A system of interconnected local networks. General principles of organizing local networks. Let's take a closer look at the classification of LAN

3.5. Local area networks

Local area network (LAN) called the joint connection of several separate computers to a single data transmission channel. The concept of LAN (English LAN - Local Area Network) refers to geographically limited (territorially or production) hardware and software complexes in which several computer systems are interconnected using appropriate communications means.

A LAN provides the ability to simultaneously use programs and databases by several users, as well as the ability to interact with other workstations connected to the network. Through a LAN, the system combines personal computers located at many remote workplaces, which share equipment, software and information. Employees' workplaces are no longer isolated and are combined into a single system.

The most important characteristic of a LAN is the speed of information transfer. Ideally, when sending and receiving data over a network, the response time should be almost the same as if it were received from the user's PC rather than from somewhere else on the network. This requires data transfer speed 10 Mbit/s and higher. The following speeds are actually achieved:

· Coaxial cable – 10¸ 50 Mbaud;

· Twisted pair – up to 10 Mbaud;

· Special twisted pair cable of category 5 – up to 100 Mbaud;

· Optical fiber - up to 1 Gbaud;

· Telephone line - from 2400 baud to 56 kbaud;

· Satellite 10,000 computers at the same time and the speed is about 1 Mbaud.

LAN components: network devices and communications.

The LAN implements the principle of modular organization, which allows you to build networks of various configurations with different functionality. The main components from which the network is built are the following:

transmission medium – coaxial cable, telephone cable, twisted pair, fiber optic cable, radio broadcast, etc.;

workstations– PC, workstation or network station itself. If the workstation is connected to a network, it may not require a hard drive or floppy disks. However, in this case, a network adapter is required - a special device for remotely loading the operating system from the network;

interface boards – network cards for organizing interaction of workstations with the network;

servers– separate computers with software that perform the functions of managing shared network resources;

network software .

Let's look at some of the listed network components in more detail.

Servers

A network may have one or more servers. Various servers can be used to manage the network ( network servers), storing information in the form of files ( file servers), searching and retrieving information from databases ( database servers), information distribution ( mail servers), network printing ( print servers) etc. Server disks are accessible from all other workstations on the network if users have the appropriate permissions.

The interaction of the server with workstations occurs approximately according to the following scheme. As necessary, the workstation sends a request to the server to perform any actions: read data, print a document, send an email, etc. The server performs the requested action and issues a confirmation.

Transmission medium

Transmission media are characterized by the speed and range of information transmission and reliability.

The most commonly used means of communication in a LAN are twisted pair cables, coaxial cables, and fiber optic lines. When choosing a transmission medium, the following indicators must be taken into account:

· information transfer speed;

· range transfer of information;

· security of information transmission;

· reliability of information transmission ;

· cost of installation and operation.

Simultaneously meeting the requirements for the transmission medium is a difficult task. For example, high data transmission speeds are often limited by the maximum permissible distance for reliable data transmission, while ensuring the necessary level of protection of the transmitted data. The cost of communication means affects the ability to build and expand the network.

Let us consider in more detail the properties of some transmission media.

twisted pair

Twisted pair wire connection, the cheapest among transmission media. Allows you to transmit information at speeds of up to 10 Mbit/s, easy to expand, low noise immunity. The cable length does not exceed 1000 m with a transmission speed of 1 Mbit/s. To increase the noise immunity of information, shielded twisted pair cable is used, placed in a shell similar to the screen of a coaxial cable. The price of such a pair is close to the price of a coaxial cable.

Coaxial cable

Coaxial cable is used for communication over distances of up to several kilometers, has good noise immunity at an average price. Information transfer speeds range from 1 to 10 Mbit/s, in some cases reaching 50 Mbit/s. Coaxial cable can be used for broadband transmission of information.

Broadband coaxial cable.

This coaxial cable is weakly susceptible to interference, is easy to expand, but has a high price. The information transfer speed reaches 500 Mbit/s. To transmit information over a distance of more than 1.5 km in the base frequency band, it is necessary repeater(amplifier), while the stable transmission distance increases to 10 km. For a LAN with a bus or tree topology, the cable must have a terminator (terminating resistor).

Ethernet cable

Thick Ethernet

Coaxial cable with a characteristic impedance of 50 Ohms (thick Ethernet. or yellow cable). Uses 15-pin standard connection. The maximum permissible transmission distance without a repeater does not exceed 500 m, and the total length of the Ethernet network is 3000 m. Thick Ethernet, due to the backbone topology, uses only one terminator at the end. In terms of noise immunity, it is an expensive alternative to conventional coaxial cable.

Thin Ethernet

Coaxial cable with a characteristic impedance of 50 ohms (thin Ethernet) and an information transfer rate of 10 7 bps, cheaper than thick Ethernet.

LANs with thin Ethernet cable are characterized by low cost, minimal expansion costs and do not require additional shielding. The cable is connected to the network cards of workstations using tee connectors ( T-connectors ) with small-sized bayonet connectors (CP-50). Repeaters are required when connecting thin Ethernet segments. The distance between workstations without repeaters cannot exceed 300 m, and the total length of the network is 1000 m.

Fiber optic cable

The most expensive transmission medium for a LAN is fiber optic cable, also called fiberglass cable. The speed of information transfer through it reaches several gigabits per second with an allowable length of more than 50 km. The noise immunity of fiber optic cable is very high, so LANs based on it are used where electromagnetic interference occurs and information transmission over long distances is required without the use of repeaters. The networks are resistant to eavesdropping because the branching technique in fiber optic cables is very complex. Typically, LANs based on fiber optic cable are built in a star topology.

Characteristics of typical transmission media are given in the table.

IVS topology

Topology, i.e. configuration of connection of elements in a LAN , attracts attention to a greater extent than other network characteristics. This is due to the fact that it is the topology that largely determines the most important properties of the network, such as reliability and performance.

There are different approaches to classifying LAN topologies. According to one of them, local network configurations are divided into two main classes: broadcast And sequential .

IN broadcast configurations, each PC transmits signals that can be perceived by other PCs. Such configurations include a common bus, a tree (connecting several common buses using repeaters), and a star with a passive center. The advantages of configurations of this class are the simplicity of network organization.

IN consecutive configurations, each physical sublayer transmits information to only one PC. Such configurations include a star with an intellectual center, a ring, a hierarchical connection, and a snowflake. The main advantage is the simplicity of the software implementation of the connection.

To prevent collisions in information transmission, it is most often used time division method , according to which each connected workstation at certain points in time is granted the exclusive right to use the information transmission channel. Therefore, the requirements for network throughput under increased load, i.e. when new workstations are introduced, they decrease.

Different topologies implement different principles of information transfer . In broadcast it is selection of information, in successive – information routing.

In a broadband LAN, workstations are assigned a frequency on which they can send and receive information. The transmitted data is modulated at the appropriate carrier frequencies. The technology of broadband messages makes it possible to simultaneously transport a fairly large amount of information in a communication environment.

Star topology .

Network topology in the form stars with an active center inherited from the area mainframes , where the head machine receives and processes all data from the terminal devices as the active data processing node. All information between peripheral workstations passes through the central node of the computer network.

Network throughput is determined by the computing power of the central node and is guaranteed for each workstation. Collisions, i.e. There are no collisions in data transmission.

The cabling of the topology is relatively simple, since each workstation is connected to a central node, but the cost of laying communication lines is high, especially when the central node is not geographically located in the center of the topology.

When expanding a LAN, it is impossible to use previously made cable connections: a separate cable from the central network node must be laid to the new workstation.

Star topology with good central node performance is one of the fastest topologies LAN, since the transfer of information between workstations occurs over dedicated lines used only by these workstations. The frequency of requests to transfer information from one station to another is low compared to other topologies.

Figure 1. Star topology

The performance of a star topology LAN is primarily determined by the parameters of the central node, which acts as network server. It may turn out to be a network bottleneck. If the central node fails, the operation of the network as a whole is disrupted.

In a LAN with a central control node, it is possible to implement an optimal mechanism for protecting against unauthorized access to information.

Ring topology.

In a ring network topology, LAN workstations are connected to each other in a circle. The last workstation is connected to the first, i.e. the communication link is closed in a ring.

Laying communication lines between workstations can be quite expensive, especially if the workstations are located far from the main ring.

Messages in the LAN ring circulate in a circle. The workstation sends information to a specific address, having previously received a request from the ring. The transmission of information turns out to be quite efficient since messages can be sent one after another. For example, you can make a ring request to all stations. The duration of information transfer increases in proportion to the number of workstations included in the LAN.

Rice. 2. Ring topology

The main problem of the ring topology is that each workstation must participate in the transfer of information, and if at least one of them fails, the entire network is paralyzed. Faults in the cable system are easily localized.

Expanding a network with a ring topology requires stopping the network because the ring must be broken. There are no special restrictions on the size of the LAN.

A special form of ring topology is logical ring .

Physically, it is mounted as a connection of star topologies. Individual stars are switched on using special switches (English: Hub), which in Russian are also sometimes called “hubs”. Depending on the number of workstations and the length of the cable between workstations, active or passive hubs are used. Active hubs additionally contain an amplifier for connecting from 4 to 16 workstations. The passive hub is purely a splitter device (for a maximum of three workstations). Managing an individual workstation in a logical ring network is the same as in a regular ring network. Each workstation is assigned an address corresponding to it, through which control is transferred (from senior to junior and from junior to senior). The connection is broken only for the downstream (closest) node of the computer network, so that only in rare cases can the operation of the entire network be disrupted.

Bus topology

In a LAN with a bus topology, the main transmission medium ( tire) – common for all workstations. The functioning of the LAN does not depend on the state of the individual workstation, i.e. workstations can be connected to or disconnected from the bus at any time without disrupting the operation of the network as a whole.

Rice. 3. Bus topology

However, in the simplest Ethernet network with a bus topology, a thin Ethernet cable with a tee connector is used as the transmission medium ( T -connector), therefore expanding such a network requires breaking the bus, which leads to disruption of the functioning of the network. More expensive solutions involve installation instead T - connectors of passive plug boxes.

Since the expansion of a LAN with a bus topology can be carried out without interrupting network processes and breaking the communication medium, the removal of information from the LAN and, accordingly, eavesdropping on information is quite easy, as a result of which the security of such a LAN is low.

Characteristics of computer network topologies are given in the table.

Tree topology.

It is formed by various combinations of the LAN topologies discussed above. The base of the tree (root) is located at the point where communication lines (tree branches) gather.

Networks with a tree structure are used where direct application of basic network structures is not possible. To connect workstations, devices called hubs .

There are two types of such devices. Devices to which a maximum of three stations can be connected are called passive concentrators. To connect more devices you need active hubs with the possibility of signal amplification.

Types of LAN construction based on information transfer methods.

Token Ring Network

This standard was developed by IBM. Unshielded or shielded twisted pair or optical fiber are used as the transmission medium. Data transfer rates from 4 Mbit/s to 16 Mbit/s. As access control method workstations to the transmission medium are used marker ring (Token Ring). Basic principles of the method:

¨ ring LAN topology;

¨ the workstation can transmit data only after receiving the token, i.e. permission to transmit information;

¨ At any given time, only one station in the network has this right.

In LAN To k e n Ring uses three main types of packets:

¨ control/data package (Data/Command Frame);

¨ token;

¨ reset package (Abort).

Management/Data Package . Using such a packet, data or network control commands are transmitted.

Marker.The station can start transmitting data only after receiving such a packet. There can be only one marker in the ring and, accordingly, only one station with the right to transmit data.

Reset Pack.Sending such a packet causes the transmission of information to cease.

Network To k e n Ring allows you to connect computers in a star topology.

Arknet local network.

Arknet (Attached Resource Computer NETWork) is a simple, inexpensive, reliable and flexible LAN architecture. Developed by Datapoint Corporation in 1977. Subsequently, the license for Arcnet was acquired by SMC Corporation (Standard Microsistem Corporation), which became the main developer and manufacturer of equipment for Arcnet networks. Twisted pair, coaxial cable with a characteristic impedance of 93 Ohms and fiber optic cable are used as transmission media. Data transfer speed is 2.5 Mbit/s. When connecting devices to bus and star topologies are used. Access control method stations to the transmitting medium – marker tire (Token Bus). The method provides the following rules:

¨ devices connected to the network can transmit data only after receiving permission to transmit (token);

¨ at any given time, only one station in the network has this right;

Operating principles

Each byte is transmitted to Arcnet by sending an ISU (Information Symbol Unit) consisting of three service start/stop bits and eight data bits. At the beginning of each packet, the initial separator AB (Alegt Burst), which consists of six service bits, is transmitted. The start delimiter acts as a packet preamble.

Arcnet defines 5 types of packets:

1. ITT package(Information To Transmit) – an invitation to transmit. This message transfers control from one network node to another. Station that received the packet ITT , receives the right to transfer data.

2. FBE package(Free Buffеr Enquiries) – request for readiness to receive data. This packet checks the node's readiness to receive data.

3. Data package.Data transmission is carried out using this parcel.

4. ASK package (ACKnowledgments) – confirmation of reception. Confirmation of readiness to receive data or confirmation of receipt of a data packet without errors, i.e. response to FBE and data packet.

5. Package NAK(Negative AcKnowledgments) unpreparedness to receive. The node is not ready to receive data in response to an FBE or received a packet with an error.

Ethernet LAN

The Ethernet specification was proposed by Xerox in the late seventies. Later, Digital Equipment Corporation (DEC) and Intel joined this project. In 1982, the Ethernet specification version 2.0 was published. The IEEE 802.3 standard was developed based on Ethernet.

Basic operating principles

¨ bus topology at the logical level;

¨ all devices connected to the network have equal rights, i.e. any station can start transmitting at any time (if the transmitting medium is free);

¨ Data transmitted by one station is available to all stations in the network.

Introduction

One of the basic human needs is the need for communication, which becomes possible when people understand each other. To do this, they study languages, master the culture of communication, and use modern means and methods of communication. Communication in a broad sense refers to the process, path and means of transmitting an object or message from one place to another. Communications can be organized using different transmission media, for example, water and air communications, gas pipelines, railways and highways, etc.
Computer networks provide invaluable assistance to people, the emergence of which marked a new era in the history of communications development. With the advent of computer networks, people began to talk about computer communications, meaning the exchange of all kinds of information using computers. They are increasingly entering our lives, in some cases displacing, and in others complementing existing ones. Being far away from each other, you exchange letters by mail - in a computer network this type of communication is known as e-mail. To discuss some important problem, you organize a meeting, meeting, conference. A corresponding type of communication exists in a computer network. This is a teleconference. Computer communications are in many ways reminiscent of traditional ones, but at the same time, mail delivery time is significantly reduced, communications are organized more quickly, the ability to communicate with a larger circle of people expands, and prompt access to global information repositories appears.
Computer communications are provided using computer networks: local, regional, corporate, global.
At the lecture you will learn how they differ from each other and what their hardware is, namely: what components ensure the network operates, what communication channels are used, what a modem and network adapter are, what role protocols play in computer networks, and much more.

Computer networks. Basic information.

Telecommunication(from Greek tele - “afar”, far ~ and Lat. communicato - “communication”) - this is the exchange of information at a distance.
Radio transmitter, telephone, teletype, fax machine, telex and telegraph are the most common and familiar examples of telecommunications technology to us today.
Later, another means was added to them - computer communications, which are now becoming increasingly widespread. They promise to supplant fax and teletype communications, just as the latter supplanted the telegraph.

Computer communications– exchange of information at a distance using computer networks.

Nowadays, computer networks are becoming increasingly important in the life of mankind, and their development is very promising. Networks can unite and make available the information resources of both small enterprises and large organizations occupying premises remote from each other, sometimes even in different countries.

Computer networks– a system of computers connected by information transmission channels.

The purpose of all types of computer networks is determined by two functions:
- ensuring the sharing of hardware and software network resources;
- providing shared access to data resources.
For example, all participants in a local network can share one common printing device - a network printer or, for example, the hard drive resources of one dedicated computer - a file server. Software can be shared in the same way. If there is a special computer on a network that is dedicated to sharing among network members, it is called a file server.

Networks by dimension are divided into local, regional, corporate, global

local network(LAN - Local Area Network) - a connection of computers located at short distances from each other (from several meters to several km). PCs in such networks are located in the same room, in the same enterprise, in nearby buildings.
Local networks do not allow shared access to information among users located, for example, in different parts of the city. They come to the rescue regional networks, connecting computers within one region (city, country, continent).

regional net(MAN - Metropolitan Area Network) - combining PCs and local networks to solve a common problem on a regional scale. Regional A computer network connects computers located at a considerable distance from each other. It may include computers within a large city, economic region, or individual country. Typically, the distance between subscribers of a regional computer network is tens to hundreds of kilometers.
Many organizations interested in protecting information from unauthorized access (for example, military, banking, etc.) create so-called corporate networks. A corporate network can unite thousands and tens of thousands of computers located in different countries and cities (an example is the Microsoft Corporation network)

corporatenetworks - association of local networks within one corporation.

The needs of forming a single world information space led to the creation of the global computer network Internet.

global networks(WAN - Wide Area Network) – a system of interconnected local networks and user PCs located at remote distances for the common use of world information resources .
Information networks create a real opportunity for quick and convenient user access to all the information accumulated by humanity throughout history.

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

Wired (coaxial cable, twisted pair, fiber optic);
- wireless with information transmission via radio channels or in the infrared range.
According to the method of organizing the interaction of network computers divided into peer-to-peer and dedicated server (hierarchical networks).
All computers in a peer-to-peer network have equal rights. Any network user can access data stored on any computer.
The main advantage of peer-to-peer networks is the ease of installation and operation. The main disadvantage is that in peer-to-peer networks it is difficult to resolve information security issues. Therefore, this method of organizing a network is used for networks with a small number of computers and where the issue of data protection is not fundamental.
In a hierarchical network, when setting up a network, one or more servers are allocated in advance - computers that manage data exchange over the network and resource distribution. Any computer that has access to the server's services is called a network client or workstation.

The general scheme for connecting computers into local networks is called network topology. There are only 5 main types of network topologies:

1. BUS topology. In this case, the connection and data exchange is carried out through a common communication channel, called a common bus. The bus type structure is simpler and more economical, since it does not require an additional device and consumes less cable. But it is very sensitive to cable system faults. If the cable is damaged in even one place, then problems arise for the entire network. The location of the fault is difficult to locate.

2. STAR topology. In this case, each computer is connected by a separate cable to a common device called a hub, which is located in the center of the network. The star cable system is more resistant to faults in the cable system. A damaged cable is a problem for one specific computer; it does not affect the operation of the network as a whole. No effort is required to isolate the fault. The disadvantages of a star topology include the higher cost of network equipment due to the need to purchase a hub. In addition, the ability to increase the number of nodes in the network is limited by the number of hub ports. Currently, this structure is the most common type of connection topology in both local and global networks.

3. RING topology. In networks with a ring topology, data in the network is transmitted sequentially from one station to another along the ring, usually in one direction. If the computer recognizes the data as intended for it, then it copies it to its internal buffer. In a network with a ring topology, it is necessary to take special measures so that in the event of a failure or disconnection of any station, the communication channel between the remaining stations is not interrupted. The advantage of this topology is ease of management, the disadvantage is the possibility of failure of the entire network if there is a failure in the channel between two nodes.

4. Mesh topology. The mesh topology is characterized by a computer connection scheme in which physical communication lines are established with all adjacent computers. In a network with a mesh topology, only those computers between which intensive data exchange occurs are directly connected, and for data exchange between computers that are not directly connected, transit transmissions through intermediate nodes are used. The mesh topology allows the connection of a large number of computers and is typically characteristic of global networks. The advantages of this topology are its resistance to failures and overloads, because There are several ways to bypass individual nodes.
5. Mixed topology. While small networks typically have a typical star, ring, or bus topology, large networks typically have random connections between computers. In such networks, it is possible to distinguish separate arbitrary subnetworks that have a standard topology, which is why they are called networks with a mixed topology.

The operating principles of various electronic networks are approximately the same:

1. The network consists of interconnected PCs
In most cases, the network is built on the basis of several powerful computers called servers. Servers and, accordingly, second-order (regional), third-order (corporate), fourth-order (local) networks are usually connected to global network servers, and users of individual computers are connected to them - subscribers(clients) networks. Note that not all intermediate levels (for example, corporate) networks are required.

2. PCs are interconnected by communication channels
The main purpose of creating any computer network is to ensure the exchange of information between objects (servers and clients) of the network. To do this, it is necessary to communicate between computers. Therefore, the mandatory components of any network are all kinds of communication channels (wired and wireless), for which they use different physical media. In accordance with this, networks distinguish between such communication channels as telephone and fiber optic lines, radio communications, space communications, etc.
The purpose of communication channels in a computer network is easy to understand if you compare them with the transport channels of a freight or passenger transportation system. Transportation of passengers can occur by air, by railways or by water (sea or river) routes. Depending on the transportation environment, the vehicle is chosen. Information is transported through computer networks. The environments in which network computers communicate determine the means by which computers are connected. If it is an environment that requires telephone communication, then the connection is made through a telephone cable. Connecting computers using electrical cables, radio waves, fiber optic cables, etc. is widely used.

Let's look at the main types of channels. Some of them are mutually exclusive, some may describe one channel from different angles.
There are channels digital and analog.
TO analog channels can be considered an ordinary telephone channel. To use it, you need a special device - a modem that converts digital information into analog. Analogue channels are highly susceptible to interference and have low bandwidth (several tens of kilobytes per second). Now there is a trend to replace all analog channels with digital ones, not only in computer networks, but also in telephone networks.
Channels are also divided into dedicated And switched.
When using dial-up line, the connection is formed during the data transfer, and at the end of this transfer it is disconnected. Dial-up is communication via a regular telephone line.

Dedicated the line works differently:
The connection is permanent and always allows you to transfer data from one computer to another. Leased lines differ from dial-up lines in their high speed (up to tens of Megabits per second) and high rental price.
Channels are divided by physical device on electric wired, optical and radio channels.
Wired channels represent a connection with an electrical cable, possibly complexly arranged. All such channels use data transmission using electrical impulses.

Optical channels communications are based on light guides. The signal is transmitted using lasers.

Radio channels operate on the same principle as radio and television.
These are all different communication channels. The efficiency of communication in computer networks significantly depends on the following main characteristics (parameters) of communication channels:
- throughput (data transfer rate), measured by the number of bits of information transmitted over the network per second (bits per second is called baud);
Average throughput – measured on average over a certain period of time (for a large file)
Guaranteed bandwidth – the minimum bandwidth that the channel provides (for video files)
- reliability - the ability to transmit information without distortion and loss;
- cost;
- expansion possibilities (connecting new computers and devices).

To transmit information over communication channels, it is necessary to convert computer signals into signals from physical media.
For example, when transmitting information via a fiber optic cable, the data presented in the computer will be converted into optical signals, for which special technical devices are used - network adapters.

Network adapters (network cards) - technical devices that perform the functions of pairing computers with communication channels.
If the communication channel – telephone line, then a modem is used to receive and transmit information.

Modem– (modulator – demodulator) – a device for converting digital PC signals into audio (analog) telephone line signals and vice versa.
The main characteristic of the modem is the speed of receiving and transmitting information (measured in bits per second). Modern modems have a data transmission and reception speed of 33600 bits per second, 57600 bits per second.

3. The network operates using protocols
In order for information transmitted by one PC to be understood by another PC, it was necessary to develop uniform rules called protocols.

Protocol– a set of agreements on the rules for generating and transmitting messages, on methods of exchanging information between PCs, on the rules of operation of various equipment on the network

There are 2 types of Internet protocols: basic and application protocols.

basic protocols responsible for physical sending electronic messages of any type between Internet computers (IP and TCP). These protocols are so closely related that they are most often referred to as the TCP/IP protocol;

applied higher-level protocols responsible for the functioning of specialized Internet services: HTTP protocol (transfer of hypertext messages), FTP protocol (file transfer), email protocols, etc.
In a technical sense, TCP/IP is not one, but two network protocols. TCP is a transport layer protocol. It controls how information is transferred. The IP protocol is addressable. It determines where data transfer occurs.

4. The operation of a PC on a network is ensured by network programs, usually organized according to the client-server model:

server- a program that provides services, client– a program that consumes server services - programs

IP-addresses

The information exchanged between PCs is divided into packages. A PACKET is a “piece” of information containing the address of the sender and recipient.
A. Many packets form a stream of information that is received by the user's PC
B. Then the “scattered packets” arriving from the network are collected into a single “bundle” by the client program of your PC (for example, the Microsoft InterNet Explorer browser)
C. In order for the packet to find its recipient, each PC is assigned an IP address (upon registration with the provider). An IP address contains 4 bytes (32 bits) separated by dots or 4 numbers from 0 to 255. It is easy to calculate that the total number of different IP addresses is more than 4 billion: 232 = 4294967296.

The lP address is “read” from right to left. Typically, the rightmost number indicates a specific computer, and the remaining numbers indicate network and subnet numbers (i.e. local networks).
Sometimes this may not be the case, but in any case, if the address is represented in binary form, then some of the rightmost bits identify a specific computer, and the rest indicate the networks and subnets to which the computer belongs.

Example. 192.45.9.200. Network address - 192.45; subnet address - 9; computer address - 200.
The packet contains the recipient's address and the sender's address, and is then thrown into the network.
Routers determine the route that packets take.

Domain name system

Computers can easily communicate with each other using a numeric IP address, but it is not easy for a person to remember a numeric address, and for convenience, the Domain Name System (DNS) was introduced.
Domain name system assigns a unique domain name to the numerical IP address of each computer. Domain addresses are assigned by the Internet Network Information Center (InterNIC).

Domain (domain– region, district) – defines a set of PCs belonging to any part of the Internet, within which computers are united according to one characteristic.

Domain address defines an area representing a range of host computers. Unlike a digital address, it is read in reverse order. First comes the name of the computer, then the name of the network on which it is located.
A computer name includes at least two levels of domains. Each level is separated from the other by a dot. To the left of the top-level domain are subdomains for the general domain.
In the Internet address system, it is common to represent domains as geographic regions. They have a name consisting of two letters.
Example. Geographic domains of some countries: France - fr; Canada- sa; USA - us; Russia - ru; Belarus - by.
There are also domains divided by thematic signs. Such domains have three-letter abbreviated name.
Example. Educational institutions - edu. Government agencies - gov. Commercial organizations – com:

tutor.sp tu.edu . Here edu- a common domain for schools and universities. Tutor- subdomain sp tu , which is a subdomain edu.

World Wide Web

The most popular Internet service is the World Wide Web (abbreviated WWW or Web), also called the World Wide Web. The presentation of information on the WWW is based on the capabilities of hypertext links. Hypertext- this is text that contains links to other documents. This makes it possible, when viewing a document, to easily and quickly move to other information related to it in meaning, which can be text, an image, an audio file, or have any other form accepted in the WWW. At the same time, linked documents can be scattered throughout the globe.
Numerous intersecting connections between WWW documents and the computer web span the planet - hence the name. Thus, there is no dependence on the location of a specific document.
The World Wide Web service is designed for access to a special kind of electronic documents, which are called Web documents or, more simply, Web pages. A Web page is an electronic document that, in addition to text, contains special formatting commands, as well as embedded objects (drawings, audio and video clips, etc.).
Browse Web pages using special programs called browsers, so the browser is not just a WWW client used to interact with remote Web servers, it is also a means of viewing Web documents. For example, if a Web page was saved to your hard drive, you can view it using a browser without an Internet connection. This type of browsing is called offline browsing.
Unlike printed electronic documents, Web pages are not absolute, but relative formatting, that is, they are formatted at the time of viewing in accordance with the screen and browser they are viewed on. Strictly speaking, the same Web page can look different when viewed in different browsers - it depends on how the browser reacts to the commands that its author has built into the Web page.
Each Web document (and even each object embedded in such a document) on the Internet has its own unique address - it is called uniform resource locator URL (Uniformed Resource Locator) or, for short, URL. By contacting this address, you can obtain the document stored there.
There are many, many Web documents stored on the Internet. Over the past seven years, WWW content has doubled every year and a half. Apparently, in the coming years this rate will decrease somewhat, but will remain quite high, at least until the 10 billion mark. Due to such a huge number of Web documents, there is an important problem of searching and selecting them on the Web today - we will look at it separately, but for now let’s get acquainted with how a URL formally looks like.
Example URL: http://klyaksa.net/htm/exam/answers/images/a23_1.gif
Here is the URL to the picture located on one of the Web pages of the portal www.klyaksa.net.
A document URL has three parts and, unlike domain names, is read from left to right. The first part indicates the name of the application protocol through which this resource is accessed. For the World Wide Web service, this is the HyperText Transfer Protocol (HTTP). Other services have different protocols. The protocol name is separated from the rest of the address by a colon and two forward slashes.
The second element is the domain name of the computer on which the document is stored. We are already familiar with the structure of a domain name - its elements are separated by dots. The domain name is followed by a slash.
The last element of the address is the path to access the file containing the Web document on the specified computer. We are already familiar with recording the path to a file in the Windows operating system, but there is an important difference. In Windows, it is customary to separate directories and folders with the backslash character "\", while on the Internet it is recommended to use a regular forward slash "/". This is due to the fact that the Internet originated on computers running the UNIX operating system, and there it was customary to divide directories this way.
Each hyperlink on the Internet is associated with the Web address of some document or object (a file with a picture, sound recording, video clip, etc.). When you click on a hyperlink, a request is sent to the Network to supply the item pointed to by the hyperlink. If such an object exists at the specified address, it is downloaded and played. If it does not exist in nature (for example, it ceased to exist for some reason), an error message is displayed - then you can return to the previous page and continue working.

Basic Internet services

1. Electronic mail (E-mail).
Electronic mail (E-mail - Electronic mail, English mail - "mail") is the most common and, until recently, the most popular use of the Internet. According to estimates by the International Telecommunication Union, the number of e-mail users exceeds 50 million. The popularity of e-mail is explained not only by its capabilities, but also by the fact that it can be used with any type of Internet access, even the cheapest.
When using email, each user is assigned a unique email address, which is usually formed by appending the username to the name of the computer itself. The username and computer name are separated by the special symbol @. For example, if the user's login name is emsworth on the computer blandings.corn, then their email address will be: [email protected].

3. Teleconferencing service (Usenet)
Another widely used service provided by the Internet is Usenet news- Usenet news, which is also often called newsgroups (they have nothing to do with television, and the prefix “tele” means “remote”, “operating at a long distance”). They provide the opportunity to read and post messages to public (open) discussion groups.
Usenet is a virtual, imaginary network through which news is transmitted between computers - news servers using a special protocol NNTP (Network News Transfer Protocol).

4. File Transfer Service (FTP) deals with the reception and transmission of large files. The FTP service has its own servers on the global network where data archives are stored. These archives may be commercial or restricted, or may be publicly available.

5. Access to a remote computer (Telnet)
If we recall the history of the development of computers, there was a time when the computer itself was large and stood in a special computer room. The terminals (i.e., displays with keyboards) that allowed the computer to be used were located in another room. The displays were alphanumeric, so the dialogue with the computer consisted of entering symbolic commands, in response to which the computer printed the corresponding data on the screen.
When creating a remote access system, it was decided to preserve this method of dialogue with a computer.
The remote access program is called Telnet.
For its functioning, like all Internet services, it is necessary to have two parts - a server program installed on a remote computer, and a client program on the local computer.
To connect to a remote system, you must be a registered user, that is, have a login name and password. To establish a connection, you must specify the name of the remote computer. After a successful connection, you can do the same operations on the remote computer as on the local computer, i.e. browse directories, copy or delete files, run various programs that have an alphanumeric interface.

6. IRC (Internet Relay Chat) service designed for direct communication between several people in real time. This service is also called chat conferencing or simply chat.

7. ICQ service. Its name comes from the expression I seek you - I am looking for you. The main purpose is to enable communication between two people, even if they do not have a permanent IP address.
8. World Wide Web (WWW) service is a single information space consisting of hundreds of millions of interconnected electronic documents stored on Web servers. Individual documents are called Web pages. Groups of thematically linked Web pages are called Web sites or Web sites.

2.List 3 basic network topologies:

Write down the answer: __________________________________________

3. Select the transmission speed of the medium-speed network.

1) up to 100Mbit/s

2) up to 100MB/s

3) up to 1000Mbit/s

4. Global network is.

1. system of interconnected local networks

2. a system of interconnected computers

3. system of interconnected local telecommunications networks

4. a system of interconnected local networks and computers of individual users

5. To connect two computers via telephone lines you must have:

1. Modem

2. two modems

3. telephone, modem and special software

4. via modem on each computer and special software

6. Which of the following computer connection diagrams is a closed chain?

1. Tire

2. Ring

3. Star

4. There is no right answer

7. Which cable provides data transfer speeds of up to 10 Mbit/s?

1. Coaxial

2. twisted pair

3. fiber optic

4. there is no right answer

8.The largest network size (up to 20 km) has the topology:

Star

Ring

Tire

9.The smallest network size (up to 200 m) has a topology:

Star

Ring

Tire

10. The topology of a computer network in which all computers on the network are connected to a central node is called

1. Tire

2. Ring

3. Star

4. There is no right answer

11. Protocol is

1. the ability of a computer to send files through communication channels

2. device for local network operation

3. standard for data transmission via a computer network

4. standard for sending messages via email

12. Highest level of security

Star

Ring

Tire

13.For general access of network users, the following is used:

1) workstation

2) server

3) client

14. Multiport devices to connect PC using network cable?

Write down the answer: __________________________________________

15. Communication channel providing high-speed transmission?

Write down the answer: __________________________________________

16. A computer that uses server resources is called...

Write down the answer: __________________________________________

17. Data on the network is transmitted in packets no larger than:

1.5 GB

1.5 KB

1.5 Bytes

18. Types of computer networks:

Personal, local, corporate, territorial, global

Personal, local, corporate, city, global

Personal, satellite, 4-G

19. Communication lines are of two types:

Satellite and Glonass

Wireless and WAN

Wireless and wired

20. Bluetooth operates within a radius

10 meters 2) 20-30 meters 3) 100 meters

21. The LAN can connect as much as possible

1000 computers

100 computers

20 computers

22. Eight-pin connector with a latch for connecting a PC to the network:

COM 2. R.J.-48 3. R.J.-45

23 Switches or switches used for:

to select a route

connecting computers into a single network

signal amplification

24. The capacity of the information channel is measured:

1.Hertz 2.Seconds 3.Mbps

25.The lowest throughput and noise immunity is:

Coaxial cable

Telephone cable 3. Twisted pair

Answers

Local area network

Star, Tire, ring

Hub (switch) and switch (hub)

Optical fiber

Client

Grade

21-25 "5"

16-21 "4"

| §4.1 Local and global computer networks

Lesson 24
§4.1 Local and global computer networks

Key words:

Message
communication channel
computer network
information transfer rate
local network
global network

4.1.1. Transfer of information

We have already said earlier that information transfer is one of the most important information processes. Information is transmitted from the source to the receiver in the form of a certain sequence of signals, symbols, signs. For example, during direct conversation between people, sound signals are transmitted - speech; When reading a text, a person perceives graphic symbols - letters. The transmitted sequence of signals, symbols, signs is called a message.

A communication channel (information transmission) is a system of technical means and a signal propagation medium for transmitting messages from a source to a receiver. When people communicate directly, information is transmitted using sound waves, when talking on the phone - using acoustic and electrical signals distributed along communication lines, when reading - using light waves.

Any transformation of information coming from a source into a form suitable for its transmission over a communication channel is called coding. Currently, digital communications are widely used, when the transmitted information is converted into binary code.

Insufficient technical quality of communication channels and some other reasons can lead to distortion of the transmitted signal and loss of information. To avoid such situations, the code transmitted over the communication line is made redundant. Due to this, the loss of some part of the information during transmission can be compensated. In addition, in modern digital communication systems, all messages are divided into parts (packets, blocks). For each block, a checksum (the sum of binary digits) is calculated, which is transmitted along with this block. At the receiving site, the checksum of the received block is recalculated, and if it does not coincide with the original sum, then the transmission of this block is repeated.

For centuries, humanity has used postal services to transmit letters; in the second half of the 19th century, sound transmission technology (telephone) was invented; Since the 30s of the 20th century, telefax began to be used to transmit images. Nowadays, computers are widely used to transmit text, images, sound and many other types of information. computer networks- two or more computers connected by information transmission lines. With the advent of computer networks, it became possible to send a letter that reaches faster than a telegram, receive an answer, find out the latest news, talk to a friend sitting at a computer hundreds of kilometers away as if he were in the next room, book a plane ticket or a room in hotel, “download” the desired program, melody or movie.

An important characteristic of a computer network is information transfer rate, or channel capacity. This value is defined as the amount of information in bits per second (bps) and in derived units: kilobits per second (1 Kbps = 1000 bps), megabits per second (1 Mbps = 1000 Kbps), gigabits per second (1 Gbit/s = 1000 Mbit/s).

There are local and global computer networks.

4.1.2. What is a local computer network

A local computer network unites computers installed in one room (for example, a school computer lab) or in one building (for example, all computers located in a school building can be combined into a local network). A local network allows users to share access to computer resources, as well as to peripheral devices (printers, scanners, disks, modems, etc.) connected to the network.

Local networks are peer-to-peer and dedicated server.

In small local networks, all computers have equal rights, that is, each of them can use the resources of the other. Users independently decide which resources of their computer (files, folders, disks) to make available to the entire network. Such networks are called peer-to-peer.

In networks with a large number of users, it is not desirable for all of them to have access to all computers on the network. When combining more than 10 computers, it is advisable to select the most powerful computer - the server. The server's hard drive contains files (data and programs) that can be accessed by other computers on the network - clients. In addition, peripheral equipment connected to the server (for example, a printer or scanner) can be available to all network users.

Each computer connected to a local network must have a special card - a network adapter. Its function is to transmit and receive signals distributed through communication channels.

Connecting computers (their network cards) to a local network is carried out using various types of cables (twisted pair, optical fiber - Fig. 4.1) or via wireless channels (such as Wi-Fi).

Rice. 4.1. Cables:
twisted pair and optical fiber

A twisted pair consists of two insulated copper wires twisted relative to each other. Twisting the wires this way reduces the effect of interference on the signals transmitted over the cable. A twisted pair connection consists of several twisted pairs (2 or 4) covered with a plastic sheath. Data transfer speed - from 10 Mbit/s to 1000 Mbit/s.

A fiber optic cable transmits light through a glass fiber. This type of connection provides a very high transmission speed, the length of the channel is hundreds and thousands of kilometers, and it is absolutely not susceptible to electromagnetic interference. Data transfer speed - from 100 Mbit/s to 10 Gbit/s.

Wireless Wi-Fi connection provides data transfer speeds of up to 300 Mbps.

4.1.3. What is a global computer network

Local networks, connecting dozens of computers in a small area, do not provide shared access to information to users located at a considerable distance from each other (for example, in different localities).

Global computer network is a system of interconnected computers located at an arbitrarily large distance from each other (for example, in different countries and on different continents).

Examples of global computer networks include regional and corporate networks. Regional computer networks provide the unification of computers within one region (city, region, region, country). Corporate computer networks are created to support the activities of various types of corporate structures that have geographically remote divisions (for example, banks with their branches).

The best known and most extensive global computer network is the Internet. This network unites numerous local, regional and corporate networks, as well as individual user computers distributed throughout the world.

The basis of any global computer network is computer nodes and communication channels.

A node is a powerful computer that is constantly connected to the network. Subscribers - personal computers of users or local networks - are connected to the nodes of the computer network.

To transmit data in global networks, a wide variety of physical channels are used: electrical cable; radio communication through repeaters and communication satellites; infrared rays (as in television remote controls); modern fiber optic cable; regular telephone network.

An organization that provides users with connections to the global network through their computers is called provider(English provider - supplier) of network services.

Task. The data transfer rate through a certain connection is 128,000 bps. How much time (in seconds) will it take to transfer a 625 KB file over this connection?

THE MOST IMPORTANT

Computer network- these are two or more computers connected by information transmission lines.

Local computer network connects computers installed in the same room or building, and provides users with the ability to share access to computer resources, as well as peripheral devices connected to the network. Local networks can be peer-to-peer or with a dedicated server.

Global computer network- this is a set of interconnected computers located at an arbitrarily large distance from each other (for example, in different countries and on different continents).

Questions and tasks

1. Read the presentation materials for the paragraph contained in the electronic appendix to the textbook. What can you say about the forms of presentation of information in the presentation and in the textbook? What slides could you add to your presentation?

2. How do you understand the meaning of the phrase: “The ability to transfer knowledge and information is the basis for the progress of society as a whole and each person individually”? Discuss this question in a group.

3. Since ancient times, people have exchanged information in various ways, notified of danger or conveyed important and urgent information. Prepare a short report about one of the previously used methods of transmitting information.

4. What is a computer network?

5. What is a communication channel? How is the bandwidth of a communication channel determined?

6. How does a peer-to-peer local network work?

7. How does a local network with a dedicated server work?

8. What type of local network is installed in your computer lab? What functions does it perform?

9. What networks are called global? Give examples of such networks.

10. What communication channels are used to transmit data in global computer networks?

11. The data transmission speed over a certain communication channel is 512 000 bit/s. Transferring a file over this channel takes 16 seconds. Determine the file size in kilobytes.

12. Find out the names of network service providers in your area.

13. Construct a graph of relations connecting the concepts discussed in this section.