By way of network management. The main components and varieties of computer networks. Main software and hardware components of the network

1.1. General characteristics of information and computing networks

The end of the 20th century was marked by an unprecedented leap in the development of global information and communication technologies - the third after the discovery of audio and video transmission channels, which radically influenced the development of the mass media system, after radio and television broadcasting were invented network technologies based on a different, digital, way of transmitting information, which led to the formation of a new environment for the dissemination of information flows.

Along with offline work a significant increase in the efficiency of using computers can be achieved by combining them into computer networks (network).

A computer network in the broad sense of the word is understood as any set of computers interconnected by communication channels for data transmission..

There are a number of good reasons for networking computers together.

Firstly, resource sharing allows multiple computers or other devices to share access to a single disk (file server), CD-ROM drive, tape drive, printers, plotters, scanners and other equipment, which reduces the cost per individual user.

Secondly In addition to sharing expensive peripherals, it is possible to similarly use network versions of application software.

Third, computer networks provide new forms of user interaction in one team, for example, when working on a common project.

Fourth, it becomes possible to use common means of communication between various application systems (communication services, data and video data transmission, speech, etc.). Of particular importance is the organization of distributed data processing. In the case of centralized storage of information, the processes of ensuring its integrity, as well as backup, are greatly simplified.

1.1.1. Main software and hardware components of the network

Computer network is a complex set of interconnected and coordinated software and hardware components.

Studying the network as a whole requires knowledge of the principles of operation of its individual elements:

Computers;
- communication equipment;
- operating systems;
- network applications.

The whole complex of software and hardware of the network can be described by a multilayer model. At the heart of any network lies the hardware layer of standardized computer platforms, i.e. the system of the end user of the network, which can be a computer or a terminal device (any input/output or information display device). Computers at the nodes of a network are sometimes referred to as host machines or simply hosts.

At present, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers in the network should correspond to the set of various tasks solved by the network.

Second layer is communication equipment. Although computers are central to the processing of data in networks, communication devices have recently begun to play an equally important role.

Cabling, repeaters, bridges, switches, routers, and modular hubs have evolved from ancillary network components to being essential, along with computers and system software, both in terms of impact on network performance and cost. Today, a communications device can be a complex, dedicated multiprocessor that needs to be configured, optimized, and administered.

third layer, which form the software platform of the network, are operating systems (OS). The efficiency of the entire network depends on what concepts of managing local and distributed resources are the basis of the network operating system.

The topmost layer of network facilities are various network applications, such as network databases, mail systems, data archiving tools, teamwork automation systems, etc.

It is important to be aware of the range of capabilities provided by applications for different applications, as well as to know how they are compatible with other network applications and operating systems.

Data transmission channels over computer networks. In order for computers to communicate with each other in a network, they must be connected to each other using some kind of physical transmission medium.

The main types of transmission media used in computer networks are:

Analog telephone channels of general use;
- digital channels;
- narrowband and broadband cable channels;
- radio channels and satellite communication channels;
- fiber optic communication channels.

Analogue communication channels were the first to be used for data transmission in computer networks and made it possible to use the developed public telephone networks that already existed at that time.

Data transmission over analog channels can be performed in two ways.

At first way telephone channels (one or two pairs of wires) through telephone exchanges physically connect two devices that implement communication functions with computers connected to them. Such connections are called dedicated lines or direct connections.

Second way is the establishment of a connection by dialing a telephone number (using switched lines).

The quality of data transmission over dedicated channels is usually higher and the connection is established faster. In addition, each dedicated channel requires its own communication device (although there are also multi-channel communication devices), and with dial-up communication, one communication device can be used to communicate with other nodes.

Digital communication channels. In parallel with the use of analog telephone networks for computer-to-computer interaction, methods for transmitting data in a discrete (digital) form over unloaded telephone channels (that is, telephone channels that are not connected to the electrical voltage used in the telephone network) began to develop - digital channels.

It should be noted that, along with discrete data, analog information (voice, video, facsimile, etc.) converted into digital form can also be transmitted via a digital channel.

The highest speeds over short distances can be obtained by using a specially twisted pair of wires (in order to avoid interaction between adjacent wires), the so-called twisted pair(TP - Twisted Pair).

Cable channels, or coaxial pairs are two cylindrical conductors on the same axis, separated by a dielectric coating. One type of coaxial cable (50 ohm impedance) is used primarily for narrowband transmission. digital signals, another type of cable (with a resistance of 75 ohms) - for the transmission of broadband analog and digital signals. Narrowband and broadband cables directly interconnecting communication equipment allow data to be exchanged at high speeds (up to several megabits / s) in analog or digital
form.

Radio channels and satellite communication channels. The use of radio waves of various frequencies as a transmission medium in computer networks is cost-effective either for communication over long and extra-long distances (using satellites), or for communication with hard-to-reach, mobile or temporarily used objects.

The frequencies on which radio networks operate abroad typically use the 2-40 GHz band (especially the 4-6 GHz band). The nodes in the radio network can be located (depending on the equipment used) at a distance of up to 100 km from each other.

Satellites usually contain several amplifiers (or transponders), each of which receives signals in a given frequency range (typically 6 or 14 GHz) and regenerates them in a different frequency range (eg 4 or 12 GHz). For data transmission, geostationary satellites are usually used, placed in equatorial orbit at an altitude of 36,000 km. Such a distance gives a significant signal delay (on average 270 ms), for which special methods are used to compensate.

In addition to data exchange in the radio range, recently for communication over short distances (usually within a room) infrared radiation.

IN fiber optic communication channels the phenomenon of total internal reflection of light, known from physics, is used, which makes it possible to transmit light streams inside a fiber-optic cable over long distances with virtually no loss. Light-emitting diodes (LEDs) or laser diodes are used as light sources in a fiber optic cable, and photocells are used as receivers.

Fiber optic communication channels, despite their higher cost compared to other types of communication, are becoming more widespread, and not only for communication on short distances but also in intra-urban and inter-urban areas.

In computer networks, three technologies can be used to transfer data between network nodes: circuit switching, message switching, and packet switching.

Circuit switching, provided by the public telephone network, allows, with the help of switches, to establish a direct connection between network nodes.

At message switching devices called switches and made on the basis of universal or specialized computers allow you to accumulate (buffer) messages and send them in accordance with a given priority system and routing principles to other network nodes. Using message switching can increase the delivery time of messages compared to circuit switching, but at the same time it smooths out network peaks and increases network survivability.

At packet switching user data is divided into smaller portions - packets, and each packet contains service fields and a data field. There are two main methods of data transmission during packet switching: a virtual channel, when a connection is established and maintained between nodes, as it were, over a dedicated channel (although in fact the physical data transmission channel is divided between several users) and datagram mode, when each packet from a set of packets, containing user data is transmitted between nodes independently of each other. The first connection method is also called contact mode(connection mode), the second - contactless(connectionless mode).

1.1.2. Classification of computer networks

The combination of the above components into a network can be done in various ways and means. According to the composition of their components, the methods of their connection, the scope of use and other features, networks can be divided into classes in such a way that the belonging of the described network to one or another class could quite fully characterize the properties and qualitative parameters of the network.

However, this kind of classification of networks is rather conditional. The most widespread today is the division computer networks based on territorial location.

On this basis, networks are divided into three main classes:

LAN - local networks (Local Area Networks);
MAN - city networks (Metropolitan Area Networks).
WAN - global networks (Wide Area Networks);

Local Area Network (LAN) is a communication system that supports one or more high-speed transmission channels within a building or some other limited area. digital information provided to connected devices for short-term exclusive use. The territories covered by the LA can vary significantly.

The length of communication lines for some networks can be no more than 1000 m, while other LANs are able to serve the whole city. Served territories can be both factories, ships, aircraft, and institutions, universities, colleges. Coaxial cables are typically used as the transmission medium, although twisted-pair and fiber optic networks are becoming more common, and wireless technology has also rapidly developed in recent years. local networks, which use one of three types of radiation: broadband radio signals, low-power microwave radiation (MW radiation), and infrared rays.

Small distances between network nodes, the transmission medium used and the associated low probability of errors in the transmitted data make it possible to maintain high exchange rates - from 1 Mbps to 100 Mbps /With).

City networks, as a rule, cover a group of buildings and are implemented on fiber optic or broadband cables. According to their characteristics, they are intermediate between local and global networks. Recently, in connection with the laying of high-speed and reliable fiber-optic cables in urban and intercity areas, and new promising network protocols, for example, ATM (Asynchronous Transfer Mode - asynchronous transfer mode), which in the future can be used both in local and wide area networks.

global networks, unlike local ones, as a rule, cover much larger territories and even most regions of the globe (the Internet can serve as an example). Currently, analogue or digital wired channels, as well as satellite communication channels (usually for communication between continents) are used as a transmission medium in global networks. Transfer rate limits (up to 28.8 Kbps on analog channels and up to 64 Kbps on user sites digital channels) and the relatively low reliability of analog channels, which requires the use of error detection and correction tools at the lower levels of the protocols, significantly reduce the speed of data exchange in global networks compared to local ones.

There are other classification features of computer networks.

By area of ​​operation networks are divided into:

banking networks,
- networks of scientific institutions,
- university networks;

According to the form of operation can be distinguished:

Commercial networks;
- free networks,
- corporate networks
- public networks;

By the nature of the implemented functions networks are divided into:

Computing designed to solve control problems based on computational processing initial information;
- informational, designed to obtain reference data at the request of users; mixed, in which computing and information functions are implemented.

By way of management computer networks are divided into:

Networks with decentralized control;
- centralized management;
- mixed control.

In the first case, each computer that is part of the network includes a complete set software tools to coordinate network operations. Networks of this type are complex and quite expensive, since the operating systems of individual computers are developed with a focus on collective access to the common memory field of the network.

In the conditions of mixed networks under centralized control, tasks with the highest priority and, as a rule, are associated with the processing of large amounts of information are being solved.

By software compatibility there are networks:

Homogeneous;
- homogeneous (consisting of software-compatible computers)
- heterogeneous or heterogeneous (if the computers included in the network are programmatically incompatible).

1.1.3. Local networks

There are two approaches to building local networks and, accordingly, two types: client/server networks and peer-to-peer networks.

In client/server networks, a dedicated computer (server) is used, on which shared files are concentrated and which provides a print service for many users (Fig. 1).

Rice. 1. Client/server networks

Server- a computer connected to a network and providing its users with certain services.

Servers can perform data storage, database management, remote job processing, job printing, and a number of other functions that network users may need. The server is the source of network resources. There can be quite a lot of servers on the network, and each of them can serve its own group of users or manage certain databases.

Work station- a personal computer connected to the network, through which the user gets access to its resources. The network workstation operates in both network and local modes. It is equipped with its own operating system (MSDOS, Windows, etc.) and provides the user with all the necessary tools for solving applied problems. Workstations connected to the server are called clients. Both powerful computers for resource-intensive spreadsheet processing and low-power PCs for simple word processing can be used as clients. In contrast, powerful computers are usually installed as servers. Due to the need to ensure simultaneous processing of requests from a large number of clients and good protection of network data from unauthorized access, the server must be managed by a specialized operating system.

Examples: Novell Net Ware, Windows NT Server, IBM OS/2 Lan Server, Banyan Vines.

Peer-to-peer networks. Dedicated servers are not used in peer-to-peer networks (Fig. 2).

Rice. 2. Location of computers in peer-to-peer networks

At the same time as serving the user, a computer in a peer-to-peer network can take on the functions of a server, executing print jobs and responding to file requests from other workstations on the network. Of course, if a computer does not share its disk space or its printer, then it is only a client in relation to other workstations that act as a server. Windows 95 has built-in capabilities for building a peer-to-peer network. If you need to connect to other peer-to-peer networks, then Windows 95 supports the following networks:

NetWare Lite
- Artisoft LANtastic.

1.1.4. Network topology

Under topology refers to the description of the network properties inherent in all its homomorphic transformations, i.e. such changes appearance network, the distances between its elements, their mutual arrangement, at which the ratio of these elements to each other does not change.

The topology of a computer network is largely determined by the way computers are connected to each other. Topology largely determines many important properties of a network, such as reliability (survivability), performance, etc. There are different approaches to classifying network topologies. According to one of them, local area network configurations are divided into two main classes: broadcast And consecutive.

IN broadcast configurations each PC (physical signal transceiver) transmits signals that can be perceived by other PCs. Such configurations include "common bus", "tree", "star with passive center" topologies. A star-with-passive-centre network can be thought of as a kind of "tree" that has a root with a branch to each connected device.

IN serial configurations each physical sublayer transmits information to only one PC. Examples of sequential configurations are: arbitrary (arbitrary connection of computers), hierarchical, "ring", "chain", "star with an intelligent center", "snowflake" and
other.

The most optimal in terms of reliability (the possibility of the network functioning in the event of failure of individual nodes or communication channels) is fully connected network, i.e. a network in which each network node is connected to all other nodes, however, with a large number of nodes, such a network requires a large number of communication channels and is difficult to implement due to technical difficulties and high cost. Therefore, almost all networks are incompletely connected.

Although for a given number of nodes in a non-mesh network, there can be a large number of options for connecting network nodes, in practice, the three most common (basic) LAN topologies are usually used:

1. common bus;
2. ring;
3. star.

Bus topology (Fig. 3), when all network nodes are connected to one open channel, usually called a bus.

Fig 3. Topology "Bus"

In this case, one of the machines serves as a system server that provides centralized access to shared files and databases, printers, and other computing resources.

networks of this type gained great popularity due to low cost, high flexibility and data transfer speed, ease of network expansion (connecting new subscribers to the network does not affect its main characteristics). The disadvantages of bus topology include the need to use rather complex protocols and the vulnerability to physical damage to the cable.

Ring topology(Fig. 4), when all network nodes are connected to one closed ring channel.

Fig 4. Topology "Ring"

This network structure is characterized by the fact that information can be transmitted along the ring in only one direction and all connected PCs can participate in its reception and transmission. In this case, the recipient subscriber must mark the received information with a special marker, otherwise "lost" data may appear that interfere normal operation networks.

As a daisy chain configuration, the ring is especially vulnerable to failures: the failure of any cable segment leads to the termination of service for all users. LAN designers have put in a lot of effort to deal with this problem. Protection against damage or failure is provided either by closing the ring on the return (backup) path, or by switching to a spare ring. In both cases, the general ring topology is preserved.

Star topology(Fig. 5), when all network nodes are connected to one central node, called host (host) or hub.

Fig 5. Topology "Star"

The configuration can be seen as a further development of the "rooted tree" structure with a branch to each connected device. A switching device is usually located at the center of the network, ensuring the viability of the system. LANs of this configuration are most often used in automated office management systems that use a central database. Star LANs tend to be less reliable than shared bus or hierarchical networks, but this problem is solved by duplicating the central site hardware. The disadvantages include significant cable consumption (sometimes several times higher than the consumption in LANs of similar capabilities with a common bus or hierarchical ones).

Networks can also be of mixed topology ( hybrid) when separate parts of the network have different topologies. An example is the local FDDI network, in which the main ( trunk) nodes are connected to the ring channel, and other nodes are connected to them in a hierarchical topology.

1.1.5. Levels of interaction of computers in networks

In a computer network, there are 7 levels of interaction between computers:

Physical;
- logical;
- network;
- transport;
- the level of communication sessions;
- representative;
- application level.

Physical layer(Physical Layer) defines electrical, mechanical, procedural, and functional specifications and provides the link layer with the establishment, maintenance, and termination of a physical connection between two computer systems that are directly connected to each other by means of a transmission medium, such as an analog telephone circuit, a radio circuit, or a fiber optic circuit.

Link layer(Data Link Layer) controls the transmission of data over a communication channel. The main functions of this layer are the division of transmitted data into portions called frames, the separation of data from the bit stream transmitted at the physical layer for processing at the network layer, the detection of transmission errors and the recovery of incorrectly transmitted data.

network layer(Network Layer) provides communication between two computer systems of a network exchanging information with each other. Another function of the network layer is the routing of data (called packets at this layer) within and between networks (internet protocol).

transport layer(Transport Layer) provides reliable transmission (transportation) of data between computer systems of the network for higher levels. To do this, mechanisms are used to establish, maintain and break virtual channels (analogous to dedicated telephone channels), identify and correct transmission errors, and control data flow (in order to prevent overflow or data loss).

session layer(Session Layer) provides the establishment, maintenance and termination of a communication session for the presentation layer, as well as the resumption of an abnormally interrupted session.

Presentation layer(Presentation Layer) provides a transformation of data from the representation used in the application program of one computer system to the representation used in another computer system. The functions of the presentation layer also include the transformation of data codes, their encryption / decryption, as well as the compression of transmitted data.

Application layer(Application Level) differs from other layers of the OSI model in that it provides services for application tasks. This level determines the availability of application tasks and resources for communication, synchronizes interacting application tasks, establishes agreements on procedures for error recovery and data integrity management. Important functions of the application layer are network management, as well as the execution of the most common system application tasks: Email, file sharing and more.

Each level, in order to solve its subtask, must ensure the performance of the functions defined by the model. given level, actions (services) for a higher level and interact with a similar level in another computer system.

Accordingly, each level of interaction corresponds to a set of protocols (i.e. rules of interaction).

Under protocol refers to a set of rules governing the format and procedures for the exchange of information.

In particular, it determines how the connection is made, noise on the line is overcome, and error-free data transfer between modems is ensured.

The standard, in turn, includes a generally accepted protocol or set of protocols. The operation of network equipment is impossible without interrelated standards. Harmonization of standards is achieved both through consistent technical solutions and through the grouping of standards. Each specific network has its own basic set of protocols.

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ACADEMIC DISCIPLINE: Computer networks

Types of computer networks

Computer networks can be classified according to various criteria.

I. According to the principles of management:

1. Peer-to-peer - not having a dedicated server. In which control functions are alternately transferred from one workstation to another;

2. Multi-rank is a network that includes one or more dedicated servers. The remaining computers in such a network (workstations) act as clients.

II. By connection method:

1. "Direct connection"- two personal computers are connected by a piece of cable. This allows one computer (master) to access the resources of another (slave);

2. "Common bus" - connecting computers to one cable;

3. "Star" - connection through the central node;

4. "Ring" - serial connection PC in two directions.

III. By area coverage:

1. The local network(a network in which computers are located at a distance of up to a kilometer and are usually connected using high-speed communication lines.) - 0.1 - 1.0 km; LAN nodes are located within the same room, floor, building.

2. Corporate network(within are within the same organization, firm, plant). The number of nodes in the CVS can reach several hundred. At the same time, the corporate network usually includes not only personal computers, but also powerful computers, as well as various technological equipment(robots, assembly lines, etc.).

The corporate network makes it possible to facilitate the management of the enterprise and the management of the technological process, to establish clear control over information and production resources.

3. Global network(a network whose elements are separated from each other by a considerable distance) - up to 1000 km.

Both specially laid (for example, transatlantic fiber optic cable) and existing communication lines (for example, telephone networks) are used as communication lines in global networks. The number of nodes in a hot water supply system can reach tens of millions. The global network includes separate local and corporate networks.

4. World Wide Web- unification of global networks (Internet).

TOPOLOGY OF COMPUTER NETWORKS

Network topology is the geometric shape and physical arrangement of computers in relation to each other. Network topology allows you to compare and classify different networks. There are three main types of topology:

1) Star;

2) Ring;

BUS TOPOLOGY

This topology uses a single transmission channel based on a coaxial cable, called a "bus". All network computers are connected directly to the bus. At the ends of the bus cable, special plugs are installed - "terminators" (terminator). They are necessary in order to extinguish the signal after passing through the bus. The disadvantages of the "Bus" topology include the following:

Data transmitted over the cable is available to all connected computers;

In the event of a bus failure, the entire network ceases to function.

TOPOLOGY "RING"

Ring topology is characterized by the absence of connection endpoints; the network is closed, forming an inextricable ring through which data is transmitted. This topology implies the following transmission mechanism: data is transmitted sequentially from one computer to another until it reaches the recipient computer. The disadvantages of the "ring" topology are the same as those of the "bus" topology:

Public availability of data;

Damage resistance of the cable system.

STAR TOPOLOGY

In a network with a "star" topology, all computers are connected to a special device called a network hub or "hub" (hub), which performs the functions of data distribution. There are no direct connections between two computers on the network. Due to this, it is possible to solve the problem of public data availability, as well as increase the resistance to damage to the cable system. However, network functionality depends on the state of the network hub.

Carrier access methods in computer networks

IN various networks there are different procedures for exchanging data between workstations.

The International Institute of Electrical and Electronics Engineers (IEEE) has developed standards (IEEE802.3, IEEE802.4, and IEEE802.5) that describe methods for accessing network data channels.

The most common implementations of access methods are Ethernet, ArcNet, and token ring. These implementations are based respectively on the IEEE802.3, IEEE802.4 and IEEE802.5 standards.

Ethernet access method

This access method, developed by Xerox in 1975, is the most popular. It provides high data transfer speed and reliability.

This access method uses a "common bus" topology. Therefore, a message sent by one workstation is received simultaneously by all other stations connected to the common bus. But the message is only for one station (it includes the address of the destination station and the address of the sender). The station to which the message is intended receives it, the rest ignore it.

The Ethernet access method is a Carter Sense Multiple Access with Collision Detection (CSMA/CD -Carter Sense Multiple Access with Collision Detection) method.

Before transmitting, the workstation determines whether the channel is free or busy. If the channel is free, the station starts transmitting.

Ethernet does not exclude the possibility of simultaneous transmission of messages by two or more stations. The hardware automatically recognizes such conflicts. After a collision is detected, the stations delay transmission for some time. This time is short and each station has its own. After a delay, transmission resumes.

In reality, conflicts lead to a decrease in network performance only if several tens or hundreds of stations are operating.

ArcNet access method

This method was developed by Datapoint Corp. It has also become widespread, mainly due to the fact that ArcNet equipment is cheaper than Ethernet or Token-Ring equipment.

ArcNet is used in local area networks with a star topology. One of the computers creates a special marker (message of a special kind), which is sequentially transmitted from one computer to another.

If a station wishes to send a message to another station, it must wait for the token and append the message to it, complete with the sender and destination addresses. When the packet reaches the destination station, the message will be "unhooked" from the marker and transferred to the station.

Token-Ring access method

The Token-Ring access method was developed by IBM and is designed for a ring network topology.

This method is similar to ArcNet in that it also uses a token passed from one station to another. Unlike ArcNet, the Token-Ring access method allows you to assign different priorities to different workstations.

Communication media, their characteristics

Coaxial cable

Coaxial cable was the first type of cable used to connect computers on a network. This type of cable consists of a central copper conductor covered with a plastic insulating material, which in turn is surrounded by a copper mesh and/or aluminum foil. This outer conductor provides grounding and shields the center conductor from external electromagnetic interference. When laying networks, two types of cable are used - "Thick coaxial cable" (Thicknet) and "Thin coaxial cable" (Thinnet). Networks based on coaxial cable provide transmission at speeds up to 10 Mbps. Maximum length segment lies in the range from 185 to 500 m, depending on the type of cable.

"Twisted Pair"

Twisted pair cable is one of the most common cable types today. It consists of several pairs of copper wires covered with a plastic sheath. The wires that make up each pair are twisted around each other, which provides protection against mutual interference. Cables of this type are divided into two classes - "shielded twisted pair" ("Shielded twisted pair") and "unshielded twisted pair" ("Unshielded twisted pair"). The difference between these classes is that shielded twisted pair is more protected from external electromagnetic interference, due to the presence of an additional shield of copper mesh and / or aluminum foil surrounding the cable wires. Twisted-pair networks, depending on the cable category, provide transmission at speeds from 10 Mbps to 1 Gbps. The length of a cable segment cannot exceed 100 m (up to 100 Mbps) or 30 m (1 Gbps).

Fiber optic cable

Fiber optic cables are the most advanced cable technology, providing high speed data transmission over long distances, resistant to interference and eavesdropping. An optical fiber cable consists of a central glass or plastic conductor surrounded by a layer of glass or plastic coating and an outer protective sheath. Data transmission is carried out using a laser or LED transmitter that sends unidirectional light pulses through the center conductor. The signal at the other end is received by a photodiode receiver, which converts the light pulses into electrical signals that can be processed by a computer. The transmission speed for fiber optic networks ranges from 100 Mbps to 2 Gbps. The segment length limit is 2 km.

A computer network is a connection between two or more computers. In general, to create a computer network, you need special hardware (network equipment) and software(network software). The simplest connection between two computers to exchange data is called a direct connection. In this case, no additional hardware or software is required. The role of the hardware connection is performed by a standard parallel port, and all the software is already in the operating system. The advantage of a direct connection is its simplicity, the disadvantage is low speed data transmission.

The networks are divided into local and global. The purpose of all types of networks has one purpose - to provide shared access to shared resources: hardware, software and information (data resources).

According to the nature of the implemented functions, the network is divided into:

On computing, designed to solve control problems based on computational processing of the source information;

Informational, designed to obtain reference data at the request of users;

Mixed, in which computing and information functions are implemented.

According to the method of network management, they are divided into networks:

With decentralized management - each computer that is part of the network includes a complete set of software tools for coordinating network operations;

With centralized management - the coordination of the work of computers is carried out under the control of a single OS;

With mixed control - under centralized control, tasks that have the highest priority and, as a rule, are associated with the processing of large amounts of information are being solved.

Communication Model Layers:

1. Application layer- the user creates a document using applications.

2. Presentation Layer- The operating system of the computer fixes where the data is located and provides interaction with the next level.

3. session layer- the computer interacts with the network: it checks the user's right to access the network and transfers the document to the transport layer protocols.

4. transport layer- the document is converted into the form in which it is supposed to transmit data in the network used.

5. network layer determines the route of data movement in the network.

6. Connection level necessary in order to modulate the signals in accordance with the data received from the network layer. In a computer, these functions are performed LAN card or modem.

7. Physical layer. At this level, the real data transfer takes place. There are no documents, no packets, no bytes - only bits. Restoration of the document occurs gradually, when moving from the lower level to the upper one. The physical layer facilities lie outside the computer. In local networks, this is the equipment of the network itself. For dial-up communication using modems, this line telephone connection, switching equipment, etc.

The different protocol layers of the server and client do not communicate with each other directly, but they do communicate through physical layer. Gradually moving from top level to the bottom, the data is continuously converted. This creates the effect of virtual interaction between the levels. However, despite the virtuality, these are still connections through which data also passes. All services are based on virtual connections modern Internet.

Local area networks (LAN). If computers are located close to each other, use a common set of network equipment and are controlled by one software package, then such a network is called a local network. The creation of local networks is typical for individual divisions of enterprises. Let's consider the organization of information exchange of the model of interaction in a LAN.

In server LANs, two models of user interaction with workstations are implemented: file server and model client-server. In the first model, the server provides access to the database files for each workstation, and this is where its work ends. For example, if a file-server database is used to obtain information about taxpayers living on a particular street in Moscow, the entire table for the territorial district will be transmitted over the network, and it is up to you to decide which entries in it satisfy the request and which do not. the workstation itself. Thus, the operation of the “file-server” model leads to network congestion.

Elimination of these shortcomings is achieved in the "client-server" model. In this case application system is divided into two parts: external, facing the user and called the client, and internal, serving and called the server. The server is a machine that has resources and provides them, and the client is a potential consumer of these resources. The role of resources can play file system(file server), processor (computing server), database (database server), printer (printer-server), etc. Since the server (or servers) simultaneously serves many clients, a multitasking operating system must function on the server computer.

In the client-server model, the server plays an active role, because its software forces the server to "think first, act later." The flow of information flowing over the network becomes smaller as the server first processes requests and then sends what the client needs. The server also controls whether records can be accessed on an individual basis, which provides greater data security.

In the "client-server" model, created on the basis of a PC, the following is proposed:

The network contains a significant number of servers and clients;

The basis of the computing system is made up of workstations, each of which functions as a client and requests information that is located on the server;

The user of the system is freed from the need to know where the information he needs is located, he simply requests what he needs;

The system is implemented as an open architecture that combines computers of various classes and types with various systems.

LAN configuration. The configuration of a local network is called a topology. The most common topologies are:

- tire- one of the machines serves as a system server providing centralized access to shared files, databases and other computing resources;

- ring- information on the ring can be transmitted only in one direction;

- star(radial) - in the center of the network there is a switching device that ensures the viability of the system;

- snowflake(multi-connected) - topology with a file server for different workgroups and one central server for the entire network;

- hierarchical(tree) - formed by connecting several buses to the root system, where the most important components of the LAN are located.

In practice, hybrid LANs are more common, adapted to the requirements of a particular customer and combining fragments of different topologies. Local networks can be combined with each other, even if there are very large distances between them. In this case, the usual means of communication are used: telephone lines, radio stations, fiber optic lines, satellite connection etc. When connecting two or more networks, a global network is formed. The global network can cover a city, a region, a country, a continent, and the entire globe. In cases where networks operating on different protocols intersect, it becomes necessary to transfer data from the format accepted in one network to the format accepted in another network. Computers or programs that perform this function are called gateways. If networks that use the same protocols are connected, then the equipment standing between them is called bridges.

LAN access methods. According to the methods in the network, the most common networks such as Ethernet, ArcNet, Token Ring are distinguished.

ethernet- multiple access method. Before transmitting, the workstation determines whether the channel is free or busy. If free, the station starts transmission. The bus topology is used for this method. A message sent by one workstation is received simultaneously by all other stations connected to the common bus. The message is ignored by all stations, except for the sender and recipient.

Arcnet- used in a LAN with a star topology. One of the PCs creates a special token that is passed sequentially from one PC to another. If a station sends a message to another station, it must wait for the token and append the message to it, complete with the sender and destination addresses. When the packet reaches the destination station, the message will be stripped from the token and passed to the station.

token ring- Designed for a ring structure and also uses a marker transmitted from one station to another. But with it, it is possible to assign different priorities to different workstations. In this method, the token moves around the ring, giving successive computers on it the right to transmit.

Ensuring the security of information in computer networks. When connecting a local network to a global network, the concept plays an important role network security. Access to the local network for unauthorized persons from the outside should be limited, as well as access outside the local network for employees of the enterprise who do not have the appropriate rights. To ensure network security between local and global network install firewalls - computers or programs that prevent unauthorized movement of data between networks.

Global information network Internet. The Internet in the narrow sense is a combination of networks. However, in recent years, the word has taken on a broader meaning: the World Wide Web. The Internet can be viewed in the physical sense as several million computers connected to each other by all sorts of communication lines. However, this physical view is very narrow.

The Internet is a kind of information space within which data is continuously circulated. In this sense, it can be compared with television and radio, although there is an obvious difference in that no information can be stored on the air, while on the Internet it moves between computers that make up network nodes and is stored on hard drives for some time. Consider the principles of the functioning of the Internet.

The year 1983 is considered to be the birth of the Internet. This year saw revolutionary changes in computer communication software. Birthday in the modern sense of the word was the date of standardization of the TCP / IP communication protocol, which underlies World Wide Web to the present day.

TCP is a transport layer protocol. It controls how information is transferred. According to the TCP protocol, the sent data is "cut" into small packets, after which each packet is marked so that it contains the data necessary for the correct assembly of the document on the recipient's computer.

The IP protocol is addressable. He belongs network layer and determines where the transfer takes place. Its essence is that each member of the World Wide Web must have its own unique address (IP address). This address is expressed in four bytes. Each computer through which a TCP packet passes can determine from these four numbers which of the nearest neighbors the packet needs to be sent so that it is “closer” to the recipient. As a result of a finite number of transfers, the packet reaches the desired address.

Main informational resources Internet:

1. Remote access to TELNET network resources. Historically, one of the earliest is the service remote control Telnet computer. By connecting to a remote computer using the protocol of this service, you can control its operation. Such control is also called console or terminal. Telnet protocols are often used to remote control technical objects.

2. Email:

- E-mail (E-Mail). Mail servers receive messages from clients and forward them along the chain to the recipient's mail servers, where these messages are accumulated. When a connection is established between a destination and its mail server incoming messages are automatically transferred to the recipient's computer. The mail service is based on two protocols: SMTP and POP3. According to the first one, correspondence is sent from the computer to the server, and according to the second one, the received messages are received. There is a wide variety of client fasting programs.

- Mailing Lists. These are special thematic servers that collect information on certain topics and forward it to subscribers in the form of e-mail messages. Mailing lists allow you to effectively address the issues of regular data delivery.

- Teleconferencing service (Usenet). A teleconferencing service is similar to an e-mail broadcast, in which one message is sent to a large group. Such groups are called newsgroups or newsgroups. Messages directed to a newsgroup server are sent from it to all servers to which it is associated, if they do not have the message. On each of the servers, the received message is stored for a limited time, and everyone can read it. Around a million newsgroup posts are created every day around the world. The entire teleconferencing system is divided into thematic groups.

3. World Wide Web (WWW) technology. World Wide Web (WWW) service. It is the most popular service on the Internet today. This is a single information space, consisting of hundreds of millions of interconnected electronic documents stored on web servers. The individual documents that make up a Web space are called Web pages. Groups of thematic Web pages are called Web sites. One physical Web server can contain quite a few Web sites, each of which is usually assigned a separate directory on the server's hard drive. Programs for viewing Web pages are called browsers or browsers. The browser displays the document on the screen, guided by the commands that the author has embedded in the text. Such commands are called tags. The rules for writing tags are contained in the specification of a special markup language called the Hypertext Markup Language - HTML. There is a possibility of embedding graphic and multimedia documents into hypertext.

The most important feature of Web pages are hypertext links. Any piece of text can be associated with another Web document, that is, a hyperlink can be set. The hypertext link between hundreds of millions of documents is the basis for the existence of the logical space of the World Wide Web. The address of any file on a worldwide scale is determined by a uniform resource locator - URL. The URL consists of three parts:

Specifies the protocol of the service that accesses this resource. For the WWW, the HTTP protocol (http://…) is applied;

Indication of the domain name of the server on which this resource is stored (http://www.abcde.com);

Specifying the full path to the file on this computer (http://www.abcde.com/Files/New/abcdefg.zip).

It is in the form of a URL that the resource address is associated with hypertext links on Web pages. When a hyperlink is clicked, the browser sends a request to find and deliver the resource specified in the link.

4. Domain name service (DNS). An IP address is convenient for a computer, but inconvenient for people, so there is a more convenient form of notation that uses the domain system. For example: www.microsoft.com, microsoft- Domain name server - received during registration, com - suffix that determines the ownership of the domain. The following suffixes are most common: com – server of a commercial organization; gov - government organization server; edu is the school server. Such a system is adopted in the USA, in other countries, instead of the server type, indicate the country code, for example, Russia - ru. It is necessary to translate domain names into IP addresses. This is what Domain Name Service servers do.

4. FTP file exchange:

- File Transfer Services (FTP). Receiving and sending files is a significant percentage of other Internet services. The FTP service has its own servers that store data archives.

- IRC service (chat rooms, chat conferences). Designed for direct communication of several people in real time.

- ICQ service. This service is designed to find the network IP address of a person connected in this moment to the Internet. The need for such a service is due to the fact that most users do not have a permanent IP address. To use this service, you must register on its central server and obtain an identification number (UIN). Knowing the UIN of the recipient, but not knowing his current IP address, you can send him a message. In this case, the ICQ service takes on the character of an Internet pager.

Today's article opens a new section on the blog, which will be called " networks". This section will cover a wide range of issues related to computer networks. The first articles of the rubric will be devoted to explaining some of the basic concepts that you will encounter when working with the network. And today we will talk about what components will be required to create a network and which ones exist. types of networks.

Computer network is a set of computer and network equipment connected via communication channels into a single system. To create a computer network, we need the following components:

• computers that have networking capabilities (for example, a network card that is in every modern PC);
• transmission medium or communication channels (cable, satellite, telephone, fiber-optic and radio channels);
• network equipment (for example, a switch or router);
• network software (usually included with the operating system or supplied with network equipment).

Computer networks are usually divided into two main types: global and local.

Local networks(Local Area Network - LAN) have a closed infrastructure before reaching Internet service providers. The term "local area network" can describe both a small office network and the network of a large factory covering several hectares. In relation to organizations, enterprises, firms, the term is used corporate network – local network of a separate organization ( legal entity) regardless of the territory it occupies.
Corporate networks are networks of a closed type, access to them is allowed only to a limited circle of users (for example, company employees). Global networks are focused on serving any users.

Global network(Wide Area Network - WAN) spans large geographic regions and consists of many local area networks. Everyone is familiar with the global network, which consists of several thousand networks and computers - this is the Internet.

The system administrator has to deal with local (corporate) networks. A typical user computer connected to a local network is called workstation . A computer that shares its resources with other computers on a network is called server ; and the computer accessing the shared resources on the server is client .

There are various types of servers: file (for storing shared files), database servers, application servers (providing remote operation of programs on clients), web servers (for storing web content) and others.

Network load is characterized by a parameter called traffic. Traffic is the flow of messages in a data network. It is understood as a quantitative measurement of the number of data blocks passing through the network and their length, expressed in bits per second. For example, the data transfer rate in modern local networks can be 100Mbps or 1Gbps

Currently, the world has a huge amount of all kinds of network and computer equipment that allows you to organize a variety of computer networks. The whole variety of computer networks can be divided into several types according to various criteria:

By territory:

• local - cover small areas and are located inside individual offices, banks, corporations, houses;
• regional - are formed by combining local networks in separate territories;
• global (Internet).

By way of connecting computers:

• wired (computers are connected via cable);
• wireless (computers exchange information via radio waves. For example, by WI-FI technologies or bluetooth).

Control method:

• with centralized management - one or more machines (servers) are allocated to manage the process of data exchange in the network;
• decentralized networks - do not contain dedicated servers, network management functions are transferred in turn from one computer to another.

According to the composition of computing facilities:

• homogeneous - combine homogeneous computing tools (computers);
• heterogeneous - combine various computing tools (for example: PCs, trading terminals, webcams and network storage).

By type of transmission medium networks are divided into fiber-optic, with the transmission of information via radio channels, in the infrared range, through satellite channel etc.

You may come across other classifications of computer networks. Usually, system administrator one has to deal with local wired networks with centralized or decentralized control.

These models define the interaction of computers in the local computer network. In a peer-to-peer network, all computers are equal to each other. In this case, all information in the system is distributed between individual computers. Any user can allow or deny access to data stored on his computer.

The working group is independent solution organization of a computer network for a small number of computers, which has a peer-to-peer architecture and the authentication process in which takes place on the basis of a local database stored on each of the computers in the workgroup

In a peer-to-peer network, a user working on any computer has access to the resources of all other computers on the network. For example, while sitting at one computer, you can edit files located on another computer, print them on a printer connected to the third, run programs on the fourth.

The advantages of this LAN organization model include ease of implementation and cost savings, since there is no need to purchase an expensive server.

Despite the ease of implementation, this model has a number of disadvantages:

• 1. Low performance with a large number of connected computers;
• 2. Lack of a unified information base;
• 3. Absence unified system information security;
• 4. The dependence of the availability of information in the system on the state of the computer, i.e. If the computer is turned off, then all information stored on it will be inaccessible.

Active Directory

Active Directory allows administrators to manage all declared resources from one workplace: files, peripherals, databases, server connections, Web access, users, services.

In networks where DNS is deployed to support the Active Directory directory service, it is highly recommended to use directory service-integrated core zones, which provide the following benefits:

• 1. Master server upgrade and advanced security features based on Active Directory capabilities.
• 2. Replication and synchronization of zones with new domain controllers is performed automatically each time a new controller is added to the Active Directory domain.
• 3. By storing the DNS zone databases in Active Directory, you can streamline database replication across your network.
• 4. Directory replication is faster and more efficient than standard DNS replication.

Because Active Directory replication occurs at the per-property level, only the necessary changes are propagated. However, directory-integrated zones use and send less data.

The advantages of such a model are:

• 1. High network performance;
• 2. Availability of a unified information base;
• 3. Availability of a unified security system.

However, this model also has disadvantages. The main disadvantage is that the cost of creating a client-server network is much higher due to the need to purchase a special server. Also, the disadvantages include the presence of an additional need for service personnel - the network administrator.

For this organization, a local area network was chosen based on the client-server model. The server in this organization will be presented in the form of a computer from class No. 2, to which only the management personnel of the Internet cafe will have access. The server will be placed in a special computer cabinet for protection.