According to the method of network management. Main components and types of computer networks. Basic software and hardware components of the network

1.1. General characteristics of information and computer 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 channels for transmitting audio and video signals, which radically influenced the development of the media system, after radio and television broadcasting were invented network technologies, based on a different, digital, method of transmitting information, which led to the formation of a new environment for the dissemination of information flows.

Along with autonomous operation A significant increase in the efficiency of using computers can be achieved by combining them into computer networks.

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

There are a number of good reasons for connecting computers together in a network.

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

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

Third, computer networks provide new forms of interaction between users 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 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 significantly simplified.

1.1.1. Basic 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 presupposes knowledge of the operating principles of its individual elements:

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

The entire network hardware and software complex can be described by a multilayer model. At the heart of any network is a 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 on network nodes are sometimes called host machines or simply hosts.

Currently, computers of various classes are widely and successfully used in networks - from personal computers to mainframes and supercomputers. The set of computers on the network must correspond to the variety of tasks solved by the network.

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

Cabling systems, repeaters, bridges, switches, routers, and modular hubs have gone from being ancillary network components to becoming essential components, along with computers and system software, in both their impact on network performance and cost. Today, a communications device may be a complex, specialized multiprocessor that must be configured, optimized, and managed.

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

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

It is important to understand the range of capabilities that applications provide for different applications and how compatible they are 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 physical transmission medium.

Main types of transmission media used in computer networks are:

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

Analog communication channels were the first to be used for data transmission in computer networks and made it possible to use the then already developed public telephone networks.

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

At first method 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– this 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 discrete (digital) form over unloaded telephone channels (i.e. telephone channels to which the electrical voltage used in the telephone network is not supplied) began to develop - digital channels.

It should be noted that along with discrete data, analog information (voice, video, fax, etc.) converted into digital form can also be transmitted over 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(TR – Twisted Pair).

Cable channels, or coaxial pairs They are two cylindrical conductors on the same axis, separated by a dielectric coating. One type of coaxial cable (with a resistance of 50 ohms), used primarily for transmission of narrowband digital signals, another type of cable (with a resistance of 75 Ohms) - for transmitting broadband analog and digital signals. Narrowband and broadband cables that directly connect communication equipment to each other allow you to exchange data 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 in computer networks as a transmitting medium is cost-effective either for communication over long and ultra-long distances (using satellites), or for communication with hard-to-reach, mobile or temporarily used objects.

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

Satellites typically contain multiple amplifiers (or transponders), each of which receives signals in a given frequency range (usually 6 or 14 GHz) and regenerates them in a different frequency range (for example, 4 or 12 GHz). For data transmission, geostationary satellites are usually used, placed in an equatorial orbit at an altitude of 36,000 km. This 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 it has been used 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 losses. 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 increasingly widespread, and not only for communication on short distances, but also on intracity and intercity sections.

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 message delivery time compared to circuit switching, but it smooths out network peaks and improves 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 if 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), second – contactless(connectionless mode).

1.1.2. Classification of computer networks

The combination of the components discussed above into a network can be done in various ways and means. Based on the composition of their components, methods of their connection, scope of use and other characteristics, networks can be divided into classes in such a way that the belonging of the described network to a particular class can sufficiently fully characterize the properties and quality parameters of the network.

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

Based on this feature, networks are divided into three main classes:

LAN – local area networks;
MAN – Metropolitan Area Networks.
WAN – global networks (Wide Area Networks);

Local network (LAN) is a communications 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 areas covered by the drug may vary significantly.

The length of communication lines for some networks can be no more than 1000 m, while other networks are able to serve an entire city. The serviced areas can be factories, ships, airplanes, as well as institutions, universities, and colleges. As a rule, coaxial cables are used as a transmission medium, although twisted pair and fiber optic networks are becoming increasingly widespread, and recently wireless technology has also been rapidly developing. local networks, which use one of three types of radiation: broadband radio signals, low-power microwave radiation (microwave radiation) and infrared rays.

The short 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 Mbit/s to 100 Mbit/s (at present there are already industrial designs of LANs with speeds of the order of 1 Gbit /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), which in the future can be used in both local and global networks.

Global networks, unlike local ones, as a rule, cover much larger territories and even most regions of the globe (an example is the Internet). Currently, analogue or digital wire channels, as well as satellite communication channels (usually for communication between continents), are used as transmission media in global networks. Limitations on transmission speed (up to 28.8 Kbit/s on analogue channels and up to 64 Kbit/s on user sections digital channels) and the relatively low reliability of analog channels, requiring the use of error detection and correction tools at the lower levels of 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 functioning can be distinguished:

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

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

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

By control method computer networks are divided into:

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

In the first case, each computer included in the network includes a complete set software to coordinate ongoing 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 mixed networks, under centralized control, tasks that have the highest priority and, as a rule, are associated with the processing of large volumes of information, are 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 software 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.

Client/server networks use a dedicated computer (server) that hosts shared files and provides printing services to many users (Figure 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 few 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 a network through which the user gains access to its resources. A 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 processing of spreadsheets 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 operate under the control of a specialized operating system.

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

Peer-to-peer networks. Peer-to-peer networks do not use dedicated servers (Figure 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, performing 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 perform server functions. Windows 95 has built-in capabilities for building a peer-to-peer network. If you need to connect to other peer-to-peer networks, Windows 95 supports the following networks:

Net Ware Lite
- Artisoft LANtastic.

1.1.4. Network topology

Under topology is understood as a description of the properties of a network inherent in all its homomorphic transformations, i.e. such changes appearance network, the distances between its elements, their relative positions, at which the relationship between these elements 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 network configurations are divided into two main classes: broadcast And sequential.

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 a passive center” topologies. A star-type network can be thought of as a type of “tree” that has a root with a branch to each connected device.

IN sequential configurations Each physical sublayer transmits information to only one PC. Examples of sequential configurations are: random (random connection of computers), hierarchical, ring, chain, smart star, snowflake, and
other.

The most optimal from the point of view of reliability (the ability of the network to function in the event of failure of individual nodes or communication channels) is mesh 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 partial network there may be a large number of options for connecting network nodes, in practice the three most widely used (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.

Figure 3. Bus topology

In this case, one of the machines serves as a system service device, providing centralized access to shared files and databases, printing devices and other computing resources.

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

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

Figure 4. Ring topology

This network structure is characterized by the fact that information along the ring can be transmitted only in 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 interferes with the normal operation networks.

As a daisy-chain configuration, the ring is particularly vulnerable to failure: failure of any cable segment results in loss of service to all users. LAN developers have put a lot of effort into dealing with this problem. Protection against damage or failure is provided either by closing the ring to the reverse (redundant) path, or by switching to a spare ring. In both cases, the general ring topology is maintained.

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

Fig 5. Star topology

The configuration can be thought of as a further development of a rooted tree structure with a branch to each connected device. At the center of the network is usually a switching device that ensures the viability of the system. LANs of this configuration are most often used in automated institutional control systems that use a central database. Star LANs are generally less reliable than bus or hierarchical networks, but this problem can be solved by duplicating the equipment at the central node. Disadvantages can also include significant cable consumption (sometimes several times higher than the consumption in LANs with a common bus or hierarchical ones with similar capabilities).

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

1.1.5. Levels of interaction between computers in networks

There are 7 levels of interaction between computers in a computer network:

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

Physical layer(Physical Layer) defines the electrical, mechanical, procedural, and functional specifications and provides the link layer with the establishment, maintenance, and termination of physical connections between two computer systems directly coupled through a transmission medium, such as an analog telephone circuit, a radio circuit, or a fiber optic circuit.

Data Link Layer(Data Link Layer) controls the transfer of data over the communication channel. The main functions of this layer are to split the transmitted data into pieces called frames, extract data from the stream of bits transmitted at the physical layer for processing at the network layer, detect transmission errors and recover incorrectly transmitted data.

Network layer(Network Layer) provides communication between two computer systems on a network that exchange information with each other. Another function of the network layer is to route 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 on the network for higher layers. For this purpose, mechanisms are used to establish, maintain and terminate virtual channels (analogous to dedicated telephone channels), detect and correct transmission errors, and control the data flow (in order to prevent overflow or data loss).

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

Data presentation layer Presentation Layer provides the transformation of data from the presentation used in an application program on one computer system to the presentation used in another computer system. The functions of the presentation layer also include conversion of data codes, their encryption/decryption, as well as 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 layer determines the availability of application tasks and communication resources, synchronizes interacting application tasks, and establishes agreements on error recovery procedures and data integrity management. Important functions of the application layer are network management, as well as performing the most common system application tasks: Email, file sharing and others.

Each level, in order to solve its subtask, must ensure the execution of the functions defined by the model this 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 certain set of rules governing the format and procedures for exchanging information.

Specifically, it determines how connections are made, line noise is overcome, and data transmission between modems is ensured without errors.

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

BELARUSIAN NATIONAL TECHNICAL UNIVERSITY

INTERNATIONAL INSTITUTE OF DISTANCE EDUCATION

TEST

IN THE ACADEMIC DISCIPLINE: Computer networks

Types of computer networks

Computer networks can be classified according to various criteria.

I. According to management principles:

1. Peer-to-peer - without a dedicated server. In which control functions are alternately transferred from one workstation to another;

2. Multi-peer is a network that includes one or more dedicated servers. The remaining computers of 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 the other (slave);

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

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

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

III. By territory 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, or building.

2. Corporate network(within the limits of one organization, company, plant). The number of nodes in a FAC 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.).

A corporate network makes it easier to manage and manage a business technological process, establish clear control over information and production resources.

3. Global network(a network whose elements are located at a considerable distance from each other) - 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 WAN can reach tens of millions. The global network includes separate local and corporate networks.

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

COMPUTER NETWORK TOPOLOGY

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 coaxial cable-based transmission channel, called a "bus". All network computers are connected directly to the bus. At the ends of the bus cable, special plugs are installed - “terminators”. 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;

If the “bus” is damaged, the entire network stops functioning.

RING TOPOLOGY

The ring topology is characterized by the absence of connection endpoints; the network is closed, forming an unbroken ring through which data is transmitted. This topology implies the following transmission mechanism: data is transferred 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;

Instability to cable system damage.

STAR TOPOLOGY

In a network with a star topology, all computers are connected to a special device called a network hub or hub, which performs data distribution functions. There are no direct connections between two computers on the network. Thanks to this, it is possible to solve the problem of public data availability, and also increases the resistance to damage to the cable system. However, network functionality depends on the status of the network hub.

Carrier access methods in computer networks

IN various networks There are various 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 widely used specific implementations of access methods are Ethernet, ArcNet and Token Ring. These implementations are based on the IEEE802.3, IEEE802.4 and IEEE802.5 standards, respectively.

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 intended for only 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 others ignore it.

The Ethernet access method is a multiple access method that listens to the carrier and resolves conflicts called collisions (CSMA/CD -Carter Sense Multiple Access with Collision Detection).

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

Ethernet does not exclude the possibility of simultaneous transmission of messages by two or more stations. The equipment automatically recognizes such conflicts. After detecting a conflict, stations delay transmission for some time. This time is short and different for each station. After a delay, transmission resumes.

In reality, conflicts lead to a decrease in network speed 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 equipment or Token-Ring.

ArcNet is used in local networks with a star topology. One of the computers creates a special token (a special type of message), 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 a message to it, complete with the source and destination addresses. When the packet reaches the destination station, the message will be “unhooked” from the token and transmitted 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, as 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.

Data transmission media, their characteristics

Coaxial cable

Coaxial cable was the first type of cable used to connect computers to a network. This type of cable consists of a central copper conductor covered with a plastic insulating material, which in turn is surrounded by copper mesh and/or aluminum foil. This outer conductor provides grounding and protects 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 speeds of up to 10 Mbit/s. Maximum length segment ranges from 185 to 500 m depending on the cable type.

"Twisted Pair"

Twisted pair cable is one of the most common types of cable 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” and “Unshielded twisted pair”. The difference between these classes is that shielded twisted pair cable 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 speeds from 10 Mbit/s – 1 Gbit/s. The cable segment length cannot exceed 100 m (up to 100 Mbps) or 30 m (1 Gbps).

Fiber optic cable

Fiber optic cables are the most advanced cabling technology, providing high speed data transmission over long distances, resistant to interference and eavesdropping. A fiber optic 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 a central conductor. The signal at the other end is received by a photodiode receiver, which converts light pulses into electrical signals that can be processed by a computer. Transmission speeds for fiber optic networks range from 100 Mbit/s to 2 Gbit/s. 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.

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).

Based on the nature of the implemented functions, networks are divided into:

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

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

Mixed, in which computational and information functions are implemented.

Based on the management method, networks are divided into networks:

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

With centralized control - coordination of computer operation is carried out under the control of a single OS;

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

Communication model levels:

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

2. Presentation layer– The computer operating system records where the data is and provides interaction with the next level.

3. Session layer– the computer interacts with the network: checks the user’s right to access the network and transmits the document to the transport layer protocols.

4. Transport layer– the document is converted into the form in which data is supposed to be transmitted on the network being used.

5. Network layer determines the route of data movement on the network.

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

7. Physical layer. This layer is where the actual data transfer occurs. There are no documents, no packets, no bytes - only bits. Document recovery 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 remote communication using modems, this line telephone communication, switching equipment, etc.

The different protocol layers of the server and client do not communicate with each other directly, but they communicate through physical layer. Gradually moving from top level to the bottom, the data is continuously transformed. 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 computer networks (LAN). If computers are located close to each other, use a common set of network equipment and are controlled by the same software package, then such a network is called local. The creation of local networks is typical for individual departments of enterprises. Let's consider the organization of information exchange of the interaction model on a LAN.

Server LANs implement two models of user interaction with workstations: model file server and model client-server. In the first model, the server provides access to database files for each workstation, and that's where its work ends. For example, if a file server type database is used to obtain information about taxpayers living on a specific street in Moscow, the entire table for the territorial district will be transmitted over the network, and it is necessary to decide which records 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 be played file system(file server), processor (computing server), database (database server), printer (printer server), etc. Since the server (or servers) serves many clients simultaneously, 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 and act later.” The flow of information across the network becomes smaller because the server processes requests first and then sends what the client needs. The server also controls whether records can be accessed on an individual basis, providing greater data security.

The client-server model, created on a PC, offers the following:

· 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 located on the server;

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

· the system is implemented in the form of an open architecture that combines computers of various classes and types with various systems.

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

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

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

- star(radial) - a switching device is located in the center of the network, ensuring 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, tailored to the requirements of a specific customer and combining fragments of various topologies. Local networks can be connected to each other, even if there are very large distances between them. In this case, conventional means of communication are used: telephone lines, radio stations, fiber optic lines, satellite connection etc. When two or more networks are connected to each other, a global network is formed. A global network can cover a city, region, country, continent and the entire globe. In cases where networks operating using different protocols intersect, there is a need 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 using the same protocols are connected, then the equipment located between them is called bridges.

LAN access methods. Based on the network methods, the most common networks are identified as Ethernet, ArcNet, Token Ring.

Ethernet- multiple access method. Before transmission begins, the workstation determines whether the channel is free or busy. If free, the station starts transmitting. This method uses a bus topology. 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 the sender and destination.

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

Token Ring- designed for a ring structure and also uses a token transmitted from one station to another. But it allows you to assign different priorities to different workstations. With this method, the token moves around the ring, giving the computers located in series on it the right to transmit.

Ensuring information security 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 must be limited, and access outside the local network must be limited for enterprise employees 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, this word has acquired a broader meaning: the World Wide Web. The Internet can be considered in a physical sense, as several million computers connected to each other by all kinds of communication lines. However, this physical view is very narrow.

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

The birth of the Internet is considered to be 1983. This year has seen revolutionary changes in computer communications software. The birthday in the modern sense of the word was the date of standardization of the TCP/IP communication protocol that underlies World Wide Web to this day.

The TCP protocol 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 level and determines where the transfer occurs. Its essence is that each participant in 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 its closest neighbors needs to forward the packet so that it is “closer” to the recipient. As a result of a finite number of transfers, the packet reaches the desired address.

Basic 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. This type of control is also called console or terminal. Telnet protocols are often used for remote control technical objects.

2. Email:

- Electronic mail (E-Mail). Mail servers receive messages from clients and forward them along the chain to the recipients' mail servers, where these messages are accumulated. When a connection is established between the recipient and his mail server Incoming messages are automatically transferred to the recipient's computer. The mail service is based on two protocols: SMTP and POP3. The first method is used to send correspondence from the computer to the server, and the second method is used to receive incoming messages. There is a wide variety of client post programs.

- Mail Lists. These are special topical servers that collect information on certain topics and forward it to subscribers in the form of email messages. Mailing lists allow you to effectively solve problems of regular data delivery.

- Teleconferencing service (Usenet). A teleconferencing service is similar to broadcast email, sending one message to a large group. These groups are called newsgroups or newsgroups. Messages sent to a newsgroup server are sent from it to all servers with which it is connected, if they do not have the message in question. On each of the servers, the received message is stored for a limited time, and anyone can read it. About a million newsgroup posts are created every day around the world. The entire teleconference system is divided into thematic groups.

3. World Wide Web (WWW) technology. World Wide Web (WWW) service. This is the most popular service on the modern Internet. 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 the Web 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 allocated 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 hypertext markup language - HTML. It is possible to embed graphic and multimedia documents into hypertext.

The most important feature of Web pages is hypertext links. You can link another Web document to any piece of text, that is, set a hyperlink. Hypertext communication between hundreds of millions of documents is the basis of the existence of the logical space of the World Wide Web. The worldwide address of any file is determined by the Uniform Resource Locator (URL). The URL consists of three parts:

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

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 linked to 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 recording 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 most common suffixes are: com – server of a commercial organization; gov – server of a government organization; edu – educational institution server. This system is adopted in the USA; in other countries, instead of the server type, they 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. File exchange via FTP:

- File Transfer Services (FTP). Receiving and transmitting files makes up a significant percentage of other Internet services. The FTP service has its own servers on which data archives are stored.

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

- ICQ service. This service is designed to find the network IP address of a person connected to 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 a user identification number (UIN). Knowing the recipient's UIN, 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 in the section 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 are required to create a network and what exist types of networks.

Computer network is a collection 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 the ability to connect to a network (for example, a network card, which is found 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 accessing Internet service providers. The term “local area network” can describe both a small office network and a 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 closed networks; access to them is allowed only to a limited number of users (for example, company employees). Global networks are focused on serving any users.

Global network(Wide Area Network - WAN) covers large geographic regions and consists of many local 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. An ordinary user computer connected to a local network is called workstation . A computer that makes its resources available for shared use by other computers on the network is called server ; and the computer accessing shared resources on the server is client .

There are various types of servers: file servers (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 a 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 speed in modern local networks can be 100 Mbit/s or 1 Gbit/s

Currently in the world there is a huge amount of all kinds of network and computer equipment that allows you to organize a wide variety of computer networks. The entire 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 – formed by combining local networks in separate territories;
• global (Internet).

According to the method of computer connection:

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

By control method:

• with centralized control - one or more machines (servers) are allocated to manage the data exchange process on 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 tools:

• homogeneous – combine homogeneous computing means (computers);
• heterogeneous - combines various computing tools (for example: PCs, trading terminals, web cameras and network data storage).

By type of transmission medium networks are divided into fiber optics, 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 you have to deal with local wired networks with centralized or decentralized control.

These models determine the interaction of computers in the local computer network. In a peer-to-peer network, all computers have equal rights with each other. In this case, all information in the system is distributed between separate computers. Any user can allow or deny access to data stored on their computer.

Workgroup is independent decision organizing a computer network for a small number of computers, which has a peer-to-peer architecture and the authentication process in which occurs 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, sitting at one computer, you can edit files located on another computer, print them on a printer connected to a third, and run programs on a fourth.

The advantages of this model of organizing a LAN include ease of implementation and savings in material resources, 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. 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, peripheral devices, databases, connections to servers, access to the Web, users, services.

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

• 1. Main server update and advanced security features based on Active Directory capabilities.
• 2. Replication and synchronization of zones with new domain controllers occurs automatically each time a new controller is added to the Active Directory domain.
• 3. By storing 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 individual property level, only necessary changes are propagated. However, directory service-integrated zones use and send less data.

The advantages of this model include:

• 1. High network speed;
• 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 significantly higher due to the need to purchase a special server. Another disadvantage is the presence of an additional need for service personnel - a network administrator.

For this organization, a local area network was chosen based on a 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 staff of the Internet cafe will have access. The server will be placed in a special computer cabinet for protection.