3.5. Local area networks

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

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 integrates personal computers located at many remote workstations 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 it was received from the user's PC, and not from another place on the network.... This requires a data transfer 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 of 5 categories - up to 100 Mbaud;

· Optical fiber - up to 1Gbaud;

· Telephone line - from 2400 baud to 56 kbaud;

· Satellite 10,000 computers simultaneously and the speed of about 1 Mbaud.

LAN components: network devices and communications.

The principle of modular organization is implemented in the LAN, which allows you to build networks of different configurations with different functionality... The main components from which the network is built are as follows:

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

workstations - PC, workstation or the actual network station. 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 remote loading of the operating system from the network;

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

servers - individual computers with software that perform the functions of managing shared network resources;

network software .

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

Servers

A network can have one or more servers. Various servers can be used to control network operation ( network servers), storing information in the form of files ( file servers), search and retrieval of 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 is approximately as follows. As necessary, the workstation sends a request to the server to perform any action: read data, print a document, send an email, etc. The server performs the requested action and issues a confirmation.

Transmission medium

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

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

· information transfer rate;

· range transmission of information;

· security of information transfer;

· reliability of information transmission ;

· the cost of installation and operation.

The simultaneous fulfillment of the requirements for the transmission medium is a difficult task. So, for example, high data transfer rates are often limited by the maximum allowable distance of reliable data transmission, while ensuring the required level of protection of the transmitted data. The cost of communication means affects the scalability and expansion of the network.

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

Twisted pair

Twisted pair, the cheapest transmission medium. It allows you to transfer information at a speed of up to 10 Mbit / s, it is easily expandable, and the noise immunity is low. The cable length does not exceed 1000 m at a transmission speed of 1 Mbit / s. To increase the noise immunity of information, a shielded twisted pair is used, placed in a sheath similar to the shield 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 Mbit / s, in some cases it reaches 50 Mbit / s. Coaxial cable can be used for broadband transmission of information.

Broadband coaxial cable.

Such a coaxial cable is weakly susceptible to interference, easy to grow, but has a high price. The information transfer rate reaches 500 Mbit / s. To transmit information over a distance of more than 1.5 km in the base frequency band, you need repeater (amplifier), while the stable transmission distance is increased to 10 km. For a LAN with a bus or tree topology, the cable must be terminated terminator (terminating resistor).

Ethernet - cable

Thick Ethernet

Coaxial cable with a characteristic impedance of 50 Ohm (thick Ethernet. Or yellow cable). Uses 15-pin standard connection. The maximum allowable 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 trunk topology, uses only one terminator at the end. In terms of noise immunity parameters, it is an expensive alternative to conventional coaxial cable.

Thin Ethernet

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

LANs with thin Ethernet cables are low cost, minimal overhead, and require no additional shielding. The cable connects to the workstation network cards using tee connectors ( T-connectors ) with small bayonet connectors (SR-50). Repeaters are required when connecting thin Ethernet segments. The distance between workstations without repeaters cannot exceed 300 m, and the total network length 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 a fiber-optic cable is very high, therefore, a LAN based on it is used where electromagnetic interference occurs and information transfer is required over long distances without using repeaters. The networks are resistant to eavesdropping, as the fiber optic branching technique is very complex. Typically, fiber-optic LANs are built in a star topology.

The characteristics of typical transmission media are shown in the table.

Indicators	Transmission medium
	Twisted pair	Coaxial cable	Fiber optic cable
Price	Low	Average	High
Build-up	Very simple	Problematic	Problematic
Eavesdropping protection	Bad	Good	Very good
Earthing	No	Required	No
Interference immunity	Low	High	Very high

IVS topology

Topology, i.e. configuration of connecting elements in the LAN , 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 reliability and performance.

There are different approaches to classifying LAN topologies. According to one of them the configuration local area networks are divided into two main classes: broadcast and consistent .

AT broadcast In configurations, each PC transmits signals that can be perceived by other PCs. These configurations include a common bus, a tree (connection of several common buses using repeaters), a star with a passive center. The advantages of this class of configurations are the ease of networking.

AT consecutive In configurations, each physical sublayer transmits information to only one PC. Such configurations include a star with an intelligent 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 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 at increased load, i.e. when new workstations are introduced, are reduced.

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

On a broadband LAN, workstations receive a frequency on which they can send and receive information. The transmitted data is modulated at the respective carrier frequencies. Broadband communication technology 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 area mainframes where the host receives and processes all data from the terminal devices as an active data processing node. All information between peripheral workstations passes through the central node of the computer network.

Network bandwidth is determined by the processing 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 straightforward as each workstation is connected to a central site, but the cost of cabling is high, especially when the central site is not geographically located at the center of the topology.

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

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

Figure 1. Star topology

The performance of a LAN in a star topology is primarily determined by the parameters of the central node, which acts as server network... It can be a bottleneck in the network. In case of failure of the central node, the operation of the network as a whole is disrupted.

An optimal protection mechanism against unauthorized access to information can be implemented in a LAN with a central control unit.

Ring topology.

In a ring network topology, LAN workstations are connected in a circle. The last workstation is linked 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 geographically located far from the main ring.

Messages on the LAN ring circulate in a circle. The workstation sends information to a specific address, having previously received a request from the ring. The transfer of information turns out to be quite effective, 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.

Figure: 2. Ring topology

The main problem of the 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.

Extending 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 .

It is physically mounted as a star topology junction. Individual stars are switched on with the help of special switches (English Hub - hub), which in Russian is also sometimes called a “hub”. Depending on the number of workstations and the length of the cable, active or passive concentrators are used between workstations. Active hubs additionally contain an amplifier for connecting 4 to 16 workstations. The passive hub is a purely branching device (maximum three workstations). A single workstation in a logical ring is controlled in the same way as in a conventional ring. Each workstation is assigned an address corresponding to it, at which control is transferred (from the oldest to the youngest and from the youngest to the oldest). The disconnection occurs only for the downstream (closest) node of the computer network, so only in rare cases can 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 at any time can be connected to the bus or disconnected from it without disrupting the network as a whole.

Figure: 3. Bus topology

However, in the simplest Ethernet network with a bus topology, a thin Ethernet cable with a T-piece connector (T -connector), therefore the expansion of such a network requires a bus break, which leads to a malfunction of the network. More expensive solutions involve installation instead ofT - connectors of passive plug boxes.

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

The characteristics of the topologies of computer networks are given in the table.

Characteristic	Topology
	Star	Ring	Tire
The cost enlargement	Low	Average	Average
Connection of subscribers	Passive	Active	Passive
Defence from refusals	Low	Low	High
Eavesdropping protection	Good	Good	Bad
Behavior at high	Good	Bad	Bad
Real-time operation	Good	Good	Bad
Wiring cable	Good	Bad	Good

Tree topology.

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

Treelike networks are used where direct application of the underlying 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 hubs... To connect more devices, you need active concentrators with the possibility of signal amplification.

Types of building a LAN 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 Mbps to 16 Mbps. As access control method workstations to the transmission medium used marker ring (Token Ring). The main provisions of the method:

¨ ring LAN 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.

In LAN To k e n Ring, there are three main types of packets used:

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

¨ marker (Token);

¨ reset package (Abort).

Management / Data package ... With the help of such a package, 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 termination of information transmission.

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 Arcnet license was acquired by the Standard Microsistem 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 use bus and star topologies. Access control method stations to the transmission medium - marker line (Token Bus). The method provides the following rules:

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

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

Work principles

The transfer of each byte to Arcnet is performed 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 (Аlegt Burst) is transmitted, which consists of six service bits. The leading delimiter acts as the packet preamble.

Arcnet defines 5 packet types:

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

2. FBE package (Free Buffe? Inquiries) - a request for readiness to receive data. This packet checks the readiness of the node to receive data.

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

4. ACK package (ACKnowledgments) - acknowledgment of receipt. 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. NAK package(Negative AcKnowledgments) unwillingness to receive. Unavailability of the node to receive data in response to the FBE or receive 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 has been developed on the basis of Ethernet.

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.

. Questions + on the topic computer networks

2016-02-10

Questions + on the topic of computer networks

Tests on the topic "Local area networks" - informatics, lessons

Testing on the topic "Computer networks"

Option number 2.

Question 1. (Difficulty - A) The global network is.
Answer 1.a system of interconnected computers
Answer 2.a system of interconnected local networks
Answer 3.a system of interconnected local telecommunication networks
* Answer 4.a system of interconnected local networks and computers of individual users
Question 2. (Difficulty - A) To connect two computers via telephone lines, you must have:
Answer 1. Modem * Answer 2.two modems
Answer 3.phone, modem and special software
Answer 4. by modem on each computer and special software
Question 3. (Difficulty - A) E-mail is:
Answer 1.search program Answer 2.mail server name
Answer 3.mail program
* Answer 4. Exchange of letters in computer networks (e-mail)
Question 4. (Complexity - A) The HTTP protocol serves to:
* Answer 1.Hypertext transfer Answer 2.File transfer
Answer 3: messaging control
Answer 4.Run the program from a remote computer
Question 5. (Difficulty - A) What components of a computer network are required to organize a peer-to-peer local area network?
* Answer 1. modem, server computer
Answer 2.Network card, network software
Answer 3. Server computer, workstations,
Answer 4.Communication lines, network card, network software
Question 6. (Difficulty - A) To view the WEB-pages are intended:
Answer 1.search servers * Answer 2.browsers
Answer 3. Teleconferencing Answer 4. Providers
Question 7. (Difficulty - A) Which of the following schemes for connecting computers is a closed chain?
Answer 1. Bus * Answer 2. Ring
Answer 3. Star Answer 4. There is no correct answer
Question 8. (Difficulty - A) Which cable provides data transfer rates up to 10 Mbps?
* Answer 1. Coaxial Answer 2. twisted pair Answer 3. fiber optic
Answer 4. there is no correct answer
Question 9. (Difficulty - A) A protocol is used to transfer files over the network.
Reply 1. POP3 Reply 2. HTTP Reply 3. CMPT * Reply 4. FTP

Question 10. (Difficulty - A) Choose the correct address email:
Answer 1. [email protected]
Answer 2.ivan_petrov.mail.ru
Answer 3.ivan petrov.mail.ru
* Answer 4. [email protected]
Question 11. (Difficulty - A) The data transfer rate is 6000Mb / min. This amounts to. Mbps
Answer 1.10
* Answer 2.100
Answer 3.3600
Answer 4.36000
Question 12. (Difficulty - A) An Internet email address is set: [email protected] What is the name of the mail server?
Answer 1. [email protected]
Answer 2.fortuna
* Answer 3.list.ru
Answer 4.list
Question 13. (Difficulty - A) The computer connected to internet networks, necessarily has
Answer 1. URL;
* Answer 2. IP address
Answer 3. WEB-page;
Answer 4. domain name;
Question 14. (Difficulty - A) Choose the correct IP address of the computer on the network
* Answer 1.08.214.198.112
Answer 2.18.274.198.0
Answer 3. 1278.214.198
Answer 4.10,0,0,1225
Question 15. (Complexity - A) The topology of a computer network in which all computers on the network are connected to a central node is called
Answer 1. Bus
Answer 2. Ring
* Answer 3. Star
Answer 4. There is no correct answer
Question 16. (Difficulty - A) Which top-level domain does "educational site" mean?
Answer 1. mil
Answer 2.gov
Answer 3. Com

* Answer 4.edu
Question 17. (Difficulty - A) Determine the level 2 domain www.klyaksa.inform.net
Answer 1.klyaksa
Answer 2.inform
Answer 3.www
* Answer 4. net
Question 18. (Difficulty - B) Determine the computer number on the network by IP 215.128.255.106
Answer 1.215.128.255.106
Answer 2.128.255.106
Answer 3.255.106
* Answer 4.106
Question 19. (Difficulty - A) Protocol is.
Answer 1.The ability of a computer to send files through information channels
Answer 2.device for working local network
* Answer 3.Standard for data transmission over a computer network
Answer 4.The standard for sending messages via email
Question 20. (Difficulty - B) How long will it take to transfer a 128KB file over a 128Kbps network?
* Answer 1.8 p.
Answer 2.1 s.
Answer 3.1 min.
Answer 4.10 p.

Test: Local network

Question number 1: Local area network is.

• Fishing net
• A computer network that unites a group of computers that are located in one place
• A computer network connecting all computers Answer: 2;

Question number 2: In order to connect the computer to the network, you must have.

• Network cable
• Keyboard
• Network adapter
• Network drive Answer: 1.3;

Question number 3: There are networks.

Please select several answer options:

• Local
• Global
• High
• Deep
• Regional Answer: 1,2,5;

Question number 4: If there are several computers, then an additional device is needed to connect them to the network.

Choose one of the answer options:

• Connector
• Switch
• Networker Answer: 2;

Question number 5: The baud rate is characterized by.

Choose one of the answer options:

• kb / cm
• kb / s Answer: 4;

Question number 6: Specify the procedure for opening access to your folder.

Indicate the order of the answer options:

• Enable the item to share this folder
• Right click on your folder
• select Properties
• select tab access
• click Apply Answer: 2-3-4-1-5;

Question number 7: What cannot be done with a local network?

Choose one of the answer options:

• To communicate
• To play together
• Work on one document
• Pass candy Answer: 4;

Question number 8: What is the name of the icon in windows, with which we can go to other computers in our local network?

Choose one of the answer options:

• Network folder
• network environment
• Network Icon Answer: 2;

Viewing Document Contents Tests on Local Area Networks

A local network can be considered the connection of two or more devices using a cable, radio waves or optical signals, in which it becomes possible to exchange data between them. Devices located in the same room or building and connected to each other 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 cable system and network equipment used.

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 the network components that provide the physical connection between computers. These components are usually: network interface (network card or network adapter card, standard or extended communication or parallel port or multiport card), network media (coaxial, two-wire, so-called twisted pair or fiber optic cable) and nodal elements (routers, hubs, repeaters (repeaters, hubs), switches) and final elements (terminators, connectors, connectors, plugs).

At present, there is a clear structuring of networks into local and global, the process of integrating the former into the latter, where networks with several hundred computers are still considered local, while global networks have tens of thousands of connected computer systems. The information exchange rate reaches 200 Mbit / s, and 10 Mbit / s is considered the basic initial and low-cost configuration. Now computer networks allow not only transmitting or receiving information in the literal sense of this concept, but also provide many service opportunities, the list of which is constantly expanding. These are remote administration, distributed file systems, remote program execution, e-mail, remote printing, distributed databases, remote access systems and distributed control systems, search engines, teleconferencing and much more.

Devices that are used both as control centers in the network and as storage devices are called servers. If the 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 corporate in nature, both in terms of its operation and the nature of the system software.

The principles of organization and software protocols of local and global computer systems can be both different and absolutely identical. Therefore, the network cannot be classified as local or global only on the basis of the type of network interaction and basic software.

Topic 1.3: Open Systems and the OSІ Model

Topic 1.4: LAN Basics

Topic 1.5: Basic technologies of local networks

Topic 1.6: Basic software and hardware components of a LAN

Local networks

1.4. LAN Basics

1.4.1. Basic LAN concepts

LAN classification

Computer network Is a set of computers united by data transmission channels. Depending on the distance between computers, the following computer networks are distinguished:

• local area networks - LAN;
• territorial area networks, which include regional MAN and global WAN networks;
• corporate networks.

A local area network is a LAN in which a PC and communication equipment are located a short distance from each other. A LAN is usually designed to collect, store, transfer, process and provide users with distributed information within a division or firm. In addition, a LAN usually has an Internet connection.

Local networks can be classified by:

• control level;
• appointment;
• uniformity;
• administrative relations between computers;
• topology;
• architecture.

Let's take a closer look at the LAN classification

The following LANs are distinguished by the control level:

• Workgroup LANs, which consist of several PCs running the same operating system. In such a LAN, as a rule, there are several dedicated servers: file server, print server;
• LAN of structural divisions (departments). The LAN data contains several dozen PCs and servers of the following types: file server, print server, database server;
• LAN of enterprises (firms). These LANs can contain over 100 computers and servers of type: file server, print server, database server, mail server and other servers.

By purpose, networks are subdivided into:

• computer networks intended for settlement work;
• information and computer networks, which are intended both for conducting settlement operations and for providing information resources;
• information and advisers who, based on data processing, generate information to support decision-making;
• information and control networks, which are designed to control objects based on information processing.

The types of computers used can be distinguished:

• homogeneous networks that contain the same type of computers and system software;
• heterogeneous networks that contain different types of computers and system software.

Administrative relations between computers can be distinguished:

• LAN with centralized control (with dedicated servers);
• LANs without centralized management (decentralized) or peer-to-peer (peer-to-peer) networks.

By topology (main topologies) LANs are divided into:

• bus topology;
• star topology;
• ring topology.

By architecture (main types of architectures) LANs are divided into:

1. Ethernet.
2. Arcnet.
3. Token ring.
4. FDDI.

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 means and methods of communication. Communication in a broad sense is understood as a process, path and means of transferring 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 help to people, the appearance of which marked a new era in the history of the development of communications. With the advent of computer networks, they began to talk about computer communications, understanding by this the exchange of all kinds of information using computers. They are increasingly entering our life, in some cases displacing, and in others - supplementing the existing ones. When you are far apart, you exchange letters by mail - on a computer network, this type of communication is known as e-mail. To discuss some important problem, you organize a meeting, meeting, conference. There is a corresponding type of communication in the computer network. This is a teleconference. Computer communications are in many ways reminiscent of traditional ones, but at the same time the time for mail delivery is significantly reduced, communication is organized more quickly, the ability to communicate with a large circle of people expands, and quick access to the world's information storages appears.
Computer communications are provided using computer networks: local, regional, corporate, global.
In the lecture, you will learn how they differ from each other and what their hardware is, namely: what components ensure the operation of the network, what communication channels are used, what is a modem and a network adapter, what role protocols play in computer networks, and much more.

Computer networks. Basic information.

Telecommunications(from the Greek. tele - "into the distance", far ~ and Latin communicato - "communication") is the exchange of information at a distance.
The radio transmitter, telephone, teleprinter, fax machine, telex and telegraph are the most common and familiar examples of telecommunications technology today.
Later, another means was added to them - this is computer communications, which are now becoming more widespread. They promise to squeeze out facsimile 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 gaining 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 information transmission channels.

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

Networks by dimensionare 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, in the same enterprise, in closely located buildings.
Local networks do not allow to provide shared access to information for users located, for example, in different parts of the city. Come to the rescue regional networksthat unite computers within one 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 can include computers within a large city, economic region, or a single country. Typically, the distance between subscribers of a regional computer network is tens - 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 (as an example, the Microsoft Corporation network)

corporate networks -consolidation of local networks within one corporation.

The needs for the formation of a single world information space have led to the creation of a 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.

By the type of transmission medium, networks are divided into:

Wired (coaxial, twisted pair, fiber optic);
- wireless with the transmission of information over radio channels or in the infrared range.
By the way 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 are equal. Anyone on the network can access data stored on any computer.
The main advantage of peer-to-peer networks is ease of installation and operation. The 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 the network is used for networks with a small number of computers and where the issue of data protection is not critical.
In a hierarchical network, when installing the network, one or more servers are pre-allocated - computers that control the exchange of data over the network and the allocation of resources. Any computer that has access to the server's 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, connection and data exchange is carried out through a common communication channel called a common bus. The bus structure is simpler and more economical because it does not require an additional device and uses less cable. But it is very sensitive to cabling faults. If the cable is damaged in even one place, then problems arise for the entire network. The fault location is difficult to locate.

2. STAR topology... In this case, each computer is connected with a separate cable to a common device called a hub (hub), which is located in the center of the network. The star is more resistant to malfunctions of 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 grow the number of nodes in the network is limited by the number of hub ports. Currently, such a structure is the most common type of link topology in both local and wide area 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 itself into an internal buffer. In a network with a ring topology, special measures must be taken so that in the event of failure or disconnection of a station, the communication channel between the other stations is not interrupted. The advantage of this topology is simplicity of management, the disadvantage is the possibility of failure of the entire network in case of a failure in the link 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 there is an intensive exchange of data are directly connected, and for the exchange of data between computers that are not connected by direct links, transit transmissions are used through intermediate nodes. A mesh topology allows a large number of computers to be connected and is typical, as a rule, of wide area networks. The advantages of this topology are its resistance to failures and overloads. there are several ways to bypass individual nodes.
5. Mixed topology... While small networks tend to have a typical topology - star, ring, or shared bus, large networks tend to have arbitrary connections between computers. In such networks, separate arbitrary subnets can be distinguished, having a typical topology, therefore they are called mixed topology networks.

The principles of functioning 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.To servers global network usually connected servers and, accordingly, networks of the second order (regional), third order (corporate), fourth order (local), and to them - users of individual computers - subscribers(clients) networks. Note that not all intermediate tiers (eg corporate) networks are required.

2. PCs are interconnected by communication channels
The main goal 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 connect the computers to each other. Therefore, all kinds of communication channels (wired and wireless), for which different physical media are used, are mandatory components of any network. In accordance with this, such communication channels are distinguished in networks 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. Transportation of passengers can take place by air, using railways or water (sea or river) routes. Depending on the environment of transportation, a vehicle is selected. Information is transported through computer networks. The environments in which network computers communicate determine the means of connecting computers. If it is an environment that requires telephone connection, then the connection is made via a telephone cable. Connections of computers using electrical cables, radio waves, fiber optic cables, etc. are widely used.

Let's consider the main types of channels. Some of them are mutually exclusive, some may describe the same channel from different angles.
Channels are digital and analog.
TO analog channels include an ordinary 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 low bandwidth (several tens of kilobytes per second). Now there is a tendency to replace all analogue channels with digital ones, not only in computer networks, but also in telephone ones.
Channels are also divided into allocated and dial-up.
Using dial-up line, the connection is formed for the duration of the data transmission, and at the end of this transmission, it is disconnected. Dial-up is a connection using the usual telephone line.

Dedicated the line works differently:
the connection is permanent, it always allows data to be transferred 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 onelectrical wire, optical and radio channels.
Wired channels represent a connection with an electric cable, possibly complex. All such channels use electrical impulse data transmission.

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 substantially 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 a channel can provide (for video files)
- reliability - the ability to transmit information without distortion and loss;
- cost;
- expansion possibilities (connecting new computers and devices).

To transmit information through communication channels, it is necessary to convert computer signals into signals of physical media.
For example, when transmitting information over a fiber-optic cable, the data presented in a 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 interfacing computers with communication channels.
If the communication channel – telephone line, then when receiving - transmitting information, a modem is used.

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: speed of reception - transmission of information (measured in bits per second). Modern modems have a data transfer rate of 33600 bits per second, 57600 bits per second.

3. The network operates using 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 of operation of various equipment in the network

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

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

applied higher-level protocols responsible for the operation of specialized Internet services: HTTP (hypertext messaging), FTP (file transfer), e-mail protocols, etc.
Technically, TCP / IP is not one, but two network protocols. TCP is a transport layer protocol. It controls how information is transmitted. The IP protocol is addressable. It defines where the data transfer takes place.

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

server - a program providing services, client - program consuming server services - programs

IP-addresses

Information exchanged between PCs is divided into packages.A PACKAGE is a "piece" of information containing the address of the sender and recipient.
A. Multiple packets form a data stream that is received by the user 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 browser InterNet Explorer)
C. In order for the packet to find its addressee - 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 over 4 billion: 232 \u003d 4294967296.

the lР -address is "read" from right to left. Typically, the rightmost digit represents a specific computer, and the remaining digits show the network and subnet (i.e. local area networks) numbers.
Sometimes this may not be the case, but in any case, if the address is represented in binary form, then some part of the rightmost bits identify a specific 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 is thrown into the network.
Routers determine the route of packets.

Domain name system

Computers can easily communicate with each other using a numeric IP address, but it is not easy for humans to remember a numeric address, and the Domain Name System (DNS) was introduced for convenience.
Domain name system maps the numeric IP address of each computer to a unique domain name. Domain addresses are assigned at 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 by one attribute.

Domain address defines an area that represents 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 in which it is located.
A computer name includes at least two levels of domains. Each level is separated from the other by a point. To the left of the top-level domain are the subdomains for the general domain.
In the Internet addressing system, it is common to represent domains by geographic regions. They have a two-letter name.
Example. Geographic domains of some countries: France - fr; Canada- sa; USA - us; Russia - ru; Belarus - by.
There are also domains separated by thematic featured. Such domains have 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 capabilities of hypertext links. Hypertext is a text that contains links to other documents. This makes it possible, when viewing a document, to easily and quickly switch to other information related to it, which can be text, an image, a sound file, or any other form accepted in the WWW. In this case, linked documents can be scattered around the globe.
Numerous overlapping links between WWW documents and the computerized web span the planet - hence the name. Thus, the dependence on the location of a specific document disappears.
The World Wide Web service is designed to provide access to a special kind of electronic documents called Web documents or, more simply, Web pages. A web page is an electronic document that contains, in addition to text, special formatting commands, as well as embedded objects (pictures, audio and video clips, etc.).
They browse web pages using special programs called browsersso a 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. This is called offline browsing.
Unlike printed electronic documents, Web pages have not absolute, but relative formatting, that is, they are formatted at the time of viewing in accordance with the screen and with which browser they are viewed. Strictly speaking, the same Web page may look different when viewed in different browsers, depending on how the browser responds to the commands that the author has embedded in the Web page.
Every Web document (and even every 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 many, many Web documents on the Internet. In the past seven years, the content of the WWW has doubled every 18 months. Apparently, in the coming years this rate will decrease somewhat, but it will remain quite high, at least until the turn of 10 billion. In connection with such a huge number of Web-documents, the Internet today has an important problem of their search and selection - we will take a closer look at it, but for now we will get acquainted with how the URL looks formally.
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.
The document URL has three parts and, unlike domain names, reads from left to right. The first part specifies the name of the application protocol used to access this resource. 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 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 periods. A slash is placed after the domain name.
The last element of the address is the path to the file containing the Web document on the specified computer. Writing the path to the file in operating system We are already familiar with Windows, but there is an important difference. On Windows, it is customary to separate directories and folders with a backslash "\\", but on the Internet, it is customary to use the normal forward slash "/". This is due to the fact that the Internet originated on computers running on the UNIX operating system, and there it is customary to separate directories that 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 you click on the hyperlink, a request is sent to the Network to supply the object that the hyperlink points to. 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. Electronic mail (E-mail).
Electronic mail (E-mail - Electronic mail, English mail - "mail") is the most widespread and until recently the most popular application of the Internet. According to the estimates of 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 e-mail, each user is assigned a unique mailing address, which is usually formed by attaching the user's name to the name of the computer itself. The username and computer name are separated by the @ symbol. For example, if a user has the login name еmswоrth on blandings.corn, their email address will look like [email protected]

3. Teleconference service (Usenet)
Another widely used service provided by the Internet is Usenet news - Usenet news, which are also often called teleconferences (they have nothing to do with television, and the prefix "tele" means "remote", "operating at a long distance"). They provide the ability to read and post 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 in the world network, which store data archives. These archives can be commercial, restricted, or publicly available.

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 computer room. The terminals (ie displays with keyboards) allowing the computer to operate were located in a different 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 keep this method of dialogue with a computer.
The remote access program is called Telnet.
For its functioning, as for all Internet services, two parts must exist - a server program installed on a remote computer and a client program on local computer.
To connect to a remote system, you must be a registered user, that is, have an input 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, that is, browse directories, copy or delete files, run various programs that have an alphanumeric interface.

6. IRC service (Internet Relay Chat) is intended 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. Its main purpose is to enable communication between two people, even if they do not have a permanent IP address.
8. World Wide Web Service (WWW) Is one information spacecomposed 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.