the most modern cell phones (for example, by Nokia), the Chat function appeared (in the Russian version - "dialogue"). With its help, you can communicate in real time with other cell phone owners, as is done on the Internet. Essentially, this is a new kind of SMS messaging. To do this, you compose a message to your interlocutor and send it. The text of your message appears on the displays of both cell phones - yours and your interlocutor. Then he answers you and his message is displayed on the displays. Thus, you are conducting an electronic dialogue. But if your interlocutor's cell phone does not support this function, then he will receive regular SMS messages.

Cellular phones have also appeared that support high-speed Internet access via GPRS (General Packet Radio Service) - a standard for packet data transmission over radio channels, in which the phone does not need to "dial": the device constantly maintains a connection, sends and receives data packets. Cellular telephones with built-in digital cameras are also produced.

According to the research company Informal Telecoms & Media (ITM), the number of mobile communication users in the world in 2007 is 3.3 billion people.

Finally, the most complex and expensive devices are smartphones and communicators that combine the capabilities of a cell phone and a pocket computer.

Internet telephony: Internet telephony has become one of the most modern and economical types of communication. Her birthday can be considered February 15, 1995, when VocalTec released its first soft-phone - a program that serves to exchange voice over an IP network. Microsoft then released the first version of NetMeeting in October 1996. And already in 1997, it became quite common to connect via the Internet to two ordinary telephone subscribers located in completely different places on the planet.

Why is ordinary long-distance and international telephone communication so expensive? This is explained by the fact that during a conversation you occupy a whole communication channel, not only when you are talking or listening to the interlocutor, but also when you are silent or distracted from the conversation. This is what happens when voice is transmitted over the telephone in the usual analog way.

With the digital method, information can be transmitted not continuously, but in separate "packets". Then one communication channel can send information simultaneously from many subscribers. This principle of packet transmission of information is similar to the transportation of many letters with different addresses in one mail car. After all, they don't "drive" one mail car to transport each letter separately! Such temporary "packet compaction" makes it possible to use existing communication channels much more efficiently, "compress" them. At one end of the communication channel, information is divided into packets, each of which, like a letter, is provided with its own individual address. Packets of many subscribers are transmitted "mixed" over the communication channel. At the other end of the communication channel, packets with the same address are again combined and sent to their destination. This packet principle is widely used on the Internet.

Through a personal computer, you can send and receive letters, texts, documents, drawings, photographs over the Internet. But Internet telephony (IP telephony) works exactly the same way - a telephone conversation between two users of personal computers.

To do this, both users must have microphones connected to the computer and headphones or speakers, and their computers must have sound cards (preferably for two-way communication). In this case, the computer converts the analog "voice" signal (the electrical analogue of sound) into a digital one (combinations of pulses and pauses), which is then transmitted over the Internet.

At the other end of the line, your interlocutor's computer performs the reverse conversion (digital signal to analog), and the voice is reproduced as in a regular phone. Internet telephony is much cheaper than long-distance and international calls on a regular phone. After all, with IP-telephony you need to pay only for using the Internet.

Having a personal computer, a sound card, a compatible microphone and headphones (or speakers), you can use Internet telephony to call any subscriber who has a regular landline phone. During this conversation, you will also pay only for using the Internet.

Before using Internet telephony, a subscriber who owns a personal computer must install a special program on it.

To use Internet telephony services, it is generally not necessary to have a personal computer. To do this, it is enough to have a regular phone with touch-tone dialing. In this case, each dialed digit goes into the line not in the form of a different number of electrical impulses, as when the disk rotates, but in the form of alternating currents of different frequencies. This tone mode is found in most modern telephones.

To use Internet telephony using a telephone set, you need to buy a credit card and call a powerful central server computer at the number indicated on the card. Then the server's automat voice (optionally in Russian or English) announces the commands: dial the serial number and card key using the telephone buttons, dial the country code and the number of your future interlocutor.

Next, the server turns the analog signal into digital, sends it to another city, country or another continent to a server located there, which again converts the digital signal into analog and sends it to the desired subscriber. The interlocutors talk like on a regular phone, however, sometimes there is a small (for a fraction of a second) delay in the answer. Recall once again that to save communication channels, voice information is transmitted in "packets" of digital data: your voice information is divided into segments, packets, called Internet protocols (IP).

TCP/IP (Transmission Control Protocol / Internet Protocol) is the main Internet protocol, or data transmission format on the Internet. At the same time, IP ensures the promotion of the packet over the network, and TCP guarantees the reliability of its delivery. They provide a breakdown of the transmitted data into packets, the transfer of each of them to the recipient along an arbitrary route, and then - assembly in the correct order and without loss.

Not only your packets are sequentially transmitted over the communication channel, but also the packets of several other subscribers. At the other end of the communication line, all your packets are combined again, and your interlocutor hears all of your speech. In order not to feel a delay in the conversation, this process should not exceed 0.3 seconds. This is how information is compressed, thanks to which Internet telephony is several times cheaper than conventional long-distance and even more so international calls.

Skype (www.skype.com) was created in 2003. It is completely free and requires almost no knowledge from the user to install or use it. It allows you to talk with video support to interlocutors sitting at their computers in different parts of the world. In order for the interlocutors to see each other, the computer of each of them must be equipped with a web-camera.

This is such a long way in the development of communications has been made by mankind: from signal fires and drums to a cellular mobile phone, which allows you to almost instantly contact two people located anywhere on our planet.

4. Organization of access to information networks

4.1 Structureterritorialnetworks

The Internet is the largest and the only network of its kind in the world. Among global networks, it occupies a unique position. It is more correct to consider it as an association of many networks that retain their independent value.

Indeed, the Internet has neither a clear owner nor a national identity. Any network can be connected to the Internet and, therefore, be considered part of it, if it uses the Internet-accepted TCP/IP protocols or has converters to TCP/IP protocols. Almost all national and regional networks have access to the Internet.

A typical territorial (national) network has a hierarchical structure.

The upper level is federal nodes interconnected by trunk communication channels. Trunk channels are physically organized on FOCL or on satellite communication channels.

Middle level - regional nodes that form regional networks. They are connected to the federal nodes and possibly to each other by dedicated high or medium speed circuits such as T1, E1, B-ISDN circuits or microwave links.

The lower level is local nodes (access servers) connected with regional nodes, mainly switched or dedicated telephone communication channels, although there is a noticeable trend towards the transition to high- and medium-speed channels.

It is to the local nodes that the local networks of small and medium-sized enterprises, as well as the computers of individual users, are connected. Corporate networks of large enterprises are connected to regional nodes by dedicated high- or medium-speed channels.

4.2 Mainkindsaccess

4.2. 1 Telecommunication technology service

The main services provided by telecommunication technologies are:

Email;

File transfer;

teleconferences;

Reference services (bulletin boards);

Videoconferencing;

Access to information resources (information databases) of network servers;

Mobile cellular communication;

Computer telephony.

The specificity of telecommunications is manifested primarily in application protocols. Among them, the protocols associated with the Internet and the protocols ISO-IP (ISO 8473), related to the seven-layer model of open systems, are the most well-known. Internet application protocols include:

Telnet - a terminal emulation protocol, or, in other words, a remote control implementation protocol is used to connect a client to a server when they are located on different computers, the user has access to the server computer through his terminal;

FTP - file exchange protocol (remote host mode is implemented), the client can request and receive files from the server, the address of which is specified in the request;

HTTP (Hypertext Transmission Protocol) - protocol for communication between WWW servers and WWW clients;

NFS is a network file system that provides access to files of all UNIX machines on a local network, i.e. node file systems appear to the user as a single file system;

SMTP, IMAP, POP3 - email protocols.

These protocols are implemented using the appropriate software. For Telnet, FTP, SMTP, fixed numbers of protocol ports are allocated on the server side.

4.2. 2 Email

E-mail (E-mail) - a means of exchanging messages by electronic communications (in off-line mode). You can forward text messages and archived files. The latter may contain data (for example, program texts, graphic data) in various formats.

4.2. 3 File sharing

File sharing - access to files distributed across different computers. The Internet uses the FTP protocol at the application layer. Access is possible in off-line and on-line modes.

In off-line mode, a request is sent to the FTP server, the server generates and sends a response to the request. In the on-line mode, interactive viewing of the FTP-server directories, selection and transfer of the necessary files is carried out. An FTP client is required on the user's computer.

4.2. 4 Newsgroups and bulletin boards

Teleconferences - access to information allocated for group use in separate conferences (newsgroups). Global and local teleconferences are possible. Inclusion of materials in newsgroups, distribution of notifications about new incoming materials, fulfillment of orders are the main functions of the teleconferencing software. E-mail and on-line modes are possible.

The largest teleconferencing system is USENET. In USENET, information is organized hierarchically. Messages are sent either in an avalanche, or through mailing lists.

Teleconferences can be with or without a moderator. Example: the work of a team of authors on a book on mailing lists.

There are also means of audio conferencing (voice teleconferencing). A call, a connection, a conversation take place for the user as in a regular phone, but the connection goes through the Internet.

Electronic "bulletin board" BBS (Bulletin Board System) - a technology close in functionality to a teleconference, allows you to centrally and promptly send messages to many users.

The BBS software combines email, teleconferencing and file sharing. Examples of programs that have BBS tools are Lotus Notes, World-group.

4.2. 5 Access to distributed databases

In "client/server" systems, the request must be formed on the user's computer, and the organization of data search, their processing and the formation of a response to the request belong to the server computer.

In this case, the necessary information can be distributed to different servers. There are special database servers on the Internet, called WAIS (Wide Area Information Server), which can contain collections of databases controlled by various DBMS.

A typical scenario for working with a WAIS server:

Selecting the required database;

Formation of a query consisting of keywords;

Sending a request to the WAIS server;

Receiving from the server the titles of documents corresponding to the given keywords;

Selecting the desired header and sending it to the server;

Get the text of the document.

Unfortunately, WAIS is not currently being developed, so it is little used, although indexing and searching by indexes in large arrays of unstructured information, which was one of the main functions of WAIS, is an urgent task.

4.2. 6 WWW information system

WWW (World Wide Web - World Wide Web) - hypertext information system of the Internet. Its other short name is Web. This more modern system provides users with more options.

Firstly, it is a hypertext - a structured text with the introduction of cross-references into it, reflecting the semantic connections of parts of the text. Link words are highlighted in color and/or underlined. Selecting a link brings up the text or image associated with the link word. You can search for content by keywords.

Secondly, the presentation and acquisition of graphic images is facilitated. Information accessible via Web technology is stored on Web servers.

The server has a program that constantly monitors the arrival of requests from clients on a specific port (usually port 80). The server satisfies requests by sending the client the content of the requested Web pages or the results of the requested procedures. WWW client programs are called browsers.

There are text and graphic browsers. Browsers have commands for paging, moving to the previous or next document, printing, following a hypertext link, and so on.

For the preparation of materials and their inclusion in the WWW database, a special HTML language (Hypertext Markup Language) and software editors that implement it, such as Internet Assistant as part of the Word editor or Site Edit, have been developed; preparation of documents is also provided as part of most browsers.

For communication between Web servers and clients, the HTTP protocol has been developed, which works on the basis of TCP / IP. The web server receives a request from the browser, finds the file that matches the request, and passes it to the browser for viewing.

Conclusion

Intranet and Internet technologies continue to evolve. New protocols are being developed; old ones are being revised. NSF has greatly complicated the system by introducing its backbone network, several regional networks, and hundreds of university networks.

Other groups also continue to join the Internet. The most significant change was not due to the addition of additional networks, but due to additional traffic.

Physicists, chemists, and astronomers work and exchange more data than the computer scientists who make up the bulk of the traffic of the early Internet.

These new scientists brought about a significant increase in Internet downloads when they started using it, and downloads steadily increased as they increasingly used it.

To accommodate the increase in traffic, NSFNET's backbone network bandwidth has been doubled, resulting in current bandwidth approximately 28 times greater than the original; Another increase is planned to bring this ratio up to 30.

At the moment, it is difficult to predict when the need for additional throughput increases will disappear. The growth in demand for networking was not unexpected. The computer industry has taken great pleasure in the constant demands for more processing power and more storage space for data over the years.

Users have just begun to understand how to use networks. In the future, we can expect a constant increase in the need for interaction.

Therefore, interaction technologies with greater bandwidth will be required to accommodate this growth.

The expansion of the Internet lies in the complexity that has arisen from the fact that several autonomous groups are part of the unified Internet. The original designs for many subsystems assumed centralized control. It took a lot of effort to finalize these projects to work under decentralized governance.

So, for the further development of information networks, more high-speed communication technologies will be required.

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5. Telecommunication systems. - URL: http://otherreferats.allbest.ru/radio. - (Date of access: 12/18/2015).

6. Finaev V.I. Information exchanges in complex systems: Textbook. - Taganrog: Publishing House of TRTU, 2001.

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Introduction. 2

Digital telecommunication system. 5

Telecommunication. 5

1.2) Telecommunication system. 9

1.3) Digital transmission system. 12

1.3.1) Secondary digital transmission system PCM120. 21

1.3.2) Tertiary digital transmission system PCM480. 25

1.3.4. STM-N.. 32

1.4) Types of DH.. 43

1.5) Digital transmission systems PCM and STM.. 56

Main advantages of SDH technology: 57

Disadvantages of SDH technology: 58

2.2. Determine the quantization step by amplitude. 66

2.3. Develop a diagram of the time spectrum of the PZT. 71

2.4) Develop an enlarged block diagram of the PZT, consisting of temporary multiplexing equipment, equipment of the linear path of the terminal station and intermediate stations of the linear path. 86

Conclusion. 91

Bibliography. 92

Introduction

Scientific and technological progress at the end of the 20th century opened the way for the creation of a global information society, in which information and telecommunication technologies are of particular importance, developing into the infocommunication sector.

Humanity is moving to a new level of communication and information transfer. Now, in order to transmit a message, there is no need to be at close range. It is possible to transmit information from different parts of the world. Telecommunication systems have a great impact on all spheres of human life. Russia needs to finance the development of telecommunication systems, because the state is one step lower in comparison with global trends.

The development of communications at the beginning of the 21st century is characterized by the following concepts: universalization, integration, intellectualization - in terms of technical means and in the network plan; globalization, personalization - in terms of services. Progress in the field of communications is based on the development and mastering of new telecommunication technologies, as well as on the further development and improvement of existing ones that have not yet exhausted their potential.

The development of the infocommunication sector in the world occurs simultaneously in several directions. At the same time, in the field of telecommunications and information, it is characterized by the creation of global infocommunication systems, which are based on digital transmission systems (DTS) for various purposes with the widespread use of modern fiber optic technologies and digital switching systems of various types and levels.

Digital communication is actively developing all over the world - this is the main trend in the development of telecommunications. The quality of digital communication has a number of advantages over conventional communication. On the basis of digital transmission systems, extended transport networks of almost any purpose are built. Thanks to scientific progress, modern digital data transmission systems allow the simultaneous transmission of audio, video and digital signal.

Recent years in Russia in terms of the development of telecommunications have not been stable. They were preceded by a global telecommunications crisis that led to slower growth. Nevertheless, even during this period, new telecommunication technologies were developed and introduced. During this period, OJSC "Svyazinvest" carried out the structuring of the former telecommunications networks towards their consolidation, created strong, highly capitalized, profitable and competitive companies. As a result, there are seven interregional companies (RTOs) in Russia, and about 6,500 registered new operators in the telecommunications market. In June 2003, the State Duma of the Russian Federation adopted a new federal law "On Communications", which entered into force on January 1, 2004. With this, in essence, the completion of one stage in the development of communications in Russia and the beginning of a new stage are connected.

The modernization of terrestrial broadcasting networks through the transition to digital technologies is a global trend followed by the Russian Federation. The transition to digital broadcasting in Russia will not only provide the population with multi-program broadcasting of a given quality, but will also have a stimulating effect on the development of media markets, communications and the production of domestic television and radio equipment, the creation of infrastructure for production and innovation, marketing and service organizations, the further development of small and medium-sized businesses and the development of competition in this area. The main goal, according to the Concept for the Development of Television and Radio Broadcasting in the Russian Federation for 2008-2015, is to provide the population with multi-program broadcasting with guaranteed provision of publicly available television and radio channels of a given quality, which will allow the state to fully realize the constitutional right of citizens to receive information.

In accordance with this goal, the following tasks have been set:

Explore the basic principles of a digital data transmission system;

Consider what digital transmission systems exist;

To study the features of building digital transmission systems.

Digital telecommunication system

Telecommunication

Telecommunication (Greek tele - far, far and Latin communicatio - communication) - data transmission over long distances.

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

A telecommunications network is a set of telecommunication means interconnected and forming a network of a certain topology (configuration). Telecommunication networks are:

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

Radio networks for the transmission of audio data;

Television networks for video data transmission;

Digital (computer) networks or data transmission 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:

continuous;

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:

Electrical (electrical current);

Optical (light);

Electromagnetic (electromagnetic radiation field - radio waves).

To transmit electrical and optical signals, cable communication lines are used, respectively:

Electrical (ELS);

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:

continuous;

Discrete.

In this case, 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:

Cable (electrical and fiber-optic communication lines);

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 can consist of several serial communication lines, forming a composite channel, for example: a communication channel is formed between subscribers A1 and A2, including telephone (TfLS) and fiber-optic (FOCL) communication lines. At the same time, in one communication line, as will be shown below, several communication channels can be formed that provide simultaneous data transmission between several pairs of subscribers.

Telecommunication system

Under telecommunication systems (TS) it is customary to understand structures and means designed to transmit large amounts of information (usually in digital form) through specially laid communication lines or radio. At the same time, it is supposed to serve a significant number of system users (from several thousand). Telecommunication systems include such information transmission structures as television broadcasting (collective, cable, satellite, cellular), public telephone networks (PSTN), cellular communication systems (including macro- and micro-cellular), paging systems, satellite communication systems and navigation equipment, fiber networks for information transmission.

It should be noted that the main requirement for communication systems is the absence of the fact of communication interruption, but some deterioration in the quality of the transmitted message and waiting for the connection to be established are allowed.

By purpose, telecommunication systems are grouped as follows:

broadcasting systems;

communication systems (including paging);

· computer networks.

By type of information transfer medium used:

cable (traditional copper);

fiber optic;

ethereal;

satellite.

By way of information transfer:

· analog;

digital.

We will consider transmission methods: analog and digital.

There are two classes in telecommunication communication systems (switching). These are analog and digital systems.

Analog transmission and communication systems (switching).
In analog systems, all processes (reception, transmission, communication) are based on analog signals. There are many examples of such systems: television broadcasting, radio, telephone switching (communication).
Digital transmission and communication systems (switching).
In digital systems, all processes originate from digital (discrete) signals. Examples are - modern communication facilities, digital telephony, digital television. The evolutionary process of transition from analog systems to digital is connected with:

1. the age of new technologies, respectively, microprocessor signal processing technologies are increasingly spreading in technology;

2. a high-speed web of digital telecommunication networks is being created;
The connecting threads of the web are highways, which are a set of digital switching (communication) channels on a global and local scale. Access to these channels is allowed to various government agencies, businesses, and private users. The transmission and communication quality is correspondingly very high.
Let's give the advantages of digital transmission and data processing systems over analog systems:
1. Reliability of data transmission, as well as high noise immunity;
2. Data storage at the highest level;
3. Tied to computing;
4. Minimizing the occurrence of errors in the processing, transmission, switching (communication) of data;

Digital transmission system

Controls, an automatic control system in which signals are quantized in terms of level and time. Continuous signals (influences) arising in the analog part of the system (which usually includes the control object, actuators and measuring converters) are converted in analog-to-digital converters, from where they are digitally received for processing in a digital computer. The results of data processing are subjected to inverse transformation in the form of continuous signals (influences) fed to the actuators of the control object. The use of a digital computer makes it possible to significantly improve the quality of control and optimize the control of complex industrial facilities. An example is an automated process control system (APCS).

The concept of "digital transmission" is quite broad and includes many issues, such as the choice of pulse parameters in a particular transmission medium, the conversion of a digital sequence to a transmission code, etc.

Synchronization In digital transmission systems, it is necessary to ensure that all digital signal processing operations are performed synchronously and sequentially. If these operations happened locally and were synchronized from one source, then there were no problems. In this case, strict requirements would not be imposed on the stability of the master oscillator, since the same changes in the clock frequency would occur in all sections. But since any digital transmission system can be considered as consisting of two or more receiving and transmitting half-sets separated by considerable distances, synchronization requirements become fundamental. Highly stable and therefore expensive, clocks can become useless due to line noise causing clock jitter. Essentially, jitter causes a change in the number of bits transmitted over the line. To combat this phenomenon, elastic memory devices are used, in which recording is carried out at the clock frequency of the received signal, and reading at the clock frequency of the local generator. Such a memory allows you to compensate for even large, but short-term deviations in the clock frequency. However, elastic memory does not cope with long, even small deviations. It can overflow or underflow depending on the ratio of clock frequencies. In this case, the so-called slippage occurs. Recommendation ITU-T G.822 normalizes the frequency of slips depending on the quality of service and establishes the distribution of the duration of work with reduced and unsatisfactory quality. Thus, the ITU-T recommendation allows some synchronization violations on synchronous digital networks. Recommendation ITU-T G.803 describes the following modes of digital networks for synchronization: • synchronous mode, in which there are practically no slips, having a random character. This mode of operation of networks with forced synchronization, when all elements of the network receive a clock frequency from one reference generator. · pseudosynchronous mode occurs when there are several highly stable generators (their instability is not more than 10-11 according to G.811). One slip per 70 days is allowed. This mode takes place at the junctions of networks with synchronous modes of different operators. · The plesiochronous mode appears on the digital network when the network element loses external forced synchronization. On a network with synchronous mode, this can happen if the main and backup paths of the clock signal fail or if the reference oscillator fails. To ensure in this case an acceptable slip level, 1 slip per 17 hours, the network element generators must have an instability of no more than 10-9. · The asynchronous mode is characterized by one slip per 7 seconds, which allows to have generators with instability not worse than 10-5. This mode is practically not used on digital networks. Currently, all digital transmission systems used on digital networks are usually divided into PDH (Plesiochronous Digital Hierarchy - plesiochronous digital hierarchy) and SDH (Synchronous Digital Hierarchy - synchronous digital hierarchy) systems. They owe their names to the corresponding modes of operation for synchronization. In this article, we will consider PDH in detail; a separate article is devoted to the principles of SDH. Plesiochronous Digital Hierarchy PDH systems were the first to emerge and were based on time division channel (TMD) and PCM coding systems. For historical reasons, two types of plesiochronous hierarchy have emerged - the North American one, used mainly in the USA, Canada and Japan, and the European one, used in most countries. The base rate or zero level in both types of hierarchy (PDH and SDH) is 64 kbps, which refers to one standard telephone channel. The next step in the plesiochronous hierarchies are the primary digital transmission systems. ITU-T G.732 describes the European system (PCM30), while G.733 describes the North American system (PCM24). A frame or cycle of the PCM30 system has a duration of 125 µs and consists of 32 bytes, each of which refers to a specific channel of the system. Fig 1.1) Cycle structure. The figure shows the structure of the cycle. The zero channel is intended for transmission of service signals and synchronization signals. Channels 1 to 15 and 17 to 31 are information or telephone. In each cycle, 32 * 8 = 256 bits are transmitted, which ultimately gives a speed of 2048 kbps. Channel number 16 is called a signaling channel and can be used in two ways: • to transmit signaling information for telephone channels. In this case, in each cycle, the signaling channel byte is split into two halves. In the first half, signal information is transmitted sequentially for 15 cycles from 1 to 15 telephone channels, in the second - from 16 to 31 channels. In frame zero, a multi-frame synchronization signal is transmitted on the signaling channel. Thus, signaling information is transmitted through the signaling channel for each telephone channel at a rate of 2 kbps. · the signaling channel of the PCM30 system can be used to provide signaling transmission over a common channel, for example, SS No. 7, or for data transmission. Let us explain some notation in the figure. In all overhead bytes, the bit marked “X” is reserved for international use. The "Y" bits are reserved for national use. The “Z” bit is used to signal multiframe failures. Bit "A" is used to signal the presence of important messages. This signal occurs (bit takes the value “1”) in the following cases: · power supply failure; frame synchronization failure; failure of line coding equipment; · the presence of errors in the incoming signal 2.048 Mbps; · the frequency of occurrence of serial frame synchronization errors exceeds the value of 10-3. The PCM24 cycle also has a duration of 125 µs, but consists of 24 bytes and one additional bit. Each byte refers to a specific system channel. Rice. 1.2. Cycle structure. The figure shows the structure of the cycle. In one cycle, 24 * 8 + 1 = 193 bits are transmitted, which gives a speed of 1544 kbps. Frame and multi-frame synchronization is provided by a specific combination of an extra bit, counted over 12 cycles. The signaling information of telephone channels is transmitted over two subchannels A and B, formed by the least significant bits of all channels, respectively, in 6 and 12 cycles. These channels provide signaling for each telephone channel at 1.333 kbps. The absence of a separate signal channel, compared to the European hierarchy, allows for more efficient use of bandwidth. However, there is a slight decrease in channel speed. Due to the multiplicity of the signal channel formation cycle, equal to 6, the rate decrease “floats” between the channels, which practically does not affect the speech quality, but does not allow simultaneous data transmission over individual PCM24 channels. Thanks to frame and multi-frame synchronization, the requirements of plesiochronous operation in primary digital systems are supported. To synchronize slave generators in the European hierarchy, a clock frequency of 2048 kHz is used, allocated from a digital stream at a speed of 2048 kbps. The subsequent steps of the North American and European plesiochronous digital hierarchies are based on their primary digital systems. The tables show the ratio of the number of channels and speeds. Tab. 1.1. European plesiochronous digital hierarchy Tab 1.2. North American Plesiochronous Digital Hierarchy Unlike the European one, the North American plesiochronous digital hierarchy has a number of variations that have not been standardized by the ITU-T. Another 3152 kbit/s (T1C) DS1C signal is used, providing 48 telephone channels. In Japan, 32,064 kbps (480 channels) is used instead of 44,736 kbps, and 97,728 kbps (1,440 channels) instead of 274,176 kbps. As can be seen from the tables in the North American hierarchy, the signals are given the name DS, which stands for very simply - digital signal (Digital Signal). Very often, alphanumeric combinations are used to indicate the speed of digital signals, which are shown in the tables. The primary digital stream is formed by byte-by-byte combining of channels. At the next levels, the combining occurs on the basis of bit-by-bit multiplexing of the primary streams. Due to the plesiochronous nature of the primary flows, slips are inevitable when they are combined. To reduce the likelihood of their occurrence, a procedure for matching or leveling speeds (stuffing) is used. Its essence lies in adding "empty" bits at the transmitting end and excluding them at the receiving end. This is a positive stuffing procedure. The ability to insert additional bits is provided by using a slightly higher combined stream rate than the sum of the original ones. Of course, in addition to additional bits, service signals and frame synchronization signals are also transmitted. The main disadvantages of the plesiochronous digital hierarchy (PDH) are the impossibility of direct access to channels, without demultiplexing/multiplexing procedures for the entire line signal, and the practical absence of network monitoring and control tools. The need for higher speeds of digital transmission systems, increased quality requirements led to the creation of synchronous digital hierarchy (SDH) systems.

1.3.1) Secondary digital transmission system PCM120

The secondary DSP with PCM, which meets the recommendations of the CCITT on the European hierarchy, is the serial PCM-120 system. It is designed to organize channels in local and zonal sections of the primary network using cables of the ZKNAP and MKS types. The main node of the PCM-120 system is a device for generating a typical secondary digital stream with a transmission rate of 8448 kbps from four primary ones with a transmission rate of 2048 kbps (Fig. 1.3). In this case, as in the primary DSPs, all options for organizing PDI, SV channels, etc., instead of PM channels, are saved.

1.3. Structure of DSP IKM-120

Rice. 1.4. Time spectrum of TsSP IKM-120

Table 1.3. Time spectrum of DSP IKM-120.

The linear path is organized according to a two-cable scheme, but single-cable is also allowed in local sections of the network. Nominal scheme of the cable section l uch = 5 km, the maximum length of the remote power section l dptah= 200 km. The maximum length of the receiving section of the PM L max = 600 km, which corresponds to the maximum length of the zone section of the primary network.

The digital stream at the SS 2 network junction between the VVG and the OLT of the IKM-120 system has parameters that comply with the recommendations of the CCITT, and therefore can be used to organize communication using standard equipment via RRL and FOCL.

The secondary digital stream is divided into cycles of duration T c = 125µs, consisting of 1056 bit intervals. The cycle is subdivided into four subcycles of equal duration (Fig. 1.4.). The first eight positions of the I subcycle are occupied by the clock signal of the combined stream (111001100), and the remaining 256 positions (from the 9th to the 264th inclusive) - by the information of the symbolically combined source (four) streams. In the figure, at the corresponding positions, the numbers of symbols of the source streams are marked. The first four positions of the II subcycle are occupied by the first symbols of the rate matching commands (KCC), and the next four positions are occupied by the SS signals. The second and third symbols of the KSS (the command for positive matching has the form 111, and the negative one - 000) occupy the first four positions of III and IV subcycles.

The distribution of the CSS symbols allows you to protect commands from the effects of bursts of impulse noise. Positions 5,...,8 of subcycle III are used for transmission of CI signals (two positions), emergency signals (one position) and call service communication (one position). In the IV subcycle, at positions 5,..., 8, information of the combined flows is transmitted with negative speed matching. With positive speed matching, the transmission of information at positions 9, ..., 12 IV of the subcycle is excluded. Thus, the total number of information symbols in the cycle is 1024+4. Since the rate matching operation is performed no more often than after 78 cycles, positions 5,...,8 of subcycle IV are occupied very rarely, and therefore they are used to transmit information about intermediate values ​​and the nature of the change in the rates of the combined flows.