Lecture
A computer network or a computer network (CS) is usually called a set of interacting stations organized on the basis of a computer (including a PC), called network nodes. The communication centers interact with each other through data transmission channels (EFF), which form the data transmission medium (SPD). Each CSS can process information offline and exchange informational messages with other CSS.
Network operations are governed by a set of rules and agreements (called a network protocol ) that defines: types of connectors and cables, types of signals, data formats, algorithms of network interfaces, ways to monitor and correct errors, the interaction of application processes, etc.
To date, a significant number of varieties of the organizational and architectural structure of the Armed Forces have been developed. They can be classified according to the following criteria:
1) in scale - local and global;
2) by the method of organization - centralized and decentralized;
3) by topology (configuration) - star-shaped, ring, tire, mixed.
Local aircrafts (LAN) are networks whose nodes are located at small distances from each other (in different rooms of the same building, in different buildings located on the same territory).
In global aircrafts (GVS), network nodes are located at significant distances from each other (in different parts of a large city, in remote settlements, in different regions of the country, and even in different countries).
Centralized aircraft are networks in which a main node is provided, through which all information exchanges take place and which manages all the processes of interaction between nodes.
Decentralized aircraft are networks with relatively equal peers, control of access to data transmission channels in these networks is distributed between nodes.
Varieties of the sun on topology
As noted above, there are four types of aircraft configurations: star, ring, tire and combined (see Fig. 1). Their distinctive signs are as follows.
Star configuration (Fig . 1 a). The network provides for a central node (NCC) through which all messages are transmitted. Such networks appeared earlier than others when a developed network of remote user terminals was created on the basis of a large central computer.
The disadvantages of star networks:
1) the complete dependence of the reliability of the network on the reliability of the NOC, the failure of which uniquely leads to the failure of the entire network;
2) the complexity of the NOC, which is entrusted with almost all network functions.
Ring configuration (Fig. 1b) . In the ring network, the node controlling the transmission of messages is not allocated, they are transmitted in one direction through special repeaters, to which all network nodes are connected.
Advantages of ring aircraft:
1) the lack of dependence of the network on the functioning of its individual nodes, and disabling any node does not disrupt the network;
2) easy identification of faulty nodes and the ability to reconfigure the network in case of failure or malfunction.
Disadvantages:
1) the reliability of the network depends entirely on the reliability of the cable system, since the failure of this system in any one place completely disables the entire network;
2) the solution of information security problems becomes more complicated, since messages pass through all network nodes during transmission.
Fig. 1. Basic LAN topologies
Bus structure (Fig. 1 c). The bus is a closed data transmission medium. All network nodes are connected to the bus in the same way through amplifiers, repeater signals, as the signals on the bus attenuate. Signals on the bus from the transmitting node are distributed in both directions at a speed comparable to the speed of light. Since all receiving nodes receive the transmitted messages almost simultaneously, special attention should be paid to access control to the transmission medium.
Advantages of the bus structure:
1) ease of organization, especially when creating a LAN;
2 ease of connecting new nodes;
The main disadvantages are:
1) passivity of the transmission medium, which is why it is necessary to amplify signals that fade in the medium;
2) the complication of solving problems of information protection;
3) with an increase in the number of CSS increases the danger of saturation of the transmission medium, which leads to a decrease in throughput.
Combined networks, as the name implies, are organized by combining individual network fragments with different topologies into a common network.
On the basis of even such a cursory examination of possible structures of the Armed Forces, it is easy to conclude that for those objects (enterprises, institutions, other organizations) in which significant amounts of information to be protected are regularly processed, the combined structure of the LAN will be most appropriate. For example, an independent subnet organized according to a star-shaped scheme can be created for processing confidential information, and a subnet organized according to a bus diagram can be created for processing the publicly accessible one, and the NOC of the first subsystem can be connected to the common bus of the second subsystem as its full node.
Currently, three types of cables are predominantly used to create a physical data transfer medium: twisted pair, coaxial, and fiber optic cable.
Twisted pair consists of two insulated wires, spirally intertwined with each other. Such cables have been used for a long time in telephone communication. They provide reliable data transfer at relatively low speeds (a few Mbit / s) and short transmission distances (several tens of meters). Therefore, they should be used in compact LANs with not very large data streams.
There are two types of cables of this type: unshielded and shielded, and in the shielded cables side electromagnetic radiation is extinguished, so they are protected from interception of the transmitted information by non-contact connection.
Coaxial cable contains two conductors: one is used to transmit signals, the second is for grounding. The external conductor always plays the role of grounding. The space between the conductors is filled with insulating material.
A coaxial cable is capable of transmitting broadband signals, i.e., many signals simultaneously, each at its own frequency, which ensures a high data transfer rate. In addition, coaxial cables are highly robust.
The industry produces standard (thick) and cheap (thin) coaxial cables. A thick cable is notable for increased noise immunity and low attenuation of transmitted signals; however, for its connection, special connectors are necessary - connections. Thin cable is inferior to thick noise immunity and attenuation of the signal, but it connects to standard connectors - connections. In addition, these types of cable differ in maximum length between network nodes: thick - up to 2500 m, thin - up to 925 m.
Fiber optic cable allows you to solve all the problems of creating an effective data transmission medium with a high transmission rate (up to 50 Mbps), no transmission losses, almost complete immunity to interference, no restrictions on transmission distance and bandwidth. Its disadvantages are the complexity of installation and diagnostics, as well as the high cost. In addition, there is currently little experience in its application. However, despite these drawbacks, the fiber optic cable is very promising for the organization of the data transmission medium of the aircraft.
A workstation is a computer connected to a network through which a user gains access to its resources. Often a workstation (a network user or an application task performed on a network) is called network clients.
A server is a multi-user computer dedicated to processing requests from all network workstations, providing these stations with access to common system resources (computing power, database, program libraries, printers, fax machines) and distributing these resources. Of the most important requirements for the server, high performance and reliability should be emphasized. examples of specialized servers:
1. file server is designed to work with databases, has a large disk storage devices;
2. backup server performs daily automatic archiving of information from servers and workstations;
3. fax server - a dedicated workstation for the organization of effective facsimile communication;
4. mail server - a dedicated workstation for organizing electronic correspondence;
5. print server - designed for efficient use of system printers;
6. Proxy server - a popular tool for connecting local corporate networks to the Internet.
If there are several computers in the same room, building or complex of nearby buildings, the users of which must jointly solve some tasks, exchange data or use common data, then these computers should be combined into a local network.
A local area network (local area network, LAN) is a group of several computers interconnected by means of cables (sometimes also telephone lines or radio channels) used to transfer information between computers. To connect computers to a local network, you need network equipment and software.
Local networks allow you to provide:
1. collective processing of data by users of computers connected to a network and data exchange between these users;
2. sharing programs;
3. sharing of printers, modems and other devices;
Therefore, almost all firms with more than one computer, combine them into a local network.
To merge computers into a local network requires:
1. insert a network controller (network adapter or network card) into each computer connected to the network, which allows the computer to receive information from the local network and transmit data to the network;
2. Connect computers with a cable that transfers data between computers, as well as other devices connected to the network (printers, scanners, etc.). Cable connection is carried out through special devices - hubs (or hubs), switches, etc.
To ensure the functioning of a local network, a special computer is often identified - a server, or several such computers. On server drives are shared programs, databases, software distributions. The rest of the local network computers are often called workstations. In networks consisting of more than 20-25 computers, the presence of a server is mandatory - otherwise, as a rule, network performance will be unsatisfactory. The server is also necessary when working together intensively with a database.
Many servers are much more expensive (5-10 times) of ordinary computers. It is not surprising - after all, they are not only very powerful computers with a large amount of RAM, but in addition they provide exceptional reliability, high performance and duplication of devices and stored data.
To ensure the functioning of the local network requires appropriate software.
Windows 95, 98, NT Workstation operating systems have built-in capabilities for organizing local area networks without a dedicated server. Typically, such networks are called peer-to-peer, because in them all computers are equal, each of them performs both the role of the user's workplace and the server's role in providing access to their resources and data.
But often a peer-to-peer network is not the best solution. After all, the user OS is little adapted to perform the functions of a network server that it has to perform. And if a user plays DOOM on any computer or draws a picture in Paint, and other users work with files on the same computer, they will interfere with each other - their speed will drop dramatically.
Local networks with a dedicated server on the server use special operating systems that ensure reliable and efficient processing of many requests from user workstations. On workstations such a local network can use any operating system, for example, DOS. Windows, and should be running a driver that provides access to the local network.
Local networks are called local because they connect computers that are in the same place, that is, close to each other. But many enterprises (corporations, banks) have divisions located in different parts of the city or even in different cities and countries. For effective work, they, as a rule, need to combine their units into a single network. Such networks are commonly referred to as distributed.
Naturally, in all cases, to create a distributed network, you need the appropriate equipment (modems, routers, etc.).
In the very near future, the Internet will radically change the economic and social conditions around the world. These changes can not affect you personally. The Internet will become a very important part of your life. This will affect your work, leisure and communication with other people. The Internet helps to conduct research, improves the interaction of employees in the office, provides an opportunity to work at home, increases the chances of finding work.
Soon the Internet will change our whole life, affect our work, rest, how we shop, how we communicate with each other, how we read newspapers, how we will see the weather for tomorrow and for many, many other things.
At the end of the 60s, by the request of the US Department of Defense, a distributed ArpaNet network was created for communication between the computers of the Ministry. In the development of the ARPAnet network, the task was to provide communication between multiple sets of disparate computers that were remote from each other, and this connection should not be broken if the network was partially damaged (for example, when one or several network nodes were bombarded). The principles developed for the organization of such networks (equal rights of nodes, TCP / IP protocols, routing algorithms) proved so successful that many other organizations (especially universities and government agencies) began to create networks on the same principles. These networks began to unite with each other, forming a single network with a common address space (just as all telephone exchanges in one city support a single system of telephone numbers). This unified network (or network of networks, a collection of networks) became known as the Internet.
Currently, the Internet is a global, intercontinental network, it brings together tens of millions of computers and local area networks, and its services are used from 100 to 250 million people.
Until the mid-90s, mainly:
1. e-mail - with its help, you can send an e-mail (test or arbitrary file) to any Internet user. The delivery time is usually no more than several hours, and sometimes several minutes.
2. newsgroups ( USENET ) - an exchange of views through e-mails about those or other topics. Each Internet user can subscribe to newsgroups of interest to him (there are several tens of thousands of them and they are devoted to a variety of topics - from the problems of using laser printers to psychological disorders). In this case, the user will receive all the letters sent to the relevant newsgroups, and maybe he himself will express his opinion or answer someone's question.
3. news servers - they send news on certain topics in the form of emails. An Internet user may subscribe, for example, to receiving stock reports, political gossips, etc.
4. FTP service - file storage. Texts of documents, programs, tests of books are stored on them. Each Internet user can receive a table of contents of FTP servers or any of the files stored on it as an e-mail or archive.
5. search service - allows you to find the desired document on the FTP-servers included in the Internet. Search can be conducted by keywords and other characteristics of the document. You can ask a search service online or by sending a specially designed letter to it.
6. electronic bulletin boards ( BBS) is a place where all information to be exchanged is flown. С помощью BBS можно опубликовать объявление для общего ознакомления, или оставлять на доске информацию, которую адресат может забрать в удобное для него время.
7. Internet Relay Chat (IRC) – для поддержания так называемого живого диалога. Недавно выпущенное программное обеспечение позволяет проводить реальные видео- и аудиконференции.
8. служба ICQ (аська, I seek you “Я ищу тебя”) - предназначена для обмена короткими текстовыми сообщениями между пользователями, одновременно находящимися на связи.
9. The Word Wide Web (WWW) is a huge collection of statistical and interactive documents related to each other. To view these documents - Web pages - Web browsers are used. Web pages are located on hundreds of thousands of Web servers located throughout the world. To go from one Web page to another, just click on one of the hypertext links that are found on almost every page.
Each computer connected to the Internet is called a node. Some nodes provide programs and data to other nodes, they are called servers. Other computers use the information provided by the server, they are called clients. Systems whose components interact in this way are called client / server systems.
The same terminology applies to programs running on computers connected to the Internet.
A client program, or simply a client, is a user-friendly program that runs on your computer and has access to Internet resources. When a client program needs to get information from the Internet, it accesses the server program. The server program, or simply the server, returns to the client program a response to its request.
To exchange information over the Internet, every computer (no matter which OS it uses, Windows, Mac, or Unix, and regardless of whether it is a supercomputer or laptop) must support TCP / IP. TCP / IP protocol standardizes the exchange of information between systems and determines how to present data as a packet and how to transfer each packet to a remote computer.
To be precise, TCP / IP is two different protocols that are closely interconnected.
IP . Special computers, called routers, use the Internet protocol to transfer packets over the network. Each information packet contains the IP addresses of the sending computer and the receiving computer. An IP address is a unique number that uniquely identifies a computer on the Internet. The IP address is 4 numbers separated by dots. For example, the IP address of the host is 206.246.150.10.
TCP.Transmission Control Protocol defines how information should be broken up into packets and sent over the Internet. Imagine that you pulled out the first three sheets of this book and sent them to your friend. You entrusted the first sheet to deliver to the messenger who will reach the destination by bus, the second was sent by mail, and the third was handed over to the taxi driver. It is clear that your friend will receive these sheets at different times and in the wrong sequence. To read the text, your friend will have to check the page numbers and place them in the correct order. Similarly, the TCP protocol processes information packets. No one can guarantee that all information will arrive at the same time, but TCP places the packets in the right order, and also checks each packet for transmission errors.
Domain Names Every computer on the Internet has an IP address. But try to remember the IP addresses of at least a few computers. Fortunately, this is not necessary. There is another, more user-friendly way of addressing computers on the Internet - the domain name system. The IP address of the host is 206.246.150.10, its domain name is www.mcp.com. It is much easier to remember.
A domain name consists of two or more words, separated by dots, according to the principle: node.second_level.first_level. Top Level Domains are well defined. The first level domains, for example, com or uk, indicate the type of organization or country in which the organization is located (Table 1). The second level domain defines the organization. To transmit information, the global network uses domains of the first and second levels. The domain name of the node is used to transfer data from computer to computer within the local network.
Table 1
Name |
Type of organization, country |
Com |
Commercial organizations |
|
Org |
Diverse, mostly non-profit organizations |
|
Net |
Computers that make up the infrastructure of the Internet, and providers |
|
Edu |
Universities and other educational institutions |
|
gov |
Government organizations |
|
mil |
Military institutions |
|
ru |
Russia |
|
it |
Italy |
|
jp |
Japan |
|
us |
USA |
|
pl |
Poland |
|
su |
the USSR |
|
Ua |
Ukraine |
|
Uk |
England |
You can connect to the Internet either via a dedicated line or via a dial-up line (via the telephone network). Most users connect to the ISP via a telephone channel using a modem. The modem converts digital data to analog signals and transmits it over a telephone line. This process is called Modulation. The modem on the other end of the line receives the analog signal and converts it to digital data again. This process is called demodulation. In the process of information exchange, both external modems connected by a cable to the serial port and internal modems that are inserted into one of the computer connectors can participate.
The best way to ensure your safety is to not transmit any personal Information over the Internet. Some data must be kept confidential.
Never tell anyone your username and password, do not tempt your neighbor to commit an unseemly act.
Do not send anyone your Internet address and telephone number over the Internet.
Do not pass on the Internet the names of your children for the same reason that you would not give this information to strangers by phone.
Do not tell anyone your account number via the Internet. In particular, never buy anything based on an ad found in your mailbox.
Improper behavior on the Internet can be dangerous not only for you personally, but also for the organization in which you work.
By law, an enterprise is solely responsible for information located on any of its computers. Personal records on the service computer should not be. Therefore, do not store anything on the disk that you think the manager should not see: he has the right to check the contents of the computer at any time.
Now that the Remote Access and TCP / IP tools are installed on your computer , you need to set the parameters for connecting to your provider's server. In the My Computer folder you now have a new folder called Remote Access to the Network . Open My Computer and double-click the Remote Access Network icon . To set connection parameters with your provider, do the following:
1. Double-click the New Connection icon located in the Remote Access Network folder .
2. Specify a name for the connection being created in the Enter the connection name field , for example, the name of the computer with which the connection is established. Name the Server and click the Next button .
3. On the screen you will see the input field; set in it the phone number to connect to the server that the provider told you. Click the Next button .
4. Click the Finish button and the record of the new connection will be saved in the Remote access to the network folder.
5. In the Remote Access Network folder, right-click the new connection icon and select Properties . You will see a window displaying the parameters of the new connection.
6. Click the Server Type button.
7. From the Remote Server Type list, select PPP; Windows 98, Windows NT, Internet . Make sure that only two of the options available in this window are included: Software data compression and TCP / IP .
8. Click the TCP / IP Settings button, and you will see the TCP / IP Settings dialog box.
9. Select the value of the option Addresses are entered manually and specify the primary and secondary domain name servers (DNS) that the provider previously informed you.
For example, 199_1_11_2 - primary address
199_1_11_15 - secondary address.
10. When you finish entering the parameters, click the Ok button. Click on the Ok button again to save the changes.
Now that all the necessary software is installed, you can make a real connection with your provider's computer. To do this, do the following.
1. Double-click on the icon corresponding to this connection, located in the Remote Access Network folder . The Dialog box appears .
2. Set your username and password in the appropriate input fields. If you do not want to enter this data with each new connection, enable the Save password option .
3. Click the Connect button and the system will start dialing your provider number. A window will appear displaying call progress information.
4. If you set the connection mode to the server when setting the connection parameters, the Post-Dial Terminal Screen window will appear on the screen . In this case, register on the server manually and click the Continue button .
When you want to break the connection with the provider, click the Break Connection button in the window that displays the connection status.
PROGRAMS FOR WORK ON THE INTERNET
To work on the Internet, you need a computer, a modem, an Internet provider and a client program. To find suitable Internet programs, you do not have to engage in a long search. You can use Microsoft or Netscape software. Both companies supply a full range of required applications.
To surf the Web space, exchange messages with other users, and participate in newsgroup discussions, you need Web browsers, an email program, and a UseNet client program. If you have installed Microsoft Internet Explorer or Netscape Communicator, then all the necessary components are already on your hard disk. In addition, bundled with these products are also delivered tools that allow you to talk with other Internet users in real time.
A web page is a combination of plain text and HTML (Hypertext Markup Language) tags used to format this text. HTML is a standard defined by the World Wide Web Consortium. The latest version of HTML - version 3.2.
A URL defines the location or address of each Web page on the Internet. URL consists of three parts:
- protocol
- domain name of the node
- way.
The protocol defines the method of interaction between the client program and the server. The node's domain name identifies the server on which this web page resides. The path is the full path and possibly the file name of the Web document. The path and file name may be omitted.
Example:
http://www.host.com/files/are/here.html
protocol domain name path and name
Hypertext links together various documents using highlighted words and phrases. Hypertext links allow you to move from one Web document to another. Clicking on the link takes you to a new web page. Text links are usually highlighted with underline and color. If you put the mouse pointer on a link, it usually changes its appearance and turns into a hand with an extended index finger; click in this place, and your browser will open the Web page that this link points to.
Web pages use not only hypertext tools, but also hypermedia tools. In addition to text, hypermedia includes image and sound. Data fragments of these types can also refer to each other. For example, you can associate text with a picture so that when you click on an image in the browser window, text is displayed commenting on this image.
When viewing some Web documents, the browser window is divided into several panels, also called frames, or frames. Different frames display different data. Frames make it easier to work with Web pages, for example, a table of contents can be displayed in one frame, and the text of a document in another. If you click on one of the items of the table of contents, the contents of the other frame will change, but the table of contents will remain on the screen; looking at it, you can decide which document to call next.
The easiest way to open a Web page is to specify its URL in the Browser Address field and press Enter . If you click on the down arrow located to the right of the Address field, you will see a list of addresses entered earlier. Click on one of them, the corresponding document will open in the browser window. An alternative way to open a page is to select the File-Open item in the main menu , enter the address and click Enter .
It may happen that you will need to re-refer to the page you recently visited. Click on the Back button on the toolbar of your browser, thus you will open the penultimate page of those with which you worked during this session. Continue clicking this button and you will get to the very first page opened in the current session. From this point on, the Back button will become unavailable and you will not be able to use it again.
You can also move forward in the history list of the current session; To do this, use the Forward button . If you use the Next button to get to the very last page opened during the session, the browser will disable the Next button.
If clicking on the Forward and Back buttons, you can’t find the page you need, use the browser history list.
In Internet Explorer, to list all sites that you visited earlier, do the following:
1. Click on the Log button located on the toolbar and you will see a list of history
2. Click on the icon representing one of the days and a set of folders will appear under it, each of which corresponds to a Web site.
3. Click on one of the folders; the folder opens and shows the list of Web pages you viewed on this site.
To bookmark a page listed in the history list, right-click on the page, and select Add to bookmarks .
Each time you start using Internet Explorer, the same Web page appears on the screen. It is called the original browser page. You can customize your browser so that it is the page you selected. When working with Internet Explorer, do the following:
1. Navigate to the page you want to use as the source.
2. On the View menu, select Internet Options and the General tab.
3. Click on the button With the current. Internet Explorer will enter the URL of the current page in the Address field above the button.
4. Click on the OK button . The next time you start Internet Explorer, this page will be displayed first.
If during the process of copying a web page you realize that the rest of the document does not interest you, you do not need to wait until the transfer is complete. Regardless of which browser you are working with, click the Stop button located on the toolbar. In this case, the Web page will not look perfect: after all, the browser has not completed the download process.
To prohibit copying images to your browser, in Internet Explorer, select the following items in the main menu: View - Options. Click on the Advanced tab, turn off the Show pictures option.
Hypertext is a system of information objects, interconnected by directional semantic links that form a network. Each object is associated with a screen information panel, on which the user can associatively select one of the links.
Hypertext technology involves moving from one object to another, taking into account their semantic, semantic relatedness. The processing of information according to the rules of formal inference in hypertext technology corresponds to the memorization of the path of movement along the hypertext network. The user himself determines the approach to the study of the material based on their individual abilities, knowledge, skill level and training. Hypertext contains not only information, but also the apparatus of its effective search.
Structurally, hypertext consists of information material, a hypertext thesaurus, a list of main topics and an alphabetical dictionary.
Information material is divided into informational articles, consisting of the title of the article and the text. The title contains the subject or name of the object being described. The informational article contains traditional definitions and concepts, should occupy one panel and be easily observable so that the user can understand whether it is worth reading it carefully or moving on to other articles that are close in meaning. The text included in the information article may be accompanied by explanations, examples, graphs, documents and video images of real-world objects. Keywords to link to other informational articles should be visually different.
A hypertext thesaurus is an automated dictionary that displays semantic relationships between lexical units of an information retrieval language and is designed to search for words by their semantic content.
The term "thesaurus" was introduced in the XIII century by Florentine Brunetto Lotiki for the name of the encyclopedia. In Latin, this term is translated as treasure, stock, wealth. Hypertext thesaurus consists of thesaurus articles. The thesaurus article has a heading and a list of headings of related thesaurus articles, which indicate the type of relationship and the headings of thesaurus articles. The title of the thesaurus article coincides with the title of the information article and is the name of the object described in the information article. Forming a hypertext thesaurus article means indexing the text.
The list of main topics contains the headings of all reference articles for which there are no links with gender-type, part-whole relations. It is desirable that the list occupied no more than one screen panel.
The alphabet dictionary contains a list of names of all information articles in alphabetical order.
By studying the information provided in the form of hypertext, the user can get acquainted with the sequence of data blocks. The process of selecting the sequences of these blocks, i.e. The method of driving a user from one object to another is called navigation. At the same time, terminological navigation , consistent movement in terms of each other, and thematic navigation are distinguished , through which the user must obtain for reading all the articles necessary to study the topic he needs.
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Information technology in research
Terms: Information technology in research