According to the method of management, computer networks are divided into: Network management method. Types of computer networks

LAN classification

Local networks can be classified according to:

management level;
purpose;
homogeneity;
administrative relations between computers;
topology;
architecture.

The following LANs are distinguished by management level: :

Workgroup LANs, which consist of several PCs running the same operating system. In such a LAN, as a rule, there are several dedicated servers: a file server, a print server;
LAN of structural units (departments). These LANs contain several dozen PCs and servers such as: file server, print server, database server;
LAN of enterprises (firms). These LANs can contain over 100 computers and servers such as: file server, print server, database server, mail server and other servers.

Based on their purpose, networks are divided into :

computer networks designed for computational work;
information and computer networks, which are intended both for conducting settlement work and for providing information resources;
information-advising, which, based on data processing, generate information to support decision-making;
information-control networks, which are designed to manage objects based on information processing.

The types of computers used can be distinguished:

homogeneous networks that contain the same type of computers and system software;
heterogeneous networks that contain different types of computers and system software.

According to the administrative relationships between computers, we can distinguish:

LAN with centralized management (with dedicated servers);
LANs without centralized control (decentralized) or peer-to-peer (single-level) networks.

In local networks with centralized management, the server ensures interactions between workstations, performs the functions of storing public data, organizes access to this data and transmits the data to the client. The client processes the received data and provides the processing results to the user. It should be noted that data processing can also be carried out on the server.

Local networks with centralized management, in which the server is intended only for storing and issuing information to clients upon requests, are called networks with a dedicated file server. Systems in which the server, in addition to storing information, also processes information are called “client-server” systems.

It should be noted that in server local networks, only server resources are directly available to the client. But workstations that are part of a centrally managed LAN can simultaneously organize a peer-to-peer local network with all its capabilities among themselves.

Software, work manager A centrally controlled LAN consists of two parts:

network operating system installed on the server;
software on a workstation, which is a set of programs running under the operating system that is installed on the workstation. At the same time, different operating systems can be installed on different workstations on the same network.

In large hierarchical local networks, UNIX and LINUX are used as network operating systems, which are more reliable. For medium-sized local networks, the most popular network OS is Windows 2008 Server.

Depending on how the server is used in hierarchical networks, the following types of servers are distinguished:

File server. In this case, shared files and/or shared programs are located on the server.
Database server. The server hosts a network database.
Print server. A fairly powerful printer is connected to the computer, on which information can be printed from several workstations at once.
Mail server . The server stores information sent and received both by local network.

Advantages:

higher data processing speed;
has a reliable system for protecting information and ensuring secrecy;
easier to manage compared to peer-to-peer networks.

Flaws:

the network is more expensive due to a dedicated server;
less flexible compared to a peer-to-peer network.

All computers on the local network are connected by communication lines. The geometric location of communication lines relative to network nodes and the physical connection of nodes to the network is called physical topology. Depending on the topology, networks are distinguished: bus, ring, star, hierarchical and arbitrary structures.

There are physical and logical topologies. Logical and physical network topologies are independent of each other. Physical topology is the geometry of the network, and logical topology determines the directions of data flows between network nodes and methods of data transmission.

All existing configurations can be divided into two main classes: broadcast and serial.

In the case of a broadcast LAN configuration, signals transmitted by one device connecting to the physical medium are perceived by all others. In a broadcast LAN, only one station can operate at any time. All workstations can directly come into contact with any workstation available on the web.

To build a broadcast configuration, it is necessary to use relatively powerful receivers and transmitters. Consequently, there is a need to limit the length of cable segments and the number of connections. If limits are exceeded, an analog amplifier or digital repeater is used. In addition, the means of connection to the physical medium are selected such that they do not cause significant signal attenuation.

The main types of broadcast topologies "bus", "tree" and "star" are shown in the diagrams (Figure 3).

Figure 3 - Types of broadcast topologies:

a) “tire”; b) “tree”; c) "star"

In the case of a sequential LAN configuration, each device connecting to the physical medium transmits information to only one device. At the same time, the requirements for transmitters and receivers are reduced, since all stations actively participate in the transmission.

The main types of sequential topologies: “ring”, “chain”, “snowflake” and “mesh” are shown in the diagram (Figure 4).

Figure 4 - Types of sequential topologies “ring”, “chain”, “snowflake” and “mesh”

Consider the following physical topologies:

physical “bus” (bus);
physical “star” (star);
physical “ring” (ring);

Bus topology

easy to connect a new PC;
there is the possibility of centralized management;
The network is resistant to failures of individual PCs and to interruptions in the connection of individual PCs.

Disadvantages of star topology networks:

hub failure affects the operation of the entire network;
high cable consumption;

Ring topology

In a network with a ring topology, all nodes are connected by communication channels into a continuous ring (not necessarily a circle) through which data is transmitted. The output of one PC is connected to the input of another PC. Having started the movement from one point, the data ultimately ends up at its beginning. Data in a ring always moves in the same direction.

The receiving workstation recognizes and receives only the message addressed to it. A network with a physical ring topology uses token access, which grants a station the right to use the ring in a specific order. The logical topology of this network is a logical ring.

This network is very easy to create and configure. The main disadvantage of ring topology networks is that damage to the communication line in one place or PC failure leads to the inoperability of the entire network.

As a rule, the “ring” topology is not used in its pure form due to its unreliability, so in practice various modifications of the ring topology are used.

In general, the IT infrastructure of various enterprises can be distinguished by:

scale;
composition of components;
equipment level, etc.

Based on this, certain types of IT infrastructures can be presented in the form of basic configurations, which are shown in Figures 5, 6 and 7.

Figure 5 - Small local network.

Small local network. Typically consists of 1-3 servers, network switches, 5-30 workstations.

Figure 6 - Local network and telephone network with MiniPBX.

Local network and telephone network with MiniPBX. Includes all the components of a “small local network” with the addition of an internal MiniPBX for switching phones within the office

Figure 7 - Local network and digital telephone network at several sites.

Local network and digital telephone network at several sites, combined into a virtual private network. The organization's local network is used for IP telephony. It is possible to combine the digital telephone networks of an organization's divisions via the Internet using virtual private networks.

The level of training of specialists servicing the IT infrastructure of enterprises must be very high, requiring responsibility for work on which the functioning and security of corporate computer networks.

Connecting computers and devices into a network can be done different ways and means. Based on the composition of their components, the methods of their connection, the 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 of computer networks today is based on territorial location. Based on this feature, networks are divided into three main classes:

LAN (Local Area Networks) – local networks;

MAN (Metropolitan Area Networks) – regional (city or corporate) networks;

WAN (Wide Area Networks) – global networks.

A local area network (LAN) is a communications system that supports, within a building or some other limited area, one or more high-speed channels for transmitting 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 networks using twisted pair and optical fiber are becoming increasingly widespread, and in Lately Wireless local area network technology is also rapidly developing, using one of three types of radiation: broadband radio signals, low-power microwave radiation (microwave radiation), and infrared rays.

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/s ).

Regional 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.

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 wired channels are used as a transmission medium in global networks, as well as satellite channels communications (usually for communication between continents). Limitations on transmission speed 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. For example:

– according to the area of operation, the networks can be divided into banking research institutions and universities;

– based on the form of operation, one can distinguish between commercial and free networks, corporate and public;

– according to the nature of the implemented functions, networks are divided into computational ones (intended to solve control problems based on computational processing of initial information); informational (intended to obtain reference data at the request of users); mixed (they implement computational and information functions);

– according to the control method computer networks are divided into networks with decentralized, centralized and mixed control. In the first case, each computer included in the network includes full 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, tasks that have the highest priority and, as a rule, are associated with the processing of large volumes of information, are solved under centralized control.

Local networks

A local network is created, as a rule, to share computer resources or data (usually within the same organization). From a technical point of view, a local network is a collection of computers and communication channels that unite computers into a structure with a specific configuration, as well as network software that controls the operation of the network. The method of connecting computers into a local network is called topology.

Topology largely determines many important properties of a network, such as reliability (survivability), performance, etc. There are different approaches to classifying network topologies. Based on performance, they are divided into two main classes: broadcast and serial.

In broadcast configurations, each computer transmits signals that can be received by other computers. 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”, “star with an intellectual center”, “snowflake” and others.

Bus topology

Figure 10.2. Local network bus topology

With such a connection, exchange can be carried out between any computers on the network, regardless of the others. If the connection of one computer to the common bus is damaged, this computer is disconnected from the network, but the entire network is operational. In this sense, the network is quite stable, but if a bus is damaged, the entire network fails.

Ring topology

Figure 10.3. Ring LAN topology

This connection also transfers data serially from computer to computer, but compared to a simple serial connection, data can be transferred in two directions, which makes it more resilient to network problems. One break does not disable the network, but two breaks make the network inoperable. The ring network is widely used, mainly due to high speed data transmission. Ring networks are the fastest.

Star topology

Figure 10.4. Star-shaped local network topology

When connected by a star, the network is very resistant to damage. If one of the connections is damaged, only one computer is disconnected from the network. In addition, this connection scheme allows the creation of complex branched networks. Devices that allow you to organize complex network structures are called hubs and switches.

Communication network– a system of nodes and connections between them. The nodes carry out the functions of creating, transforming, storing and consuming a communication product. Connections (transmission channels, communication lines) serve to transfer the product between nodes. Depending on the type of product, material, energy, and information networks are distinguished. Examples of physical networks: road and railway communications; water and gas supply.

Information network – communication network, in which the product of communication is information. Examples: telephone networks, television, radio broadcasting.

Computing, or computer network – information network, the nodes of which are computers and other computing equipment. In addition to special network equipment, networking software is also required. Thanks to the interaction of computers on a network, a number of new opportunities become available.

The first is the sharing of hardware and software resources. Yes, when public access to an expensive peripheral device (printer, plotter, scanner, fax, etc.), the costs for each individual user are reduced. Network versions of application software are used in the same way.

The second is shared access to data resources. With centralized storage of information, the processes of ensuring its integrity are significantly simplified, as well as Reserve copy, which ensures high reliability. Having alternate copies on two machines at the same time allows you to continue working if one of them is unavailable.

Third, accelerating data transfer and providing new forms of interaction between users in one team when working on a common project.

Fourth, the use of common means of communication between various application systems (communication services, data transmission, video, speech, etc.).

One of the important classification features of networks is their size. The size of the network influences the choice of equipment used and transmission technologies used.

Local computing network(LAN, or LAN - Local Area Network) unites nearby computers within a limited area, room, building. Distinctive features of the LAN are minimal delay time and low level errors. LANs can be elements of larger-scale entities: campus, or corporate network(CAN - Campus Area Network), connecting local networks of nearby buildings; municipal network, or city-scale network (MAN - Metropolitan Area Network); regional or wide-area network (WAN - Wide Area Network), covering a large area; wide area network(WAN, or GAN - Global Area Network), which has the size of a country and a continent.

Based on the management method, networks are divided into peer-to-peer and with dedicated server(centralized control). In peer-to-peer networks, all nodes have equal rights - each node can act as both a client and a server. Under client refers to a hardware and software object that requests some services. And under server– a combination of hardware and software that provides these services. A computer connected to a local network, depending on the tasks solved on it, is called a workstation or server.

Peer-to-peer LANs are quite easy to maintain, but cannot provide adequate information protection if the network size is large. The costs of organizing peer-to-peer computer networks are relatively small. However, as the number of workstations increases, the efficiency of network use decreases sharply. Therefore, peer-to-peer LANs are used only for small workgroups - no more than 20 computers.

A dedicated server implements network management (administration) functions in accordance with specified policies - sets of rules for dividing and limiting the rights of network participants. LANs with a dedicated server have good data security features and are capable of supporting thousands of users, but require constant qualified maintenance by a system administrator.

Depending on the data transmission technology used, there are different broadcast networks and networks with transmission from node to node. Broadcast transmission is used mainly in small networks, and in large networks it is used for transmission from node to node.

In broadcast networks, all network nodes share a single communication channel. Messages sent by one computer, called packets, are received by all other machines. Each packet contains the address of the message recipient. If the packet is addressed to another computer, it is ignored. Thus, after verifying the address, the recipient processes only those packets that are intended for it.

Networks with transmission from node to node consist of pairwise connected machines. In such a network, to get to its destination, a packet passes through a number of intermediate machines. However, there are often alternative paths from the source to the recipient.

The method of connecting computers together in a network is called topology. There are three most common topologies used in LANs. These are the so-called tire, ring And star-shaped structures.

In the case of a bus (linear) structure, all computers are connected in a chain using one common coaxial cable. If at least one of the sections of the network with a bus structure is damaged, the entire network as a whole becomes inoperable. The fact is that then there is a break in the only physical channel necessary for the movement of the signal.

The ring structure is used mainly in Token Ring networks and differs from the bus structure in that all computers are connected in pairs to each other, forming a closed loop. Also, if one of the network segments malfunctions, the entire network goes down.

In a star network, the central node to which all others connect is hub(Hub – “hub”). Its main function is to ensure communication between computers on the network. This structure is preferable because if one of the workstations or the cable connecting it to the hub fails, all the others remain operational.

When building networks, cellular ( fully connected) a topology in which each node is connected to all other individual links. The costs of creating redundant channels are offset by high reliability - there are almost always several paths for signals to pass from sender to recipient, so if some channels are disconnected, signals can be transmitted through others.

The following are distinguished: switching methods data in information networks: circuit switching, packet switching And message switching.

When switching circuits, the entire connection path is first established - from the sender to the recipient. This path consists of several sections connected by switches and/or multiplexers. All data is transmitted along the established route. Once the transfer is complete, the connection is terminated. Example – phone conversation: The channel is busy during the entire call, even if the callers are silent. The transmission speed over such a channel is limited to the area with the lowest bandwidth.

With the second method, messages are divided into packets of a fixed length, which can be delivered across the network via independent routes, ensuring uniform network load. In this case, packets of different messages can be transmitted over one channel. As an example, let's give an analogy: during rush hour, a group of students gets from the hostel to the university using different transport, each in their own way.

Message switching is similar to packet switching, but at a higher level (message switching nodes can be connected by either a circuit-switched network or a packet-switched network). The main difference is that the size of the data block is determined not by technological limitations, but by the content of the information in the message. This can be a text document, email, file. Example - a group of tourists follows a route, and at each point the composition of the group is checked. This scheme is used to transmit messages that do not require an immediate response, for example, messages Email.

15.3 OSI/ISO network model

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 - the “language” of data transmission. Protocol– formalized rules for the interaction of several computers, which can be described as a set of procedures that determine the sequence and format of messages exchanged between network components located at the same level, but in different nodes.

The International Organization for Standardization (ISO) has proposed model computer network architecture OSI(Open System Interconnection - connection of open networks). This model, which most users try to adhere to, divides communication functions on the network into seven levels. Data exchange occurs by moving it on the sender’s computer from top level to the lower one, then transported over the communication channel and reverse conversion on the recipient's computer from the lower level to the upper one.

The highest level is application layer(Application Layer) is the interface between application programs and processes of the OSI model.

The Presentation Layer determines the format for data exchange and serves for encryption, compression and code conversion of data.

The Session Layer performs the functions of coordinating communications between workstations. The layer provides the creation of a communication session, control of the transmission and reception of message packets, and termination of the session.

The Transport Layer divides or assembles messages into packets when more than one packet is in the process of transmission or reception, as well as controls the order in which message components pass through. In addition, at this level, through gateways, the network layers of various incompatible networks are negotiated. Guarantees delivery of packets without errors, in the same sequence, without losses and duplication with confirmation of receipt.

The Network Layer provides the translation of logical address names into physical ones. Based on specific network conditions and the priority of the service, routing is carried out, that is, the choice of transmission route for a data packet in the network, and control of the data flow in the network (data buffering, error control when establishing a connection).

The Data Link layer defines the rules for using the physical layer by network nodes. This layer is divided into two sublayers: Media Access Control, associated with network access and management, and Logical Link Control, associated with the transmission and reception of user messages. It is at the Data Link level that data transmission is ensured in frames, which are blocks of data containing additional control information. Error correction is performed automatically by resending the frame. In addition, at this level the correct sequence of transmitted and received frames is ensured.

Lowest – physical layer(Physical Layer) defines the physical, mechanical and electrical characteristics of communication lines. This layer converts data coming from the data link layer into signals that are then transmitted over communication lines. In local networks this conversion is carried out using network adapters, in global networks modems are used for this purpose.

Each level actually interacts only with neighboring levels (upper and lower), and virtually only with a similar level at the end of the line. Real interaction is the direct transfer of information in which the data remains unchanged. Virtual interaction is indirect interaction and data transfer, and the data can be modified during the transfer process.

Physical connection actually takes place only at the lowest level. Horizontal connections between all other levels are virtual; in reality they are carried out by transferring and converting information first downwards, sequentially to the lowest level, where the real transfer occurs, and then at the other end - reverse transmission upwards sequentially to the appropriate level.

A computer network is a complex system through which data is transferred and exchanged according to a certain principle between several objects. Using the network has a number of advantages, mainly due to almost unlimited possibilities due to access to additional resources. The organization of a computer network allows you to install powerful units to run software that is too heavy for a weak computer. Users also get […]

Allows you to install powerful units to run software that is too heavy for a weak computer. Users also have the opportunity to exchange information with other participants in the process, and they can save on installing additional peripheral devices, for example, by connecting several computers to one printer or scanner.

Computer networks are classified according to a number of criteria, such as:

length of lines;
topology (construction method);
control method.

To better understand the different management methods that networks differ in, it is necessary to become familiar with their varieties according to the scale and specifics of their functioning.

PAN is a personal network that ensures the interaction of several devices within one project.

LAN is a local network with a closed infrastructure, regardless of scale. Access to local networks is available to a limited number of users defined by the administrator.

CAN is an association of several local networks of nearby objects.

MAN – computer networks between institutions within the same locality, connecting many local networks.

WAN is an open global network serving large-scale geographic regions, which includes both local networks and other telecommunications nodes.

There are several scenarios for constructing a computer network, providing for the order of location of individual workstations and the method of connecting them by communication highways.

This area determines the type of equipment used, cable, control methods, etc. Three network configurations are most widespread:

tire;
ring;
star.

The bus implies equal rights for all subscribers connected one by one via a single communication line. The peculiarity of this topology is the absence of a central subscriber, and the connection of new participants in the process is carried out in the most in a simple way, moreover, the least amount of low-current cable is used here.

The ring topology is characterized by the simplicity of the device, where each individual computer is connected cable line with two others. There is also no clearly defined center, and each computer has equal rights.

The star provides for the presence central computer, which bears the main burden of managing the exchange. In this case, it is the main computer that has the greatest power, and in the network itself there are no conflicts between individual subscribers. Depending on the control method, each of the topologies has distinctive characteristics, and below we will tell you what control methods the networks differ in.

Classification of computer networks by management method

Considering that a complex system requires constant monitoring and correct interaction of all nodes, it is constantly under control. Based on the management method, networks are divided into:

centralized, where the main management functions are performed by the server, providing users with access to available resources. If a server (or several servers at the same time) is powerful computer, which bears the main load, then the remaining machines are workstations;
decentralized, or as they are also called, peer-to-peer (peer-to-peer). In this case, there are no local network management tools such as servers, and all computers have equal rights, and control can be carried out from any of the machines;
mixed, in which the most complex and priority tasks are solved through centralized control.

Since networks come in different types according to the way they are managed, there are certain standards for this process.

Network management system standards are a complex, protocol-based discipline that regulate the way in which host and managed entities interact.

Considering the fact that the design and installation of local networks is a responsible and difficult process, its implementation can only be carried out by experienced professionals.

Involving competent designers and installers in setting up a computer network guarantees a high level of work and also ensures the reliable functioning of each element that is part of it.

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 the low data transfer speed.

Networks are divided into local and global. The purpose of all types of networks has one purpose - to provide shared access to common 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 – operating system The computer 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 the physical layer. Gradually moving from the 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 area 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. For this method The bus topology is used. 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, firewalls are installed between the local and global networks - 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 there have been revolutionary changes in software computer communications. The birthday in the modern sense of the word was the date of standardization of the TCP/IP communication protocol, which underlies the World Wide Web to this day.

The TCP protocol is a transport layer protocol. It controls how information is transferred. According to 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.

Main information resources on the Internet:

1. Remote access to TELNET network resources. Historically, one of the earliest is the Telnet remote computer control service. By connecting to 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 one 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 a 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.