Topic 1.4: LAN Basics

Topic 1.5: Basic LAN Technologies

Topic 1.6: Basic software and hardware components of a LAN

Local networks

1.4. LAN Basics

1.4.3. Network topologies

All computers in the local network are connected by communication lines. The geometrical arrangement of communication lines in relation to network nodes and the physical connection of nodes to the network is called physical topology. Depending on the topology, networks are distinguished: bus, ring, star, hierarchical and arbitrary structure.

Distinguish between physical and logical topology. The logical and physical network topologies are independent of each other. Physical topology is the geometry of a network, and logical topology determines the direction of data flow between nodes on the network and how data is transmitted.

Currently, the following physical topologies are used in local networks:

• physical "bus" (bus);
• physical "star" (star);
• physical “ring”;
• physical "star" and logical "ring" ( Token Ring).

Bus topology

Bus topology networks use a linear mono channel (coaxial cable) for data transmission, at the ends of which terminating resistors (terminators) are installed. Each computer is connected to a coaxial cable using a T-connector (T-connector). Data from the transmitting network node is transmitted along the bus in both directions, reflected from the terminal terminators. Terminators prevent signal reflections, i.e. are used to suppress signals that reach the ends of the data link.

Thus, information goes to all nodes, but is received only by the node to which it is intended. In the topology of the logical bus, the data transmission medium is used jointly and simultaneously by all PCs in the network, and signals from the PC are distributed simultaneously in all directions over the transmission medium. Since signaling in the topology of the physical bus is broadcast, i.e. signals propagate simultaneously in all directions, the logical topology of this local network is a logical bus.

Rice. one.

This topology is used in local area networks with Ethernet architecture (10Base-5 and 10Base-2 classes for thick and thin coaxial cable, respectively).

The advantages of bus topology networks:

• failure of one of the nodes does not affect the operation of the network as a whole;
• the network is easy to set up and configure;
• the network is resistant to malfunctions of individual nodes.

Disadvantages of bus topology networks:

• a cable break can affect the operation of the entire network;
• limited cable length and number of workstations;
• it is difficult to identify connection defects.

Star topology

In a star network, each workstation is connected with a cable (twisted pair) to a hub or hub ( hub). The concentrator provides parallel connection PCs and thus all computers connected to the network can communicate with each other.

Rice. 2.

Data from the transmitting station of the network is transmitted through the hub over all communication lines to all PCs. Information arrives at all workstations, but is received only by those stations for which it is intended. Since the signaling in the physical star topology is broadcast, i.e. signals from the PC are distributed simultaneously in all directions, then the logical topology of this local network is a logical bus.

This topology is used in local area networks with 10Base-T Ethernet architecture.

Benefits of star topology networks:

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

Disadvantages of star topology networks:

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

Ring topology

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

Rice. 3.

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

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

Typically in pure form the ring topology is not used due to its unreliability, therefore, in practice, various modifications of the ring topology are used.

Token Ring topology

This topology is based on a star-connected physical ring topology. In this topology, all workstations are connected to a central hub (Token Ring) as in a physical star topology. The central hub is an intelligent device that, using jumpers, provides serial connection exit of one station with the entrance of another station.

In other words, with the help of a hub, each station is connected to only two other stations (the previous and the next station). Thus, workstations are connected by a loop of cable, through which data packets are transmitted from one station to another, and each station relays these sent packets. In each workstation there is a transceiver for this, which allows you to control the passage of data in the network. Physically, such a network is built in a "star" topology.

The concentrator creates the primary (main) and backup rings. If a break occurs in the main ring, then it can be bypassed using the backup ring, since a four-core cable is used. A station failure or a broken workstation link does not result in a network failure as in a ring topology, because the hub will shut down the faulty station and close the data ring.

Rice. 4.

In the Token Ring architecture, the token is passed from node to node along a logical ring created by a central hub. Such marker transmission is carried out in a fixed direction (the direction of movement of the marker and data packets is shown in the figure by arrows of blue color). A station with a token can send data to another station.

To transfer data, workstations must first wait for a free token to arrive. The marker contains the address of the station that sent this marker, as well as the address of the station to which it is intended. After that, the sender transmits the token to the next station in the network so that it too can send its data.

One of the network nodes (usually a file server is used for this) creates a token that is sent to the network ring. Such a node acts as active monitor, which makes sure that the marker is not lost or destroyed.

Token Ring Topology Advantages:

• topology provides equal access to all workstations;
• high reliability, since the network is resistant to malfunctions of individual stations and to interruptions in the connection of individual stations.

Disadvantages of Token Ring topology networks: high cable consumption and, accordingly, expensive wiring of communication lines.

The local network - important element any modern enterprise, without which it is impossible to achieve maximum productivity. However, to take full advantage of the network's capabilities, it must be properly configured, bearing in mind that the location of the connected computers will affect the performance of the LAN.

Topology concept

The topology of local computer networks is the location of workstations and nodes relative to each other and the options for their connection. In fact, this is the architecture of the LAN. The location of computers determines specifications networks, and the choice of any kind of topology will affect:

• Varieties and characteristics of network equipment.
• Reliability and scalability of the LAN.
• A method for managing a local network.

There are many such options for the location of working nodes and methods for connecting them, and their number increases in direct proportion to the increase in the number of connected computers. The main LAN topologies are star, bus, and ring.

Factors to consider when choosing a topology

Before finally deciding on the choice of topology, it is necessary to take into account several features that affect the performance of the network. Based on them, you can choose the most suitable topology, analyzing the advantages and disadvantages of each of them and correlating this data with the conditions available for installation.

• The operability and serviceability of each of the workstations connected to the LAN. Some types of local network topology are entirely dependent on this.
• Serviceability of equipment (routers, adapters, etc.). A breakdown of network equipment can either completely disrupt the operation of the LAN, or stop the exchange of information with one computer.
• The reliability of the cable used. Damaging it disrupts the transmission and reception of data throughout the LAN or one of its segments.
• Limiting the length of the cable. This factor is also important when choosing a topology. If only a few cables are available, you can choose a positioning method that requires less cable.

About star topology

This type of arrangement of workstations has a dedicated center - a server to which all other computers are connected. It is through the server that data exchange processes take place. Therefore, its equipment must be more complex.

Advantages:

• Star LAN topology compares favorably with others complete absence conflicts on the LAN - this is achieved through centralized management.
• A breakdown of one of the nodes or damage to the cable will not have any effect on the network as a whole.
• Having only two subscribers, the main and the peripheral, allows you to simplify network hardware.
• The accumulation of connection points in a small radius simplifies the process of monitoring the network, and also allows you to increase its security by restricting access to unauthorized persons.

Flaws:

• Such a local network, in the event of a central server failure, becomes completely inoperative.
• The cost of a star is higher than other topologies because much more cable is required.

Bus topology: simple and cheap

In this method of connection, all workstations are connected to a single line - a coaxial cable, and data from one subscriber is sent to the rest in half-duplex mode. Local network topologies of this kind assume the presence of a special terminator at each end of the bus, without which the signal is distorted.

Advantages:

• All computers are equal.
• The ability to easily scale the network even while it is running.
• Failure of one node does not affect the rest.
• The cable consumption is significantly reduced.

Flaws:

• Insufficient network reliability due to cable connector problems.
• Low performance due to channel sharing between all subscribers.
• Complexity of management and troubleshooting due to parallel-connected adapters.
• The length of the communication line is limited, so these types of local network topologies are used only for a small number of computers.

Ring topology characteristics

This type of communication involves the connection of a working node with two others, from one of them data is received, and the second is transmitted. The main feature of this topology is that each terminal acts as a repeater, excluding the possibility of signal attenuation in the LAN.

Advantages:

• Quickly create and configure this LAN topology.
• Easy scaling, however, requiring a network shutdown while a new node is being installed.
• A large number of potential subscribers.
• Resistant to congestion and no network conflicts.
• Possibility to expand the network up to huge size by relaying the signal between computers.

Flaws:

• The unreliability of the network as a whole.
• Lack of resistance to cable damage, so a parallel backup line is usually provided.
• Large cable consumption.

Types of local networks

The choice of the topology of local networks should also be made based on the type of LAN available. The network can be represented by two models: peer-to-peer and hierarchical. They are not very different functionally, which allows you to switch from one of them to another if necessary. However, there are still a few differences between them.

With regard to the peer-to-peer model, its use is recommended in situations where the ability to organize big network is absent, but the creation of any communication system is still necessary. We recommend that you create it only for a small number of computers. Centralized control communication is commonly used in various enterprises to control workstations.

Peer-to-peer network

This type of LAN assumes equality of rights for each workstation, distributing data between them. Access to information stored on a site can be allowed or denied by its user. As a rule, in such cases the topology of local computer networks "bus" will be the most suitable.

Peer-to-peer networking means that workstation resources are available to other users. This means the ability to edit a document from one computer while working after another, print remotely and launch applications.

Advantages of a peer-to-peer LAN type:

• Ease of implementation, installation and maintenance.
• Low financial costs. This model eliminates the need to purchase an expensive server.

Flaws:

• The network performance decreases in proportion to the increase in the number of connected working nodes.
• Missing one system security.
• Availability of information: when the computer is turned off, the data in it will become inaccessible to others.
• There is no single information base.

Hierarchical model

The most commonly used LAN topologies are based on this type of LAN. It is also called "client-server". The essence of this model is that in the presence of a certain number of subscribers, there is one main element - a server. This control computer stores all data and processes them.

Advantages:

• Excellent network performance.
• United reliable system security.
• One, common for all, information base.
• Simplified management of the entire network and its elements.

Flaws:

• The need for a special personnel unit - an administrator who monitors and maintains the server.
• Large financial costs for the purchase of a host computer.

The most frequently used configuration (topology) of a local computer network in a hierarchical model is a "star".

The choice of topology (layout of network equipment and workstations) is solely important point when organizing a local network. The selected type of communication should ensure the most efficient and safe work LAN. It is also important to pay attention to financial costs and the possibility of further network expansion. Finding a rational solution is not an easy task, which is accomplished thanks to careful analysis and a responsible approach. It is in this case that correctly selected local network topologies will ensure maximum performance of the entire LAN as a whole.

Topology of local computer networks

Topology, i.e. the configuration of the connection of elements in a LAN attracts attention to a greater extent than other characteristics of the network. This is due to the fact that it is the topology that largely determines a number of important properties of the network, for example, as reliability (survivability), performance, etc.

Exists different approaches to the classification of LAN topologies. According to one of them, LAN configurations are divided into two main classes: broadcast and sequential. V broadcast configurations each subscriber (transceiver physical signals) transmits signals that can be perceived by other subscriber systems. These configurations include common bus, tree, star with a passive center. In sequential configurations, each physical sublayer transmits information to only one subscriber system. It is clear from this that broadcast configurations are typically information-selective LANs, while sequential configurations are information-routed LANs.

Broadcast configurations must employ relatively powerful transmitters and receivers that can handle a wide range of signal levels. This problem is partially solved by the introduction of restrictions on the length of the cable segment and on the number of connections or the use of repeaters (amplifiers). Since only one station (subscriber system) can operate on a broadcast LAN at any given time, the transmitted service information is used to establish control of the station over the network during the propagation of the signal through the network, its processing in the network.

The basic type of broadcast configuration is common bus(Figure 4.2). The advantages of a LAN with a bus topology are: ease of expanding the network and the control methods used, the ability to work in parallel code (if available additional channels data transmission), no need for centralized control, minimum cable consumption.

The common bus is a passive medium and therefore has a very high reliability. The bus cable is very often laid in false ceilings of buildings, and to each network station special branches are made. It is desirable that the branch connections are passive, since in this case the intensity decreases. physical access to the main bus. To increase reliability, along with the main cable, a spare cable is also laid, to which the stations switch in case of a failure of the main one.

Rice. 4.2. Bus topology

Configuration type " tree"(Fig. 4.3.) is a more developed version of the configuration of the type " tire". A tree is formed by connecting several buses with active repeaters or passive "breeders" - concentrators (a concentrator is a device used to combine data transmission channels of individual network sections). It has the flexibility to span multiple floors in a building or multiple buildings in a single location using a LAN. If there are active repeaters, the failure of one segment does not lead to the failure of the rest. In the event of a repeater failure, the tree branches into two subtrees or two buses.

Rice. 4.3. Tree topology

Broadband LANs with " tree" often have so-called root - the control position in which the most important components networks. High demands are placed on the reliability of this equipment, since the operation of the entire network depends on it. For this reason, equipment is often duplicated.

Type configuration development "tree"- network type "star"(Fig. 4.4.), which can be thought of as a tree that has a root with branches to each connected device. In a LAN, a passive connector or an active repeater can be located in the center of the star - these are fairly simple and reliable devices. Star LANs are generally less reliable than network topologies such as "tire" or "tree", but they can be protected against cable disturbances by a central relay that shuts off the failed cable beams. Note that a topology like "star" requires more cable to implement than "tire" or "ring".

Figure 4.4. Star topology

V sequential configurations each physical sublayer transmits information to only one workstation. To the transmitters or receivers of the station, more low requirements than in broadcast configurations, and different parts of the network can use different types physical transmission medium.

The simplest way to build a LAN is to directly connect all devices that need to interact with each other, using data transmission channels from device to device. Each channel can use, in principle, different transmission methods and various interfaces, the choice of which depends on the structure and characteristics of the connected devices. This method of connecting devices is quite satisfactory for a LAN with limited number connections. The main advantages of this method are the need to connect nodes only on physical level, in simplicity software implementation connections, in the simplicity of the structure of the interfaces. However, there are also disadvantages such as high price, big number channels, the need for routing information.

Another common way of connecting subscriber systems in a LAN when they small number- hierarchical connection. In him intermediate nodes work according to the principle of “accumulate and transfer”. The main advantages of this method lie in the possibility of optimal connection of computers included in the network. The disadvantages are mainly related to the complexity of the logical and software LAN structures... In addition, in such a LAN, the speed of information transfer between subscribers of various hierarchical levels is reduced.

The most common sequential configurations are - "ring"(fig. 4.5.), Chain, star with an intellectual center, snowflake.

Rice. 4.5. Ring topology

In configurations "ring" and "chain" for the proper functioning of the LAN, it is necessary Full time job all blocks. To reduce this dependence, a relay is switched on in each of the blocks, which blocks the block in case of malfunctions. Signals usually travel around the ring in one direction only. Each LAN station has memory ranging from a few bits to a whole packet. The presence of memory slows down the transfer of data in the ring and causes a delay, the duration of which depends on the number of stations. Information is transmitted from station to station, returning again to the sending station, the sender during the processing of the packet can set some confirmation indicator. This indicator is used for flow control and / or handshaking. Flow control involves the removal of packets from the ring by the receiving station or after a full circle is completed by the sending station. Since any station may fail and the packet may not reach its intended destination, it is necessary to install a special "garbage collector", which recognizes and destroys such "Lost" packages.

As a sequential configuration, "ring" especially vulnerable to rejections. Failure of cable segments terminates service to all users. At the same time, the ring structure provides many functionality LAN with high efficiency of using a mono-channel, low cost and sufficient reliability of the LAN. The ring structure retains the advantages of the bus: ease of expanding the LAN and control methods, high throughput with low power consumption and average speed of LAN elements and nodes. In addition, in a ring LAN, a number of drawbacks of a common bus are eliminated due to the ability to monitor the operability of a mono-channel by sending a message around the ring.

It should also be noted that in broadcast configurations and in most sequential configurations (with the exception of a ring), each cable element must provide data transmission in different directions with two directional cables and use in broadband systems different carrier frequencies to transmit signals in two different directions.

The presence of a single cable leads to additional loading systems due to the need to “reverse” the direction of transmission in the cable. V large systems when working at high speeds, this disadvantage can become very significant. Full duplex transmission must maintain the same transmission characteristics, which can cause certain technical difficulties. For example amplifiers cable TV and fiber optic connectors usually only carry information in one direction. In this respect, ring topology LANs have an advantage, since they make it possible to use unidirectional signal amplifiers and unidirectional optoelectronic information channels in both directions.

Thus, for local computer networks, the following characteristic features can be distinguished: relative simplicity of network configuration; use of high-speed digital channels data transmission; high level functional interaction of network users; placement of the network in a limited area, where all the main information flows are closed; relatively low cost of network equipment, including network adapters.

LAN configuration

LAN configuration is a method of connecting computers to each other. The term "network topology" is sometimes used. One of the three main types of connection is most commonly used.

1. 1. Bus - all computers are, as it were, built in one line, that is, there are taps from one cable to each of the computers in the network, and the ends of the cable are open. Most often, such a scheme is used to connect several computers installed in one room, for which a thin or thick coaxial cable is used. The most striking drawback of this topology is that any break in the cable loses communication between all computers.
2. This topology is used for networks such as Ethernet, when all the computers on the network are connected to each other in parallel with sequential inclusion into the main line, and the ends of the line are closed with "terminators". If to network board the T-connector is already connected, then the cable can be connected to it using the BNC-connector (Bayonet Nut Connector).
3. When using this topology, keep in mind that adding new computers may require a complete network reorganization.
4. 2. Ring - all computers, as in the previous case, are connected to each other using one cable, the ends of which are connected between them. This scheme is primarily used to create IBM Token Ring networks. Now any break in the cable no longer leads to the loss of communication between computers. To connect several rings, special devices are used.
5. 3. Star - each computer on the network is connected with a separate cable to one PC, which plays the role of file server... The most common scheme - a local network is created on the basis of "twisted pair" with the use of a hub. A break in the cable leads to the loss of contact with only one computer or a network segment, but to create a network according to this topology, a special distributor is required (the connection of computers is completely parallel).

Depending on the size of the local network, the location of computers and their role in this network, a combined connection scheme may be encountered.

Before purchasing network equipment, you should carefully study the conditions in which the local network will be operated in order to accurately calculate both the cable length required to create it and the bandwidth, which determines the type of cable used and the need for purchase. additional devices... It is better even to display the diagram on paper. future network with all its parameters - the length of the segments, the location of computers, etc.

Based on the physical location of each computer, you should compile detailed plan laying the cable taking into account the architectural features of the room, such as corners, projections, bends (you are not going to throw the wire in the middle of the room, because it takes a long time). It should be noted that the total cable length (from computer to computer) should not exceed 100 m, otherwise you will have to use a device that amplifies the signal, the so-called "repeater" (repeater), which in turn allows you to create networks of any length ...

It should be noted that the price of the entire kit usually consists of the following components:

• cable required to cover the distance between computers and all additional devices (hub, switch, repeater) plus 20-30 cm for each connection in addition, so that it remains possible to move system unit or the same hub, if the need arises;
• connectors at each end of the cable, both on the computer side and on the side of additional devices (hub, switch, repeater);
• fasteners for a cable, which can be either plastic boxes or ordinary "squiggles" with studs;
• a special crimping tool, without which, most likely, you will not be able to get a stable working local network. You can, of course, do with pliers, but in this case you will have to try very hard so that the quality of the compression is sufficient.

When laying the network, one should adhere to the rule that the cable must be well protected from any external influences (heels, animals, precipitation), which is usually achieved by laying it in a special plastic casing. When laying the cable outdoors, keep in mind that it is highly discouraged for the cable to sag under its own weight, i.e. when laying the cable, for example, from the roof of one house to another, steel wire should be used. At the same time, when laying the cable outdoors, it should be borne in mind that in winter, due to the temperature difference with strong tension, a rupture can occur. It should be noted that due to some sagging of the cable when it is thrown, for example, from the roof of one building to the roof of another building, its length should be increased by about 1.5 times in the calculations.

Before fixing the connectors to the ends of the cable, pay attention to the specifics of its routing. For example, if you have to pull the cable through holes drilled somewhere, then it is better to first stretch it, and only then install the connectors, otherwise you will have to expand the holes.

Lesson topic: Local computer networks.

Lesson objectives:

1. Master the types of local computer networks;
2. Have an idea of ​​their capabilities

Lesson Objectives

Educational:

• to give an idea of ​​the purpose of computer networks, their types.
• to acquaint students with the structure of local networks.
• teach selection different types topologies of local networks.

Developing:

• develop students' ability to exchange files in a local computer network.
• to instill in students the basic techniques of working in the network.
• develop skills for identifying network topology.

Educational

• instill an interest in the subject.
• to form the skills of independence and discipline, the basics of communicative communication.

Students must:

1. Know the concept of computer networks, their types.
2. Know the concept of a local network, its purpose and organization.
3. Be able to correctly determine the topology of a local network, identify the shortcomings of each topology.

Equipment: Classroom LAN, computer, screen, projector, presentation on the topic.

Lesson plan:

1. Organizational moment - 2 min.
2. Explanation new topic- 25 minutes
3. Fastening of new material - 8 min.
4. Summing up the lesson and homework- 5 minutes.

Introduction

The arising problem of transferring information between users over a certain distance is solved by applying different channels transmission of information that can use different physical principles. So for example, for direct communication people, information can be transmitted using sound waves, when talking on the phone - using electrical signals propagating through communication lines. Using communication channels of various physical nature (cable, fiber-optic, radio channels, etc.), it is possible to transfer information between computers. Practical need quick access To information resources other computers, printers, and other peripheral devices has been the cause of the emergence of computer networks. By way mutual arrangement networked computers, networks are divided into two types:

• Local area networks.
• Global networks.

Let's get acquainted with the concept and capabilities of local computer networks.

I. Let's define a computer network:

A computer network is a system of computers linked by channels transmission of information.

Small-scale computer networks operating within the same room, building, on a relatively short distance are called local area networks(LS).

An example of a local computer network is a computer network in an informatics classroom, a school-wide LAN that unites computers installed in subject rooms. Also, various departments of enterprises, firms, institutions are united in the LAN. Typically, LAN computers are located at a distance of no more than 1 km from each other.

Let's answer the question "How does autonomous work on a PC differ from work on the same PC that is part of the LAN?" on the example of drugs at school.

(There is a discussion that should lead to a conclusion about the benefits of networking).

So there are two basic goals in use LS:

• File exchange between network users;
• Using public resources: large space disk memory, printers, centralized base data, software and etc.

Users of a shared local area network are usually called working group , and computers connected to the network - workstations .

If all computers on the network are equal, i.e. the network consists only of workstations (PC) - then it is called peer-to-peer.

II. Computer network topology

Local area networks (LAN), depending on the purpose and technical solutions, can have various structures combining computers. This structure is called configuration, architecture, network topology.

The general scheme for connecting computers in a local network is called a network topology.

There are two main classes of networks, distinguished by the way computers are connected:

1. broadcast configuration (each computer transmits information that can be perceived. by all other computers on the network);
2. sequential configuration (a computer can transmit information only to its nearest neighbor). The most common network topologies are:

• Bus topology;
• Star topology;
• Ring topology.

In broadcast configurations each personal computer transmits signals that can be picked up by other computers. These configurations include bus, tree, and star passive center topologies. A star with a passive center can be thought of as a kind of tree, rooted with a branch to each connected device.

In sequential configurations each physical sublayer transmits information to only one personal computer... Examples of sequential configurations are: arbitrary (arbitrary connection of computers), hierarchical, "ring", "chain", "star with an intelligent center", "snowflake", etc.

Let's take a quick look at the three most common (basic) LAN topologies: star, bus, and ring.

Consider the following schemes:

1. Topology of the "linear bus" type.

A variant of connecting computers to each other, when a cable passes from one computer to another, connecting computers in series and peripherals each other (as shown in Fig. 1) is called linear bus.

Rice. 1. Bus topology

An example of such a configuration is the following connection. Information via the bus is transmitted to all PCs in the network, but it is received only by the PC for which this information is intended.

2. Topology of the "ring" type.

Type topology "ring" implies the connection of computers in a network with a closed curve - a transmission medium channel. The output of one host is connected to the input of another. Closed loop information is transmitted from PC to PC. In a relatively small space, such a topology is preferable, although the failure of one of the computers in the "ring" violates the integrity of the network.

At ring topology data is transferred from one computer to another over the relay (Fig. 2). If some computer receives data that is not intended for him, he transfers it further along the ring. The addressee does not transmit the data intended for him anywhere.

Rice. 2. Ring topology

A special form of ring topology is the logical ring local area network. It is physically mounted as a star topology junction. Individual stars are switched on using special switches (eng. Hub- hub), which in Russian is also sometimes called “hub”. Depending on the number of workstations and the length of the cable, active or passive concentrators are used between workstations. Active hubs additionally contain an amplifier for connecting 4 to 16 workstations. The passive hub is a purely branching device (for a maximum of three workstations). Management of an individual workstation in a logical ring local computer network occurs in the same way as in a conventional ring local area network. Each workstation is assigned an address corresponding to it, at which control is transferred (from the oldest to the youngest and from the youngest to the oldest). The disconnection occurs only for the downstream (closest) node of the local computer network, so only in rare cases the operation of the entire local computer network can be disrupted.

3. Star topology.

The connection option, when a separate cable is connected to each computer, from one central site, is called a "star" configuration.

When star topology each computer through a special network adapter connected with a separate cable to the central unit (Fig. 3). The central node is a passive connector or an active repeater.

Rice. 3. Star topology

Typically, with this connection scheme, the more powerful computer is the central node. A variation of the star topology is the radial topology.

4. Tree topology.

Computers on the network can be located on different levels(floors). In this case, such a configuration can be applied, which is often called "Snowflake".

Let's consider the possibilities of networks with different topologies.

Network topology	Dignity	Flaws
Bus topology	simplification of the logical and software architecture of the network; ease of expansion; simplicity of management methods; minimum cable consumption; no need for centralized management; reliability (failure of one PC will not disrupt the operation of others).	the cable connecting all stations is one, therefore, the PCs can only "communicate" in turn, which means that special means to resolve conflicts; it is difficult to find cable faults; if it is broken, the operation of the entire network is disrupted.
Star topology	reliability (failure of one station or cable will not affect the operation of others).	a large amount of cable is required; reliability and performance are determined by the central node, which can be a bottleneck (therefore, this equipment is often duplicated).
Ring topology	low cost; high efficiency of using a mono channel; ease of expansion; simplicity of management methods.	if at least one computer fails, the entire network is paralyzed; a buffer for intermediate storage is required on each workstation transmitted information that slows down data transfer; connecting a new station requires disconnecting the network, so special devices are being developed to block circuit breaks.

The structure of the network influenced the creation of the system itself information support, called information space, which also has a network structure. Everything informational space can be imagined as navigation system, a certain set of programs that allow the user to navigate in all the variety of information posted on the network and find the factual data he needs, historical information, useful programs... Most often, the navigation system is organized through a system of nested menus. The user does not need to remember the address or the name of the resource and the sequence of commands required to access it: moving through the program menu, you can navigate the contents of various computers connected to the network.

The topology of a real LAN can be exactly the same as one of the above, or include a combination of them. Network structure in general case is determined by the following factors: the number of combined computers, requirements for the reliability and efficiency of information transfer, economic considerations, etc.

Combining computers into single network provides network users with new opportunities incomparable with the possibilities individual computers... The network is not an addition, but a multiplication of the capabilities of individual computers. A local network allows organizing the transfer of files from one computer to another or others, sharing computing and hardware resources, combining distributed data processing on several computers with centralized storage of information, and much more. With the help of a computer local network, the collective use of technical resources is carried out, which has a beneficial effect on the psychology and behavior of the user, not only in the network, but also in real life.

Network hardware resources

Network hardware resources- it optional equipment that can be connected to the network and shared between users. Hardware resources expand the capabilities of the network.

Printers, scanners, modems and fax modems, CD-ROMs are all hardware resources on the network.

Peer-to-peer, decentralized or peer-to-peer(from the English. peer-to-peer, P2P- equal to equal) networks are computer networks based on the equality of participants. In such networks, there are no dedicated servers, and each peer is both a client and a server. Unlike a client-server architecture, such an organization allows the network to remain operational for any number and any combination of available nodes. So to say "face to face."

The phrase "peer-to-peer" was first used in 1984 by Parbawell Yohnuhuitsman when developing the IBM Advanced Peer to Peer Networking architecture.

To prepare the lesson, the following literature was used:

1. Informatics and ICT 8. Textbook for grade 8. Ugrinovich N.D. - M .: BINOM, 2008;
2. Workshop on Informatics and information technology: Tutorial... / Ugrinovich N.D. and others - M.: BINOM. Knowledge laboratory, 2007.
3. Teaching the course "Informatics and ICT". Ugrinovich N.D. Methodical manual - 4th ed., Revised - M .: BINOM, 2007;
4. Simonovich S.V., Evseev G.A., Alekseev A.G. General Informatics: A Textbook for High School. - M .: Ast-press, Informkom-press, 2001 .-- 592 p.
5. Methods of teaching informatics: Textbook. manual for stud. ped. universities / M.P. Lapchik, I. G. Semakin, E.K. Henner; Ed. M.P. Lapchik. - M .: Publishing Center "Academy", 2001. - 624 p.