| §4.1 Local and global computer networks

Lesson 24
§4.1 Local and global computer networks

Keywords:

Message
link
computer network
information transfer rate
the local network
global network

4.1.1. Transfer of information

Earlier, we have already said that communication is one of the most important information processes . Information is transmitted from the source to the receiver in the form of some sequence of signals, symbols, signs. For example, in a direct conversation between people, a transmission occurs sound signals- 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 (transfer of information) is a system 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 over 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. At present, digital communication is widely used when transmitted information converted to 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, modern systems digital communication 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 place of reception, the checksum of the received block is recalculated, and if it does not match the original amount, then the transmission of this block is repeated.

For centuries, for the transmission of letters, mankind has used the services of postal service; in the second half of the 19th century, sound transmission technology (telephone) was invented; Since the 1930s, telefax has been used to transmit images. Nowadays, for the transmission of texts, 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 arrives faster than a telegram, get 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 desired program, ringtone 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 Gbps = 1000 Mbps).

Distinguish between 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 class) or in one building (for example, all computers located in a school building can be connected to a local network). A local area network allows users to share access to computer resources, as well as to peripherals(printers, scanners, disks, modems, etc.) connected to the network.

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

In small local networks, all computers are equal, 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 undesirable that all of them have access to all computers on the network. When combining more than 10 computers, it is advisable to allocate the most powerful computer- server (English server - serving). On the hard drive of the server, files (data and programs) are placed that are accessed by other network computers - clients. In addition, all network users can have access to peripheral equipment connected to the server (for example, a printer or scanner).

Each computer connected to the local network must have a special board - a network adapter. Its function is the transmission and reception of signals propagated through communication channels.

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

Rice. 4.1. Cables:
twisted pair and fiber

A twisted pair is two insulated copper wires that are twisted together relative to each other. This twisting of the wires reduces the effect of interference on the signals transmitted over this cable. A twisted pair connection consists of several twisted pairs (2 or 4) covered with a plastic sheath. Data transfer rate - from 10 Mbps to 1000 Mbps.

Fiber optic cable transmits light through glass fibre. 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 subject to electromagnetic interference. Data transfer rate - from 100 Mbps to 10 Gbps.

Wireless wifi connection provides data transfer rates up to 300 Mbps.

4.1.3. What is a global computer network

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

global computer network- 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 are regional and corporate networks. Regional computer networks provide an association of computers within the same region (city, region, region, country). Corporate computer networks are created to ensure the activities of various types of corporate structures that have territorially remote divisions (for example, banks with their branches).

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

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

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

A wide variety of physical channels are used for data transmission in global networks: electric cable; radio communication through repeaters and communication satellites; infrared rays(as in television remote controls); modern fiber optic cable; conventional telephone network.

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

Task. The data transfer rate through some 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 THING

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

local computer network combines computers installed in the same room or building, and provides users with the ability to share access to computer resources, as well as to peripheral devices connected to the network. Local networks are peer-to-peer and 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. Familiarize yourself with the presentation materials for the paragraph contained in the electronic supplement to the textbook. What can you say about the forms of presentation of information in the presentation and in the textbook? What slides would you like to add to your presentation?

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

3. Since ancient times, people different ways exchanging information, advising of a danger, or transmitting important and urgent information. Prepare a short report about one of the previously used ways 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 LAN work?

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

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

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

10. What communication channels are used for data transmission in global computer networks?

11. The data transfer rate over a certain communication channel is 512,000 bps. Transferring a file over this channel takes 16 seconds. Determine the size of the file in kilobytes.

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

13. Build a graph of relationships connecting the concepts discussed in this paragraph.

A local area network can be considered a connection of two or more devices using a cable, radio waves or optical signals, which makes it possible to exchange data between them. Devices located in the same room or building and interconnected are called a local computer network (LAN - Local Area Network). The number of devices connected to such a network is limited by the capabilities of the applied cable system and network equipment.

The connection between devices can be direct or using additional communication nodes.

Networks are backbone information structures, consisting of logical and physical layers or components, the main purpose of which is the exchange of information.

The physical layer is represented by network components that provide a physical connection between computers. These components are typically: network interface (network card or network adapter card, standard or advanced communication or parallel port or multiport card), network media (coaxial cable, two-wire so-called twisted pair or fiber optic) and node elements (routers, hubs, repeaters (repeaters, hubs (hub)), switches (switch)) and end elements (terminators, connectors, connectors, plugs).

Currently, there is a clear structuring of networks into local and global ones, the process of integrating the first into the second, where networks with several hundred computers are still considered local, and global ones have tens of thousands of connected computer systems. The information exchange rate reaches 200 Mbps, and 10 Mbps is considered the basic initial and low-cost configuration. Now computer networks allow not only to transmit or receive information in the literal sense of this concept, but also provide many service capabilities, the list of which is constantly expanding. This and remote administration, distributed file systems, remote program execution, e-mail, remote printing, distributed bases data, systems remote access and distributed systems management, search engines, teleconferencing and more.

Devices that are used both as control centers in the network and as storage media are called servers. If devices are located relatively close to each other and are connected using high-speed network adapters, then such networks are called local networks. When using a local area network, devices are usually located within the same room, building, or in several closely spaced houses. A local computer network, as a rule, unites no more than a hundred computer systems belonging to any one structure, and is of a corporate nature, both in terms of its operation and the nature of the system software.

The principles of organization and software protocols for local and global computer systems can be both different and absolutely the same. Therefore, it is impossible to classify a network as local or global only on the basis of the type of network interaction and underlying software.

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 (eng. LAN - Local Area Network) refers to geographically limited (territorially or production) hardware and software complexes in which several computer systems are interconnected using appropriate means of communication.

LAN provides the possibility of simultaneous use of 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 that share equipment, software and information. Workplaces of employees 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 the network, the response time should be almost the same as if they were received from the user's PC, and not from anywhere else on the network.. This requires data transfer at a rate 10 Mbps and above. The following speeds are actually achieved:

· Coaxial cable - 10¸ 50 Mbaud;

· Twisted pair - up to 10 Mbaud;

· Special twisted pair 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 a speed of about 1 Mbaud.

LAN components: network devices and means of communication.

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

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

workstations- PC, workstation or actual network station. If the workstation is connected to the network, it may not require either a hard drive or floppy disks. However, in this case, you need a network adapter - a special device for remote booting the operating system from the network;

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

servers– separate computers with software that perform control functions network resources public access;

network software .

Let's take a closer look at some of these network components.

Servers

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

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

Transmission medium

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

Twisted pair, coaxial cable, fiber optic lines are most often used as means of communication in a LAN. When choosing a transmission medium, the following indicators must be taken into account:

· information transfer rate;

· range transfer of information;

· security of information transfer;

· reliability of information transmission ;

· the cost of installation and operation.

Simultaneous fulfillment of the requirements for the transmission medium is a difficult task. So, for example, a high data transfer rate is often limited by the maximum allowable distance for reliable data transfer, while ensuring the necessary level of protection of the transmitted data. The cost of communication means affects the possibility of building and expanding the network.

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

twisted pair

Twisted two-core wire connection (twisted pair), the cheapest among transmission media. Allows you to transfer information at speeds up to 10 Mbps, easy to grow, low noise immunity. The cable length does not exceed 1000 m at a transmission rate of 1 Mbps. To increase the noise immunity of information, shielded twisted pair placed in a sheath 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. The information transfer rate is from 1 to 10 Mbps, in some cases it reaches 50 Mbps. Coaxial cable can be used for broadband transmission of information.

Broadband coaxial cable.

Such a coaxial cable is weakly susceptible to interference, easy to build, but has a high price. The data transfer rate reaches 500 Mbps. To transmit information over a distance of more than 1.5 km in the base band, it is necessary repeater(amplifier), while the distance of stable transmission is increased 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 wave impedance of 50 ohms (thick Ethernet. or yellow cable (yellow cable)). Uses a 15-pin standard switch. The maximum allowed 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.

Slim Ethernet

50 ohm coaxial cable ( thin Ethernet) and a data transfer rate of 10 7 bps, cheaper than thick Ethernet.

LANs with thin Ethernet cable are low cost, require minimal scalability, and do not require additional shielding. The cable is connected to network cards workstations using tee connectors ( T connectors ) with compact bayonet connectors (SR-50). When connecting thin Ethernet segments, repeaters are required. The distance between workstations without repeaters cannot exceed 300 m, and the total length of the network cannot exceed 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 a fiber optic cable is very high, so LANs based on it are used where electromagnetic interference occurs and information is required to be transmitted over long distances without the use of repeaters. 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 on a star topology.

Characteristics of typical transmission media are given in the table.

Topology of the IVS

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

There are different approaches to the classification of LAN topologies. According to one of them, local area network configurations are divided into two main classes: broadcast and consecutive .

V 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), a star with a passive center. The advantage of this class of configurations is the ease of networking.

V successive configurations, each physical sublayer transmits information to only one PC. Such configurations include a star with an intellectual center, a ring, a hierarchical connection, a snowflake. The main advantage is simplicity software implementation connections.

To prevent collisions in the transmission of information, it is most often used temporary separation 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 bandwidth under increased load, i.e. with the introduction of new workstations, are reduced.

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

In a broadband LAN, workstations receive a frequency on which they can send and receive information. The transmitted data is modulated on the respective carrier frequencies. The technology of broadband communications allows you to simultaneously transport a fairly large amount of information in a communication medium.

Star topology .

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

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

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

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

Fig 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 can 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 interconnected in a circle. The last workstation is connected to the first one, i.e. communication link is closed in a ring.

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

Messages in the LAN ring circulate in a circle. The workstation sends information to a certain address after receiving a request from the ring. The transfer of information turns out to be quite efficient, since messages can be sent one after another. So, 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 ring topology is that each workstation must participate in the transmission of information, and if at least one of them fails, the entire network is paralyzed. Faults in the cable system are easily localized.

Network expansion with ring topology requires the network to stop working, since 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 with the help of special switches (eng. Hub - concentrator), which in Russian is also sometimes called a “hub”. 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 exclusively a branching device (for a maximum of three workstations). The management of a single workstation in a logical ring is the same as in a normal ring. Each workstation is assigned an address corresponding to it, to which control is transferred (from the oldest to the youngest and from the youngest to the oldest). The disconnection occurs only for the downstream (nearest) 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 an individual workstation, i.e. workstations can be connected to the bus or disconnected from it at any time without disturbing 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 a transmission medium ( T -connector), so the expansion of such a network requires a bus break, which leads to disruption of the network. More expensive solutions involve installing 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 environment, the removal of information from the LAN and, accordingly, the listening of information are carried out quite easily, as a result of which the security of such a LAN is low.

Characteristics of topologies of computer networks 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 it is impossible to directly apply the basic network structures. To connect workstations, devices called concentrators .

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

Types building a LAN methods of information transfer.

Token Ring network

This standard was developed by IBM. Unshielded or shielded twisted pair or optical fiber is used as the transmission medium. Data transfer rate from 4 Mbps to 16 Mbps. As access control method workstations to the transmission medium used marker ring (Token Ring). The main provisions of the method:

¨ LAN ring topology;

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

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

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

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

¨ marker (Token);

¨ reset packet (Abort).

Control/Data package . With the help of 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 package.Sending such a packet causes the transmission of information to stop.

Network To k e n Ring allows computers to be connected 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 SMS Corporation (Standard Microsystem Corporation), which became the main developer and manufacturer of equipment for Arcnet networks. Twisted pair, 93 ohm coaxial cable and fiber optic cable are used as transmission medium. The data transfer rate is 2.5 Mbps. When connecting devices in apply bus and star topologies. Access control method stations to the transmission medium - marker tire (Token Bus). The method includes the following rules:

¨ devices connected to the network can only transmit data if they receive permission to transmit (token);

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

Work principles

The transfer of each byte in Arcnet is carried out by sending an ISU (Information Symbol Unit - information transfer unit), consisting of three service start / stop bits and eight data bits. At the beginning of each packet, the initial delimiter AB (Alegt Burst) is transmitted, which consists of six service bits. The start delimiter functions as the preamble of the packet.

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. The station that received the packet ITT , acquires the right to transfer data.

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

3. data package.This message is used to transfer data.

4. ASK package (ACKnowledgements) - confirmation of receipt. Acknowledgment of readiness to receive data or confirmation of receipt of a data packet without errors, i.e. response to FBE and data packet.

5. NAK package(Negative AcKnowledgements) unwillingness to receive. Node not ready to receive data in response to FBE or receiving 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 specification for Ethernet version 2.0 was published. Based on Ethernet, the IEEE 802.3 standard was developed.

Basic principles of work

¨ bus topology at the logical level;

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

¨ data transmitted by one station is available to all stations on 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, use modern facilities and communication methods. Communication in a broad sense is understood as the process, way and means of transferring an object or message from one place to another. Communications can be organized using different transmission media, such as water and air communications, gas pipelines, railways and highways, etc.
Invaluable help to people is provided by computer networks, the appearance of which marked new era in the history of communications. With the advent of computer networks, they began to talk about computer communications, meaning by this the exchange of all kinds of information using computers. They are increasingly entering our lives, in some cases crowding out, and in others - supplementing those already available. Being far apart, you exchange letters by mail - in a computer network, this type of communication is known as e-mail. To discuss some important issue, you organize a meeting, meeting, conference. There is a corresponding type of communication in a computer network. This is a teleconference. Computer communications in many ways resemble traditional ones, but at the same time, mail delivery time is significantly reduced, communication is organized more quickly, the ability to communicate with a large circle of people expands, and there is quick access to the world's information repositories.
Computer communications are provided with the help of computer networks: local, regional, corporate, global.
At the lecture, you will learn how they differ from each other and what constitutes them. Hardware, namely: what components ensure the operation of the network, 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 - "far", far ~ and Latin communicato- "communication") - this is the exchange of information at a distance.
The radio transmitter, telephone, teletype, facsimile, telex and telegraph are the most common and familiar examples of telecommunications technology today.
Later, another tool was added to them - these are computer communications, which are now becoming more widespread. They promise to oust facsimile and teletype communications, just as the latter ousted the telegraph.

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

Nowadays, computer networks are becoming more and more importance in the life of mankind, their development is very promising. Networks can connect and make available informational resources 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 channels of information transmission.

The purpose of all types of computer networks is determined by two functions:
- provision sharing hardware and software resources of the network;
- providing shared access to data resources.
For example, all members of the local network can share one common printing device - network printer or, for example, resources hard drives one dedicated computer - file server. Similarly, software can be shared. If the network has a special computer dedicated to the sharing of network members, it is called a file server.

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

the local network(LAN - Local Area Network) - connection of computers located at short distances from each other (from several meters to several kilometers). PCs in such networks are located in the same room, at the same enterprise, in closely spaced buildings.
Local networks do not allow sharing information with users located, for example, in different parts of the city. They come to help regional networks, connecting computers within the same region (city, country, continent).

regional net(MAN - Metropolitan Area Network) - combining PCs and local networks to solve a common problem of 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, an economic region, a separate 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 (Microsoft's network can be used as an example)

corporatenetworks - association of local networks within one corporation.

The need for the formation of a single global 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 PCs of users located at remote distances for the general use of world information resources .
Information networks create a real opportunity for quick and convenient user access to all the information accumulated by mankind throughout history.

According to the type of transmission medium, networks are divided into:

Wired (on coaxial cable, twisted pair, fiber optic);
- wireless with the transmission of information via radio channels or in the infrared range.
By the method of organizing the interaction of network computers are divided into peer-to-peer and with a dedicated server (hierarchical networks).
All computers in a peer-to-peer network are equal. 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. Main disadvantage is that in the conditions of peer-to-peer networks it is difficult to solve 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 a matter of principle.
In a hierarchical network, when installing a network, one or more servers are pre-allocated - computers that manage the exchange of data over the network and the distribution of resources. Any computer that has access to server services is called a network client or workstation.

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

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 failures. If the cable is damaged in at least one place, then there are problems for the entire network. The fault location is difficult to locate.

2. Topology STAR. In this case, each computer is connected by a separate cable to a common device called a hub (hub), which is located in the center of the network. To malfunctions of the cable system, the "star" is more resistant. A damaged cable is a problem for one particular computer; it does not affect the operation of the network as a whole. No troubleshooting effort required. 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. At present, such a structure is the most common type of communication topology in both local and global networks.

3. Topology RING. 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 itself in an internal buffer. In a network with a ring topology, special measures must be taken so that in the event of a failure or disconnection of a station, the communication channel between the other stations is not interrupted. The advantage of this topology is the 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. A 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 takes place are directly connected, and for data exchange between computers that are not connected by direct connections, transit transmissions through intermediate nodes are used. Mesh Topology Allows Connection a large number computers and is typical, as a rule, for global networks. The advantages of this topology are in its resistance to failures and overloads, since there are several ways to bypass individual nodes.
5. Mixed topology. While small networks tend to have a typical star, ring, or bus topology, large networks tend to have random connections between computers. In such networks, it is possible to single out separate arbitrary subnets with a typical topology; therefore, they are called networks with a mixed topology.

The principles of operation 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, networks of the second order (regional), third order (corporate), fourth order (local) are usually connected to the servers of the global network, and users of individual computers are connected to them - subscribers(clients) networks. Note that networks of not all intermediate levels (for example, corporate) 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 the objects (servers and clients) of the network. To do this, you need to connect computers to each other. Therefore, the mandatory components of any network are all kinds of communication channels (wired and wireless), for which various physical media are used. In accordance with this, networks distinguish 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 we compare them with the transport channels of a freight or passenger transportation system. Passengers can be transported by air, with the help of railways or water (sea or river) routes. Depending on the medium of transportation, a means of transportation is chosen. Information is transported through computer networks. The environments in which network computers communicate determine the means by which computers are connected. If this is an environment that requires telephone communication, then the connection is made through a telephone cable. Computer connections are widely used with electrical cables, radio waves, fiber optic cables, etc.

Consider the main types of channels. Some of them are mutually exclusive, some may describe one channel from different angles.
Channels are digital and analog.
TO analog channels can be attributed to the usual telephone channel. To use it, you need a special device - a modem that converts digital information into analog. Analog channels are highly susceptible to interference and have a low bandwidth (several tens of kilobytes per second). Now there is a tendency 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.
Using switched line, the connection is formed for the duration of the data transfer, and at the end of this transfer, it is disconnected. Switched communication is conventional telephone line.

Dedicated line works differently:
The connection is permanent, always allowing you to transfer data from one computer to another. Leased lines differ from dial-up lines in high speed (up to tens of megabits per second) and high rental prices.
By physical device channels are subdivided on the electrical wire, optical and radio channels.
Wired channels are a connection with an electrical cable, possibly complex. All such channels use data transmission using electrical impulses.

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

radio channels operate on the same principle as radio and television.
All these are different channels of communication. The efficiency of communication in computer networks essentially 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 big file)
Guaranteed bandwidth - the minimum bandwidth that the channel provides (for video files)
- reliability - the ability to transmit information without distortion and loss;
- cost;
- expandability (connection of new computers and devices).

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

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

Modem- (modulator - demodulator) - a device for converting digital PC signals into audio (analogue) signals of a telephone line and vice versa.
The main characteristic of the modem: the speed of receiving - transmitting information (measured by bits per second). Modern modems have a speed of receiving and transmitting information - 33600 bits per second, 57600 bits per second.

3. Network operation is carried out according to protocols
In order for the 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 the formation and transmission of messages, on the methods of exchanging information between PCs, on the rules for the operation of various equipment in the network

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

basic protocols for physical forwarding 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 protocols over high level responsible for the functioning of specialized Internet services: HTTP protocol(hypertext messaging), FTP (file transfer), email protocols, etc.
Technically speaking, TCP/IP is not one, but two. network protocol. TCP is a transport layer protocol. It controls how information is transferred. The IP protocol is addressable. It defines where the data is transferred to.

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

server- a program that provides services, customer- 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. A lot of packets form a stream of information that is received by the user's PC
B. Then the "scattered packets" that arrived 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 package to find its destination, each PC is assigned an IP address (when registering 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.

lP address is "read" from right to left. Typically, the rightmost digit indicates a specific computer, while the remaining digits indicate the numbers of networks and subnets (i.e., local networks).
Sometimes this may not be the case, but in any case, if the address is representable in binary form, then some of the rightmost bits identify a particular computer, and the rest designate 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 address of the recipient and the address of the sender, and then thrown into the network.
Routers determine the route that packets take.

Domain name system

Computers can easily communicate with each other by a numeric IP address, however, it is not easy for a human to remember a numeric address, and for convenience, Domain System Names (DNS - Domain Name System).
Domain name system maps each computer's numeric IP address to a unique domain name. Domain addresses are assigned by the Internet Network Information Center (InterNIC).

Domain (domain- region, district) - defines a set of PCs belonging to any section of the Internet, within which computers are combined according to one attribute.

Domain address defines an area representing a set of host computers. Unlike a numeric 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 next by a dot. To the left of the top-level domain are the subdomains for the general domain.
In the Internet address system, it is customary to represent domains by geographic regions. They have a two-letter name.
Example. Geographical domains of some countries: France - fr; Canada- sa; USA - us; Russia - en; Belarus - by.
There are also domains divided by thematic signs. Such domains are three-letter abbreviation.
Example. Schools - 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 as WWW or Web), also called the World Wide Web. The presentation of information on the WWW is based on the possibilities of hypertext links. Hypertext is text that contains links to other documents. This makes it possible, when viewing a certain document, to easily and quickly switch to other information related to it, which can be text, an image, a sound file, or have any other form accepted in the WWW. At the same time, linked documents can be scattered around the globe.
Numerous cross links between WWW documents span the planet - hence the name. Thus, the dependence on the location of a particular document disappears.
The World Wide Web service is designed to access a special kind of electronic documents called Web documents or simply Web pages. A web page is an electronic document that, in addition to text, contains special formatting commands, as well as embedded objects (pictures, audio and video clips, etc.).
They surf the Web with the help of special programs called browsers, so the browser is not just a WWW client for interacting with remote Web servers, it is also a Web document viewer. So, for example, if a Web page has been saved to your hard drive, you can view it using a browser without an Internet connection. Such browsing is called offline.
Unlike printed electronic documents, Web pages do not have absolute, but relative formatting, that is, they are formatted at the time of viewing in accordance with what screen and with what browser they are viewed. Strictly speaking, the same Web page, when viewed in different browsers may look different - it depends on how the browser responds to the commands that the author has embedded in 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 get the document stored there.
There are a lot of Web documents stored on the Internet. In the last seven years, the content of the WWW has doubled every year and a half. Apparently, this rate will decrease somewhat in the coming years, but will remain quite high, at least until the 10 billion milestone. With so many Web documents on the Web today, there is an important problem of finding and selecting them - 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 of a picture located on one of the Web pages of the www.klyaksa.net portal.
A document URL consists of three parts and, unlike domain names, is read from left to right. The first part specifies the name of the application protocol by which the 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 this document. 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 the file containing the Web document on the specified computer. With a record of the path to the file in the operating Windows system we are already familiar with, but there is an important difference. On Windows, it is customary to separate directories and folders with a backslash character "\", while on the Internet it is customary to use a normal forward slash "/". This is due to the fact that the Internet originated on computers running in operating system UNIX, and there it is customary to separate directories in this way.
Each hyperlink on the Web is associated with the Web address of some document or object (file with a picture, sound recording, video clip, etc.). When a hyperlink is clicked, a request is sent to the Web for the supply of the object pointed to by the hyperlink. If such an object exists at the specified address, it is loaded 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. E-mail (E-mail).
E-mail (E-mail - Electronic mail, English mail - "mail") is the most common and until recently the most popular use of the Internet. According to 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 one.
When using e-mail, each user is assigned a unique mailing address, which is usually formed by appending the user's name to the name of the computer itself. The username and computer name are separated by the @ special character. For example, if the user has the login name emsworth on the computer blondings.corn, then their email address will look like [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 allow you 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) is engaged in receiving and transmitting files of large volumes. The FTP service has its own servers in the world wide web, where data archives are stored. These archives may be commercial or restricted, or they may be public.

5. Access to a remote computer (Telnet)
If we recall the history of the development of computers, then there was a time when the computer itself was large and stood in a special machine room. The terminals (i.e. keyboard displays) that allowed the computer to be operated were located in a different room. The displays were alphanumeric, so the dialogue with the computer consisted in 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 keep this method of dialogue with a computer.
The remote access program is called Telnet.
For its functioning, as for all Internet services, the existence of two parts is necessary - a server program installed on remote computer, and the client program - on 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 on the remote computer, you can do the same operations as on the local computer, i.e. browse directories, copy or delete files, run various programs, which have an alphanumeric interface.

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

7. ICQ service. Its name comes from the expression I seek you - I'm 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- it is one informational space, consisting of hundreds of millions of interconnected electronic documents stored on Web servers. Individual documents are called Web pages. Groups of thematically grouped Web pages are called Web sites or Web sites.

Net is a group of computers connected to each other by a communication channel. The channel provides data exchange within the network (that is, data exchange between computers in a given group). The network may consist of two or three computers, or it may unite several thousand PCs. Physically, data exchange between computers can be carried out via a special cable, telephone line, fiber optic cable, or radio channel.

Computers on the network can be connected:

• directly with each other (so-called point-to-point compound);
• through intermediate communication nodes.

Computers connected to a network can perform two functions: they can be workstations or servers.

Work station- this is any working computer on the network that is not a server, as a rule, users work behind them. Requirements for workstations are determined by the range of tasks of the station. Typically, the main requirements are the requirements for speed and the amount of RAM.

Servers- these are computers that manage the entire network and accumulate all the data of workstations. Servers can work in automatic mode - they stand without a keyboard and sometimes even without a monitor, but in any case, the servers perform the functions of network management and data concentration. Network Administrator- a person whose responsibilities include all issues related to the installation and operation of the network, as well as solving all problems related to the rights and capabilities of network users.

Typically, the largest and most powerful computer on the network is selected as the server. However, development computer technology obviously leads to a reduction in internal components - the computer becomes faster and more economical. Therefore, in a short time, the server can become outdated faster than conventional computers, which are not subject to such high requirements.

It is customary to distinguish between local and global networks. In essence, the main difference between them is already clear from the names, but there are also some significant technological differences.

Local networks(from English local - local) - these are networks consisting of closely spaced computers, most often located in the same room, in the same building or in closely spaced buildings. Local computer networks covering a certain enterprise or firm and uniting heterogeneous computing resources in a single environment are called corporate(from English corporate - corporate, general). Examples: banking network, educational institution network.

The most important characteristic of local networks is the data transfer rate, so computers are connected using high-speed adapters with a data transfer rate of at least 10 Mbps. Local networks use high-speed digital communication lines. In addition, local networks must be adaptable and flexible: users must be able to locate computers connected to the network where they need to, add or move computers or other devices, and disconnect them as needed without interrupting the network.

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

Topology of local networks

Topology is a network configuration, a way of connecting network elements (i.e. computers) to each other. There are three most common ways to connect computers to a local network: "star", "common bus" and "ring".

Star connection(Fig. 1). Each computer through a special network adapter is connected by a separate cable to the merging device. If necessary, you can combine several networks with a star topology together, while the network configuration is branched.

Advantages: With a star connection, it is easy to troubleshoot the network.

Disadvantages: The connection is not always reliable, because the failure of the central node can bring the network to a halt.

Common bus connection(Fig. 2). All computers on the network are connected to the same cable; this cable is shared by all workstations in turn. With this type of connection, all messages sent by each individual computer are received by all other computers on the network.

Advantages: in a "common bus" topology, the failure of individual computers does not bring the entire network to a halt.

software file operating driver

Flaws: it is somewhat more difficult to find a fault in the cable, and if the cable (single for the entire network) breaks, the operation of the entire network is disrupted.

Ring connection(Fig. 3). Data is transferred from one computer to another; at the same time, if one computer receives data intended for another computer, then it transfers them further (along the ring).

Advantages: load balancing, the possibility and convenience of cable laying.

Flaws: physical restrictions on the total length of the network.

The composition of the hardware and software depends on the scheme. The topology is chosen based on the needs of the enterprise. If the enterprise occupies a multi-storey building, then the scheme can be applied in it. "Snowflake"(Figure 4), which has file servers for different workgroups and one central server for the entire enterprise.