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What is a local area network (LAN). Local area networks

Local computer networks(LAN) became most widespread with the advent of personal computers. They made it possible to raise the management of production facilities to a new level, increase the efficiency of using computer resources, improve the quality of processed information, begin the introduction of paperless technology, and create new technologies for distributed information processing. Merging LAN and global networks allowed access to global information resources.

Local networks are focused on combining computers and peripheral devices concentrated in a small space (for example, within one room, building, group of buildings within several kilometers).

Advantages of a local network:

Use of shared network resources (disks, modems, printers, programs and data) in multi-user mode;

- the ability to transfer information from one computer to another;

- relatively low cost;

- high survivability and ease of integration;

- equipped with modern operating systems for various purposes;

- high data transfer speed.

The main hardware components of a LAN are:

workstations

servers

interface boards

cables

Workstations- These are, as a rule, personal computers, which are the workplaces of network users.

Servers LANs perform the functions of distributing network resources. Usually its functions are assigned to a sufficiently powerful PC, minicomputer, mainframe computer or special computer- server. There can be one or more servers on one network.

Server LANs implement two models of user interaction with workstations (PCs): the “file-server” model and the “client-server” model.

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 database is used to obtain information about taxpayers living on a particular street, the entire table for the city 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.

In the client-server model, the 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, the server computer must have a multitasking operating system.

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

The network contains a significant number of servers and clients;

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

The user of the system is freed from the need to know where the information he 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.

The most important parameters that should be taken into account when choosing a server computer are: processor type, volume RAM, type and volume hard drive and disk controller type. Values specified parameters depend on the tasks being solved, the organization of computing on the network, network load, the OS used and other factors.

Workstations and servers on the network are connected to each other through data lines, which are cables. Computers are connected to the cable using interface cards - network adapters. In wired networks, the following are used as physical connections in channels:

Flat two-core cable,

Twisted pair of wires,

coaxial cable,

Light guide (fiber optic cable).

Most networks use three main groups of cables:

Coaxial cable;

Twisted pair, unshielded and shielded;

Fiber optic cable.

Coaxial cable until recently it was the most common. Inexpensive, lightweight, flexible, convenient, safe and easy to install.

There are two types of coaxial cables: thin (10Base2 specification) and thick (10Base5 specification). Thin - flexible, diameter 0.64 cm (0.25). Easy to use and suitable for almost any type of network. Connects directly to the board network adapter. Transmits a signal over 185 m with virtually no attenuation. Characteristic impedance- 50 ohm. Thick - hard, diameter 1.27 cm (0.5). It is sometimes called standard Ethernet(the first cable in the popular network architecture). The vein is thicker, the attenuation is less. Transmits a signal without attenuation over 500 m. Used as a backbone connecting several small networks. Characteristic impedance - 75 ohms.

To connect to a thick coaxial cable, a special device is used - transceiver(transceiver - transceiver). It is equipped with a connector called vampire or piercing coupler. The transceiver is connected to the network card using a cable with a connector. To connect a thin coaxial cable, BNC connectors (British Naval Connector) are used. BNC-T connectors are used for connection network cable with a computer network card, BNC barrel connectors for splicing two sections of cable, BNC terminators for absorbing signals at both ends of the cable in networks with a bus topology.

Twisted pair - These are two twisted insulated copper wires. Some twisted pairs wires are often placed in a single protective sheath. Interweaving the wires allows you to get rid of electrical interference induced by neighboring wires and other external sources, for example motors, transformers, powerful relays.

Unshielded twisted pair (UTP) is widely used in LAN, the maximum length is 100 m. UTP is defined by a specific standard that specifies the regulatory characteristics of cables for various applications, which ensures product consistency.

Shielded twisted pair(STP) is enclosed in copper braid. In addition, pairs of wires are wrapped in foil. Therefore, STP is less susceptible to electrical interference and can transmit signals at higher speeds and distances. long distances.

The advantages of twisted pair cable are low cost and ease of connection. Disadvantages - cannot be used when transmitting data over long distances at high speed.

IN fiber optic cable digital data is distributed along optical fibers in the form of modulated light pulses. This is a reliable method of transmission because no electrical signals are transmitted. Hence, fiber optic cable the data cannot be opened or intercepted.

Fiber optic lines are designed for moving large amounts of data at very high speeds, since the signal in them is practically not attenuated or distorted. Fiber transmits signals in only one direction, so the cable consists of two fibers with separate connectors: one for transmission, the other for reception.

Data transfer speeds currently range from 100 Mbit/s. Meanwhile, speeds of 1 Gbit/s are becoming increasingly widespread, theoretically up to 200 Gbit/s. The distance is many kilometers. The cable is not subject to electrical interference. A significant disadvantage of this technology is the high cost and complexity of installation and connection.

A typical optical network consists of a laser light transmitter, a multiplexer/demultiplexer to combine optical signals of different wavelengths, optical signal amplifiers, demultiplexers, and receivers that convert the optical signal back to electrical signal. All these components are usually assembled by hand.

To transmit encoded signals via cable, two technologies are used - unmodulated and modulated transmission.

Unmodulated systems transmit data in the form of digital signals, which are discrete electrical or light pulses. With this method digital signal uses the entire bandwidth of the cable (bandwidth is the difference between the maximum and minimum frequency that can be transmitted along the cable). A device on a baseband network sends data in both directions. In order to avoid signal attenuation and distortion in unmodulated systems, use repeaters, which amplify and relay the signal.

Modulated systems transmit data in the form of an analog signal (electrical or light) occupying a certain frequency band. If there is enough bandwidth, then one cable can simultaneously use several systems (for example, broadcast cable television and transfer data). Each transmission system is allocated a portion of the bandwidth. To restore the signal in modulated systems, use amplifiers In a modulated system, devices have separate paths for receiving and transmitting signals, since transmission occurs in one direction. In order for devices to both transmit and receive data, they use the division of the bandwidth into two channels, which operate at different frequencies for transmission and reception, or the laying of two cables - for transmission and reception.

IN lately began to appear wireless networks where they are used frequency channels data transmission (the medium is ether). The main advantage of wireless technologies is the capabilities it provides to users laptop computers. However, the transmission speed in wireless technologies cannot yet be compared with the throughput of cable.

Collocation wireless environment does not mean a complete absence of wires in the network. Typically, wireless components communicate with a network that uses cable as the transmission medium. Such networks are called hybrid.

The wireless environment provides a temporary connection to the existing cable network, guarantees a certain level of mobility and reduces restrictions on network length. It is used in office premises where employees do not have a permanent workplace, in isolated rooms and buildings, in buildings where laying cables is prohibited.

The following types of wireless networks exist: LAN, extended LAN And mobile networks(laptop computers). The main differences between them are the transmission parameters. LANs and extended LANs use transmitters and receivers from the organization in which the network operates. For laptop computers, the transmission medium is public networks(for example, telephone or Internet).

A LAN looks and functions much the same as a cable network, except for the transmission medium. A wireless network adapter with a transceiver is installed in each computer, and users work as if their computers were connected by a cable. A transceiver or access point allows signals to be exchanged between wirelessly connected computers and a cable network. Small wall-mounted transceivers are used that establish radio contact with portable devices.

The operation of wireless LANs is based on four methods of data transmission: infrared radiation, laser, narrow-band radio transmission (single-frequency transmission), and scattered spectrum radio transmission.

By LAN access methods The most common networks such as Ethernet, ARCnet, and Token Ring are highlighted.

Ethernet access method, which is the most popular, provides high data transfer speeds and reliability. It uses a “common bus” topology, so a message sent by one workstation is received simultaneously by all other stations connected to the common bus. But since the message includes the addresses of the sender and destination stations, other stations ignore this message. This is a multiple access method. With it, before starting transmission, the workstation determines whether the channel is free or busy. If free, the station starts transmitting.

ARCnet access method became widespread due to the low cost of equipment. It is used in a LAN with a star topology. One of the PCs creates a special marker (message special type), which is sequentially transmitted from one PC to another. If a station sends a message to another computer, 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 access method designed for a ring topology and also uses a token passed 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. If a computer receives an empty token, it can fill its message with a frame of any length, but only during the period of time that is allocated by a special timer for finding the token at one point in the network. The frame moves across the network and each PC regenerates it, but only the receiving PC copies this frame into its memory and marks it as received, but does not remove the frame itself from the ring. This function is performed by the sending computer when its message is returned to it. This confirms that the message has been transmitted.

Telecommunications systems

Telecommunications – communication at a distance (lat.)

Communication( the process of information exchange) is a necessary condition for the existence of living organisms, ecological communities and human society. Social development is accompanied by the development of telecommunications technologies. Telecommunication technologies have been developing especially intensively over the past few decades.

Telecommunications can be defined as technologies dealing with communication at a distance and this can be explained in various ways. Figure 8.2 shows one possible representation of the various sections of telecommunications.

Figure 8.2. Telecommunications: forms and types

Telecommunications are divided into two types: unidirectional and bidirectional. Unidirectional, such as mass radio and television broadcasting, involve the transmission of information in one direction - from the center to subscribers. Bidirectional supports dialogue between two subscribers.

Telecommunications use mechanical and electrical means because historically telecommunications have evolved from mechanical to electrical forms, using increasingly complex electrical systems. This is the reason why many traditional operators in telecommunications such as national postal, telegraph and telephone companies use both forms. The share of mechanical telecommunications such as mail and newspapers is expected to decrease, while the share of electrical, especially bidirectional, will increase and become dominant in the future. Already in our time, corporations and the press are interested, first of all, in electrical telecommunications (telecommunications) as a profitable business opportunity.

Along the edges of Figure 8.2. telecommunications services are shown, initially mechanical: press (newspaper forwarding), post office; then electrical: telegraph, telex (subscriber telegraph), telephone, radio, television, computer networks, leased networks, cable television and mobile telephone.

Telecommunications have historically developed approximately in this order.

Telecommunication system– a set of technical objects, organizational measures and subjects that implement processes consisting of: connection processes, transmission processes and access processes.

Telecommunication systems use natural or artificial environments to exchange information. Telecommunication systems, together with the medium that is used for transmission, form telecommunication networks. The most important telecommunication networks are (Fig. 8.2.): postal services; public telephone network (PSTN); mobile telephone networks; telegraph network; Internet – a global network of interaction of computer networks; wired radio broadcasting network; cable television networks; television and radio broadcasting networks; departmental communication networks that provide communication services to public service bodies, air and maritime traffic control systems, and large industrial complexes; global rescue and safety networks.

The telecommunication systems listed above, as a rule, closely interact with each other and use common resources to implement communications. To organize such interaction, in each state and on a global scale there are special bodies that regulate the use of common resources; determine the general rules of interaction (protocols) of telecommunication systems; developing promising telecommunications technologies.

To implement communication at a distance, telecommunication systems use: switching systems; transmission systems; access and control systems for transmission channels.


Related information.


FACULTY OF FINANCE.

Coursework

in the discipline "Informatics"

Local computer networks.

Introduction........................................................ ............................................... 3

1. Local area network.................................................... ..... 4-5

2. Classification of LAN.................................................... ....................... 5-10

3. Features LAN organizations................................................. 10-14

4. Methods of access to the transmission medium.................................................... 14-15

5. Basic exchange protocols in computer networks.............................. 15-17

Conclusion................................................. ..................................... 18

Practical part................................................... ........................... 19-27

List of used literature......................................................... .27-28

INTRODUCTION

Not long ago I wondered how many computers there are in the world today? The answer interested me very much. The Gartner portal keeps a count of the number of computers on Earth. According to this agency, for every 5-6 earthlings there is one PC, i.e. total number exceeded the 1 billion mark. Many of them are combined into various information and computer networks, from small local networks in offices and homes, to global networks such as the Internet. Therefore, the topic of my work is local networks. In my opinion, this topic is especially relevant now when mobility, speed and convenience are valued all over the world, while wasting as little time as possible! The worldwide trend towards connecting computers into networks is due to a number of important reasons, such as speeding up the transmission of information messages, the ability to quickly exchange information between users, receiving and transmitting messages (faxes, E-Mail letters, etc.) without leaving the workplace, the ability to instantly exchange information between computers.

Such huge potential opportunities that the computer network carries and the new potential rise that the information complex experiences at the same time, as well as the significant acceleration of the production process, do not give us the right not to accept this for development and not to apply it in practice.

In the theoretical part, I would like to consider what a local area network is, its classification and methods of access to the transmission medium.

In the practical part, an algorithm for solving the economic problem of the company’s financial activities in the field of lending will be described.

1. LOCAL COMPUTER NETWORK.

A local area network is understood as the joint connection of several separate computer workstations (workstations) to a single data transmission channel. The most simple network(English network) consists of at least two computers connected to each other by cable. This allows them to share data. All networks (regardless of complexity) are based on this simple principle. The birth of computer networks was caused by a practical need - to be able to share data.

The concept of “local area network” - (English LAN - Local Area Network) refers to geographically limited (territorially or production) hardware and software implementations, in which several computer systems connected with each other using appropriate means of communication.

Depending on the territorial location of subscriber systems, computer networks can be divided into three main classes:

· global networks (WAN – Wide Area Network);

· regional networks (MAN – Metropolitan Area Network);

· local networks (LAN – Local Area Network).

A local area network unites subscribers located within a small area. Currently, there are no clear restrictions on the territorial dispersion of subscribers. Typically, such a network is tied to a specific location. The length of such a network can be limited to 2 – 2.5 km.

The main purpose of any computer network is to provide information and computing resources users connected to it.

Thanks to computer networks, we have the opportunity to simultaneously use programs and databases by several users.

IN industrial practice LANs play a very important role. Through a LAN, the system combines personal computers located at many remote workplaces, which share equipment, software and information. Employees' workplaces are no longer isolated and are combined into a single system.

All LANs operate in the same standard accepted for computer networks - the Open Systems Interconnection (OSI) standard.

2. LAN CLASSIFICATION,

All the many types of LANs can be divided into four groups.

TO first group include LANs aimed at the mass user. Such LANs mainly connect personal computers using data transmission systems that are low cost and provide information transmission over a distance of 100 - 500 m at a speed of 2400 - 19200 baud (baud rate is the number of signals sent per second; in one baud you can encode several bits, so the bit rate is greater than the baud rate).

Co. second group include LANs that combine, in addition to PCs, microprocessor technology built into technological equipment (design automation tools, document information processing, cash registers etc.), as well as means email. The data transmission system of such LANs provides information transmission over a distance of up to 1 km at speeds from 19200 baud to 1 Mbaud. The cost of data transmission in such networks is approximately 30% higher than the cost of this work in the networks of the first group.

TO third group include LANs that combine personal computers, minicomputers and middle-class computers. These LANs are used to organize the management of complex production processes using robotic systems and flexible automated modules, as well as to create large design automation systems, scientific research management systems, etc. Data transmission systems in such LANs are of average cost and provide information transmission over a distance of up to several kilometers at a speed of 120 Mbaud.

For LAN fourth group characterized by the unification of all classes of computers. Such LANs are used in complex management systems for large production and even a separate industry: they include the main elements of all previous LAN groups. Within this group, LANs can be used various systems data transmission, including those providing information transmission at speeds from 10 to 50 Mbaud over a distance of up to 10 km. In terms of their functionality, LANs of this group differ little from regional computer networks serving large cities, districts, and regions. They may contain an extensive network of connections between various subscribers - senders and recipients of information.

Based on topological characteristics, LANs are divided into networks of the following types: with a common bus, ring, hierarchical, radial multiconnected.

On LAN with common bus one of the machines serves as a system service device, providing centralized access to shared files and databases, printing devices and other computing resources. LAN of this type have gained great popularity due to their low cost, high flexibility and data transfer speed, 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 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.

Hierarchical LAN(tree configuration) is a more developed version of the LAN structure, built on the basis of a common bus. A tree is formed by connecting several buses to a root system where the most important components of the LAN are located. It has the necessary flexibility to cover several floors in a building or several buildings on the same territory with LAN means, and is implemented, as a rule, in complex systems numbering tens or even hundreds of subscribers.

Radial (star-shaped) 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).

The most complex and expensive is multiply connected topology, in which each node is connected to all other nodes in the network. This LAN topology is used very rarely, mainly where exceptionally high network reliability and data transfer speed are required.

In practice, they are more common hybrid LANs adapted to the requirements of a specific customer and combining fragments of bus, star and other topologies.

LAN access methods. According to network access methods, the most common networks are distinguished: Ethernet, ArcNet, TokenRing.

The most popular access method, Ethernet, provides high data transfer speeds and reliability. It is used for "common bus" topology therefore, a message sent by one workstation is received simultaneously by all other stations connected to the common bus. But since the message includes the addresses of the sender and destination stations, other stations ignore this message. This is a multiple access method. With it, before starting transmission, the workstation determines whether the channel is free or busy. If free, the station starts transmitting. The ArcNet access method has become widespread due to the low cost of equipment. It is used in LAN with star topology. One of the PCs creates a special token (a message of a special type), 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.

The TokenRing access method is designed for ring topology and also uses a token passed 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. If a computer receives an empty token, it can fill its message with a frame of any length, but only during the period of time that is allocated by a special timer for finding the token at one point in the network. The frame moves through the network, and each PC regenerates it, but only the receiving PC copies that frame into its memory and marks it as received, but does not remove the frame itself from the ring. This function is performed by the sending computer when its message is returned to it. This ensures confirmation that the message has been transmitted.

Let's return to the question of ways to connect personal computers into a single computing complex. The simplest one is to connect computers via serial ports. In this case, it is possible to copy files from the hard drive of one computer to another if you use the program from the NortonCommander operating shell. To gain direct access to the hard drive of another computer, special network cards (adapters) and software have been developed. In simple local networks, functions are performed not on a server basis, but on the principle of connecting workstations to each other, so the user does not have to purchase special file servers and expensive network software. Each PC in such a network can perform the functions of both a workstation and a server.

In a LAN with a developed architecture, management functions are performed by a network operating system installed on a computer (file server) that is more powerful than workstations. Server networks are divided into middle-class networks (up to 100 workstations) and powerful (corporate) networks, combining up to 250 workstations or more. The main developer of network software products for the LAN server is Novell.

It should be noted here that there is a trend towards accelerating data transfer to gigabit speeds. In addition, data such as high-quality audio, speech and images must be transmitted. All this leads to the gradual displacement of such “old” LANs as TokenRing, ArcNet, but allowing the use of new IT. Microsoft's WindowsNT operating system is ousting Unix from the market.

“Virtual” LAN VLANs have become very popular. Their difference from conventional LANs is that they do not have physical limitations. VLANs determine which workstations are included in physical groups based on protocol addressing, allowing them to be located anywhere on the network.

Server LANs implement two models of user interaction with workstations: model file server and model client-server.

3. FEATURES OF ORGANIZING LOCAL NETWORKS.

The main purpose of any computer network is to provide information and computing resources to users connected to it.

From this point of view, a local area network can be considered as a collection of servers and workstations.

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.

Workstation- a personal computer connected to a network through which the user gains access to its resources.

Workstation The network 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.

Computer networks implement distributed data processing. Data processing in this case is distributed between two objects: client and server.

Client – ​​a task, workstation or computer network user. During data processing, the client can create a request to the server to perform complex procedures, read files, search for information in a database, etc.

The server defined earlier fulfills the request received from the client. The results of the request are transmitted to the client. The server provides storage of public data, organizes access to this data, and transmits the data to the client.

The client processes the received data and presents the processing results in a form convenient for the user. For such systems, the terms adopted are systems or client-server architecture.

The client-server architecture can be used both in peer-to-peer networks and in networks with a dedicated server.

A peer-to-peer network that does not single center management of interaction between workstations and there is no single center for data storage. The network operating system is distributed across workstations. Each network station can perform the functions of both a client and a server. It can service requests from other workstations and forward its own service requests to the network. The network user has access to all devices connected to other stations.

Advantages of peer-to-peer networks:

· low cost;

· high reliability.

Disadvantages of peer-to-peer networks:

· dependence of network efficiency on the number of stations;

· complexity of network management;

· difficulty in ensuring information security;

· difficulties in updating and changing station software.

The most popular are peer-to-peer networks based on the network operating systems LANtastic and NetWareLite.

In a network with a dedicated server, one of the computers performs the functions of storing data intended for use by all workstations, managing interaction between workstations, and a number of service functions.

Such a computer is usually called a network server. A network operating system is installed on it, and all shared devices are connected to it. external devices– hard drives, printers and modems.

Interaction between workstations on a network is usually carried out through a server.

Advantages of a network with a dedicated server:

· reliable information security system;

· high performance;

· no restrictions on the number of workstations;

· ease of management compared to peer-to-peer networks.

Network disadvantages:

· high cost due to the allocation of one computer to the server;

· dependence of performance and reliability on the server;

· less flexibility compared to peer-to-peer networks.

Dedicated server networks are the most common among computer network users. Network operating systems for such networks are LANServer (IBM), WindowsNTServer versions 3.51 and 4.0, and NetWare (Novell).

Particular attention should be paid to one of the types of servers - file server(FileServer). In common terminology, the abbreviated name is accepted for it - file server.

The file server stores the data of network users and provides them with access to this data. This is a computer with a large capacity of RAM, hard drives large capacity and additional magnetic tape drives (streamers).

It operates under a special operating system that provides simultaneous access for network users to the data located on it.

The file server performs the following functions: data storage, data archiving, synchronization of data changes by different users, data transfer.

For many tasks, using a single file server is not enough. Then several servers can be included in the network. It is also possible to use mini-computers as file servers.

METHODS OF ACCESS TO THE TRANSMISSION MEDIUM.

The transmission medium is shared resource for all network nodes. To be able to access this resource from a network node, special mechanisms are needed - access methods.

Media access method- a method that ensures the implementation of a set of rules by which network nodes gain access to a resource.

There are two main classes of access methods: deterministic and non-deterministic.

At deterministic In access methods, the transmission medium is distributed between nodes using a special control mechanism that guarantees the transmission of node data within a certain, fairly small time interval.

The most common deterministic access methods are the polling method and the transfer method. The survey method was discussed earlier. It is used primarily in star topology networks.

The transfer of rights method is used in networks with ring topology. It is based on the transmission of a special message - a token - over the network.

Marker- a service message of a certain format into which network subscribers can place their information packets.

The token circulates around the ring, and any node that has data to transmit places it in the free token, sets the token's busy flag, and transmits it around the ring. The node to which the message was addressed receives it, sets the confirmation flag for receiving the information, and sends a token to the ring.

The sending node, having received confirmation, releases the token and sends it to the network. There are access methods that use multiple tokens.

Indeterministic up ovated- random access methods involve competition between all network nodes for the right to transmit. Simultaneous transmission attempts by several nodes are possible, resulting in collisions.

The most common non-deterministic access method is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This is essentially the competitive mode described earlier. Carrier sensing is where a node wanting to transmit a message “listens” on the transmission medium, waiting for it to become free. If the medium is free, the node starts transmitting.

It should be noted that the network topology, the transmission medium access method and the transmission method are closely related to each other. The defining component is the network topology.

5. BASIC EXCHANGE PROTOCOLS IN COMPUTER NETWORKS.

To ensure consistent operation in data networks, various data communication protocols are used - sets of rules that the sending and receiving parties must adhere to for consistent data exchange. Protocols are sets of rules and procedures that govern how some communication occurs.

There are many protocols. And although they all participate in the implementation of communication, each protocol has different goals, fulfills various tasks, has its advantages and limitations.

Protocols work on different levels OSI/ISO open systems interaction models. The functions of a protocol are determined by the layer at which it operates. Multiple protocols can work together. This is the so-called stack, or set, of protocols.

Just as network functions are distributed across all layers of the OSI model, protocols operate together at different layers of the protocol stack. The layers in the protocol stack correspond to the layers of the OSI model. Taken together, the protocols provide full description functions and capabilities of the stack.

Data transmission over the network, with technical point vision, must consist of successive steps, each of which has its own procedures or protocol. Thus, a strict sequence in performing certain actions is maintained.

In addition, all these steps must be performed in the same sequence on each network computer. On the sending computer, actions are performed in a top-down direction, and on the receiving computer, from bottom to top.

The sending computer, in accordance with the protocol, performs the following actions: Breaks the data into small blocks called packets that the protocol can work with, adds address information to the packets so that the receiving computer can determine that this data is intended for it, prepares the data for transmission through the network adapter card and then via the network cable.

The recipient computer, in accordance with the protocol, performs the same actions, but only in reverse order: receives data packets from the network cable; transmits data to the computer via the network adapter card; removes from the packet all service information added by the sending computer, copies the data from the packet into a buffer - to combine them into source block, passes this block of data to the application in the format it uses.

Both the sending computer and the receiving computer need to perform each action in the same way so that the data received over the network matches the data sent.

If, for example, two protocols have different ways of breaking up data into packets and adding information (packet sequencing, timing, and error checking), then a computer running one of those protocols will not be able to successfully communicate with a computer running the other protocol. .

Until the mid-80s, most local networks were isolated. They served separate companies and rarely merged into large systems. However, when local networks reached a high level of development and the volume of information transmitted by them increased, they became components of large networks. Data transmitted from one local network to another along one of the possible routes is called routed. Protocols that support data transfer between networks over multiple routes are called routed protocols.

Among the many protocols, the most common are the following:

· IPX/SPX and NWLmk;

· OSI protocol suite.

CONCLUSION

Based on the progress network technologies have been able to achieve in recent years, it is not difficult to guess that in the near future the speed of data transfer over a local network will at least double. The usual ten-megabit Ethernet, for a long time occupying a dominant position, at least looking from Russia, is actively being replaced by more modern and significantly more fast technologies data transfer. There is hope that the price of fiber optic cable - which is by far the most expensive, but high-speed and most noise-resistant conductor - will soon become more affordable and acceptable; all houses will be united by their own local network, and the need to run a dedicated line to each apartment will also be left behind!

Practical part

The NBC company carries out financial activities in Russia according to the types of loans in rubles presented in Fig. 11.1. Each loan has a fixed price.

The company has its branches in several cities (Fig. 11.2.) and encourages the development of each branch by providing a certain discount (discount). The discount is revised monthly based on the total amounts of contracts for branches.

At the end of each month it is compiled general register contracts for all branches (Fig. 11.3).

1. Build tables (Fig. 11.1 – 11.3).

2. Organize inter-table connections to automatically fill out the register columns (Fig. 11.3): “Name of branch”, “Name of loan”, “Loan amount, rubles”, “Loan amount at discount, rubles”.

3. Organize the calculation of the total amount of loans by branches:

1) summarize in the register table;

2) build an appropriate pivot table, providing the ability to simultaneously track results by type of loan.

4. Construct a histogram based on the data in the pivot table.

Rice. 11.1. Types of loans

Rice. 11.2. List of NBC affiliates

Branch code Branch name Credit code Loan name Loan amount, rub. discount, %. Discount amount, rub.
1 001
3 003
2 005
4 002
6 006
5 002
2 005
3 004
3 002
5 001
4 006
6 003
1 005
1 005
6 003

Rice. 11.3. Register of contracts

Description of the algorithm for solving the problem

1. Let's launch table processor MSExcel.

2. Rename sheet 1 to sheet with the title Loans .

3. On the worksheet Loans MSExcel will create a table of loan types.

4. Fill in the table with initial data (Fig. 1).

Rice. 1. Table location "Types of loans" on the MSExcel Credits worksheet

5. Rename sheet 2 to sheet with the title Branches .

6. On the worksheet Branches MSExcel will create a table that will contain a list of branches of the NBC company.

7.
Let's fill out the table with the list of branches of the NBC company with the initial data (Fig. 2).

Rice. 2. Table location "List of company branches" NBC » on the worksheet Branches MSExcel

8. Rename sheet 3 to sheet with title Treaties .

9. On the worksheet Treaties MSExcel create a table that will contain the register of contracts.

10. Fill out the table "Register of contracts" initial data (Fig. 3).

Rice. 3. Table location "Register of contracts" on the worksheet Treaties MSExcel

11. Fill in the column Branch name tables "Register of contracts" located on the sheet Treaties as follows:

IF(A3="";"";VIEW(A3,Branches!$A$3:$A$8,Branches!$B$3:$B$8)).

Let us multiply the formula entered in cell B3 for the remaining cells (from B3 to B17) of this column.

Thus, a loop will be executed whose control parameter is the line number.

12. Fill in the column Loan name tables "Register of contracts" located on sheet D slander as follows (Fig. 4):

Rice. 4. Table location "Register of contracts" on the worksheet Treaties MSExcel

loan

Let's enter the formula in cell D3:

IF(C3="";"";VIEW(C3,Credits!$A$3:$A$8,Credits!$B$3:$B$8))

Let's multiply the formula entered into cell D3 for the remaining cells of this column (from D5 to D17).

13. Fill in the column Sum loan, rub. located on the sheet Treaties, as follows (Fig. 5):

Rice. 5. Table location "Register of contracts" on the worksheet Treaties MSExcel

Let's enter the formula in cell F3:

VIEW(C3,Credits!$A$3:$A$8,Credits!$C$3:$C$8).

Let's multiply the formula entered in cell F3 for the remaining cells (from F4 to F17) of this column.

14. Fill in column D iscont, % located on the sheet Treaties, as follows (Fig. 6):

Rice. 6. Table location "Register of contracts" on the worksheet Treaties MSExcel

Let's enter the formula in cell G3:

VIEW(Agreements!A3;Branches!$A$3:$A$8;Branches!$C$3:$C$8)

Let's multiply the formula entered into cell G3 for the remaining cells of this column (from G4 to G17).

15. Fill in the column discount amount, rub., located on the Contracts sheet as follows (Fig. 7)

Fig.7 Table location "Register of contracts" on the worksheet " Treaties".

16. In the table “Register of Contracts” we will calculate the total amount of policies by branches (Fig. 8).

Rice. 8. Calculation of the total amount of policies by branches in the table "Register of contracts"

Let's sort the data by branch name

Menu Data->Sorting->by branch name

Let's calculate the total for each branch and the grand total

Menu Data->Results…

17. On the Contracts (2) MSExcel worksheet, create a pivot table

Menu Data- > Pivot table

We complete all 3 steps and drag the data into a new sheet

Rice. 9 Positioning a PivotTable on a Sheet Agreements (2)

18. Let's present the calculation results graphically:

Using the Chart Wizard, we build a chart based on the data in the pivot table (Fig. 10):

List of used literature:

1. Computer science. – 3rd revision ed. / Ed. Prof. N.V. Makarova. – M.: Finance and Statistics, 2000.

2. Pyatibratov A.P.

Computing systems, networks and telecommunications: Textbook. – 2nd ed., revised. and additional / Ed. A. P. Pyatibratova. – M.: Finance and Statistics, 2001.

3. Economic Informatics: Textbook / Ed. V. P. Kosareva. – 2nd ed., revised. and additional – M.: Finance and Statistics, 2005.

4. Computer science. Basic course/ Simonovich S.V. and others - St. Petersburg: Publishing House "Peter", 1999.

5. Economic informatics. Textbook for universities. Ed. Doctor of Economics, prof. V.V. Evdokimov. SP: Peter. 1997.

6. Information Technology. Shafrin Yu.A. – M.: Laboratory of Basic Knowledge, 1998.

7. Dubina A.G., Orlova S.S., Shubina I.Yu., Khromov A.V. Excel for economists and managers. - St. Petersburg: Peter, 2004.-295 p.

8. Kotsyubinsky A.O., Groshev S.V. Excel for an accountant in examples.-M.: ZAO Publishing House Glav-bukh.-2003.-240 p.

9. http://excel.szags.ru/All_sourse/Funktsija_If.htm


Http://www.vologda.ru/~slivin/doc/linux-how-to/linux/howto/Serial-HOWTO-13.html

Local area network is a concept that is familiar to many firsthand. Almost every enterprise uses this technology, so it can be said that every person has come across it in one way or another. Local networks have significantly accelerated production processes, thereby giving a sharp boost to their further use throughout to the globe. All this allows us to predict further growth and development similar system data transfer, up to the implementation of a LAN in every, even the smallest enterprise.

The concept of a local network

A local area network is a number of computers connected to each other by special equipment that allows for the full exchange of information between them. Important feature This type of data transmission is a relatively small area where communication nodes are located, that is, the computers themselves.

Local networks not only greatly facilitate interaction between users, but also perform some other functions:

  • Simplify work with documentation. Employees can edit and view files at their workplace. At the same time, there is no need for collective meetings and meetings, which saves valuable time.
  • They allow you to work on documents together with colleagues, when everyone is at their own computer.
  • Allows access to applications installed on the server, which allows you to save free space on the installed hard drive.
  • Save hard drive space by allowing you to save documents on your host computer.

Types of networks

A local area network can be represented by two models: a peer-to-peer network and a hierarchical one. They differ in the ways communication nodes interact.

A peer-to-peer network is based on the equality of all machines, and data is distributed between each of them. Essentially, a user of one computer can access the resources and information of another. The efficiency of the peer-to-peer model directly depends on the number of worker nodes, and its level of security is unsatisfactory, which, coupled with a rather complex management process, makes such networks not very reliable and convenient.

The hierarchical model includes one (or more) main server, where all data is stored and processed, and several client nodes. This type of network is used much more often than the first, having the advantage of speed, reliability and security. However, the speed of such a LAN largely depends on the server, which under certain conditions can be considered a disadvantage.

Drawing up technical requirements

Designing a local area network is a rather complex process. It starts with development terms of reference, which should be carefully considered, since shortcomings in it threaten subsequent difficulties in building a network and additional financial costs. Primary design can be done using special configurators that will allow you to select the optimal network equipment. Such programs are especially convenient because they can be corrected different meanings and parameters directly during operation, as well as generate a report at the end of the process. Only after these steps can you proceed to the next stage.

Schematic design

This stage consists of collecting data about the enterprise where it is planned to install a local area network, and analyzing the information received. The quantity is determined:

  • Users.
  • Workstations.
  • Server rooms.
  • Connection ports.

An important point is the availability of data on the routes for laying highways and the planning of a specific topology. In general, it is necessary to adhere to a number of requirements imposed by the IEEE 802.3 standard. However, despite these rules, sometimes it may be necessary to make calculations of signal propagation delays or consult with network equipment manufacturers.

Basic LAN characteristics

When choosing a method for placing communication nodes, you must remember the basic requirements for local networks:

  • Productivity, which combines several concepts: throughput, reaction time, transmission delay.
  • Compatibility, i.e. ability to connect various local area network equipment and software.
  • Safety, reliability, i.e. capabilities to prevent unauthorized access and complete data protection.
  • Scalability - the ability to increase the number of workstations without degrading network performance.
  • Manageability - the ability to control the main elements of the network, prevent and eliminate problems.
  • Network transparency, which consists of presenting a single computing device to users.

Basic local area network topologies: advantages and disadvantages

The topology of a network represents its physical layout, significantly affecting its basic characteristics. In modern enterprises, three types of topologies are mainly used: “Star”, “Bus” and “Ring”.

The “Star” topology is the most common and has many advantages over others. This installation method is highly reliable; If any computer fails (except the server), this will not affect the operation of the others.

The “Bus” topology is a single backbone cable with connected computers. Such an organization of a local area network saves money, but is not suitable for unification large quantity computers.

The “Ring” topology is characterized by low reliability due to the special arrangement of nodes - each of them is connected to two others using network cards. The failure of one computer leads to the shutdown of the entire network, so this type of topology is used less and less.

Detailed network design

The enterprise local area network also includes various technologies, equipment and cables. Therefore, the next step will be the selection of all these elements. Making a decision in favor of one or another software or hardware determined by the purpose of creating the network, the number of users, the list of programs used, the size of the network, as well as its location. Currently, fiber optic highways are most often used, which are distinguished by their high reliability, speed and availability.

About cable types

Cables are used in networks to transmit signals between workstations; each of them has its own characteristics, which must be taken into account when designing a LAN.

  • A twisted pair consists of several pairs of conductors covered with insulation and twisted together. Low price and ease of installation are beneficial advantages, which makes this cable the most popular for installing local networks.
  • A coaxial cable consists of two conductors inserted one inside the other. A local area network using coax is no longer so common - it was replaced by twisted pair, but it is still found in some places.
  • Optical fiber is a glass thread that can carry light by reflecting it off walls. A cable made of this material transmits data over long distances and is characterized by high performance compared to twisted pair and coaxial cables, but it is not cheap.

Required equipment

Network equipment of local area networks includes many elements, the most commonly used of which are:

  • Hub or hub. It connects a number of devices into one segment using a cable.
  • Switch. Uses special processors for each port, processing packets separately from other ports, due to which they have high performance.
  • Router. This is a device that makes decisions about sending packets based on data about routing tables and some rules.
  • Modem. Widely used in communication systems, providing contact with other workstations via a cable or telephone network.

End network equipment

Local area network hardware in mandatory includes server and client parts.

The server is powerful computer, having high network significance. Its functions include storing information, databases, serving users and processing program codes. The servers are located in special rooms with a controlled constant air temperature - server rooms, and their housing is equipped with additional protection from dust, accidental shutdown, as well as a powerful cooling system. As a rule, only system administrators or company managers.

A workstation is a regular computer connected to a network, that is, it is any computer that requests services from the main server. To ensure communication at such nodes, a modem and a network card are used. Since workstations usually use server resources, the client part is equipped with weak memory sticks and small hard drives.

Software

Local area network equipment will not be able to fully perform its functions without suitable software. The software part includes:

  • Network operating systems on servers that form the basis of any network. It is the OS that controls access to all network resources, coordinates packet routing, and resolves device conflicts. Such systems have built-in support for the TCP/IP, NetBEUI, IPX/SPX protocols.
  • Autonomous operating systems that manage the client side. They are common operating systems, for example, Windows XP, Windows 7.
  • Network services and applications. These software elements allow you to perform various actions: viewing remote documentation, printing on a network printer, sending email messages. Traditional services HTTP, POP-3, SMTP, FTP and Telnet are the basis of this category and are implemented using software.

Nuances of designing local networks

Designing a local area network requires a long and leisurely analysis, as well as taking into account all the subtleties. It is important to provide for the possibility of enterprise growth, which will entail an increase in the scale of the local network. The project must be drawn up in such a way that the LAN is ready at any time to connect a new workstation or other device, as well as upgrade any of its nodes and components.

Security issues are no less important. The cables used to build the network must be reliably protected from unauthorized access, and the lines must be located away from potentially dangerous places where they can be damaged - accidentally or intentionally. LAN components located outside the premises must be grounded and securely secured.

Developing a local area network is a fairly labor-intensive process, but with the right approach and due responsibility, the LAN will operate reliably and stably, ensuring uninterrupted user experience.

Local area network(LAN, LAN; LAN, Local Area Network) - a group of personal computers, as well as peripheral equipment, united by one or more autonomous (not rented) high-speed digital data transmission channels (including wired, fiber-optic, radio-microwave or IR -range) within one or more buildings. The LAN is used to solve a set of interrelated functional and/or information tasks (within the organization or its automated system), as well as the sharing of combined information and computing resources. Depending on the principles of construction, LANs are divided into types: “client-server”, “file-server”, “peer-to-peer”. LANs may include means for accessing distributed and global computer networks. Options for building local area networks:

AppleTalk is a technology and software tool for creating cable peer-to-peer LANs of small organizations (for example, publishing houses with several PCs and printers in the same building) based on Apple Macintosh PCs. The distance between the most distant nodes in this network should not exceed 500 m.
ARCnet (Attached Resource Computing Network) - non-standard network architecture, developed by Datapoint Corporation in the mid-1970s. The access method is based on passing a token in a network with a bus topology; Coaxial and fiber optic cables are supported, as well as twisted pair. Network devices ARCnet is used in local networks of small organizations.
Broadband LAN is a broadband local network designed for data transfer rates of over 600 Mbit/s.
Bus network - a LAN with a bus topology, all stations of which are connected to one cable. Each station, receiving signals transmitted by one of the stations, has the ability to recognize the packets intended for it and ignore the rest.
CD-ROM based LAN - local network based on the use of CD-ROM.
ESA (Enterprise Systems Architecture) is the architecture of enterprise-scale computing systems, as well as the operating system of IBM Corporation.
FireWire (fire wire) - architecture for building “home LANs”, based on the use of the IEEE 1394 standard; also known as OP i.Link. The architecture is designed to connect consumer electronic devices into a local network for the purpose of exchanging audio, video and other multimedia data. Its interface allows the use of a single-core plastic fiber optic cable and an LED laser.
LocalTalk - developed by the company Apple architecture cable system based on shielded twisted pair, designed for networking Macintosh, IBM PC and peripheral equipment; uses the CSMA access method with collision avoidance (CSMA/CA).
NetWare is a network operating system for local networks developed by Novell. Its version of Personal NetWare is intended for peer-to-peer LANs, other versions of NetWare are for networks with a client-server architecture. Later versions of the system were called IntranetWare.
Token Ring is an architecture and network technology developed by IBM, according to which stations included in the LAN can transmit data only when they own a token that continuously circulates around the ring. The existing two versions of this technology provide data transfer rates from 4 to 16 Mbit/s. Token Ring adapters typically support both modes of operation. In this case, it is possible to combine up to 8 rings with connecting bridges. One ring can contain no more than 260 network nodes (including computers, printers, scanners, plotters). Token Ring technology performs the same functions as Ethernet, but implements them in a different way. Most small businesses install Ethernet networks in preference to Token Ring due to its relative simplicity. The IEEE 802.5 standard defines the type of cable that Token Ring networks operate with (STP, UTP, or fiber optic cable).
USB LAN architecture is an architecture for building a home local area network based on the use of the universal serial bus (USB). Its simplest option is to connect two PCs with a regular cable via a USB port. If necessary, on this basis you can create a peer-to-peer network that connects up to 17 PCs with a star topology via a USB hub. The PC to which the USB hub is connected on this network acts as the manager. Another option for building a LAN is based on the use of USB transceivers. It allows you to connect to a regular network through drivers that act as bridges. The data transfer speed in an Ethernet network for USB 1.1 is 10 Mbit/s, and for USB 2.0 - up to 100 Mbit/s.
XNS (Xerox Network System) - network architecture developed by Xerox; contains a set of protocols that form the basis of the routing protocols (IPX/SPX) of the NetWare network. One of the features of the XNS architecture is that it allows network users to use files located on other computers.
Print server - software and hardware for connecting a printer to a network and providing network printing. Based on their design, they are divided into built-in and on-board servers (“network printers”), as well as external ones (providing the connection of several printers).

Local computer networks. LAN types and characteristics

Local area network is a distributed data processing system covering a small area (up to 10 km in diameter) within institutions, research institutes, universities, banks, offices, etc., it is a system of interconnected and distributed information transmission and processing facilities over a fixed territory, oriented towards collective use general network resources - hardware, information, software. A LAN can be considered as a communication system that supports, within one building or some limited area, one or more high-speed information transmission channels provided to connected subscriber systems (AS) for short-term use.

In the generalized LAN structure a set of subscriber nodes, or systems (their number can be from tens to hundreds), servers and a communication subnetwork (CP).

Main network components are cables (transmission media), workstations (workstation of network users), network interface cards (network adapters), network servers.

Workstations (PC) In a LAN, as a rule, personal computers (PCs) are used. On PCs, network users implement applied tasks, the implementation of which is associated with the concept of a computing process.

Network servers - These are hardware and software systems that perform the functions of managing the distribution of public network resources, which can also operate as a regular subscriber system. The server hardware used is a fairly powerful PC, a minicomputer, a mainframe computer, or a computer designed specifically as a server. There may be several various servers for management network resources, however, there is always one (or more) file server (database server) for managing external storage public access and organization of distributed databases (RDB).

Workstations and servers are connected to the communication subnet cable using interface cards - network adapters (NA). The main functions of the SA: organizing the reception (transmission) of data from (to) a PC, coordinating the speed of reception (transmission) of information (buffering), forming a data packet, parallel-serial conversion (conversion), encoding (decoding) data, checking the correctness of transmission, establishing connection with the required network subscriber, organizing the actual data exchange. In some cases, the list of CA functions increases significantly, and then they are built on the basis of microprocessors and built-in modems.

In a LAN, twisted pair cables are used as cable transmission media. coaxial cable and fiber optic cable.

In addition to the above, the LAN uses the following: network equipment:

transceivers (transceivers) and repeaters (repeaters) - for combining local network segments with a bus topology;

hubs (hubs) - to form a network of arbitrary topology (active and passive hubs are used);

bridges - For combining local networks into a single whole and increasing the performance of this whole by regulating traffic (user data) between individual subnets;

routers and switches - to implement switching and routing functions when managing schedules in segmented (consisting of interconnected segments) networks. Unlike bridges, which provide network segmentation at the physical layer, routers perform a number of "intelligent" functions in managing the schedule. Switches, performing almost the same functions as routers, surpass them in performance and have lower latency (hardware time delay between receiving and sending information);

modems (modulators - demodulators) - for matching digital signals generated by a computer with analog signals of a typical modern telephone line;

analyzers - to control the quality of network functioning;

network testers - to check cables and find faults in the installed cable system.

Main characteristics of LAN:

Territorial extent of the network (length of the common communication channel);

Maximum data transfer rate;

Maximum number AC online;

The maximum possible distance between workstations on the network;

Network topology;

Type of physical data transmission medium;

Maximum number of data transmission channels;

Type of signal transmission (synchronous or asynchronous);

Subscriber access method to the network;

Network software structure;

Capable of transmitting voice and video signals;

Conditions for reliable network operation;

Possibility of LAN communication with each other and with a higher-level network;

The ability to use the priority setting procedure while simultaneously connecting subscribers to a common channel.

To the most typical LAN application areas include the following.

Word processing - one of the most common functions of information processing tools used in a LAN. The transmission and processing of information in a network deployed at an enterprise (organization, university, etc.) ensures a real transition to “paperless” technology, completely or partially displacing typewriters.

Organization of own information systems, containing automated databases - individual and general, concentrated and distributed. Every organization or company can have such databases.

Exchange of information between AS networks is an important means of reducing paperwork to a minimum. Data transmission and communication occupy a special place among network applications, as this is the main condition for the normal functioning of modern organizations.

Ensuring distributed data processing , associated with the integration of workstations of all specialists of a given organization into a network. Despite significant differences in the nature and volume of calculations carried out on automated workstations by specialists of various profiles, the information used within one organization, as a rule, is located in a single (integrated) database. Therefore, combining such workstations into a network is an expedient and very effective solution.

Management decision support, providing managers and management personnel of the organization with reliable and timely information necessary to assess the situation and make the right decisions.

Organizing email - one of the types of LAN services that allows managers and all employees of an enterprise to quickly receive all kinds of information necessary in its production, economic, commercial and trading activities.

Sharing of expensive resources - a necessary condition for reducing the cost of work performed in order to implement the above LAN applications. We are talking about such resources as high-speed printing devices, high-capacity storage devices, powerful information processing tools, application software systems, databases, knowledge bases. It is obvious that it is impractical (due to the low utilization rate and high cost) to have such means in each subscriber system of the network. It is enough if these tools are available in one or more copies on the network, but access to them is provided for all AS.

Depending on the nature of the organization’s activities, in which one or more local networks are deployed, these functions are implemented in a certain combination. In addition, other functions specific to the organization may be performed.

LAN types. To divide LANs into groups, certain classification criteria are used.

By purpose LANs are divided into information (information retrieval), control (technological, administrative, organizational and other processes), settlement, information and settlement, processing of documentary information, etc.

By type used on the networkcomputer they can be divided into heterogeneous, where different classes (micro-, mini-, large) and models (within classes) of computers are used, as well as various subscriber equipment, and homogeneous, containing the same computer models and the same type of subscriber equipment.

By organization of management homogeneous LANs are differentiated into networks with centralized and decentralized control.

In networks with centralized control, one or more machines (central systems or authorities) are allocated to control the operation of the network. The disks of dedicated machines, called file servers or database servers, are accessible to all other computers (workstations) on the network. Servers run a network OS, usually multitasking. Workstations have access to server disks and shared printers, but generally cannot work directly with the disks of other PCs. Servers can be dedicated, and then they perform only network management tasks and are not used as a PC, or non-dedicated, when user programs are executed in parallel with the network management task (this reduces server performance and the reliability of the entire network due to a possible error in the user program , which may cause the network to stop working). Such networks are distinguished by the simplicity of providing interaction functions between LAN AS, but their use is advisable when the number of AS in the network is relatively small. In networks with centralized control, most of the information and computing resources are concentrated in the central system. They are also distinguished by a more reliable information security system.

If the information and computing resources of a LAN are evenly distributed over a large number of AS, centralized management is ineffective due to a sharp increase in service (control) information. In this case, networks with decentralized (distributed) control, or peer-to-peer networks, are effective. In such networks there are no dedicated servers; network management functions are transferred in turn from one PC to another. Workstations have access to the disks and printers of other PCs. This makes it easier for groups of users to work together, but the network performance is slightly reduced. Disadvantages of peer-to-peer networks: dependence of the efficiency of the network on the number of ASs, complexity of network management, difficulty in ensuring the protection of information from unauthorized access.

By data transfer rate in the common channel there are:

LANs with low bandwidth (units of megabits per second), in which twisted pair or coaxial cable is usually used as the physical transmission medium;

LAN with average bandwidth (tens of megabits per second), which also uses coaxial cable or twisted pair;

LAN with high throughput (hundreds of megabits per second), which uses fiber optic cables (light guides). By topology, those. configurations of elements in a LAN network are divided into: common bus, ring, star, etc. By topology , i.e. configurations of elements in fuel assemblies, networks can be divided into two classes: broadcast (Fig. 1) and serial (Fig. 2). Broadcast configurations and a significant part of sequential configurations (ring, star with an “intellectual center”, hierarchical) are characteristic of LANs. For global and regional networks, the most common is a random (mesh) topology. The hierarchical configuration and the star have also found use.

Rice. 1. Broadcast network configurations: a - common bus;

b - tree; c - star with a passive center



Rice. 2. Consecutive network configurations: a - arbitrary (mesh), b - hierarchical; c - ring, d - chain; d - star with an “intellectual” center

Virtual LANs

Virtual local area network (VLAN) is a logically united group of LAN users, as opposed to a physical association based on territoriality and network topology. Such networks completely eliminate physical barriers to the formation of working groups “based on interests” on a higher-level network scale, but this is especially true on the scale of a corporate computer network (CAN), since it is possible to unite physically dispersed company employees into user groups while maintaining the integrity of communications within their groups. This ensures high organizational flexibility in company management. VLAN technology allows network administrators to group different VLAN users who share the same network resources. Dividing the network into logical segments, each of which represents a VLAN, provides significant advantages in network administration, ensuring information security, and managing broadcast transmissions from virtual network via the corporate network backbone.

To organize and ensure the functioning of the VLAN, the following main components are used:

High-performance switches designed for logical segmentation of end stations connected to them;

Routers running on network level VOS models and providing increased virtual interaction between work groups and increased compatibility with established LANs;

Transport protocols that regulate the transmission of VLAN traffic through the backbones of shared LAN and ATM networks;

Network management solutions that offer centralized management, configuration and scheduling functions.

These components allow you to combine users into virtual networks based on ports, addresses or protocols.

Port-based VLAN is the simplest way to group network devices. With this organization of a virtual network, all remote devices assigned to certain ports of a high-performance network switch are combined into one VLAN, regardless of their addresses, protocols, and applications.

An address-based virtual network can support multiple workgroups of users on a single switch port. The corresponding devices of these workgroups are combined into subnets based on their addresses.

In a protocol-based virtual network, network devices based on IP, IPX, etc. protocols are combined into various logical groups. These devices usually operate at the network level and are called routers. If they are able to combine work with several protocols, then this multiprotocol routers.