According to the method of network management. Types of computer networks and methods of managing them

Modern networks can be classified according to various criteria:

By remoteness of computers:

Local LAN (Local Area Network) is a network within an enterprise, institution, or one organization. Computers are located at a distance of up to several kilometers and are usually connected using high-speed communication lines.

Regional MAN (Metropolitan Area Network) - unites users of a region, city, and small countries. Telephone lines are used as communication channels. The distance between network nodes ranges from 10 to 1000 km.

Global WAN (Wide Area Network) - includes other global networks, local networks, as well as computers separately connected to it.

By purpose and list of services provided:

- General use of files and printers - with the help of a special computer (file server, printer server) user access to files and printers is organized.

General use of databases - using a special computer (database server), user access to the database is organized.

Application of Internet technologies - e-mail, World Wide Web, teleconferencing, video conferencing, file transfer via the Internet.

By way of organizing interaction:

- Peer-to-peer networks - all computers in a peer-to-peer network have equal rights, and any network user can access data stored on any computer. The main advantage of peer-to-peer networks is the ease of installation and operation. The main disadvantage is that in peer-to-peer networks it is difficult to resolve information security issues. Therefore, this method of organizing a network is used for networks with a small number of computers and where the issue of data protection is not fundamental.

- Dedicated server networks ( hierarchical networks) - when installing a network, one or more servers- computers that manage data exchange over the network and resource distribution. Any computer that has access to the server's services is called network client or workstation. The server itself can only be a client of a server at a higher hierarchy level. The hierarchical network model is the most preferable, as it allows you to create the most stable network structure and more rationally distribute resources. Another advantage of a hierarchical network is a higher level of data protection.

The disadvantages of a hierarchical network, compared to peer-to-peer networks, include:

The need for an additional OS for the server.

Greater complexity of network installation and upgrades.

The need to allocate a separate computer as a server

Based on server usage technology:

File-server architecture networks use a file server on which most programs and data are stored. At the user's request, the necessary program and data are sent to him. Information processing is performed at the workstation.

Networks with a client-server architecture - data is exchanged between the client application and the server application. Data is stored and processed on a powerful server, which also controls access to resources and data. The workstation receives only the results of the query.

By information transfer speed computer networks are divided into low-, medium- and high-speed:

Low-speed networks - up to 10 Mbit/s;

Medium-speed networks - up to 100 Mbit/s;

High-speed networks - over 100 Mbit/s.

Based on the type of transmission medium, networks are divided into:

Wired (coaxial cable, twisted pair, fiber optic);

Wireless with information transmission via radio channels or in the infrared range.

By topology (how computers are connected to each other):

Common bus;

Network topology

Network topology refers to the physical or electrical configuration of the network's cabling and connections.

Several specialized terms are used in network topology:

Network node - a computer or network switching device;

A network branch is a path connecting two adjacent nodes;

A terminal node is a node located at the end of only one branch;

Intermediate node - a node located at the ends of more than one branch;

Adjacent nodes are nodes connected by at least one path that does not contain any other nodes.

Any computer network can be considered as a collection of nodes. The configuration of physical connections is determined by the electrical connections between computers and may differ from the configuration of logical connections between network nodes. Logical connections represent data transmission routes between network nodes, formed by appropriately configuring the equipment.

There are three main types of physical local area network topology:

Ring topology provides for the connection of network nodes in a closed curve, i.e. transmission medium cable. In such a network, each node has two and only two branches attached to it. Information along the ring is transmitted from node to node, usually in one direction. Each intermediate node between the transmitter and the receiver relays the sent message.

The receiving node recognizes and receives only messages addressed to it. In a network with a ring topology, it is necessary to take special measures so that in the event of a failure or disconnection of any station, the communication channel between the remaining stations is not interrupted. The advantage of this topology is ease of management, the disadvantage is the possibility of failure of the entire network if there is a failure in the channel between two nodes.

Bus topology one of the simplest, implemented using a cable to which all computers are connected. All signals transmitted by any computer on the network travel along the bus in both directions to all other computers.

Star topology uses a separate cable for each computer, running from a central device called hub or hub. A hub broadcasts signals received on any one of its ports to all other ports, causing signals sent by one node to reach the rest of the computers. In such a network there is only one intermediate node. A star-based network is more resilient to damage than a bus-based network, since cable damage directly affects only the computer to which it is connected, and not the entire network.

While small networks typically have a typical star, ring, or bus topology, large networks typically have random connections between computers. In such networks, individual subnetworks can be identified at random, having a standard topology, which is why they are called networks with mixed topology. The choice of a particular topology is determined by the area of ​​application of the network, the geographical location of its nodes and the size of the network as a whole.

Open systems interconnection model. The main task solved when creating computer networks is to ensure compatibility of equipment in terms of electrical and mechanical characteristics and ensure compatibility of information support (programs and data) in terms of coding system and data format. The solution to this problem belongs to the field of standardization. One example of solving this problem is the so-called open systems interconnection model OSI (Model of Open System Interconnections).

According to the OSI model, the architecture of computer networks should be considered at different levels (the total number of levels is up to seven). The highest level is applied. At this level the user interacts with the computing system. The lowest level is physical. It ensures the exchange of signals between devices. Data exchange in communication systems occurs by moving it from the upper level to the lower one, then transporting it and, finally, playing it back on the client's computer as a result of moving from the lower level to the upper one.

Let's consider how in the OSI model data exchange occurs between users located on different continents.

1. At the application level, using special applications, the user creates a document (message, drawing, etc.).

2. At the presentation level, the operating system of his computer records where the created data is located (in RAM, in a file on the hard drive, etc.), and provides interaction with the next level.

3. At the session level, the user's computer interacts with a local or global network. Protocols at this level check the user's rights to “go on the air” and transmit the document to the transport layer protocols.

4. At the transport layer, the document is converted into the form in which data is supposed to be transmitted on the network being used. For example, it can be cut into small standard size bags.

5. The network layer determines the route for data movement in the network. So, for example, if at the transport level the data was “cut” into packets, then at the network level each packet must receive an address to which it should be delivered regardless of other packets.

6. The connection layer (Link layer) is necessary in order to modulate the signals circulating at the physical layer in accordance with the data received from the network layer. For example, in a computer these functions are performed by a network card or modem.

The actual data transfer occurs at the physical layer. There are no documents, no packets, not even bytes - only bits, that is, the elementary units of data representation. Restoring a document from them will occur gradually, when moving from the lower to the upper level on the client’s computer.

The physical layer facilities lie outside the computer. In local networks, this is the equipment of the network itself. For remote communication using telephone modems, these are telephone lines, switching equipment of telephone exchanges, etc.

On the computer of the information recipient, the reverse process of converting data from bit signals to a document occurs.

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 upper level to the lower, the data is continuously transformed, “overgrown” with additional data, which is analyzed by the protocols of the corresponding levels on the adjacent side. This creates an effect virtual interactions between levels.

In order for different computers on a network to communicate with each other, they must “speak” the same language, that is, use the same protocol. A protocol is a “language” used to exchange data when operating on a network.

There are many protocols, each performing different tasks. Different protocols are used at different layers of the OSI model.

Ethernet is a Connection Layer protocol used by most modern local area networks. The Ethernet protocol provides a unified interface to the network transmission medium, which allows the operating system to use several Network layer protocols simultaneously to receive and transmit data. Token Ring is an alternative to the “classic” Ethernet protocol at the Connection Level.

To be able to transmit information over network communication channels, it is necessary to install a messaging (packet) protocol. There are several such protocols. The most widely used are: NetBEUI , IPX/SPX , TCP/IP . Protocols NETBEUI And IPX/SPX- used in local networks. Protocols TCP/IP are the basic protocols of the global Internet.

network hardware

The main components of the network are workstations, servers, transmission media (cables) And network hardware.

Workstations are called network computers on which network users implement applied tasks.

Network servers- these are hardware and software systems that perform the functions of managing the distribution of public network resources. A server can be any computer connected to a network that contains resources used by other devices on the network. Quite powerful computers are used as server hardware.

The following types are distinguished network equipment:

Network cables (coaxial, consisting of two concentric conductors insulated from each other, the outer one of which has the appearance of a tube; cables on twisted pairs, formed by two wires intertwined with each other; fiber optic and etc.).

Network cards (Network interface adapters)- these are controllers connected to the computer motherboard, designed to transmit signals to the network and receive signals from the network. A network cable is connected to the adapter connectors.

Hubs (Hub) are the central devices of a cable system or a star physical topology network, which, when receiving a packet on one of its ports, forwards it to all the others. A hub with a set of different types of ports allows you to combine network segments with different cable systems. You can connect either a separate network node or another hub or cable segment to the hub port.

The following devices are used to connect local networks to each other:

Bridges- network devices that connect two separate segments limited by their physical length. Bridges also amplify and convert signals for other types of cable. This allows you to expand the maximum network size.

Bridges transfer data between networks in packet form without making any changes to them. The figure below shows three local networks connected by two bridges. In addition, bridges can filter packets, protecting the entire network from local data flows and allowing only data that is intended for other network segments to pass through.

Gateways (Gateway) - hardware and software systems connecting heterogeneous networks or network devices. Gateways allow you to solve problems of differences in protocols or addressing systems. A gateway, unlike a bridge, is used in cases where the connected networks have different network protocols. A message from one network arriving at the gateway is converted into another message that meets the requirements of the next network.

Routers (Router) - standard network devices that operate at the network level and allow you to forward and route packets from one network to another. It allows, for example, large messages to be split into smaller pieces, thereby ensuring the interaction of local networks with different packet sizes. A router can forward packets to a specific address (bridges can only filter out unnecessary packets), choose the best path for the packet to pass through.

Firewalls (firewall, firewalls ) - this is a software and/or hardware barrier between two networks, allowing the establishment of only authorized internet connections, implementing control over information entering and exiting the local network, and ensuring protection of the local network by filtering information.

Most firewalls are built on classical access control models, according to which a subject (user, program, process or network packet) is allowed or denied access to any object (file or network node) upon presentation of some unique element inherent only to this subject. In most cases, this element is a password. For a network packet, such an element is the addresses or flags found in the packet header, as well as some other parameters.

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 opening of channels for transmitting audio and video signals, which radically influenced the development of the mass media system; after radio and television broadcasting, network technologies based on different, digital, a 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 (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) is used primarily for transmitting narrowband digital signals, the other type of cable (with a resistance of 75 ohms) is used for transmitting wideband 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, not only for short-distance communications, but also in intracity and intercity areas.

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

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

At message switching devices called switches and made on the basis of universal or specialized computers allow you to accumulate (buffer) messages and send them in accordance with a given priority system and routing principles to other network nodes. Using message switching can increase 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 today is the division of 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, within a building or some other limited area, one or more high-speed digital information transmission channels made available 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 on twisted pair and optical fiber are becoming increasingly widespread, and recently the technology of wireless local networks has also been rapidly developing, which uses one of three types of radiation: broadband radio signals, low-power radiation ultrahigh frequencies (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 both in 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 analog channels and up to 64 Kbit/s on user sections of 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 exchange rate data 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:

Computational, designed to solve control problems based on computational processing of initial 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 that is part of the network includes a full set of software tools for coordinating 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.

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 run 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 in the appearance of the network, the distances between its elements, their relative position, in 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.

Fig 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 of the network.

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 the execution of 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 of this level defined by the model, actions (services) for the 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.

These models determine the interaction of computers in a local area 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.

A workgroup is an independent solution for 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. Lack of a unified information security system;
• 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 workstation: 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.

Depending on the scalability of the network, it will depend on how the network will be managed at a given enterprise. There are several control methods. Based on the management method, local computer networks are divided into two subgroups: peer-to-peer and hierarchical (multi-level) networks.

Peer-to-peer networks

In a peer-to-peer network, all computers have equal rights: there is no hierarchy among computers and there is no dedicated server. Typically, each computer functions as both a client and a server; in other words, there is no single computer responsible for administering the entire network. All users independently decide what data on their computer to make publicly available over the network.

Peer-to-peer networks are also called workgroups. A work group is a small team, so peer-to-peer networks most often have no more than 30 computers. Peer-to-peer networks are relatively simple.

Because each computer is both a client and a server, there is no need for a powerful central server or other components required for more complex networks.

Peer-to-peer networks are usually cheaper than server-based networks, but require more powerful (and more expensive) computers. In a peer-to-peer network, the performance and security requirements for network software are generally lower than in networks with a dedicated server.

Figure 5. Peer-to-peer network

Hierarchical networks

Hierarchical networks have one or more servers that store information shared by different users. In order to increase the reliability of information storage, two disks operating in parallel and duplicating each other can be installed on the server,

Moreover, if one of them fails, the other is automatically switched on. 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. One example of using a file server is hosting MS Office programs on it. In this case, only a small (client) part of these programs is located on workstations, requiring insignificant resources. Programs that allow this mode of operation are called programs with the ability to be installed on a network.

Database server. In this case, the server hosts a database (for example, Consultant Plus, Garant, Bank Client Accounts, etc.). The database on the server can be replenished from various workstations and/or provide information upon requests from the workstation.

Clients of the Hierarchical Network can use the following operating systems: Windows XP, Windows Vista, Windows 7; servers require special server versions of operating systems.

Figure 6. Hierarchical network

Our service center will use a hierarchical network. For our case, this is the most suitable option. To prevent our network from turning into an information dump, and also to increase the reliability of information storage, it is necessary to have several servers. In this case, a file server, an Internet server and a database server. The server will host MS Office, 1C and other programs, and the workstations will contain only a small (client) part of these programs, requiring little resources. It is also necessary for each user to allocate his rights on the local network.

The basis FA classification the most characteristic functional, informational and structural features are laid down.

According to the degree of territorial dispersion Network elements (subscriber systems, communication nodes) are distinguished between global (state), regional and local computer networks (WAN, DVR and LAN).

By the nature of the functions being implemented networks are divided into computing (the main functions of such networks are information processing), information (to obtain reference data on user requests), information-computing, or mixed, in which computing and information functions are performed in a certain, variable ratio.

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

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

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

According to the topology, those. configurations of elements in TVS, networks are divided into two classes: broadcast (Fig. 11.1) and serial (Fig. 11.2). Broadcast configurations and a significant part of sequential configurations (ring, star with an intelligent center, hierarchical) are characteristic of LANs. For global and regional networks, the most common is the arbitrary (mesh) topology. The hierarchical configuration and the “star” have also found application.

IN broadcast configurations At any given time, only one workstation (subscriber system) can transmit a frame. The rest of the PCs on the network can receive this frame, i.e. Such configurations are typical for LANs with information selection. The main types of broadcast configuration are common bus, tree, star with passive center. The main advantages of a LAN with a common bus are the ease of network expansion, the simplicity of the management methods used, the absence of the need for centralized management, and minimal cable consumption. A LAN with a tree topology is a more developed version of a network with a bus topology. A tree is formed by connecting several buses with active repeaters or passive multipliers (“hubs”), each branch of the tree representing a segment. The failure of one segment does not lead to the failure of the rest. In a LAN with a star topology, there is a passive connector or an active repeater in the center - fairly simple and reliable devices. To protect against disturbances in the cable, a central relay is used, which turns off failed cable beams.

Rice. 11.1. Broadcast network configurations: A - common bus; b- tree; V - star with passive center

Rice. 11.2. Consecutive network configurations: a - arbitrary (mesh); b- hierarchical; V - ring; G - chain; d - star with an intellectual center; e - snowflake

In sequential configurations, characteristic of networks with information routing, data is transferred sequentially from one PC to an adjacent one, and different types of physical transmission media can be used in different parts of the network.

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

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

The ring structure provides fairly broad LAN functionality with high efficiency of using a monochannel, low cost, simplicity of control methods, and the ability to monitor the performance of a monochannel.

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

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