So lvs. Local Area Networks

A local area network (LAN, LAN - Local Area Network) is a combination of hardware and software that allows you to combine computers into a single distributed system for processing and storing information. All services and additional devices are also important, but they will not work in the absence of a properly designed and mounted local network. To hardware include computers with network adapters installed on them, repeaters, hubs, switches, bridges, routers, etc., interconnected by network cables. Software includes network Operating Systems and communication protocols. The distance between computers connected in a LAN usually does not exceed several kilometers, which is due to the attenuation of the electrical signal in the cables. The technology of virtual private networks (VPN - Virtual Private Network) allows through the Internet or lines telephone communication combine several LANs separated by thousands of kilometers into a single LAN.

Main features of local (computer) networks:

• File transfer. First, it saves paper and printer ink. Secondly, the electrical signal along the cable from department to department moves much faster than any employee with a document.
• Sharing data files and programs. Now there is no need to duplicate data on each computer. If the accounting data is needed by the management and the planning and economic department at the same time, there is no need to take time and nerves from the accountant, distracting him from costing every three seconds. The network allows users to work with the program at the same time and see the data contributed by each other.
• Sharing printers and other equipment. Funds for the purchase and repair of equipment are significantly saved, because. there is no need to install a printer on each computer, it is enough to install a network printer.
• E-mail and exchange systems instant messaging. In addition to saving paper and prompt delivery, problems such as "Was there, but just left. Come in (wait) in half an hour", "I was not handed over" are excluded. Whenever a busy comrade returns, a letter will be waiting for him.
• Collaboration coordination. When solving problems together, everyone can stay in the workplace, but work "in a team". For the project manager, the task of controlling and coordinating actions is greatly simplified, because. the network creates a single, easily observable virtual space with a high speed of interaction between geographically dispersed participants.
• Organizing paperwork, controlling access to information, protecting information: The fewer potential opportunities to lose (forget, put in the wrong folder) a document, the fewer such cases will be. In any case, it's much easier to find the document on the server ( automatic search, the authorship of the document is always known) than in a pile of papers on the table. The network also allows you to implement a unified security policy at the enterprise, relying less on the consciousness of employees: you can always clearly define access rights to documents and record all actions of employees.

Recently, the so-called. wireless networks based on the transmission of information over secure radio channels. This kind of equipment is used where it is not possible to lay a cable, to connect separate buildings, to connect from mobile and pocket computers, etc. Mixed systems (simultaneous use of cable and wireless technologies in a LAN) is the most promising option for building local area networks of an enterprise.

The very name Local Area Network already contains the purpose, functions and limitations of the system. Let's break down the name into parts. Local, derived from English local - local, that is, the network is tied to a specific geographical location and has restrictions on the territory, computing, associated with the composition of the network (computing equipment, software) and its purpose, net- implies the integration of computing equipment and software in a certain area (local) into a network (by means of cables).

Thus, we can formulate the definition of a Local Area Network (LAN) - a system of interconnected computing resources (computers, servers, routers, software, etc.) and processing of information of various kinds.

Different local area networks can function separately or be interconnected using communication tools, for example, at enterprises with a branch network in different cities. Through this connection, the user can interact with other workstations connected to this local area network. Exist local networks, whose nodes are geographically separated at distances of more than 12,500 km (space stations and orbital centers), but they are still classified as local.

The purpose of the LAN is to provide joint and simultaneous access of a certain group of persons to data, programs and equipment (computers, printers, plotters, storage and processing devices for files and databases) and data transmission (electronic graphics, word processing, e-mail, access to remote databases). data, digital speech transmission).

For example: a manager accepts an order and enters it into a computer, then the order goes to the accounting department and an invoice is generated there, at the same time information can be sent to the legal service to create an agreement.

LAN characteristics:

• High-speed channels (1-400 Mbps), owned mainly by one user;
• The distance between workstations connected to the local network is usually from several hundred to several thousand meters;
• Data transfer between computer user stations;
• Decentralization of terminal equipment, which is used as microprocessors, displays, cash registers, etc.
• Data transmission to subscribers connected to the network via a common cable;

The main functions of the LAN are:

• Ensuring simultaneous access to equipment, software and information integrated into the network;
• Minimizing the risk of unauthorized access to information and network resources;
• Differentiation of access to information and network resources;
• Ensuring fast and confidential exchange and simultaneous work with information for a certain circle of people;
• Control over information flows, including incoming and outgoing;
• Separation of control functions and responsible persons at each node (each node is responsible System Administrator performing maintenance and, as a rule, control functions);
• Optimization of software and hardware costs due to their collective use (for example, one printer for several departments, etc.)

As a result of the use of a LAN, personal computers located at many remote workplaces are combined. Workplaces of employees are no longer isolated and are combined into single system which has its own special Benefits:

• Possibility remote access to equipment, software and information;
• Optimization of processor resources;
• Less number and intensity of errors compared to a network based on telephone channels;
• throughput is higher than global network;
• Possibility of reconfiguration and development by connecting new terminals

Application area local area networks is very wide, at present there are such systems in almost every office (for example, one printer is installed on several computers, or several computers use the same software, for example, 1C: Accounting, etc.). Every day, information flows become larger, the software used is more complex and functional, the geography of organizations' activities is expanding. The use of LAN tools is becoming not only desirable, but necessary for the successful operation and development of business, science, education of students, schoolchildren, training and retraining of specialists, implementation government programs and functions, etc.

The structure of the network functioning.

The structure of the local network is determined by the principle of management and the type of communication, often it is based on the structure of the service organization. Types of topology are used: bus, ring, radial, tree. The first two types are the most common, effective use communication channels, ease of management, flexible expansion and change.

Bus topology- all computers are connected in a chain by connecting to the main cable segment (trunk), "terminators" are placed at its ends to dampen the signal propagating in both directions. The computers on the network are connected by a coaxial cable with a tee connector. The network bandwidth is 10 Mbps, which is not enough for modern applications that actively use video and multimedia data. The advantage of this topology lies in the low cost of wiring and the unification of connections.

The bus topology is passive. The failure of a single computer does not affect network performance. Damage to the main cable (bus) leads to signal reflection and the entire network as a whole becomes inoperable. Turning off and especially connecting to such a network requires a bus break, which causes a disruption in the circulating flow of information and a system hang.

Tree topology– a more advanced “tire” type configuration. Several simple buses are connected to the common trunk bus through active repeaters or passive multipliers.

Star topology(star) - is the fastest of all topologies, information between peripheral workstations passes through the central node of the computer network. The central control node - the file server can implement the optimal protection mechanism against unauthorized access to information. The entire computer network can be controlled from its center.

The cable connection is quite simple, since each workstation is connected only to the central node. Cabling costs are high, especially when the central site is not geographically located in the center of the topology. When expanding computer networks, previously made cable connections cannot be used: a separate cable must be laid from the center of the network to a new workplace.

In the case of a serial LAN configuration, each device connecting to the physical medium transmits information to only one device. This reduces the requirements for transmitters and receivers, since all stations actively participate in the transmission.

Topology "ring"(ring) - computers are connected by segments of a ring-shaped cable, fundamentally identical to a bus cable, except for the need to use "terminators". If one of the network segments fails, the entire network fails.

Signals are only transmitted in one direction. Each station is directly connected to two neighbors, but listens to the transmission of any station. The ring consists of several transceivers and the physical medium connecting them. All stations may have equal access rights to the physical medium. In this case, one of the stations can act as an active monitor serving the exchange of information. Laying cables from one workstation to another can be quite complex and expensive, especially if the workstations are geographically located far from the ring (for example, in a line).

The main problem with a ring topology is that each workstation must actively participate in the transfer of information, and if at least one of them fails, the entire network is paralyzed. Faults in cable connections are easily localized. Connecting a new workstation requires turning off the network, since the ring must be open during installation. There is no limit on the extent of the computer network, since it is ultimately determined solely by the distance between two workstations.

Computers can be interconnected using various access media: copper conductors (twisted pair), optical conductors (optical cables) and via a radio channel (wireless technologies). Wired, optical connections are established via Ethernet, wireless - via Wi-Fi, Bluetooth, GPRS and other means. Most often, local networks are built on Ethernet or Wi-Fi technologies. It should be noted that the Frame Relay, Token ring protocols were previously used, which are becoming less common today, they can only be seen in specialized laboratories, educational institutions and services.

Components of building a simple local network are used:

• Adapter (network adapter) - a device that connects a computer (terminal) with a network segment;
• Bridge (bridge) - a device that connects local or remote network segments;
• Router (router) - a device for limiting broadcast traffic by dividing the network into segments, ensuring information protection, managing and organizing backup paths between broadcast areas;
• The switch is a narrow-purpose device that effectively segments the network, reduces collision areas and increases the throughput of each end station.
• Blocks uninterruptible power supply- Devices to ensure the system's operability in cases of disconnection of the main power source.

Installation of Local Area Network (LAN)

The choice of the type of topology, access environment and composition of the Local Computing System depends on the requirements and needs of the Customer. Modern technologies allow us to develop an individual version that meets all the requirements and tasks.

Laying LAN cables, as well as other types of cable networks, can be carried out different ways. When choosing an installation method, they are guided by individual architectural and design features buildings, his technical specifications, presence operating networks and other equipment, the order of interaction of low-current systems with other systems. Basically, two methods can be distinguished - open and hidden. For concealed wiring of LAN cables, the construction of walls, floors, ceilings is used; Better conditions for later service. Unfortunately the opportunity to do the work in a hidden way happens rarely, more often you have to work open way using plastic boxes, vertical columns and trays. Do not forget that there is another way of laying cables over the air, most often it is used for building communication when it is not possible to lay the cable into channels or if it is too expensive.

LAN installation is a complex and responsible job. , the stability and correctness of the functioning of the system as a whole, the degree of execution of the tasks assigned to it, the speed of data transmission and processing, the number of errors, and other factors depend on the quality of its implementation. This should be taken very thoroughly and seriously, since any network is the basis (skeleton and circulatory system) of the whole organism from low-current systems responsible for a large number of functions (from Email to the safety of the object). Each subsequent intervention in the operation of the existing system (expansion, repair, etc.) requires time and money, and their number directly depends on the parameters originally included in the system, the quality of the work performed, the qualifications of developers and performers. Cost savings at the stage of designing and installing a LAN can turn into much larger expenses at the stage of operation and upgrade

Local Area Networks LAN are a joint connection of network nodes to a single data transmission channel using communication tools, i.e. cable systems (or wireless technology wifi). The main components of the LAN are SCS, server systems, active network equipment, data storage and backup systems, and software.

Structured cabling system SCS is a hierarchical cabling system of a building or group of buildings, divided into structural subsystems. It consists of patch panels, copper and optical cables, connecting cords, cable connectors, information outlets and additional equipment. All elements are integrated into a single system. SCS is created in accordance with the principle of open architecture and on the basis of relevant standards and implies the introduction of additional jobs into the cable system. The number of workplaces is determined by the areas and topology of the working premises

SCS, or a structured cabling system, is a complete set of wires and switching devices that allow you to bring together information services for various purposes. For a better understanding, a simple example can be given. Often, different equipment in the same building is combined into separate cable systems.

Computers and faxes are connected by one cable, telephones by another, alarms by a third. Each of the systems has its own sockets and wires, which are serviced by separate specialized teams.

Such an arrangement of offices causes some inconvenience, especially in case of breakdowns, when the necessary team of craftsmen is not on site. In the case of equipping a SCS building, such problems do not arise, since all the wires and sockets in the building are of the same type, that is, it does not matter at all which socket to plug the phone into, and which computer or video camera.

In other words, SCS is a universal cable network that ensures the sharing of all equipment.
The main purpose of SCS is the creation of a flexible information infrastructure that does not depend on the final media and covers the entire enterprise, connecting all points of data transmission with each other. As a rule, SCS has a single switching center, to which all backbone subsystems converge from different floors.

The structured system includes cables, receptacles, patch cords and panels to help securely manage a building's entire cabling system and provide flexibility and ease of operation for all equipment.

LAN

LAN, or local area network, is one of the elements of the SCS and connects all the computer systems of the office. in plain language, it is a group personal computers and peripheral equipment that allows solving information problems of the enterprise and exchanging data.

Its installation allows office employees to transfer to each other electronic documents, graphs, tables and other necessary information without the use of removable media.

Typically, a local area network covers a small area (office, residential building, educational institution), although it is sometimes set globally. For example, orbital centers and space stations are also LANs.

Computers can be interconnected in various ways, but most often local networks are built on Wi-Fi technologies or Ethernet. Previously, other protocols were widely used in enterprises, but now they are becoming less common.

The installation of a LAN is primarily due to the need sharing resources within the same office. Resources include not only computers, but also modems, printers, scanners, hard drives and any other devices connected to the PC.

When installing a local network, employees get the opportunity to interactively connect with each other to send and receive messages by e-mail, access centrally installed programs, as well as to abandon separate storage media at each workplace.

The difference between SCS and LAN

The main difference between SCS and a local area network (LAN) is the independence of SCS from applications used later, which allows it to regulate a wide range of connected systems. Based on this, the LAN in modern conditions is organized on the basis of SCS and is included in it as a separate computer network that combines phones, computers, printers and other peripheral equipment.

Local area cable networks are capable of performing the following tasks:

Providing simultaneous access to databases, the Internet and various programs to all employees of the organization;

Providing access to certain information to a specific department of the organization. This opportunity is due to the complex structure of large companies, where access to confidential data is allowed only for the relevant people and departments;

The ability to work remotely without being at the workplace.
Proper LAN setup ensures reliable, efficient and productive network operation for a long time. This important event should be carried out by qualified specialists, because the quality of the setup depends on productive work all employees of the organization.

Structured cabling systems are an integral part of the life support of any enterprise. Not a single organization can do without them, since the main connection between employees and the external environment is carried out through e-mail, jabbers and social networks.

On the basis of modern SCS, it is possible to create such a telecommunications infrastructure through which you can exchange not only data, but also audio and video files.

The SCS includes the following elements:

Internal computer and telephone networks;

Technological devices;

Video communication and video conferencing systems;

Security and industrial video surveillance systems.

When designing the SCS, it is necessary to provide for a reserve of connections in order to ensure its uninterrupted operation even in the event of emergency and emergency situations.
LAN refers to the category of computer networks that unite computers located in a limited space.

Installation of the cable system is carried out in full compliance with accepted standards in this area (international and national). Storey and central switching centers are being created. Floor cable network ties in common system workstations, unites points of information resources (telephones, video surveillance, computer network, etc.), allows you to administer, make changes to existing configurations. Workplaces are connected to the switching node of the floor using horizontal cable lines. With the help of vertical cable lines, all storey switching nodes are connected to the switching center of the building. It also connects to external highways of global information resources (Internet, telephony, etc.). In the event that it is required to create an SCS for a group of buildings, a backbone subsystem is built that combines all the cable systems of buildings. The material for the construction of highways can be: copper cable, optical fiber, or combinations thereof.

The installation of optical fiber is based on the welding of optical fibers of the cable. This is a technology for permanent splicing of optical fibers, based on the melting of their ends with an electric arc, followed by convergence and fusion. Provides best performance connections for insertion loss and stability, but requires the use of high-tech and expensive equipment.

We are engaged in the installation and maintenance of cable systems at a professional level, as well as the supply of equipment. This includes:

• design of SCS and LAN;
• approval of the technical project by the customer;
• delivery of equipment and materials to the installation site;
• installation and debugging of SCS and LAN;
• technical advice;
• regular technical maintenance of cable systems.

Large volumes confidential information, transmitted and stored in a single LAN (SCS) of the enterprise, require high professionalism in their installation and maintenance.

In today's world, local area networks have become more than just necessary - they are actually necessary to achieve a good level of productivity. However, before you start using such a network, you should create and configure it. Both of these processes are quite difficult and require maximum concentration, especially the first of them. A poorly designed and configured LAN will not work at all, or it will function completely differently than it should, so creating a local network should be the focus of the person doing it.

What is a local network

As a rule, the creation of such communication systems is caused by the need to share data by users who work on remote computers. LAN not only allows for near-instantaneous exchange of information and simultaneous work with files, but also allows you to remotely use network printers and other devices.

A local network is a complete set of software and hardware resources aimed at creating a single information space. In fact, this is a number of computers located at a distance from each other and connected by a communication line - a cable. The main difference between a LAN and other types of networks is short distance where the workstations are located.

Pre-project preparation and design

Before you create a local network, you must first design it, that is, plan the process of its creation. This stage is one of the most significant, since the LAN includes a huge number of components and nodes.

Initially, the terms of reference are drawn up on the basis of primary data, defining several points:

• Functions and tasks of the LAN.
• Selected topology.
• List of available equipment.

Once you have these points in mind, you can start designing. The project itself must contain LAN schemes, placement points network equipment, a list of required software and hardware.

A local area network is a complex mechanism, but if it is designed correctly and the equipment is selected in accordance with the requirements, then the likelihood of problems in the operation of the communication mechanism becomes minimal.

Required Hardware

There is a list of equipment without which no LAN can function. It includes:

• Data lines. The most commonly used coaxial cable and optical fiber. In this case, the length of the coaxial cannot exceed several hundred meters, however, if it is necessary to extend the network over long distances, special repeaters are used - signal repeaters that do not allow it to fade.
• Communication equipment: network cards (devices that perform duplex exchange of information between a computer and a data transmission medium), hubs (break the network into separate segments, structuring the network physically), routers (take on the choice of the packet transmission route), switches (logically divide the LAN into segments, combining several physical circuits), repeaters (provide signal recovery, allowing you to increase the length of the transmission medium), transceivers (amplify the signal and convert it to other forms, allowing you to use different data transmission media).

List of software

No LAN is complete without software. Required LAN programs include:

• Operating systems of work nodes. The most commonly used operating system remains Windows 7, although Windows XP is also not losing ground.
• Network operating systems installed on servers are the basis of the LAN, since it is impossible to set up a local network without them. Exactly these software take control of all data flows between the main and secondary nodes, providing the possibility of collective access to network resources. As a rule, the following Microsoft operating systems are used: Windows Server 2003 or 2008.

• Network services and applications that enable users to access deleted files, print documents on a network printer, view networked work nodes, and send electronic messages. The implementation of such services is carried out using software.

Creation and installation of a LAN

Installation and commissioning work takes the most time, since it is necessary to create a local network in several stages:

• Before starting the installation of communication lines and switching devices, you must first prepare the room.
• Next, you can lay the cable, as well as install the necessary equipment.
• Devices of the server and workstations should be connected to the cable communication line.
• After that, the software is installed and configured.

Installation of cable and equipment has a number of features, therefore, if there are difficulties with how to connect a local network, better solution entrust this issue to specialists.

Joining two computers in a LAN

In some cases, it may be necessary to combine two computers into one network, for example, to create a common information space. This is not very difficult to do if you follow a certain algorithm of actions:

• Install if necessary network adapters to both computers, not forgetting the drivers.

• Purchase a crimped network cable for connection. If you have the necessary knowledge and skills, crimping can be done on your own - the local network of two computers will not become worse quality from this.
• Connect both workstations with a communication line.
• Set up the LAN in a specific order.

Algorithm for setting up a local network between two computers for Windows 7

• Select the "Start" menu, then, by right-clicking on the "Computer" icon, enter the "Properties" submenu.
• You need to find in the list "Computer name and domain", and then select the item with the change in settings.
• The working name of the computer must be changed by clicking on the appropriate icons.
• The group name should remain unchanged - "Workgroup", but the computer names are changed to "pc1" and "pc2" for the first and second subscriber, respectively.
• You can now click OK and restart your computer.

In most cases, you may want to give each host a unique IP address:

• From the Start menu, select Settings and then Network Connections.
• Right-click to open the "Properties" submenu next to the "Local Area Connection" icon.
• In the "General" tab, select "Properties" of the item "Internet Protocol".
• Make the line "Use the following IP address" active and enter the value 192.168.0.100. After that, save the changes made.

Local network and internet

Work nodes connected in a LAN can be connected to the Internet. A local network, to which the Internet can be connected in two ways, will work at a speed divided in two.

The first way to connect is to use a router, which is assigned an identifying IP address. And in the second case, you can use a wireless connection.

In this case, the local network is the interaction of two computers, master and slave, so the IP address is written in the gateway of the main one, previously connected to the worldwide network.

If the LAN is based on the use of a server, each workstation must have an individual IP address, and a proxy server is specified in the browser settings through which the Internet is accessed.

Wireless LAN

A wireless local area network is a type of LAN that uses high-frequency radio waves to transmit information. WLAN is an excellent alternative to the conventional cable communication system, having a number of advantages:

• Improving labor productivity. WLAN makes it possible to use the Internet without being tied to one room. You can freely change your location without losing your internet connection.
• Easy installation and configuration, financial savings and reliability - all these factors are due to the absence of a cable communication line.
• Flexibility. Installing a wireless network is real where there is no way to stretch the cable.
• Possibility of expansion. Network scalability is greatly simplified with wireless network adapters that can be installed on any work node.

WLAN has a certain range, which depends on the characteristics network devices and noise immunity of the building. As a rule, the range of radio waves reaches 160 m.

Necessary equipment for creating a wireless LAN

An access point is used to connect other workstations to the network. This device is equipped with a special antenna that controls duplex data transmission (sending and transmitting) using radio signals. Such a point can transmit a signal at a distance of up to 100 m indoors and up to 50 km in an open area.

Access points significantly expand computing power the entire communication system, allowing users to freely move between each of them without losing their connection to the LAN or the Internet. In fact, these radio points act as hubs, providing a connection to the network.

Using access points allows you to scale up your entire wireless LAN by simply adding new devices. The number of subscribers that one radio point can withstand generally depends on the network load, since the traffic is divided equally between each of the users.

Wireless LAN: Windows 7 Setup Flow

First prepare the ADSL modem with WiFi technology, as well as client points with connected to them wireless adapters. After that, you can start building a wireless LAN:

• Connect the modem to the electrical network.
• Run the WLAN setup wizard on the client device.
• Select the SSID from the list of found wireless networks.

Access point setup:

• The first step is to configure the TCP/IP protocol properties by specifying the IP address and subnet mask.
• Then enter the value DNS servers, since it is not possible to fully configure the local network without this parameter. In most cases, it is enough to make the automatic assignment of the DNS address active.
• It is also mandatory to configure the parameters of the wireless network itself, in which security is important.
• At this stage, you need to configure the Internet connection and filtering for the Windows 7 firewall.
• And lastly, the wires are connected and the WLAN network is tested.

To create an optimal information space, you can combine types of networks - cable and wireless, allowing you to use the advantages of each of them for the benefit of the enterprise. However, it is important to remember that in our time, it is WLANs that are increasingly being used, which have all the advantages of cable networks and are devoid of their disadvantages.

After completing the creation and configuration of the local network, it is important to provide for its administration and the possibility of maintenance. Even if the LAN is installed perfectly, during its operation various hardware or software malfunctions are almost inevitable, which is why maintenance should be regular.

Functional groups of devices in the network

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 viewed as a collection of servers and workstations.

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

Servers can store data, manage databases, remote job processing, job printing, and a number of other functions that network users may need. The server is the source of network resources.

Work station- a personal computer connected to the network, through which the user gets access to its resources.

The network workstation operates in both network and local modes. It is equipped with its own operating system (MS DOS, Windows, etc.) and provides the user with all the necessary tools for solving applied problems.

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 high capacity computer. random access memory, high-capacity hard drives, and optional tape drives (tape drives).

It runs under the control of a special operating system, which provides simultaneous access of 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.

Managing the interaction of devices in the network

Information systems built on the basis of computer networks provide the solution of the following tasks: data storage, data processing, organization of user access to data, transfer of data and results of data processing to users.

In centralized processing systems, these functions were performed by the central computer (Mainframe, Host).

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

Client- task, workstation or computer network user.

During data processing, the client can form a request to the server to perform complex procedures, read a file, search for information in the database, etc.

The server defined earlier executes the request from the client. The results of the request are sent to the client. The server provides storage of public data, organizes access to this data and transmits customer data,

The client processes the received data and presents the processing results in a user-friendly way. In principle, data processing can also be performed on the server. For such systems, the terms are accepted - systems client-server or client-server architecture.

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

peer-to-peer network. In such a network there is no single control center for the interaction of workstations and there is no single device for data storage. The network operating system is distributed across all workstations. Each network station can act as both a client and a server. It can serve requests from other workstations and forward its service requests to the network.

The network user has access to all devices connected to other stations (disks, printers).

Advantages of peer-to-peer networks: low cost and high reliability.

Disadvantages of peer-to-peer networks:

• dependence of network efficiency on the number of stations;

• complexity of network management;

• the complexity of ensuring the protection of information;

• difficulties in updating and changing station software.

Peer-to-peer networks based on LANtastic and NetWare Lite network operating systems are the most popular.

Dedicated server network. 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, all shared external devices- hard drives, printers and modems.

Interaction between workstations in the network, as a rule, is carried out through the server. The logical organization of such a network can be represented by a star topology. The role of the central device is performed by the server. In networks with centralized control, it is possible to exchange information between workstations, bypassing the file server. To do this, you can use the NetLink program. After running the program on two workstations, you can transfer files from the disk of one station to the disk of another (similar to the operation of copying files from one directory to another using Norton Commander).

Advantages of a network with a dedicated server:

• reliable information security system;

• high speed;

• 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 for the server;

• dependence of network speed and reliability on the server;

• less flexibility compared to a peer-to-peer network.

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

TYPICAL TOPOLOGIES AND LAN ACCESS METHODS

Physical transmission medium LAN

The physical environment ensures the transfer of information between subscribers of the computer network. As already mentioned, the physical transmission medium of a LAN is represented by three types of cables: twisted pair wires, coaxial cable, fiber optic cable.

A twisted pair consists of two insulated wires twisted together (Fig. 6.19). Twisting wires reduces the influence of external electromagnetic fields on transmitted signals. The simplest version of twisted pair is a telephone cable, twisted pairs have various characteristics, determined by dimensions, insulation and twisting pitch. The cheapness of this type of transmission medium makes it quite popular for LANs.

Rice. 6.19. twisted pair wires

The main disadvantage of twisted pair is poor noise immunity and low speed information transfer - 0.25 - 1 Mbps. Technological advances can increase transmission speed and noise immunity (shielded twisted pair), but at the same time increase the cost of this type of transmission medium.

A coaxial cable (Fig. 6.20), compared to twisted pair, has a higher mechanical strength, noise immunity and provides information transfer rates up to 10 - 50 Mbit / s. Two types of coaxial cables are produced for industrial use: thick and thin. A thick cable is more durable and transmits signals of the desired amplitude over a greater distance than a thin one. At the same time, thin cable is much cheaper. Coaxial cable, like twisted pair, is one of the most popular types of LAN transmission medium.

Rice. 6.20. Coaxial cable

Rice. 6.21. Fiber optic cable

Fiber optic cable is an ideal transmission medium (Fig. 6.21). It is not affected by electromagnetic fields and has practically no radiation itself. The latter property allows it to be used in networks that require increased secrecy of information.

The speed of information transmission over a fiber optic cable is more than 50 Mbps. Compared to the previous types of transmission medium, it is more expensive and less technologically advanced in operation.

LANs produced by various companies are either designed for one of the types of transmission media, or can be implemented in various versions based on various transmission media.

Basic LAN topologies

The computers that make up the LAN can be located in the most random way in the territory where the computer network is being created. It should be noted that for the method of accessing the transmission medium and network management methods, it is not indifferent how the subscriber computers are located. Therefore, it makes sense to talk about LAN topology.

LAN topology is an averaged geometric scheme of connections of network nodes.

The topologies of computer networks can be very different, but only three are typical for local area networks: ring, bus, star.

Sometimes, for simplicity, the terms are used - ring, tire and star. It should not be thought that the types of topologies under consideration represent an ideal ring, an ideal line, or a star.

Any computer network can be viewed as a collection of nodes.

Knot- any device directly connected to the transmission medium of the network.

Topology averages the connection scheme of network nodes. So, both the ellipse, and the closed curve, and the closed polyline belong to the ring topology, and the open polyline to the bus.

Ring the topology provides for the connection of network nodes with a closed curve - a transmission medium cable (Fig. 6.22). The output of one network node is connected to the input of another. Information is passed around the ring from node to node. Each intermediate node between transmitter and receiver relays the sent message. The receiving node recognizes and receives only messages addressed to it.

Rice. 6.22. Ring topology network

The ring topology is ideal for networks that occupy a relatively small space. It does not have a central node, which increases the reliability of the network. Information relaying allows using any types of cables as a transmission medium.

The consistent discipline of servicing the nodes of such a network reduces its performance, and the failure of one of the nodes violates the integrity of the ring and requires the adoption of special measures to preserve the information transmission path.

Tire topology is one of the simplest (Figure 6.23). It is associated with the use of a coaxial cable as a transmission medium. Data from the transmitting network node is distributed over the bus in both directions. Intermediate nodes do not translate incoming messages. Information arrives at all nodes, but only the one to which it is addressed receives the message. The service discipline is parallel.

Rice. 6.23. Bus topology network

This provides a high performance LAN with a bus topology. The network is easy to expand and configure, and adapt to different systems The bus topology network is resistant to possible malfunctions individual nodes.

Bus topology networks are the most common at present. It should be noted that they are short and do not allow the use of different types of cable within the same network.

star-shaped the topology (Figure 6.24) is based on the concept of a central node to which peripheral nodes are connected. Each peripheral node has its own separate communication line with the central node. All information is transmitted through the central node, which relays, switches and routes information flows in the network.

Rice. 6.24. Star topology network

The star topology greatly simplifies the interaction of LAN nodes with each other and allows the use of simpler network adapters. At the same time, the performance of a LAN with a star topology is entirely dependent on the central node.

In real computing networks, more complex topologies can be used, which in some cases represent combinations of the considered ones.

The choice of a particular topology is determined by the scope of the LAN, the geographical location of its nodes and the dimension of the network as a whole.

Media Access Methods

The transmission medium is common 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, non-deterministic.

With deterministic access methods, the transmission medium is distributed among the nodes using a special control mechanism that guarantees the transmission of node data for a certain, rather small time interval.

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

The transfer of rights method is used in networks with a 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, in which network subscribers can place their information packets.

The token circulates around the ring, and any node that has data to send puts it in the free token, sets the token's busy flag, and sends it around the ring. The node to which the message was addressed receives it, sets the flag for acknowledging the receipt of information, and sends the token to the ring.

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

Non-deterministic - random access methods provide for the competition of all network nodes for the right to transfer. It is possible for multiple nodes to transmit at the same time, resulting in collisions.

The most common non-deterministic access method is Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In essence, this is the previously described rivalry mode. Carrier frequency control consists in the fact that a node wishing to transmit a message "listens" on the transmission medium, waiting for it to be released. If the medium is free, the node starts transmission.

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

LAN assignment

Over the past five years, local area networks have become widespread in various fields of science, technology and production.

LANs are especially widely used in the development of collective projects, for example, complex software systems. Based on the LAN, you can create computer-aided design systems. This makes it possible to implement new technologies for the design of mechanical engineering products, radio electronics and computer science. In the conditions of the development of a market economy, it becomes possible to create competitive products, quickly modernize them, ensuring the implementation of the economic strategy of the enterprise.

LANs also allow the implementation of new Information Technology in systems of organizational and economic management.

In the educational laboratories of universities, LANs make it possible to improve the quality of education and introduce modern intelligent learning technologies.

LAN COMBINING

Reasons for joining a LAN

The LAN system created at a certain stage of development ceases to satisfy the needs of all users over time, and then the problem of expanding its functionality arises. It may be necessary to combine within the company various LANs that appeared in its various departments and branches at different times, at least to organize data exchange with other systems. The problem of expanding the network configuration can be solved both within a limited space and with access to the external environment.

The desire to get access to certain informational resources may require LAN connection to higher level networks.

In the very simple version LAN bonding is necessary to expand the network as a whole, but technical capabilities the existing network is exhausted, new subscribers cannot be connected to it. You can only create another LAN and merge it with already existing using one of the following methods.

Ways to join a LAN

Bridge. The simplest way to combine LANs is to combine identical networks within a limited space. The physical transmission medium imposes restrictions on the length network cable. Within the allowable length, a network segment is built - a network segment. Bridges are used to connect network segments.

Bridge- a device that connects two networks using the same data transmission methods.

The networks that moet unites must have the same network layers of the interaction model open systems, the lower levels may have some differences.

For a network of personal computers, a bridge is a separate computer with special software and additional hardware. A bridge can connect networks of different topologies, but running the same type of network operating systems.

Bridges can be local or remote.

• Local bridges connect networks located in a limited area within an already existing system.

• Remote bridges connect geographically dispersed networks using external communication channels and modems.

Local bridges, in turn, are divided into internal and external.

• Internal bridges are usually located on one of the computers of this network and combine the function of the bridge with the function of the subscriber computer. The expansion of functions is carried out by installing an additional network card.

• External bridges provide for the use of a separate computer with a special software.

Router (router). A network of complex configuration, which is a connection of several networks, needs a special device. The task of this device is to send a message to the addressee in desired network. This device is called a router.

router, or router, - device connecting networks different type, but using one operating system.

The router performs its functions on network layer, so it depends on communication protocols, but does not depend on the type of network. With the help of two addresses - the network address and the host address, the router uniquely selects a specific network station.

Example 6.7. You need to establish a connection with the subscriber telephone network located in another city. First, the address of the telephone network of this city is dialed - the area code. Then - the address of the node of this network - telephone number subscriber. The functions of the router are performed by the PBX equipment.

The router can also choose the best way to send a message to a network subscriber, filters the information passing through it, sending to one of the networks only the information that is addressed to it.

In addition, the router provides load balancing in the network by redirecting message flows over free communication channels.

Gateway. To combine LANs of completely different types, operating on significantly different protocols, special devices are provided - gateways.

Gateway- a device that allows you to organize data exchange between two networks using different communication protocols.

The gateway performs its functions at levels above the network. It does not depend on the transmission medium used, but depends on the communication protocols used. Typically, a gateway performs a conversion between the two protocols.

With the help of gateways, you can connect the local area network to the host computer, as well as connect the local area network to the global one.

Example 6.8. It is necessary to combine local networks located in different cities. This problem can be solved using a global data network. Such a network is a packet switching network based on the X.25 protocol. The gateway connects the local area network to the X.25 network. The gateway performs the necessary protocol conversions and provides data exchange between networks.

Bridges, routers, and even gateways are designed as boards that are installed in computers. They can perform their functions both in the mode of full allocation of functions, and in the mode of combining them with the functions of a computer network workstation.