Tools for exchanging information and searching for information resources on the Internet. Concept of communication
Rice. 12. Mixed topology
Only in a network with a full mesh topology there is a separate communication line to connect each pair of computers. In all other cases, the question inevitably arises of how to organize the sharing of communication lines by several computers on the network. As always when sharing resources, main goal This is a reduction in the cost of the network.
Computer networks use both individual communication lines between computers and shared ones, when one communication line is alternately used by several computers. In the case of using shared communication lines, a complex of problems arises associated with their joint use, which includes both purely electrical problems of ensuring the required quality of signals when connecting several receivers and transmitters to the same wire, and logical problems of dividing access to these lines in time .
Exist various ways solving the organization's problem sharing to shared communication lines. Inside the computer, there are problems separating communication lines between various modules also exist - an example is access to system bus, controlled either by the processor or a special bus arbiter. In networks, organizing shared access to communication lines has its own specifics due to the significantly longer time it takes for signals to propagate over long wires; moreover, this time can be different for different pairs of computers. Because of this, link access negotiation procedures can take an excessive amount of time and lead to significant losses in network performance.
Despite all these difficulties, shared communication lines are used very often in local networks. However, in last years There has been a tendency to abandon shared data transmission media in local networks. This is due to the fact that the reduction in network cost achieved in this way comes at the cost of performance.
An important problem that needs to be taken into account when connecting three or more computers is the problem of their addressing. There are several requirements for the network node address and its destination scheme.
The address must uniquely identify a computer on a network of any size.
The address must have a hierarchical structure, convenient for building large networks.
The address must be convenient for network users, which means that it must have a symbolic representation, for example, www.cisco.com.
Since all of the above requirements are difficult to combine within the framework of any one addressing scheme, in practice several schemes are usually used at once, so that the computer simultaneously has several address-names. Each address is used in the situation when the corresponding type of addressing is most convenient. And so that there is no confusion and the computer is always uniquely identified by its address, special auxiliary protocols are used that can determine addresses of other types using an address of one type.
The most widely used are two node addressing schemes.
Symbolic addresses or names. These addresses are intended to be remembered by people and therefore usually carry a semantic load. Symbolic addresses are easy to use in both small and large networks. To work in large networks a symbolic name can have a complex hierarchical structure. All individual pieces of information separated by a "." are called domains. For example, email address Minsk State Linguistic University has next view: mslu.unibel.bv. where mslu is the name of the university, unibel is the name of the provider to which the university server is connected, and by is the abbreviated name of the Republic of Belarus (Byelorussia).
Among the domains, there is a certain hierarchy. The domain at the end of the address is called a domain top level. It identifies the geographic region of the network, its type, or the type of organization to which the message is sent. Thus, in the USA, the last domain usually denotes the type of organization: commercial (.com), educational (.edu), government (.gov), etc.
Numeric compound addresses. Symbolic names are convenient for humans, but due to their variable format and potentially large length, they are not very economical to transmit over a network. Therefore, to work in large networks, numerical composite addresses of fixed and compact formats. Typical representatives of this type of address are IP address. IP address is the unique name by which each computer connected to the Internet is known to all other computers on the Internet. worldwide network. An IP address is usually written as a sequence of four regular decimal numbers, each of which is in the range from 0 to 255. When writing, numbers are separated from each other by dots. For example, 147.120.3.28 or 255.255.255.255 - these are two IP addresses. Whenever a message is sent to any computer on the Internet, the IP address is used to indicate the address of the sender and recipient. For automatic conversion Domain names of email addresses are converted into the IP address of a computer on the Internet using a special program - DNS (Domain Names System).
The main directions of using the Internet are: obtaining information and exchanging information. The means of information exchange include:
1. Email.
2. Teleconferences (newsgroups) and mailing lists.
3. File sharing.
4. Audio and video conferences.
5. Programs for communication and collaboration in real time.
6. Paging systems.
7. Internet telephony.
Information resource search tools:
1. Search engines.
2. Thematic catalogs.
3. Metasearch engines.
4. Accelerated search programs.
Email(e-mail or e-pistles) is a means of exchanging letters electronically between people who have access to computer network. The basic concept of email is message. A message is understood as text transmitted over communication lines in a network from one user to another. A message can be considered an “electronic” version of a regular letter. An email message or letter consists of two parts: the header of the letter and the body of the letter. The header of the email contains the email addresses of the recipient and sender, its subject and the date it was sent.
To be able to send and receive messages by email, you need a so-called postal package. It usually includes the following programs:
1. Mail server.
2. Transport program.
3. Mail client.
Mail server– a program that forwards messages from mailboxes to other servers or to the user’s computer upon request mail client.
The mail server program manages the transmission of messages between clients and the Internet. If you are using a UNIX network operating system, then the UUCP (Unix-to-Unix Copy Protocol) program can serve as a mail server. UNIX systems). When using other networks operating systems SMTP (Simple Mail) program is used as a mail server Transfer Protocol- mail transfer protocol). This system requires transport program . Such a program delivers messages to mail servers. They can be, for example, a POP (Post Office Protocol) server. mail center) or IMAP (Internet Mail Access Protocol) server. The mail server then delivers the message to the final recipient.
Mail client (mailer) – a program that helps you compose and send emails, receive and display letters on the user's computer.
The mail client provides the ability to read a received letter and write a response in offline mode, that is, without an Internet connection. There are quite a lot of email clients. Thus, for the operating systems OS/2, Mac OS and UNIX, the Eudora mail program is used. For computers running Windows OS, Microsoft email programs are widely used: MS OUTLOOK, INTERNET MAIL, EXCHANGE, OUTLOOK EXPRESS and Netscape NETSCAPE MESSENGER. To transmit non-text information (audio and video files, multimedia data) by e-mail, the MIME (Multipurpose Internet Mail Extensions) standard is used.
Mailing list server is a program containing data on customer email addresses, answers to the most frequently asked questions, price lists, etc. Such a server analyzes messages received at its address and makes a decision on how and what to respond. If a message arrives from a new client, the mailing list server automatically remembers the client's email address.
Operations with outgoing messages include:
· preparing a message;
· addressing;
· attaching files to messages;
· sending messages.
Operations with incoming messages include:
· notification of mail arrival;
· reading mail;
· preparing a response;
· redirection;
· organization of message storage or deletion.
Newsgroups (newsgroups) and mailing lists.
Teleconference (newsgroup) – an online forum organized for discussion and exchange of news on a specific topic. Most known system newsgroups is Usenet (User Network). Usenet global distributed system for discussion, including many newsgroups stored on servers around the world.
Teleconferences Usenet News also called newsgroups (news groups). There are several tens of thousands of such groups. Each news group has its own name and hierarchical structure (a group may include several subgroups, each subgroup, in turn, may consist of other subgroups, etc.) For example, a news subgroup containing messages about WWW information systems is included in a large group " Computer Engineering» and has the name comp.infosystems.www (i.e. the “Computer Engineering” group, the “Information Systems” subgroup, the “WWW” subgroup). The comp.infosystems.www subgroup includes 10 smaller subgroups: comp.infosystems.www.misk, comp.infosystems.www.users, comp.infosystems.www.providers, etc. There are news groups where education issues are discussed ( edu.), science (sci.), music (music.), etc. For example, fido.networks.bel-internet – a news group of the FidoNet network dedicated to discussing network problems (networks), in particular, Internet problems in Belarus (bel-internet).
Information stored in archives on newsgroup servers is the titles and descriptions of articles, but not the articles themselves. If desired, the user can also receive full articles.
Thus, newsgroups perform similar functions to mailing lists, but use specialized software and their own protocol to do so. There are several tens of thousands of newsgroups dedicated to various issues, from computer protocols to tourism. Messages sent to a newsgroup server are sent from it to all associated servers that do not already have the message. On each server, the message is stored for a limited period of time, so this Internet service has a news nature. The main purpose of using newsgroups is to be able to ask a question to many people at the same time and get an answer or helpful advice from a more experienced colleague.
Like any service on the Internet, teleconferencing uses client/server technology.
News server– a program that provides access to news groups hosted on this server. Example of a news server address news.unibel.by. There are special client programs for working with news servers.
Newsreader (client program for reading news)– a program that uses the NNTP protocol to transmit newsgroup messages from a news server.
An example is Outlook program Express included Internet Explorer, which, in addition to mail protocols, also understands the NNTP news transfer protocol. Its rival colleague from the Netscape suite is the Netscape Collabra program.
There are teleconferences in which preliminary screening of messages that are not related to the topic of the conference is carried out. Such conferences are called moderated .
Moderated (managed) newsgroups are teleconferences in which all messages and replies are monitored moderator (managers) of this group who have the right to select articles.
In contrast, in unmoderated newsgroups, any message sent to the group is posted immediately.
Unmoderated (unmanaged) newsgroups are newsgroups in which anyone can post or reply to a message in that group.
In order to send a message to a particular news group, you need to fill out the standard positions in your mail client:
1) name of the teleconference (Newsgroup);
2) topic (Subject);
3) area of distribution (Distribution);
4) keywords(Keywords);
5) abstract (Summary).
The essence of this service is that information on a certain topic is received by a small (unlike Usenet News) circle of users who have subscribed to this list newsletters.
Mailing list topics can be varied, such as studying foreign languages, information technology, etc. Mailing lists can be controlled (moderated) or uncontrolled (unmoderated). Controlled lists have a manager (moderator), whose responsibilities include reading all letters coming to this list and filtering out unnecessary information(the so-called “noise”) and the formation of a selection of letters to be sent to subscribers. The mailing itself is carried out automatically by the mailing server. In unmoderated lists, all letters arriving at the list address are sent automatically. In controlled lists, subscription requires the permission of the moderator, and in uncontrolled lists it is carried out automatically.
7 TECHNOLOGY CLIENT / SERVER Mail server - a program that forwards messages from mailboxes to other servers or to the user’s computer at the request of his mail client Mail client (meller) - a program that helps compose and send electronic messages, receive and display letters on the user’s computer
9 MODERN INTERNET MAIL Internet mail protocols: SMTP (simple mail transfer protocol) is a mail protocol used for sending messages from a client computer to a mail server, as well as for sending mail between servers. Used on outgoing information servers
10 MODERN INTERNET MAIL Internet mail protocols: POP3 (Post Office Protocol) or IMAP (Message Access Protocol) - protocols for gaining access to a mailbox on a server and sending messages to a client computer. Used on incoming mail servers
12 EMAIL ADDRESS Example address:
13 “ENVELOPE” FORMAT OF AN ELECTRONIC LETTER To: (To:) - (email address is indicated) Copy: (Cc:) - (to whom else should the same letter be sent; the addressees will see each other) Bcc: (Bcc:) - (the addressee does not finds out who else you sent this letter to) Subject: (Subject:) - (the essence of the information being sent)
14 PROBLEMS AND LIMITATIONS Type transmitted information Universally (without possible losses) transmitted only text information on English language The most commonly used Cyrillic encodings: KOI8-R (for Unix OS) CP1251 (for MS Windows) CP866 (for MS DOS)
15 CODING OF EIGHT-BIT CHARACTERS AND BINARY ETTINGS Uuencode is an encoding method that allows you to reduce an eight-bit encoding to a seven-bit encoding text ASCII MIME (Multipurpose Internet Mail Extention, multipurpose email extensions) is a standard according to which a letter can contain text, pictures, video and binary data
25 STYLE AND ETIQUETTE Checking spelling and grammar Sparing use of CAPITAL LETTERS (THE ELECTRONIC EQUIVALENT OF SCREAMING!!!) Punctuation: !!!?????!! - expression of confusion - interpreted as an impartial expression
28 CREATE A FREE MAILBOX ON WWW Open home page one of the servers: Select the command to create a mailbox Sign me Up... or Receive mailing address or Registration
30 SECURITY AND ENCRYPTION To ensure security, there are special means, such as encryption and digital signature. Both of these concepts are related to cryptography Encryption makes the contents of the letter incomprehensible to the uninitiated Digital signature verifies the author of the message
31 SECURITY AND ENCRYPTION Support for S/MIME (Secure Multipurpose Internet Mail Extention) standard Asymmetric encryption (personal and public key i) the public key can be sent to the recipient along with digital signature. It is used to encrypt messages sent to you; the private key is stored on your PC and is used to decrypt messages from your recipient
34 MAILING LISTS. DEFINITION Mailing list - a special address, the mailbox of which is processed by a special program - mailing server (dispatcher) Mailing server - a thematic server that collects information on certain topics and forwards it to subscribers in the form of emails Controlled lists Uncontrolled lists
35 WORKING WITH MAILING LISTS Mailing server address Requests for help, subscription and unsubscription are sent to the mailing server address in the form of an email (Subject field is empty!)
36 WORKING WITH MAILING LISTS Mailing list address Articles for mailing are sent to the mailing list address
37 STANDARD COMMANDS FOR WORKING WITH THE NEWSLETTER SERVER help (send a list of commands for working with the mailing server) list (get addresses available lists mailings) subscribe list_name Your_name (subscribe to a specific list) signoff (for listserv) or unsubscribe (for majordomo) list_name (to unsubscribe)
38 NEWS GROUPS (TELECONFERENCES). DEFINITION Newsgroup (teleconference) - a network forum organized for discussion and exchange of news on a specific topic Usenet - a global distributed system for discussion, including many news groups stored on servers around the world NNTP - News Transfer Protocol
39 CLIENT/SERVER TECHNOLOGY News server is a program that provides access to news groups hosted on this server. Example news server address news.unibel.by Newsreader (client program for reading news) - a program that uses the NNTP protocol to transmit news group messages from the news server Examples of news clients: Outlook Express Netscape Collabra
40 TYPES OF NEWS GROUPS Unmoderated (unmanaged) news groups - anyone can send a message there or reply to a message in this group Moderated (managed) news groups - all messages and replies are controlled by the moderator (manager) of this group, who has the right to select articles
41 TITLES OF TELECONFERENCES The title of the conference is organized hierarchically into categories and subcategories For example, fido.networks.bel-internet – a news group of the FidoNet network dedicated to the discussion of network problems (networks), in particular, Internet problems in Belarus (bel-internet)
42 MAIN FEATURES OF THE NEWS CLIENT Creation account for a news server Connect to a specific news server Browse available news groups Receive and read articles Subscribe to and unsubscribe from a group Send an article to a group or author Work offline
44 IRC. TECHNOLOGY CLIENT/SERVER IRC servers - programs that ensure the functionality of the IRC system and store information about channels and connected users IRC system - several IRC servers connected to each other IRC client - a program for connecting to an IRC server and conducting a conversation. Client example: mIRC
46 MS NETMEETING FEATURES Connect to a directory server (ILS server), view the list of users registered on the server Call a specific user through the directory server or local network by IP address or join an ongoing call. Participants have equal rights
47 MS NETMEETING FEATURES Organization of a meeting at a scheduled time (the meeting is controlled by a leader with special powers) Chat with meeting participants by entering text from the keyboard Collaboration with graphics on a common board
49 INTERNET PAGING DEFINITION Internet paging is a system that allows you to register in your server system and receive a unique paging number. Using this system you can find and call a person who has a paging number and is currently connected to the Internet
50 INTERNET PAGER ICQ. MAIN FEATURES The most popular Internet pager ICQ. A play on words I Seek You (I'm looking for you) or Registering in the ICQ server system and receiving a UIN (Universal Internet Number, universal Internet number) Every time you connect to the Internet ICQ program determines the current IP address of your computer and sends it to the central server
51 INTERNET PAGER ICQ. MAIN FEATURES Automatic search for specified people on the ICQ network and creation of your own list of subscribers for constant communication Calling the interlocutor by his UIN and sending a message with an offer to establish contact Conversation (Chat) in real time Exchange of messages, letters and files
52 INTERNET TELEPHONY. DEFINITION Internet telephony is a system that allows you to conduct a conversation in real time, one of the links of which is the Internet IP telephony service telephone communication, in which for transmission sound signal networks using the IP protocol are used
53 EXAMPLES OF PROGRAMS IMPLEMENTING INTERNET TELEPHONY MS NetMeeting A special program for entering, sending and playing audio information Speak Freely ( Internet program Phone (
54 INTERNET TELEPHONY. MAIN FEATURES Transmitting audio for one subscriber Coding (encryption) of a signal to ensure the confidentiality of conversations Transmitting audio for a group of subscribers and conducting an audio conference Working in answering machine mode Exiting to telephone network through special telephone gateways
55 LITERATURE Modern self-instruction manual for working on the Internet. Fast start. - M.: TRIUMPH Publishing House, p. A modern tutorial for working on the Internet. The most popular programs. - Ed. Komyagina V.B. - "TRIUMPH Publishing House" - Moscow, p. Bradley F. Shimmy. Effective use Email. "Enter" series. Rostov-on-Don: “Phoenix”, – 304 p.
56 REFERENCES Cox J., Kok T., Heidrick E. - Microsoft Internet Explorer 4: short course. - St. Petersburg: Peter, p. Bogumirsky B. - Windows 98 Encyclopedia (second edition). - St. Petersburg: Peter Kom, p. Using the Internet: Per. from English / J. Honeycutt et al. - 4th ed. - K.;M.; SPb: Publishing house. Williams House, p.
The student must
know:
Means of telecommunication exchange.
Means of telecommunication exchange.
Internet services: World Wide Web, email, teleconferences.
Web – industry servers.
Literature:
Additional: (4), p. 170-193; (5), p. 305-337; (3), p. 262-286
Internet resources.
The task of exchanging information has faced man at all times of his existence. With the advent of computers, people received powerful tool accumulation and processing of information and these tasks have become even more pressing. With the advent of computers, the problem of transferring information between them arose. Currently, when using personal computers, time is distributed as follows: 60% - information processing; 35% - communication between people; 5% - access to the global information network. Computer communications are necessary for the exchange of information between computers.
To transfer information from one computer to another using a computer computer network can be used:
1) email;
2) Internet information network;
3) search engines;
4) communication in real time.
The World Wide Web, the newest and fastest growing Internet service today, has almost unlimited potential in terms of collecting, distributing and studying information. Information on Web servers is stored as a set of documents. Each document contains hypertext links that allow the user to access information in other documents on the same topic. This technology allows, along with text, to include Web documents graphics, sound and video images. Hypertext links are highlighted sections of a document that allow you to navigate to another document containing related information.
Teleconferences. Real-time communication is usually understood as a process of information exchange in which those communicating have the opportunity to receive response messages with minimal delay.
Each teleconference has its own address where you can join it, and is a stream of messages visible to any of the participants. The participants themselves form this flow, since everyone can either write a message to the conference or respond to an existing one. Electronic teleconferences are thematic exchanges by email between subscribers. Programs that provide such capabilities are designed for different purposes: notification of certain events on the network, establishing audio and video contact or conversation using the keyboard.
To work in teleconferences, mail programs (Qutlook Express) are used.
Electronic mail (E-mail) is the most common Internet service. E-mail has gained wide popularity because it has several advantages over regular mail:
1) the speed of sending messages (if a letter by regular mail can take days and weeks to reach the addressee, then a letter sent by e-mail reduces the transmission time.
2) an email may contain not only text message, but also attached files (graphics, sound).
Email allows you to:
1) send a message to several subscribers at once;
2) forward letters to other addresses;
3) turn on the answering machine for everything incoming letters a response will be sent automatically;
4) create rules for execution certain actions with the same type of messages.
To work with e-mail, you need special mail programs (mail server and mail client). Mail servers run on Internet host computers, and every user must have mail clients. The simplest email client - Microsoft program Qutlook Express.
In order for an email to reach its recipient, it must, in addition to the message itself, contain the email address of the recipient of the letter. The email address is written in a specific form and consists of two parts: username @ server name.
Questions for self-control
1. What types of computer communications do you know?
2. How are teleconferences different from email?
3. Advantages and disadvantages of email. State them.
4. How does email work?
5. What means of communication does the Internet offer?
Email- a means of exchanging information prepared in electronic form between people with access to a computer network.
The main applications of e-mail are personal correspondence and work with certain Internet information resources, such as mailing lists, off-line news groups and e-mail file transfer systems.
Mail server- a program that forwards messages from mailboxes to other servers or to the user’s computer at the request of his mail client.
Mailboxes are created on the mail server for users with a specific username and password for access. The client part of the service is represented by an email client.
Mail client (mailer)- a program that helps to compose and send electronic messages, receive and display letters on the user’s computer.
Work with mail can be carried out off-line. This means that in order to receive and send mail at the appointed time, you establish a connection with the provider. Then you give a command to your mail client, by which it connects to your mail server, sends prepared letters and collects messages that have arrived in your mailbox over the past period on your local computer. You can write letters and read messages received from the server offline, that is, without an Internet connection.
Thus, email allows you to freely exchange electronic messages even when the sender and recipient are not online at the same time.
Email(e-mail – electronic mail) is one of the earliest information services Internet. It allows you to transmit letters containing text or files in binary code (graphics, sound, programs) over computer networks.
The postal service is based on two application protocols: SMTP And POP3 . By SMTP correspondence is sent from the computer to the server, and POP3 - receiving incoming messages.
Providing e-mail special mail servers handle this. Mail servers receive messages from clients and forward them along the chain to the recipients' mail servers, where these messages are accumulated. When a connection is established between the recipient and his mail server, incoming messages are automatically transferred to the recipient's computer.
There is a wide variety of client email programs. For example, Microsoft Outlook Express , included in the operating system Windows;The Bat!;Eudora Pro etc. These programs perform the following functions: text preparation; reading, saving and deleting correspondence; entering an address; commenting and forwarding correspondence; import (reception and conversion to the required format) of other files.
IN Internet There are also many free email services. For example:Hotmail( http :// www . hotmail . com );AltaVista( http :// altavista . digital . com );Usa.net( http :// www . USA . net );Mail.ru( http :// www . mail . ru ), and etc.
The difference between these services and mail received through a provider using a mail program is that working with a free mailbox occurs using a browser. The universal qualities of such mail are its accessibility from anywhere, from any computer with access to Internet which has a browser and a fairly high level of anonymity.
The email address consists of [email protected]
EXAMINATION TICKET No. 1
1. Physical and logical structure disk. Disk structure. Physical (low-level) disk formatting. Logical (high-level) disk formatting. Disk logical devices. File system FAT.
A hard drive consists of one or more metal plates, each of which is coated on one or both sides magnetic material. Thin concentric rings are marked on each side of each plate - tracks.
Track numbering starts from 0 from the outer edge to the center. As the disk spins, an element called the head reads or writes binary data.
The set of tracks of the 1st radius on all surfaces of all plates is called cylinder. Each track is divided into sectors or blocks, so that all tracks have an equal number of sectors. The sector has a fixed size – 512 bytes.
Tracks and sectors are created as a result of the procedure physical or low level formatting . The low-level disk format does not depend on the type of OS that will be installed. Disk layout under specific type file system performs the procedure high-level (logical) formatting. The OS uses its own unit when working with a disk disk space, called cluster.
Cluster it is a group of related sectors. The cluster size is a multiple of the power of 2.
At high level formatting a cluster size is created and the following is written to the disk: information necessary for the operation of the file system, information about damaged areas of the disk, OS loader - a program responsible for initializing the OS after turning on the power or restarting the computer. Before high-level formatting, the disk may be partitioned into sections.
Chapter– continuous part physical disk, which the OS provides to the user as a logical device.
Logical device functions as if it were a separate physical disk. The user accesses a logical device by symbolic names. Only one file system can be created on each logical device. On different logical devices file systems can be located on the same physical disk different types. On the FAT file system logical space is divided into two areas: system and data area. The system area is created during high-level formatting and consists of: 1. boot record, 2. reserved sectors, 3. allocation table FAT files. In the primary sector logical drive The file system loader is located. The data area of a logical disk is divided into clusters and contains directories. A file or directory in length occupies an integer number of clusters; if the file occupies more than one cluster, then these clusters are organized into a chain of clusters. The location of files in a cluster is determined using a FAT table of file sizes, in which each cluster has its own element. Elements 0 and 1 are service elements, the number of the first cluster starts with 2ki. Cluster numbers correspond to table elements. FAT elements determine the position of clusters in the chain and its state. When creating and writing a file, the OS performs the following functions: 1. Creates a description of the file in a free directory element. 2. Searches for a free FAT element and places a link to it in the directory element (this is how the first cluster of the first chain is filled. 3. The number of the next occupied cluster or the sign of the last cluster in the chain is placed in this FAT element
2. Binary decryptors. Purpose. Circuit implementation. Functional description. Circuit implementation. Increasing dimension. Arbitrary playback scheme logical functions using a decoder.
Decoder – a code converter, and specifically a binary decoder – converts a binary code into code 1 from N.
A binary decoder having n inputs must have 2 n outputs corresponding to the number different combinations in n-bit binary code.
Depending on the input binary code At the output of the decoder, one and only one of the output circuits is excited. If some of the input sets are not used, then the decoder is called incomplete, and its number of outputs is less than 2n.
Circuit implementation:
IN symbol decryptors are marked with the letters DC. The decoder inputs are usually designated by binary weights. In addition to information inputs, the decoder usually has one or more operation enable inputs designated as EN. If there is permission for this input, the decoder operates in the manner described above; if it is absent, all decoder outputs are passive. If there are several enable inputs, then the operation enable signal is formed as a conjunction of the signals of the individual inputs. Often the decoder has inverse outputs. In this case, there is only one exit null value, and all the rest are single. When the operation of the decoder is disabled, a logical one will be present at all its outputs.
The functioning of the decoder is described by a system of conjunctions:
Circuit implementation.
As is known, IC packages with a large number Leads are difficult to make and expensive. From this point of view, decoders are extremely unsuccessful circuits, since they have a simple internal structure and a small number of circuit elements, and many external pins. For placement in a regular inexpensive case, only a decoder with 4 information inputs is suitable. There are no larger “sized” decoders in the IC series.
Dimension increase:
The low capacity of standard decryptors raises the question of increasing their capacity. From low-bit decoders it is possible to construct a circuit equivalent to a decoder of higher bit capacity. To do this, the input word is divided into fields. The bit depth of the low-order bits field corresponds to the number of inputs of the available decoders. The remaining field of the most significant bits is used to receive signals to enable the operation of one of the decoders that decodes the field of the least significant bits.
As an example in Fig. A diagram of decoding a five-bit binary code using decoders “3-8” and “2-4” is shown. To obtain the required 32 outputs, a column of four “3-8” decoders is compiled. The "2-4" decoder accepts the two most significant bits of the input code. The energized single output of this decoder unlocks one of the column decoders at its enable input. The selected column decoder decrypts the least significant three bits of the input word.
Each input word corresponds to the excitation of only one output. For example, when decoding the word x 4 x 3 x 2 x 1 x 0 = 11001 2 = 25 10, at the input of the first tier decoder there is code 11, which excites its output number three (shown with a cross), which allows the operation of DC4. The code 001 is valid at the DC4 input, so one will appear at its first output, i.e. at the 25th output of the circuit as a whole, which is what is required.
Scheme for reproducing arbitrary logical functions using a decoder .
Decoders, together with OR circuits, can be used to reproduce arbitrary logical functions. By collecting the necessary terms using the OR circuit, you can obtain any function given number arguments.
In Fig. As an example, a diagram for generating two functions is shown 
And 
.
To check the correctness of the scheme, it is convenient to translate the functions F 1 and F 2 into SDNF.
3. Means of information exchange on the Internet. E-mail (The concept of “E-mail.” Methods of working with e-mail.) Search engines (The concept of a search system, the principles of operation of search engines. Organization of information search on the Internet).
Means of information exchange on the Internet:
Newsgroups (newsgroups)
IRC (Internet Relay Chat, conversation over the Internet) or Chat
Means of communication in real time (transmission of text, sound, image) and joint work with applications
Internet paging
Internet telephony
Audio and video conferences
Email- a means of exchanging electronic letters between people with access to a computer network.
Ways to work with email
Through an email client installed on the user's computer ( mail program, such as Outlook, Bat, etc.).
working directly with mail server(directly with a corporate mail server, aayandex server, mail.ru, etc.) via a web interface
Search system- a software and hardware complex with a web interface that provides the ability to search for information on the Internet. A search engine usually means a website on which the system interface is located. Software part search engine is search engine(search engine) - a set of programs that provides the functionality of a search engine and is usually a trade secret of the search engine developer company.
Related information.
