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Local computing networks. Why do I need a locally computing network locally computing network in production

LAN (local computing network) is a system of combining various telecommunication devices located both in close proximity and remote. LAN can connect several personal computers, servers, printers, scanners, etc. into one network.

The connection of devices is carried out using various access tools: copper cable (twisted pair), fiber optic cable or wireless communication channel.

Sometimes within one local network, working groups are created, combining several devices under the common name.

Most often LAN is used to create a single information space in various state and commercial organizations. Network administrators are responsible for the operation of the local network or its part. They provide stable network operation, set up equipment and software.

LAN functions

1. Access access to electronic document management systems and the Internet.

2. Providing shared access and sharing by files and network folders.

3. Storage, reservation and data protection.

4. Ensuring the access of multiple computers to office equipment, such as a printer or scanner.

5. Assignment to a network of devices that are from each other at a considerable distance. For example, LAN can combine geographically dispersed branches of one company.

Communication in LAN

Computers with each other can be combined either using the cable system and wireless way. In the first case, the devices are connected using copper or fiber-optic conduirers and ethernet packet data technology.

If the conductor acts as a wireless radio channel, then technologies such as GPRS, Wi-Fi, Bluetooth are used. One local network can be connected to the other through gateways, and also have access to global Network The Internet.

The most popular technologies for building local networks are currently Wi-Fi and Ethernet. To build a LAN, use devices such as wireless access points, routers, network adapters, switches, modems, etc.

Properties of LAN

First, the local computing network allows you to connect additional equipment without changing the software and technical parameters of all networks. Secondly, upon failure, the entire network continues to work, and access to the desired information can still be obtained. Thus, due to the technical problems of one device, the work of the whole office will not "rise." In addition, thanks to the LAN, you can distinguish the level of access to network resources of individual devices.

LAN structures

Under the structure of the LAN is meant a method for connecting network elements. These are the main types of such compounds.

1. "Tire". Information is transmitted according to the Unified Line Communication Channel. Data is available for all network workstations.

2. "Star". Using a coaxial cable, all network elements are connected to one concentrating device (hub). Information from one workstation enters the hub, and from there it becomes publicly available to all other computers.

3. "Ring". The labels are connected to each other sequentially and closed into the ring. Information passes in a circle from the first workstation to the latter.

4. Tree structure It is a combination of two or immediately all of the above communication methods.

LAN - technology providing a convenient and fast exchange of information between multiple devices. Using local networks, you can store, reserve and protect the data. Therefore, the LAN is now almost all offices of firms, banks and industrial enterprises.

Covering usually relatively small territory or a small group of buildings (house, office, firm, institute). There are also local networks, the nodes of which are geographically separated at a distance of more than 12,500 km (space stations and orbital centers). Despite such distances, similar networks are still referred to as local.

Here you should mention such essential concepts as a subscriber, server, client.

Subscriber (node, host, station) - This is a device connected to the network and actively participating in the informational exchange. Most often, the subscriber (node) of the network is a computer, but the subscriber may also be, for example, network printer Or another peripheral device that has the ability to directly connect to the network. Next, in the course, the term "computer" will be used instead of the term "Subscriber".

Server A subscriber (node) of a network is called, which provides its resources to other subscribers, but does not use their resources itself. Thus, it serves the network. Servers on the network may be several, and it is not at all necessary that the server is the most powerful computer. Dedicated) The server is a server that is only engaged in network tasks. Unrequited server. May be other tasks in addition to maintaining the network. Specific type of server is a network printer.

Client A network subscriber is called, which only uses network resources, but does not give up its resources to the network, that is, the network serves it, and he only uses it. The client computer is also often referred to as a workstation. In principle, each computer can be simultaneously both the client and the server. Under the server and the client often understand not computers themselves, but working on them software applications. In this case, the application that only gives the resource to the network is a server, and the application that only uses network resources is a client.

Objectives LS.

Local networks allow individual users to easily and quickly interact with each other. Here are just some tasks that allows LS:

  • collaboration with documents;
  • simplifying document management: you get the opportunity to view, adjust and comment on documents without leaving your workplace without organizing meetings and meetings that take a lot of time;
  • saving and archiving your work on the server so as not to use the valuable space on the hard disk of the PC;
  • easy access to applications on the server;
  • facilitate sharing in the organizations of expensive resources, such as printers, CD-ROM drives, hard drives and applications (for example, text processors or database software);

Local network components

Basic components and technologies associated with architecture of local networks may include:

Hardware:

  • Cables;
  • Servers;
  • Network Interface Boards (NIC, Network Interface Card);
  • Concentrators;
  • Servers remote access;

Software:

  • Network management

Some computer communication history

Communication for small distances in computer technician There was still long before the emergence of the first personal computers.

For large computers (mainframes), numerous terminals (or "intelligent displays" were joined). True, the intellect in these terminals was very few, practically no processing of information they did not, and the main purpose of the organization's organization was to divide the intelligence ("machine time") of a large powerful and expensive computer between users working beyond these terminals. This was called the time separation mode, since a large computer has solved the tasks of the set of users in time. In this case, a joint use of the most expensive resources at the time - computational (Fig. 1.1) was achieved.

Fig. 1.1. Connecting terminals to a central computer

Then the microprocessors and the first microcomputers were created. There was an opportunity to place a computer on the table for each user, as computational, intelligent resources fell. But all other resources remained quite expensive. What does naked intelligence mean without information storage and documentation? You won't be every time after the power is turned on again to dial the program executable or store it in a minor permanent memory. Means of communication came to the rescue. By combining several microcomputers, it was possible to organize the joint use of computer peripherals (magnetic disks, magnetic tape, printers). At the same time, all processing of information was carried out on the spot, but its results were transmitted to centralized resources. Here again, the most expensive, which is in the system, but already completely in a new way. This mode received the name of the reverse time mode (Fig. 1.2). As in the first case, the means of communication reduced the cost computer system generally.

Fig. 1.2. Combining the first microcomputer network

Then there were personal computers, which differed from the first microcomputers because they had a complete set developed enough for completely autonomous work Peripherals: Magnetic discs, printers, not to mention more advanced user interface tools (monitors, keyboards, mice, etc.). Peripherals fell and became quite comparable with a computer. It would seem why now connect the personal computers (Fig. 1.3)? What do they share when and is it all divided and is on the table for each user? Intellect on site enough, periphery too. What can network in this case?

Fig. 1.3. Combining a network of personal computers

The most important thing is again the joint use of the resource. The most reverse separation of time, but already on a fundamentally different level. Here it is already applied not to reduce the cost of the system, but for the purpose of more effective use Resources available at the disposal of computers. For example, the network allows you to combine the volume of disks of all computers, ensuring the access of each of them to the disks of all the others as your own.

But the most important advantage of the network is manifested when all users actively work with a single database, requesting information from it and enjoy a new one (for example, in a bank, in the store, in stock). No diskettes here will no longer: would have to transfer data from each computer to all other, to contain a whole staff of couriers. And everything is simple with the network: any data changes made from any computer immediately become visible and accessible to everyone. In this case, special processing on site is usually not required, and in principle it would be possible to do with cheaper terminals (return to the first considered situation), but personal computers have incomparably more convenient interface The user facilitating the work of the staff. In addition, the possibility of complex information processing on site can often significantly reduce the amount of transmitted data.

Fig. 1.4. Using a local network for organizing collaboration of computers

Without a network, it is also impossible to do in the case when it is necessary to ensure the agreed work of several computers. This situation is most often found when these computers are not used for computing and working with databases, and in management tasks, measurements, control, where the computer is conjugated with those or other external devices (Fig. 1.4). Examples include various production technological systems, as well as systems for the management of scientific installations and complexes. Here the network allows you to synchronize the actions of computers, parallelate and accelerate the data processing process accordingly, that is, not only the peripheral resources, but also the intellectual power.

It is the indicated advantages of local networks and ensure their popularity and more wider use, despite all the inconvenience associated with their installation and operation.

Topology of local networks

Under the topology (layout, configuration, structure) of the computer network Usually it is understood as the physical location of the network computers relative to each other and the method of connecting their communication lines. It is important to note that the concept of topology applies, first of all, to local networks in which the structure structure can be easily traced. In global networks, the link structure is usually hidden from users and is not too important, as each communication session can be made on its own way.

The topology determines the equipment requirements, the type of cable used, the permissible and most convenient methods for the exchange management, reliability, network expansion capabilities. And although you have to choose the topology to the network user, you need to know about the peculiarities of the main topologies, their advantages and disadvantages.

Factorsaffecting the physical performance of the network and directly related to the concept of topology.

1)Service service (subscribers)connected to the network. In some cases, the subscriber breakdown can block the operation of the entire network. Sometimes the subscriber malfunction does not affect the operation of the network as a whole, does not prevent the rest of the subscribers to exchange information.

2)Service noise equipment, that is, technical means directly connected to the network (adapters, transceivers, connectors, etc.). The failure of the network equipment of one of the subscribers can affect the entire network, but can disrupt the exchange with only one subscriber.

3)Network cable integrity. When the network cable breaks (for example, due to mechanical impacts), the exchange of information throughout the network or in one of its parts may violate. For electrical cables, the short circuit in the cable is equally critical.

4)Limit of cable lengthassociated with the attenuation of the signal spreading on it. As you know, in any medium, when distributed, the signal is weakened (faded). And the larger distance the signal passes, the more it fades (Fig. 1.8). It is necessary to ensure that the length of the cable of the network does not have larger length of the LPR, when the attenuation is exceeded, it becomes unacceptable (the receiving subscriber does not recognize the weakened signal).

Fig. 1.8. Signal attenuation when distributed over the network

There are three basic network topology:

Tire (BUS) - All computers are connected in parallel to one link. Information from each computer is simultaneously transmitted to all other computers (Fig. 1.5).

Fig. 1.5. Network topology tire

Topology tire (or, as it is also called, the total tire) itself is the identity of network equipment of computers, as well as equality of all subscribers for network access. Computers in the bus can only transmit information in turn, since the communication line in this case is the only one. If several computers will transmit information at the same time, it will distort the overlay (conflict, collision). The bus is always implemented in the so-called half-duplex (Half Duplex) metabolism (in both directions, but in turn, and not at the same time).

In the topology of the bus, there is no explicitly expressed central subscriber through which all the information is transmitted, it increases its reliability (because when the center fails to function, the entire system managed to them). Adding new subscribers to the bus is quite simple and is usually possible even during network operation. In most cases, when using a tire, a minimum number of connecting cable is required compared to other topologies.

Since the central subscriber is absent, the resolution of possible conflicts in this case falls on the network equipment of each individual subscriber. In this regard, the network equipment with a tire topology is more complicated than with other topologies. Nevertheless, due to the wide distribution of network topology networks (primarily the most popular Ethernet network), the cost of network equipment is not too high.

Fig. 1.9. Cable breaking on a network with tire topology

The important advantage of the tire is that if any of the network computers is refused, serviceable cars will be able to continue the exchange.

It would seem that when the cable breaks, two completely workable tires are obtained (Fig. 1.9). However, it should be borne in mind that due to the specifics of the distribution of electrical signals over long communication lines it is necessary to include at the ends of the special tire matching devices, Terminators shown in Fig. 1.5 and 1.9 in the form of rectangles. Without turning on the terminators, the signal is reflected from the end of the line and is distorted so that communication over the network becomes impossible. In the event of a break or damage to the cable, the communication line is violated, and the exchange is terminated even between those computers that remained interconnected. A short closure at any point of the tire cable displays the entire network.

The failure of network equipment of any subscriber in the bus can fail the entire network. In addition, such a refusal is quite difficult to localize, since all subscribers are included in parallel, and it is impossible to understand which one of them is not possible.

When passing over the communication line with the Top Topology, the information signals are weakened and not restored, which imposes rigid restrictions on the total length of communication lines. Moreover, each subscriber can receive different level signals from the network depending on the distance to the transmitting subscriber. This places additional requirements for the receiving nodes of network equipment.

If we assume that the signal in the network cable is weakened to the maximum allowable level at the LPR length, then the full length of the bus cannot exceed the values \u200b\u200bof the LPR. In this sense, the tire provides the smallest length compared to other basic topologies.

To increase the length of the network with a tire topology, several segments are often used (parts of the network, each of which is a bus) interconnected using special amplifiers and rebuilding signals - repeaters or repeaters (in fig. 1.10 shows the connection of two segments, the maximum length of the network In this case, up to 2 liters increase, as each of the segments can be the length of the LPR). However, such an increase in the length of the network cannot continue infinitely. Length limits are associated with the final speed of signal distribution over communication lines.

Fig. 1.10. Connection of a network type network segments using a repeater

Star (Star) - The remaining peripheral computers are joined to one central computer, each of them uses a separate line of communication (Fig. 1.6). Information from the peripheral computer is transmitted only by the central computer, from the central one or several peripherals.

Fig. 1.6. Network topology star

Star - This is the only network topology with a clearly dedicated center to which all other subscribers are connected. The exchange of information goes exclusively through the central computer on which a large load falls, so it can not be engaged in anything other than the network. It is clear that the network equipment of the central subscriber should be significantly more complex than the equipment of peripheral subscribers. On the equality of all subscribers (as in the bus) in this case, it is not necessary to speak. Usually the central computer is the most powerful, it is on it all the functions for the exchange management are imposed. No conflicts on the network with a topology star are in principle are impossible, since the management is fully centrally.

If we talk about the stability of the star to the refusals of computers, the failure of the peripheral computer or its network equipment does not affect the functioning of the remaining part of the network, but any denial of the central computer makes the network completely inoperable. In this regard, special measures should be taken to improve the reliability of the central computer and its network equipment.

Cable breakdown or short circuit in it with a star topology violates the exchange with only one computer, and all other computers can continue to work normally.

Unlike the tire, only two subscribers are in the star on each line of communication: the central and one of the peripherals. Most often, two links are used for their connection, each of which transmits information in one direction, that is, on each communication line there is only one receiver and one transmitter. This is the so-called point-point transmission. All this significantly simplifies network equipment compared to the tire and eliminates the need to use additional, external terminators.

The problem of attenuation of signals in the communication line is also solved in the star easier than in the case of a bus, because each receiver always receives a signal of one level. The maximum length of the network with a star topology can be twice as much as in the bus (i.e. 2 LPR), as each of the cables connecting the center with the peripheral subscriber may have the length of the LPR.

A serious lack of a star topology consists in a tough restriction of the number of subscribers. Usually the central subscriber can serve no more than 8-16 peripheral subscribers. During these limits, the connection of new subscribers is quite simple, but it is simply impossible for them. In the star, the connection is permissible instead of the peripheral one more central subscriber (as a result, topology is obtained from several interconnected stars).

Star shown in Fig. 1.6, wears the name of an active or true star. There is also a topology called a passive star, which only looks like a star (Fig. 1.11). Currently, it is spread much more widely than an active star. It is enough to say that it is used in the most popular Ethernet network today.

In the center of the network with this topology, not a computer is placed, but a special device - a hub or, as it is also called, a hub (HUB), which performs the same function as the repeater, that is, restores the incoming signals and forwards them into all other lines of communication .

Fig. 1.11. Topology passive star and its equivalent scheme

It turns out that although the scheme of the cable laying is similar to the true or active star, in fact we are talking about tire topology, since information from each computer is simultaneously transmitted to all other computers, and there is no central subscriber. Of course, a passive star is more expensive than the usual tire, as in this case there is also a hub. However, it provides a number of additional opportunities associated with the advantages of the star, in particular, simplifies the maintenance and repair of the network. That is why recently the passive star is increasingly displacing a true star, which is considered a low-job topology.

You can also allocate an intermediate type of topology between the active and passive star. In this case, the hub not only relayes the signals incoming to it, but also produces an exchange management, but does not participate in the exchange (this is done on the 100VG-AnyLAN network).

The great advantage of the star (both active and passive) is that all connection points are collected in one place. This makes it easy to control the operation of the network, localize malfunctions by easy shutdown from the center of certain subscribers (which is not possible, for example, in the case of a bus topology), as well as limit the access of unauthorized persons to a vital connection points. To the peripheral subscriber, in the case of a star, it can approach both one cable (according to which the transfer in both directions) and two (each cable transmits in one of the two oncoming directions), and the latter is much more common.

A common drawback for all topologies like a star (both active and passive) is much greater than with other topologies, cable consumption. For example, if computers are located in one line (as in Fig. 1.5), when the star is selected, the star will be needed several times more cable than with a tire topology. This significantly affects the cost of the network as a whole and significantly complicates the cable laying.

Ring (Ring) - Computers are consistently combined into the ring. The transfer of information in the ring is always performed only in one direction. Each of the computers transmits only one computer information next in the chain behind it, and receives information only from the previous one in the computer chain (Fig. 1.7).

Fig. 1.7. Network Topology Ring

Ring - This is a topology in which each computer is connected by the links with two others: it receives information from one, and another transmits. On each line, as in the case of a star, only one transmitter and one receiver (point-to-point connection) works. This allows you to abandon the use of external terminators.

An important feature of the ring is that each computer relays (restores, enhances) the signal coming to it, that is, acts as a repeater. The attenuation of the signal across the entire ring does not matter, it is important only attenuation between adjacent rings computers. If the limit length of the cable limited by attenuation is LPR, then the total length of the ring can reach the NLR, where n is the number of computers in the ring. The full size of the network in the limit will be NLP / 2, since the ring will have to be folded twice. In practice, the size of annular networks reaches tens of kilometers (for example, in the FDDI network). The ring in this regard significantly exceeds any other topologies.

There is no well-selected center with a ring topology, all computers may be the same and equal. However, quite often the special subscriber is allocated in the ring, which manages the exchange or controls it. It is clear that the presence of such a single managing subscriber reduces the reliability of the network, since its output immediately paralyzes the entire exchange.

Strictly speaking, computers in the ring are not completely equal (in contrast, for example, from tire topology). After all, one of them necessarily receives information from the computer leading to the transmission at the moment, before, and others later. It is on this particular feature of the topology and the methods of network exchange management on a network are specifically designed for the ring. In such methods, the right to the next transmission (or, as elsewhere, to capture the network) passes sequentially to the next computer in a circle. Connecting new subscribers in the ring is quite simple, although it requires a mandatory stop of the entire network to connect. As in the case of a tire, the maximum number of subscribers in the ring can be quite large (up to thousand and more). The ring topology is usually highly resistant to overloads, provides confident work with large threads transmitted over the network information, as in it, as a rule, no conflicts (unlike the tire), and there is no central subscriber (unlike a star), which It can be overloaded by large information flows.

Fig. 1.12. Network with two rings

The signal in the ring passes sequentially through all network computers, so the failure of at least one of them (or its network equipment) disrupts the operation of the network as a whole. This is a significant drawback of the ring.

Similarly, the breakdown or short circuit in any of the cables of the ring makes the work of the entire network impossible. Of the three topologies reviewed, the ring is most vulnerable to the damage to the cable, so in the case of the Topology, the ring usually provide for the laying of two (or more) parallel lines of communication, one of which is in the reserve.

Sometimes the network with the Topology ring is performed on the basis of two parallel ring communication lines transmitting information in opposite directions (Fig. 1.12). The purpose of this solution is an increase (ideally - twice) speed of information transmission over the network. In addition, during the damage of one of the cables, the network can work with another cable (though, the limit speed will decrease).

In the case of Star-ring (Star-Ring) topology in the ring, not computers themselves combine, but special hubs (depicted in Fig. 1.16 in the form of rectangles), which in turn computers are connected using star dual communication lines. In fact, all network computers are included in the closed ring, since a closed circuit is formed inside the communication contenters (as shown in Fig. 1.16). This topology makes it possible to combine the benefits of star and ring topologies. For example, hubs allow you to assemble all the network cables in one place. If we talk about the dissemination of information, this topology is equivalent to the classic ring.

In conclusion, it is also necessary to say about the grid topology (mesh), in which computers are associated with each other, but by many lines of communication forming the grid (Fig. 1.17).

Fig. 1.17. Grid topology: complete (a) and partial (b)

In full grid topology, each computer is directly related to all other computers. In this case, with an increase in the number of computers, the number of communication lines is sharply increasing. In addition, any change in the network configuration requires changes to network equipment of all computers, so complete grid topology has not been widespread.

Partial grid topology implies direct links only for the most active computers transmitting maximum amounts of information. The remaining computers are connected through intermediate nodes. The grid topology allows you to choose a route to deliver information from the subscriber to the Subscriber, bypassing faulty areas. On the one hand, it increases the reliability of the network, on the other, it requires a significant complication of the network equipment that should choose the route.

Functional groups of devices on the network

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

From this point of view, the local computing network can be mounted as a set of servers and workstations.

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

Servers can storing data, database management, remote tasks processing, print jobs and a number of other functions, the need for network users may occur. The server is a network resource source.

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

The network workstation functions both in the network and in local mode. It is equipped with its own operating system (MS DOS, Windows, etc.), provides a user by all required tools To solve applied tasks.

Particular attention should be paid to one of the types of servers - file server (File Server). In common terminology, the abbreviated name is adopted file server..

The file server stores network user data and provides them with access to these data. This is a computer with a large capacity random access memory, high-capacity hard drives and additional magnetic tape drives (strimmers).

It works under the control of a special operating system, which provides simultaneous access user access to data located on it,

The file server performs the following functions: data storage, data archiving, data change synchronization by various users, data transmission.

For many tasks, the use of one server file is insufficient. Then the network may include multiple servers. It is also possible to use mini-computer as file-servers.

Management of device interaction on the network

Information systems based on computer networks provide solving the following tasks: data storage, data processing, organization of user access to data, data transfer and data processing results.

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

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

Client - Task, workstation or computer network user.

In the process of data processing, the client can form a request to the server to perform complex procedures, reading the file, search for information in the database, etc.

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

The client processes the data obtained and represents the processing results in a convenient for the user. In principle, data processing can be performed on the server. For such systems, the terms are taken - systems client-server.or client server architecture.

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

Single Network. In such a network, there is no single operating station management center and no single device For data storage. The network operating system is distributed across all the workstations. Each network station can perform the functions of both the client and the server. It can serve requests from other workstations and send their network service requests.

The network user is available all devices connected to other stations (discs, printers).

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

Disadvantages of peer-to-peer networks:

  • dependence of the efficiency of the network from the number of stations;
  • the complexity of network management;
  • the complexity of ensuring information protection;
  • difficulties of updating and changing stations software.

Peer-based networks on the basis of network operating systems Lantastic, NetWare Lite are most popular.

Network with a dedicated server. On a network with a dedicated server, one of the computers performs data storage functions intended for all workstations, interaction management between workstations and a number of service functions.

Such a computer is usually called the network server. It establishes a network operating system, all partable external devices are connected to it - hard drives, printers and modems.

The interaction between workstations on the network is usually carried out through the server. The logical organization of such a network can be represented by the star topology. The role of the central device performs the server. In centralized control networks, there is the possibility of exchanging information between workstations, bypassing the file server. To do this, you can use the NetLink program. After starting the program on two workstations, you can send files from a single station disk to the disk of another (similar to the copying of files from one directory to another using the Norton Commander).

Network advantages with a dedicated server:

  • reliable information protection system;
  • high speed;
  • lack of restrictions on the number of workstations;
  • easy control compared to peer-to-peer networks,

Network Disadvantages:

  • high cost due to allocating one computer under the server;
  • dependence of the speed and reliability of the network from the server;
  • ligger flexibility compared to peer-to-peer network.

Networks with a dedicated server are the most common in computer network users. Network operating systems for such networks - LanServer (IBM), Windows NT Server version 3.51 and 4.0 and NetWare (Novell).

Typical topology and LAN access methods

Physical transmission Wednesday LAN

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

Twisted pair consists of two insulated wires, returned among themselves (Fig. 6.19). Wiring twisting reduces the effect of external electromagnetic fields to the transmitted signals. The easiest version of the twisted pair - telephone cable, Twisted couples have various characteristicsdefined by sizes, insulation and twisting step. The cheaper of this type of transmitting medium makes it quite popular for LAN.

Fig. 6.19. Twisted pair of wires

The main drawback of the twisted pair is a bad interference and low information transfer rate - 0.25 - 1 Mbps. Technological improvements make it possible to increase the transfer rate and noise immunity (shielded twisted pair), but at the same time the cost of this type of transmitting medium increases.

Coaxial cable (Fig. 6.20) compared with a twisted pair has a higher mechanical strength, noise immunity and provides information transfer rate up to 10-50 Mbps, two types of coaxial cables are available for industrial use: thick and thin. The thick cable is more durable and transmits the signals of the desired amplitude for a greater distance than thin. At the same time, the thin cable is much cheaper. Coaxial cable as well as twisted steam is one of the popular types of transmitting media for LAN.

Fig. 6.20. Coaxial cable

Fig. 6.21. Fiber optic cable

The fiber optic cable is an ideal transmitting medium (Fig. 6.21). It is not subject to the action of electromagnetic fields and almost does not have radiation. The last property allows you to use it in networks requiring increased security secrecy.

The speed of information on the fiber optic cable is more than 50 Mbps, compared to previous types of transmission medium, it is more expensive, less technologically in operation.

LAN, manufactured by various firms, or are designed for one of the types of transmitting medium, or can be implemented in various versions based on different transmit media.

Main topology LAN

Computing machines included in the LAN may be located the most random way on the territory where the computer network is created. It should be noted that for the method of referring to the transmitting environment and the network management methods, it is not indifferent to how subscriber computers are located. Therefore, it makes sense to talk about the LAN topology.

Topology LAN - This is averaged geometric diagram of network nodes connections.

The topology of computing networks can be the most different, but only three are typical for local computing networks: ring, tire, star-shaped.

Sometimes, the terms are used to simplify - ring, tire and star. It should not be thought that the topology types under consideration are the perfect ring, an ideal direct or star.

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

Knot - Any device directly connected to the network transmitting environment.

Topology averaged the scheme of network nodes connections. So, and the ellipse, and the closed curve, and the closed broken line belongs to the ring topology, and the unlocked broken line-to the tire.

Ring The topology involves the connection of the network nodes of a closed curve - a cable of the transmitting medium (Fig. 6.22). The output of one network node is connected to the other input. Information on the ring is transmitted from the node to the node. Each intermediate node between the transmitter and the receiver relayes the sent message. The receiving node recognizes and receives only the messages addressed to it.

Fig. 6.22. Network ring topology

The annular 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 relay allows you to use any types of cables as a transmitted medium.

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

Tire Topology is one of the simplest (Fig. 6.23). It is associated with the use of a coaxial cable as a transmit medium. Data from the transmitting node of the network is distributed over the bus in both directions. Intermediate nodes do not translate incoming messages. Information enters all nodes, but only the one that it is addressed is received. Discipline maintenance parallel.


Fig. 6.23. Network tire topology

This ensures the high speed of the LAN with a tire topology. The network is easy to increase and configure, as well as adapt to various systems, the network topology network is resistant to possible malfunctions of individual nodes.

Tire topology networks are currently the most common. It should be noted that they have a low length and do not allow using different types of cable within the same network.

Star-shaped Topology (Fig. 6.24) is based on the concept of the 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 a central node that retransmits, switches and routes information flows on the network.

Fig. 6.24. Network of star-shaped topology

Star-like topology greatly simplifies the interaction of LAN nodes with each other, allows you to use simpler network adapters. At the same time, the performance of the LAN with a star-shaped topology depends entirely on the central node.

In real computing networks, more complex topologies can be used, representing in some cases the combination of considered.

The choice of one or another topology is determined by the field of application of the LAN, the geographical location of its nodes and the dimension of the network as a whole.

Access methods for transmitting environment

Transfer Wednesday is common resource For all network nodes. To get the ability to access this resource from the network node, special mechanisms are needed - access methods.

Access method to the transmitting environment- A method that ensures the totality of the rules on which network nodes get access to the resource.

There are two main class of access methods: deterministic, non-deterministic.

During deterministic access methods, the transmitting medium is distributed between nodes using a special control mechanism that guarantees the transfer of the node data for some sufficiently small time interval.

The most common deterministic access methods are the survey method and the transfer method of transfer. The survey method was previously seen. It is used mainly in star-shaped topology networks.

The transfer method is applied in networks with an annular topology. It is based on the transmission on a network of a special message - marker.

Marker - A service message of a particular format in which network subscribers can place their information packages.

The marker is circulating over the ring, and any node that has data for transmission, places them in a free marker, sets the sign of the employment of the marker and transmits it over the ring. The node that the message was addressed receives it, sets the sign of confirming the reception of information and sends the marker to the ring.

The transmitting node, receiving confirmation, frees the marker and sends it to the network. There are access methods that use several markers.

Non-deterministic - random access methods provide for the competition of all network nodes for the right to transfer. Simultaneous attempts to transfer from several nodes, resulting in collisions arise.

The most common non-deterministic access method is the multiple access method with the control of the carrier frequency and the detection of collisions (CSMA / CD). In essence, it described earlier rivalry regimen. The control of the carrier frequency is that the node that wants to convey the message, "listens" the transmitting environment, waiting for its release. If the environment is free, the node begins to be transmitted.

It should be noted that the network topology, the access method to the transmitting medium and the transmission method is closely connected to each other. The determining component is the network topology.

Purpose of LAN

Local computing networks over the last five years have become widespread in various fields of science, technology and production.

Especially widespread LANs are used in the development of collective projects, such as complex software complexes. On the basis of the LAN, you can create automated design systems. This allows you to implement new technologies for designing products of mechanical engineering, radio electronics and computing equipment. In the context of the development of a market economy, it is possible to create competitive products, to quickly upgrade it, ensuring the implementation of the enterprise economic strategy.

LAN also allow new information Technology In the system of organizational and economic management.

In training laboratories, LAN universities allow you to improve the quality of learning and implement modern intelligent learning technologies.

Union of LAN

Causes of unification of the LAN

Created at a certain stage of development of the LAN system over time ceases to satisfy the needs of all users, and then the problem of expanding it functionality. There may be a need to unite within the company of various LANs that appeared in various departments and branches at different times, at least for the organization of data exchange with other systems. The problem of expanding the network configuration can be solved both within a limited space, and with exit to the external environment.

Desire to get out of certain informational resources May require the connection of the LAN to the high-level networks.

In the simplest version, the LAN union is necessary to expand the network as a whole, but technical capabilities existing network Exhausted, new subscribers cannot be connected to it. One can only create another LAN and combine it with an existing one, using one of the following methods.

Methods of unification of the LAN

Bridge. The easiest option of the unification of the LAN is the combination of identical networks within a limited space. Physical transmitting medium imposes limitations for length network cable. Within permissible length, a network segment is built - a network segment. Bridges are used to combine network segments.

Bridge - A device connecting two networks using the same data transfer methods.

Networks that combine washes must have the same network levels of the model of interaction of open systems, lower levels may have some differences.

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

Bridges can be local and remote.

  • Local bridges connect networks located in a limited area within the already existing system.
  • Remote bridges connect networks separated by geographically 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 computer of this network and combine the function of the bridge with the function of a subscriber computer, the extension of the functions is carried out by installing an additional network card.
  • External bridges provide for the use of separate computers with special software to perform their functions.

Router (router). A network of complex configuration, which is a connection of multiple networks, needs a special device. Task for this device - send a message to the destination to the desired network. It is called such a device by routing.

Router or router- device connecting network of different typesBut using one operating system.

The router performs its functions on the network level, so it depends on the data exchange protocols, but does not depend on the type of network. With the help of two addresses - the network addresses and the address of the node, the router unambiguously selects a specific network station.

Example 6.7. You must establish a connection with the telephone network subscriber located in another city. First, the address of the telephone network of this city is recruited - the city code. Then - the address of the node of this network - telephone number Subscriber. The router functions performs PBX equipment.

The router can also select the best path to send a network to the network subscriber, filters the information passing through it, directing into one of the networks only the information that is addressed to it.

In addition, the router provides the load balancing on the network, redirecting the flow streams on free communication channels.

Gateway. To combine the LANs of completely different types that work according to the protocols substantially different from each other, special devices are provided - gateways.

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

The gateway performs its functions at levels above the network. It does not depend on the transmitted environment used, but depends on the data exchange protocols used. Typically, the gateway performs the transformation between two protocols.

Using gateways, you can connect a local computing network to the main computer, as well as the local network to connect to the global.

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

Bridges, routers and even gateways are performed in the form of boards that are installed in computers. The functions of their own can be performed both in the mode of complete allocation of functions and in the combination mode with the functions of the computer network workstation.

According to the tradition of the first in the house they launch a cat. For organizations, the role of "cat" sometimes plays a local computing network. Without her rare company will begin work in the new office.

Installation of the LAN (local computing network, or LAN, that is, Local Area Network is a prerequisite for efficient work Modern company in which it is not without computers and other electronic devices.

But what is LAN? White cord, which passes along the walls and is inserted into the computer? You may be surprised to find out how much is included in the concept of a local network.

What is a local computing network?

LAN is a combination of software and hardware to combine computers in unified system Storage and processing of information. If easier, the local network is all: computers, modem, servers, routers, switches and other equipment. This is on the one hand. On the other - operating systems and network protocols.

The word "local" in the name LAN indicates the size of the network. Usually it serves to combine one office or building. Less often - several houses.

The distance between computers on the local network should not exceed 100 meters. This is due to the fact that the electrical signal cannot overcome the cable long distances. You can fight with this restriction. For this, active network equipment is installed between computers.

What is the LAN?

Here are the main tasks that are solved due to the local computer network:

  • Internet access. Thanks to the installation of the local network in the office, you can easily organize an Internet connection connection to the Internet.
  • Communicative. Thanks to the local network, you can configure mail server and organize a mail server or install the exchange program instant messages.
  • Joint work. Employees can work together on documents on the server without creating duplicates or printed copies. Work is much faster and more organizer.
  • Share access to printers, copiers and scanners. You can significantly save at the cost of consumables by purchasing a network printer or scanner.
  • The network opens up huge prospectsfor the organization of office work. For example, you can use corporate information system (KIS).

No, this is not all the network features, but listed enough to understand what a powerful tool can be a well-planned and configured local network.

In the title, the local computing network has already laid the assignment, function and system limitations. We will analyze the name to the components. Local, formed from the English Local - local, that is, the network is tied to a specific geographical location and has restrictions on the territory, computing, associated with network composition (computing equipment, software) and its appointment, net - implies a combination of computing equipment and software on a certain territory (local) to the network (by means of cables).

In this way, it is possible to formulate a definition of a locally computing network (LAN) - this is a system of interconnected computing resources (computers, servers, routers, software, etc.) distributed by relatively small territory (office or group of buildings), serving for receiving and transfer, storage and processing information of various kinds.

Different locally computing networks can function individually or be interconnected using communications tools, such as enterprises with a branch network in different cities. Thanks to this connection, the user can interact with other workstations connected to this locally computing network. There are local networks whose nodes are separated by geographically at a distance of more than 12,500 km (space stations and orbital centers), but they still belong to local.

The assignment of the LAN is to ensure the joint and simultaneous access of a certain group of persons to data, programs and equipment (computers, printers, charts, file storage and processing devices, and databases) and data transmission (electronic graphics, text processing, email, access to remote bases data, digital speech transmission).

For example: The manager accepts an order and introduces it to the computer, then the order enters the accounting department and there is an invoice, at the same time information may come to the legal service to create a contract.

Characteristics of LAN:

  • High-speed channels (1- 400 Mb С) belonging to predominantly one user;
  • The distance between workstations connected to the local network is usually from several hundred and several thousand meters;
  • Transmission of data between computer member stations;
  • Decentralization of terminal equipment, which uses microprocessors, displays, cash registers, etc.
  • Data transfer to subscribers connected to the network, on a shared cable;

The main functions of the LAN are:

  • Ensuring simultaneous access to equipment, software and information, combined into a network;
  • Minimizing the risk of unauthorized access to information and network resources;
  • Delimitation of access to information and network resources;
  • Ensuring fast and confidential exchange and simultaneous work with information to a certain circle of persons;
  • Control over information flows, including incoming and outgoing;
  • Remuneration of control functions and responsible persons on each node (for each node is responsible system Administratorperforming servicing and, as a rule, control functions);
  • Optimization of software and equipment costs due to their collective use (for example one printer into several departments, etc.)

As a result of the use of LAN, personal computers are combined, located on many remote workplaces. Employee jobs cease to be isolated and combined into a single system that has its own special advantages:

  • The ability to remote access to equipment, software and information;
  • Optimization of processor operation resources;
  • Fewer errors intensity compared to the telephone database network;
  • Bandwidth higher than the global network;
  • The possibility of reconfiguration and development by connecting new terminals

Application area Local networks are very wide, currently there are such systems in almost every office (for example, one printer is installed on several computers, or several computers use one software, say 1C: Accounting, etc.). Every day, information flows are becoming more, the software used is comprehensive and functional, the geography of the activities of organizations is expanding. The use of LAN funds becomes not just desirable, but necessary for the successful activity and development of business, science, training of students, schoolchildren, training and retraining of specialists, the fulfillment of government programs and functions, etc.

Network functioning structure.

The structure of the local network is determined by the principle of management and communication type, it is often based on the structure of the serviced organization. Topology types are used: tire, annular, radial, tree. The first two types are most common, due to the effective use of communication channels, ease of management, flexible expansion and change capabilities.


Topology "Tire" - All computers are binding to a chain, connecting to a trunk cable segment (barrel), "Terminators" are placed at its ends, to clean the signal spreading in both directions. Network computers are connected by a coaxial cable with a tee connector. Network bandwidth - 10 Mbps, for modern applications, actively using video and multimedia data, this is not enough. The advantage of this topology lies in the low cost of posting and unification of connections.

Tire topology is passive. A failure of one computer does not affect network performance. Damage to the main cable (tires) leads to a reflection of the signal and the entire network as a whole becomes inoperable. Turning off and especially connection to such a network requires a tire break, which causes a violation of the circulating flow of information and the system freezing.

Topology "Tree" - A more advanced configuration of the type "tire". Several simple tires are attached to the common trunk bus through active repeaters or passive multipliers.


Topology "Star" (Star) - is the most high-speed of all topologies, information between peripheral workstations passes through the central node of the computing network. Central Control Node - File Server can implement the optimal protection mechanism against unauthorized access to information. The entire computing network can be controlled from its center.

The cable connection is pretty simple, as each workstation is connected only with the central node. The cost of laying cables is high enough, especially when the central node is geographically located not in the center of the topology. When expanding computing networks, previously performed cable connections cannot be used: a separate cable from the center of the network must be used to a new workplace.

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

Topology "Ring" (Ring) - Computers are connected by a cable segments having a ring shape, fundamentally identical to the tire, except for the need to use "Terminators". In the event of a malfunction of one of the network segments, the entire network fails.

Signals are transmitted only in one direction. Each station is directly connected to two adjacent, but listens to the transmission of any station. The ring is several transceivers and the physical environment connecting them. All stations may have equal access rights to the physical environment. At the same time, one of the stations can perform the role of an active monitor serving the exchange of information. The cable laying from one workstation to another can be quite complex and expensive, especially if geographically workstations are located far from the ring (for example, in line).

The main problem with the ring topology is that each workstation should actively participate in sending information, and in case of failure, at least one of them, the entire network is paralyzed. Faults in cable connections are easily localized. Connecting a new workstation requires a network shutdown, as the ring must be open during installation. Restrictions on the length of the computing network does not exist, as it ultimately is determined exclusively by the distance between the two workstations.

Computers can be connected with each other using various access media: copper conductors (twisted pair), optical conductors (optical cables) and through radio channels (wireless technology). Wired, optical connections are installed 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, which today meet more and less often, can be seen only in specialized laboratories, educational institutions and services.

Construction components of a simple local network are used:

  • Adapter (Network Adapter) - a device connecting the computer (terminal) with a network segment;
  • Bridge (Bridge) is a device connecting local or remote network segments;
  • Router (Router) - a device for limiting broadcast traffic by splitting the network to segments, ensuring information protection, management and organization of backup paths between broadcasting areas;
  • Switch - a narrowing device, effectively segmented network, reduces collision areas and increases the bandwidth of each terminal station.
  • Uninterruptible power supply units - devices to ensure the operation of the system in cases of disconnecting the main power source.

Installation of the locally computing network (LAN)

The choice of the type of topology, the access environment and the locally computing system depends on the requirements and needs of the customer. Modern technologies allow you to develop an individual option that meets all the requirements and tasks.

The laying of LAN cables, as well as other types of cable networks can be carried out. different ways. When choosing a method of installation are guided by the individual architectural and constructive features of the building, its technical characteristics, the presence of existing networks and other equipment, the procedure for the interaction of low-current systems with other systems. You can fundamentally allocate two methods - open and hidden. For hidden wiring of LAN cables, the design of walls, floors, the ceilings are more aesthetic, the tracks are protected from extraneous influences, access to them is limited, the gasket is made immediately into special prepared places, the best conditions are provided for subsequent maintenance. Unfortunately, the ability to perform work with a hidden way is rare, more often work open way With the help of plastic boxes, vertical columns and trays. Do not forget that there is still a way of laying cables by air, most often it is used to communicate buildings, when there is no possibility to pave the cable into the channels or if it is too expensive.

Installation of LAN is a complex and responsible work. The quality of its implementation depends the stability and correctness of the functioning of the system as a whole, the degree of execution of tasks assigned to it, the rate of transmission and data processing, the number of errors, etc. Factors. It is necessary to relate to this very thoroughly and seriously, since any network is the basis (skeleton and blood system) of the whole body from low-current systems responsible for a large number of functions (from email Before the safety of the object). Each subsequent intervention in the operation of the current system (expansion, repair, etc.), requires the cost of time and means, and their number on direct depends on the parameters initially laid down in the system, the quality of work performed, the qualifications of developers and performers. Saving funds at the design and installation of the LAN, can turn out much larger spending at the stage of operation and abgrade