INTRODUCTION

1. Types of global networks

1.1 Dedicated channels

2. DTE-DCE interfaces

CONCLUSION

INTRODUCTION

Wide Area Networks (WAN), which are also called territorial computer networks, serve to provide their services a large number end subscribers scattered over a large area - within an area, region, country, continent or entire globe. Due to the large length of communication channels, building a global network requires very large costs, which include the cost of cables and work on their installation, the cost of switching equipment and intermediate amplification equipment that provides the necessary channel bandwidth, as well as operating costs for constant maintenance of in working condition network equipment scattered over a large area.

Typical subscribers of a global computer network are local networks of enterprises located in different cities and countries that need to exchange data with each other. Individual computers also use the services of global networks. Large mainframe computers typically provide access to corporate data, while personal computers are used to access corporate data and public Internet data.

WANs are usually created by large telecommunications companies to provide paid services to subscribers. Such networks are called public or public. There are also such concepts as network operator and network service provider. Network operator is the company that maintains the normal operation of the network. Service Provider, often also called provider (service provider), is the company that provides paid services network subscribers. The owner, operator, and service provider may be one company, or they may represent different companies.

In addition to global computing networks, there are other types territorial networks transfer of information. First of all, these are telephone and telegraph networks that have been operating for many decades, as well as the telex network.

Due to the high cost of global networks, there has been a long-term trend towards creating a single global network that can transmit all types of data: computer data, telephone conversations, faxes, telegrams, television images, teletex (data transfer between two terminals), videotex (receiving data stored on the network to your terminal), etc., etc. To date, no significant progress has been made in this area, although technologies for creating such networks began to be developed quite a long time ago - the first technology for integrating ISDN telecommunication services began to develop in the early 70s. So far, each type of network exists separately and their closest integration has been achieved in the area of ​​using common primary networks - PDH and SDH networks, with the help of which today permanent channels in networks with subscriber switching. However, each of the technologies, like computer networks, and telephone, today is trying to transmit “alien” traffic with maximum efficiency, and attempts to create integrated networks at a new stage of technology development continue under the successive name Broadband ISDN (B-ISDN), that is, a broadband (high-speed) network with integration of services . B-ISDN networks will be based on ATM technology as a universal transport and support various services top level for distributing various information to end users of the network - computer data, audio and video information, as well as organizing interactive interaction between users.

1. Types of global networks

Global computer network operates in the mode most suitable for computer traffic - packet switching mode. The optimality of this mode for connecting local networks is proven not only by data on the total traffic transmitted by the network per unit of time, but also by the cost of services of such a territorial network. Typically, given the same access speed, a packet-switched network turns out to be 2-3 times cheaper than a circuit-switched network, that is, a public telephone network.

However, often such a wide area network various reasons turns out to be unavailable in a particular geographical location. At the same time, services provided by telephone networks or primary networks that support dedicated circuit services are much more widespread and accessible. Therefore, when building a corporate network, you can supplement the missing components with services and equipment rented from the owners of the primary or telephone network.

Depending on what components have to be rented, it is customary to distinguish between corporate networks built using:

· dedicated channels;

· channel switching;

· packet switching.

The latter case corresponds to the most favorable case, when a packet-switched network is available in all geographical locations that need to be combined into a common one. corporate network. The first two cases require additional work in order to build a packet switching network based on the leased funds.

1.1 Dedicated channels

Dedicated (or leased) channels can be obtained from telecommunications companies that own the channels long distance communication(such as ROSTELECOM), or from telephone companies, which usually rent channels within a city or region.

You can use leased lines in two ways. The first is to build with their help a territorial network of a certain technology, for example frame relay, in which leased leased lines serve to connect intermediate, geographically distributed packet switches.

The second option is to connect only connected local networks or other types of end subscribers, such as mainframes, with dedicated lines, without installing transit packet switches operating using global network technology (Fig. 1). The second option is the simplest from a technical point of view, as it is based on the use of routers or remote bridges in interconnected local networks and the absence of protocols global technologies, such as X.25 or frame relay. By global channels the same packets of network or link layer, as in local networks.

Rice. 1 - Using dedicated channels

Today there is big choice dedicated channels - from analog channels voice frequency with a bandwidth of 3.1 kHz up to digital channels SDH technology with throughput 155 and 622 Mbit/s.

1.2 Circuit-switched wide area networks

Today, to build global connections in a corporate network, two types of circuit-switched networks are available - traditional analogue telephone networks and digital networks with the integration of ISDN services. The advantage of circuit-switched networks is their prevalence, which is typical especially for analog telephone networks. IN Lately ISDN networks in many countries have also become quite accessible to corporate users, but in Russia this statement so far applies only to large cities.

A well-known disadvantage of analog telephone networks is the low quality of the composite channel, which is explained by the use of telephone switches of outdated models operating on the principle of frequency division multiplexing (FDM technology). Such switches are heavily affected by external noise (such as lightning or running electric motors), which is difficult to distinguish from the desired signal. True, analogue telephone networks are increasingly using digital PBXs, which transmit voice to each other in digital form. In such networks, only the subscriber end remains analog. The more digital PBX in the telephone network, the higher the quality of the channel, but our country is still far from completely replacing PBXs operating on the principle of FDM switching. In addition to the quality of channels, analogue telephone networks also have the following disadvantages: big time establishing a connection, especially with the pulse dialing method typical for our country.

Telephone networks built entirely on digital switches and ISDN networks are free from many of the disadvantages of traditional analog telephone networks. They provide users with high-quality communication lines, and the connection setup time in ISDN networks is significantly reduced.

1.3 WANs with packet switching

In the 80s, to reliably connect local networks and large computers into a corporate network, almost one technology of wide area networks with packet switching was used - X.25. Today, the choice has become much wider; in addition to X.25 networks, it includes technologies such as frame relay, SMDS and ATM. In addition to these technologies, developed specifically for global computer networks, you can use the services of TCP/IP territorial networks, which are available today as an inexpensive and very common Internet networks, the quality of transport services of which is still practically not regulated and leaves much to be desired, and in the form of commercial global TCP/IP networks, isolated from the Internet and leased by telecommunications companies.

SMDS (Switched Multi-megabit Data Service) technology was developed in the USA to connect local networks across a metropolitan area, as well as provide high-speed access to global networks. This technology supports access speeds of up to 45 Mbit/s and segments MAC-level frames into cells of a fixed size of 53 bytes, which, like ATM technology cells, have a data field of 48 bytes. SMDS technology is based on the IEEE 802.6 standard, which describes a slightly broader set of functions than SMDS. SMDS standards are adopted by Bellcore, but do not have international status. SMDS networks have been implemented in many major cities in the United States, but this technology has not become widespread in other countries. Today, SMDS networks are being replaced by ATM networks, which have wider functionality Therefore, this book does not discuss SMDS technology in detail.

2. DTE-DCE interfaces

To connect DCE devices to equipment that produces data for the global network, that is, to DTE devices, there are several standard interfaces that represent standards physical level. These standards include the V series of CCITT standards, as well as the EIA RS series (Recommended Standards). The two lines of standards largely duplicate the same specifications, but with some variations. These interfaces allow you to transfer data at speeds from 300 bps to several megabits per second over short distances (15-20 m), sufficient for convenient placement, for example, of a router and modem.

Interface RS-232C/V.24 is the most popular low-speed interface. It was originally designed to transmit data between a computer and a modem at a speed of no higher than 9600 bps over a distance of up to 15 meters. Later, practical implementations of this interface began to operate at higher speeds - up to 115200 bps. The interface supports both asynchronous and synchronous operating modes. This interface gained particular popularity after its implementation in personal computers(it is supported by COM ports), where it works, as a rule, only in asynchronous mode and allows you to connect to the computer not only a communication device (such as a modem), but also many other peripheral devices - a mouse, a plotter, etc.

The interface uses a 25-pin connector or, in a simplified version, a 9-pin connector (Fig. 2).

Rice. 2 - RS-232C/V.24 interface signals

CCITT numbering is used to designate signal circuits and is called the “100 series”. There are also two-letter EIA designations that are not shown in the figure.

The interface implements a bipolar potential code (+V, -V on the lines between DTE and DCE. Typically used quite high level signal: 12 or 15 V to more reliably recognize the signal against a background of noise.

With asynchronous data transfer, the synchronizing information is contained in the data codes themselves, so there are no synchronization signals TxClk and RxClk. In synchronous data transmission, the modem (DCE) transmits synchronization signals to the computer (DTE), without which the computer cannot correctly interpret the potential code coming from the modem along the RxD line. In the case when a multi-state code is used (for example, QAM), then one clock signal corresponds to several bits of information.

Null modem interface typical for direct communication between computers a short distance using the RS-232C/V.24 interface. In this case, it is necessary to use a special null modem cable, since each computer will expect to receive data via the RxD line, which will be correct if a modem is used, but not if the computers are connected directly. In addition, a null modem cable should simulate the process of connecting and breaking through modems, which uses multiple lines (RI, CB, etc.). Therefore for normal operation two directly connected computers, a null modem cable must make the following connections:

· RI-1+DSR-1- DTR-2;

· DTR-1-RI-2+DSR-2;

· CD-1-CTS-2+RTS-2;

· CTS-1+RTS-1-CD-2;

The "+" sign indicates the connection of the corresponding contacts on one side of the cable.

Sometimes, when manufacturing a null modem cable, they are limited to only cross-connecting the RxD receiver and TxD transmitter lines, which for some software may be sufficient, but in general it can lead to incorrect operation programs designed for real modems.

Interface RS-449/V.10/V.11 supports more than high speed data exchange and greater distance between DCE and DTE. This interface has two individual specifications electrical signals. The RS-423/V.10 specification (the X.26 specification has similar parameters) supports data rates of up to 100,000 bps at a distance of up to 10 mi; speeds of up to 10,000 bps at a distance of up to 100 m. Specification RS-422/V.11 (X 27 supports speeds up to 10 Mbps at a distance of up to 10 mi, speeds up to 1 Mbps at a distance up to 100 m. Like RS-232C, the RS4 - 49 interface supports asynchronous and synchronous exchange modes between DTE and DCE. For connection A 37-pin connector is used.

V.35 interface was designed to connect synchronous modems. It provides only synchronous exchange between DTE and DCE at speeds up to 168 Kbps. To synchronize the exchange, special timing lines are used. The maximum distance between DTE and DCE does not exceed 15 m, as in the RS-232C interface.

X.21 interface designed for synchronous data exchange between DTE and DCE in X.25 packet switched networks. This is a fairly complex interface that supports connection establishment procedures in packet and circuit switched networks. The interface was designed for digital DCE. To support synchronous modems, a version of the X.21 bis interface was developed, which has several options for the specification of electrical signals: RS-232C, V.10, V.I 1 and V.35.

20L current loop interface<Л» used to increase the distance between DTE and DCE. The signal is not a potential, but a current of 20 mA flowing in a closed circuit of the transmitter and receiver. Duplex exchange is implemented on two current loops. The interface works only in asynchronous mode. The distance between DTE and DCE can be several kilometers, and the transmission speed can be up to 20 Kbps.

HSSI (High-Speed ​​Serial Interface) interface designed for connection to DCE devices operating on high-speed channels, such as TZ channels (45 Mbit/s), SONET OS-1 (52 Mbit/s). The interface operates in synchronous mode and supports data transfer in the speed range from 300 Kbps to 52 Mbps.

CONCLUSION

So, global computer networks (WAN) are used to unite subscribers of different types: individual computers of different classes - from mainframes to personal computers, local computer networks, remote terminals.

Due to the high cost of global network infrastructure, there is an urgent need to transmit over one network all types of traffic that arise in an enterprise, not just computer traffic: voice traffic of an internal telephone network running on office PBXs (PBX), traffic of fax machines, video cameras, cash registers , ATMs and other production equipment.

To support multimedia types of traffic, special technologies are being created: ISDN, B-ISDN. In addition, wide area network technologies, which were developed to transmit exclusively computer traffic, have recently been adapted to transmit voice and video. To do this, packets carrying voice measurements or image data are prioritized, and in those technologies that allow this, a connection with pre-reserved bandwidth is created to carry them. There are special access devices - “voice - data” or “video - data” multiplexers, which pack multimedia information into packets and send it over the network, and at the receiving end they unpack and convert it into its original form - voice or video.

Global networks provide mainly transport services, transferring data in transit between local networks or computers. There is a growing trend to support application level services for global network subscribers: the distribution of publicly accessible audio, video and text information, as well as the organization of interactive interaction between network subscribers in real time. These services appeared on the Internet and are successfully transferred to corporate networks, which is called intranet technology.

All devices used to connect subscribers to the global network are divided into two classes: DTE, which actually generate data, and DCE, which transmit data in accordance with the requirements of the global channel interface and terminate the channel.

WAN technologies define two types of interface: user-to-network (UNI) and network-to-network (NNI). The UNI interface is always deeply detailed to ensure connection to the network of access equipment from different manufacturers. The NNI interface may not be as detailed, since large networks may be interoperable on a case-by-case basis.

Global computer networks operate on the basis of packet, frame and cell switching technology. Most often, a global computer network is owned by a telecommunications company that rents out its network services. If there is no such network in the desired region, enterprises independently create global networks by renting dedicated or dial-up channels from telecommunications or telephone companies.

Using leased channels, you can build a network with intermediate switching based on any global network technology (X.25, frame relay, ATM) or directly connect routers or bridges of local networks with leased channels. The choice of how to use leased channels depends on the number and topology of connections between local networks.

Global networks are divided into backbone networks and access networks.

LIST OF REFERENCES USED

1. www.yandex.ru

2. http://www.klyaksa.net/htm/kopilka/uchp/p9.htm

3. http://ruos.ru/os10/index5.htm

Global networks. ^ Organization of global networks . Global computer networks connect computers located on long distances(at the scale of the region, country, world). If students can see a local network with their own eyes, then familiarity with global networks will be more descriptive. Here, as in many other topics, the method of analogies comes to the rescue. The structure of a global network can be compared to the structure of a telephone communication system - a telephone network. Subscriber phones are connected to switch nodes. In turn, all city switches are interconnected in such a way that a connection can be established between any two subscriber phones. This entire system forms the city's telephone network. City (regional) networks are interconnected via long-distance lines. Access to telephone networks of other countries occurs via international communication lines. Thus, the whole world is “entangled” in telephone networks. Two subscribers in any part of the world connected to this network can contact each other.

Having told about this, ask students to imagine that subscribers have personal computers installed instead of telephone sets; instead of switches there are powerful computer nodes and a wide variety of information circulates through such a network: from text to video and sound. This is the modern world system of global computer networks.

The first global computer network began operating in 1969 in the USA, it was called ARPANET and united only 4 remote computers. An example of a modern network for scientific and educational purposes is BITNET. It covers 35 countries in Europe, Asia and America, uniting more than 800 universities, colleges, and research centers. The largest Russian network is RELCOM, created in 1990. RELCOM is part of the European association of networks EUNET, which, in turn, is a member of the giant global INTERNET community. This hierarchy is typical for the organization of global networks.

In Fig. Figure 12.3 shows the typical architecture of a global network. The network consists of nodes host computers (U1, U2,...), PCs of network subscribers (All, All, ...), communication lines. Typically, a network node contains not one, but many computers. The functions of servers for different network services can be performed by different computers.

Host computers are always on, always ready to receive and transmit information. In this case they say that they operate in the mode on-line. Subscribers' computers connect to the network (in on-line) only for a certain time - a communication session. Having forwarded and received the necessary information, the subscriber can disconnect from the network and then work with the received information autonomously - in the off-line. The route for transmitting information to the user is usually unknown. He can only be sure that the information passes through the connection node and reaches its destination. Network system tools handle routing of transmitted data. In different sessions, communication with the same correspondent can take different routes.

A gateway is a computer that organizes the connection of a given network with other global networks.

^ Information services of global networks. Email. g history of global networks, electronic mail (e-mail) appeared as the very first information service. This service remains the main and most important in computer telecommunications. We can say that there is a process of replacing traditional paper mail with e-mail. The advantages of the latter are obvious: first of all, it is the high speed of correspondence delivery (minutes, rarely hours), and comparative cheapness. Already, huge volumes of business and personal correspondence are sent via e-mail. E-mail, combined with fax, provides the vast majority of needs for the transmission of letters and documents.

In order for a subscriber to use e-mail services, he must:

• 
  have a hardware connection of your personal computer to the mail server of a computer network node;
• 
  have your own mailbox and password on this server to access it;
• 
  have a personal email address;
• 
  Have an email client program (mailer) on your computer.

Hardware connection most often occurs via telephone lines, so the user needs access to the telephone network, i.e., his own telephone number. The organization that owns the global network node and provides network services is called provider. Recently, there are more and more of them, and the user has the opportunity to choose the provider whose conditions suit him more. Provider assigns for the user password, email address, creates for it on the mail server Mailbox- a folder for posting correspondence. Typically, the provider helps the user install and configure mail client program.

The preparation of an email is carried out by the user in the mode off-line- disconnection from the network. Using an email client program, he generates the text of the letter, indicates the recipient's address, and attaches various attachments to the letter. Then the User goes into mode on-line, those. connects to the mail server and issues the “deliver mail” command. Prepared correspondence is transferred to the server, and correspondence received at the User’s address is transferred from the server to his PC. In this case, midday letters are deleted from the mailbox, and sent ones are added to it. The mail server periodically scans subscribers' mailboxes and, having found outgoing correspondence there, organizes its poisoning.

The example of email illustrates the point well. client-server technologies, accepted in modern networks. This technology is based on the division of software functions that serve each information service between the client computer and the server. The corresponding software is called a client program and a server program (often said in short: client And server). Popular email client programs are: MAIL for MS-DOS and Outlook Express for Windows.

In the initial period of e-mail development, transmitted correspondence could only be in text format. Data of a different format (binary files) was recoded into text format using a special transcoder program UUDECOD. The Internet now uses the MIME standard, which allows a wide variety of information to be transmitted in the body of an email message without such recoding. According to this standard, the sending machine places in the header of the email message descriptions of the types of information units that make up the letter. Based on these descriptions, the receiving machine correctly interprets the received information. Now in an email, in addition to text, you can place graphic images (type image), audio information (audio), videos (video), and any applications (application).

Along with e-mail, there are other types of information services for users.

Telnet. This service allows the user to work in terminal mode of a remote computer, i.e. use programs installed on it in the same way as programs on their own computer.

FTP. This is the name of the network protocol and programs that work with directories and files on a remote machine. The FTP client has the ability to browse the directories of FTP servers and copy files of interest.

Archie. This is the name of special servers that act as search programs in the FTP server system. They help you quickly find the files you need.

Gopher. A system for searching and retrieving information from the network with developed tools for multi-level menus, reference books, index links, etc.

^ WAIS. Network information retrieval system based on distributed databases and libraries.

Usenet. Teleconference system. Another name is newsgroups. Serves subscribers of certain thematic conferences, sending them materials by e-mail.

^ Network hardware. Host computers (servers). The host computer has its own unique address on the network and acts as a host machine serving subscribers. Different types of machines are used as host computers: from powerful PCs to minicomputers and even mainframes (large computers). The main requirements are a high-speed processor and a large amount of disk memory (tens and hundreds of GB). Host computers on the Internet use the Unix operating system. All server programs that serve applications run under Unix.

From what has already been mentioned above, it follows that the concept of “server” has a software and hardware meaning. For example, a host computer that is currently running an email server program acts as a mail server. If the WWW server program starts running on the same machine, then it becomes a Web server. Often the functions of servers for various services are divided on a network node between different computers.

^ Communication lines. The main types of communication lines between computers on a network are telephone lines, electrical cables, fiber optic cable and radio communications. The main parameters of communication lines are throughput (maximum information transfer speed), noise immunity, and cost. In terms of cost, the most expensive are fiber optic lines, the cheapest are telephone lines. However, as the price decreases, the quality of the line also decreases. In table 12.1 provides comparative characteristics of lines in terms of speed and noise immunity.

Table 12.1

Characteristics of communication lines

Communication type	Speed, Mbit/s	Noise immunity
Twisted pair wires	10 -100	Low
Coaxial cable	To 10	High
Phone line	1 -2	Low
Fiber optic cable	10 -200	Absolute

Most often, dedicated telephone lines or radio communications are used for communication between host computers. If network nodes are located relatively close to each other (within a city), then communication between them can be organized via cable lines - electric or fiber optic. Recently, satellite radio communications have been actively used on the Internet.

Typically, subscribers (clients) connect to their provider's node through a telephone line. Radio communications are increasingly being used for these purposes.

To transmit information over communication channels, it is necessary to convert it from the form in which it exists in the computer into signals transmitted over communication lines. Such transformations are carried out by special devices called network adapters. There are adapters for cable and fiber optic communications. The adapter is inserted into a free socket on the motherboard and connected by a cable to the adapter of another computer. This is usually done on local networks.

In global networks connected via telephone lines, modems. The purpose of a modem is to convert information from binary computer code to a telephone signal and vice versa. In addition, the modem performs a number of other functions. For example, a network client modem must dial up to the node to which it connects.

The main characteristic of the modem is the maximum data transfer speed. Currently it ranges from 1200 bps to 112,000 bps. However, the actual speed depends not only on the modem, but also on the quality of the telephone lines. In Russian urban networks, the acceptable transmission speed is low and amounts to 2400-14400 bps. In the future, when there is a complete transition of telephone lines to digital communications, the need to use modems will disappear.

Internet. To the question of what the Internet is, you can read different answers in the literature. Most often this question is answered this way: the Internet is a supernetwork covering the whole world, which is a collection of many (more than 2000) networks that support a single TCP/IP protocol (Transmission Control Protocol/Internet Protocol).

Protocol - is a standard for representing, transforming and transferring information on a computer network. Figuratively, we can say this: a protocol is a specific network language. While the various global networks operated autonomously, they "speaked different languages." To unite them, it was necessary to come up with a common language (a kind of network Esperanto), which became the TCP/IP protocol. This protocol is supported by both software and hardware of the network. It comes down to standardizing the following procedures:

• 
  splitting the transmitted data into packets (parts);

• 
  addressing packets and transmitting them along specific routes to their destination;

• 
  assembling packages into raw data form.

In this case, the correctness of the reception-transmission of the packet and the correct assembly of all transmitted packets in the right place are monitored.

Others have been implemented based on the TCP/IP protocol Internet application protocols, forming the basis of the service on the network.

The basis of the Internet is a system of so-called IP addresses. Each host computer connected to the Internet receives a unique address within the entire network. An IP address is a sequence of four integers decimal numbers, separated by dots. For example: 195.205.31.47. Since the Internet is a network of networks, the first number determines the network to which the computer belongs, the following numbers specify the coordinates of the computer in this network.

Digital addressing is an “internal matter” of the system. It is inconvenient for users. Therefore, the alphabetic form of writing addresses is used for users - domain addresses. Domains are symbolic names separated by dots. Example of a domain address: www.psu.ru. The address is read from right to left. The first domain on the right is called a suffix. Most often, it identifies the country in which the computer is located (thus, the computer is part of a national network). For example, ru - Russia, uk - Great Britain, fr - France. US host computer addresses usually have a suffix indicating their affiliation with corporate networks: edu - scientific and educational organizations, gov - government organizations, mil - military, etc.

The following domains (there may be more than one) define the host computer in this network (PSU - Internet center of Perm State University). The last domain is the name of the server (Web server). Using a special server program, a connection is established between numeric and domain addresses.

All of the above characteristics of the Internet are most often unknown to the user. From the user's point of view, the Internet is a certain set of information services that he can receive from the network. Services include: e-mail, teleconferencing (mailing lists), file archives, directories and databases, The World Wide Web- WWW, etc. The Internet is unlimited information resources. The impact that the Internet will have on the development of human society is not yet fully realized.

^ Information services Internet. Along with the information services listed by Gillie (e-mail, teleconferences, etc.) provided to users of global networks, there are services, the emergence and development of which is associated inclusively with the development of the global Internet. The most prominent among them is the WWW.

WWW- World Wide Web - World Wide Web. This is a hypertext information system on the Internet. Recently, WWW and its software have become a universal means of information services on the Internet. They provide users with access to almost all of the resources listed above (FTP, e-mail, WAIS, Gopher, etc.). Basic concepts related to the WWW: Web page - the main information unit on the WWW with its own address;

Web server - a computer that stores Web pages and the corresponding software for working with them;

Web browser - a client program that allows you to retrieve and view Web pages;

A website is a section of data on a Web server owned by an organization or person. In this section, its owner places his information in the form of many interconnected Web pages. Typically, a site has a title - a home page, from which you can move through the pages of the site using hyperlinks or back-and-forth signs.

The most popular Web browsers are Internet Explorer and Netscape Navigator. The main task of the browser is to contact the Web server for the desired page and display the page on the screen. The simplest way to obtain the necessary information from the Internet is to indicate the address of the resource you are looking for.

To store and search information on the Internet, universal addressing is used, which is called URL - Uniform Resource Locator. A URL contains information not only about where the resource is located, but also what protocol it should be accessed through. The URL consists of two parts: the first (left) indicates the protocol used, and the second (right) indicates where exactly the resource is located on the network (the name of the corresponding server). These parts are separated by a colon, for example:

Http://servername/path/file

Ftp:// - the ftp protocol is used when accessing ftp servers;

Gopher:// - connection to Gopher servers;

Http:// - use of the hypertext protocol (Hyper Text Transfer Protocol), which underlies the WWW. This type of connection must be specified when accessing any WWW server.

Here is an example of the address of a file containing a distance learning course in German:

Http://www.scholar.urc.ac.ru/Teaher/German/main.html

In addition to direct addressing, searching for information on the Internet can be carried out using hyperlinks.

There are a number of special search programs available to help users on the Internet. They are also called search servers, search engines, search engines. Such a system is constantly in operation. Using special robot programs, it periodically crawls all Web servers on the network and collects summary information about their content. Based on the results of such views, directories and index lists are organized indicating documents where keyword definitions are found. User requests for information search are then served using these lists. The search engine provides the user with a list of document addresses that contain the keywords specified by the user.

Below are the addresses of the most popular Russian search servers:

Http://yandex.ru/ http://www.altavista.telia.com/

http://www.list.ru/

Searching for information using keywords requires certain skills from the user. Online search algorithms, like searching for information in databases, are based on logic. Let's consider this issue using the example of organizing a search using several keywords, adopted in the Alta Vista search system.

1. 
   Several keywords separated by a space correspond to the logical addition operation: OR. For example, by specifying the key: , we will get a list of all documents in which the word “School” or the word “computer science” appears. Obviously, there will be too many such documents and most of them are not needed by the user.
2. 
   Several words enclosed in quotation marks are perceived as a single whole. By specifying “School computer science” in the request, we will receive documents containing such a string.
3. 
   The “+” sign between words is equivalent to the logical Multiplication operation: AND (AND). By specifying the key in the request, we will receive all documents that contain these two words at the same time, but they can be arranged in any order and randomly.

Obviously, the second version of the request is more consistent with the goal. However, keywords in this combination may not appear in the search program lists.

In addition to WWW, among the relatively new Internet services there are the following:

^IRC. Internet Relay Chat- “chat” in real time. Allows you to conduct a written dialogue with remote interlocutors on-line;

Internet telephony. A service that supports online voice communication among network clients.

If it is possible to access the Internet, students’ practical work can be organized in the following areas:

• 
  preparing, sending and receiving email;
• 
  working with a Web browser, viewing Web pages;
• 
  contacting FTP servers, retrieving files;

• 
  searching for information on the WWW using search programs.

Acquaintance with each new type of application software that serves the corresponding information service (email program, Web browser, search program) should be carried out according to a standard methodological scheme: data, environment, operating modes, command system.

Tasks for students to perform practical work on the Internet are contained in the manual.

Global trends are such that everything is subject to integration processes. In the world of finance, mergers and acquisitions are taking place, large industrial groups are creating strategic alliances, even countries and regions are uniting. In this sense, it is not surprising that computer networks and the companies that own these networks also seek to increase their market share and reduce the cost of services provided through consolidation.

Global networks are created by large corporations (telecommunications, less often others for their own needs) to ensure information interaction between computers located in different countries, on different continents.

The company that ensures the normal functioning of the global network is calledoperator .

A company that provides paid services to network subscribers is calledprovider .

Global networks are the result of the consolidation of telecommunications companies and the unification of their networks. This is due to the need to expand the range of services provided, the cost of which depends on whether the company has its own communication channels or rents them from competitors.

The functioning of global networks is based onmulti-level message transmission principle . The message is generated at the highest level of the modelOSI and sequentially goes through all levels to the lowest. At each level, an additional header is added to the message (which is split into parts shown in Fig. 5.4 as it descends), which is required to receive the message at a similar level on the recipient’s side. On the receiving side, the message sequentially passes from the lower level to the upper one, removing the corresponding headers. Therefore, the upper level receives the original message in its “original” form.

The homogeneity of the information and communication environment of the global network is ensured by the compatibility of software and hardware, which are released in accordance withinternational standards .

The most widespread global networkInternet , the technologies of which have already penetrated into corporate networks, now calledIntranet networks.

Global networks arenodal . This means that the global network includescommunication subnet , to which local networks, individual computers and terminals (means for entering and displaying information) are connected. The subnet consists ofcommunication channels , communication nodes (designed for routing and packet switching) andcommunication node software (KU).

The typical structure of a global network is shown in Fig. 5.5.



Rice. 5.5.Global network structure

LAN - local network; M - Router; MP - multiplexer; KU - communication node; TSS - territorial communication network; RS - workstation; PBX - automatic telephone exchange.

To the global network usingrouters And KU local networks are connected.Multiplexer necessary for combination within oneterritorial communication network (TCS) computer and voice traffic fromautomatic telephone exchange (PBX).

Individuals can also connect to the global networkworkstations (PC) and home networks, as well as wireless networks.

Depending on the hardware used, global networks are distinguishedwith dedicated communication channels , circuit switched , packet switched . The most suitable mode of operation of the global network ispacket switching mode .

NOTE

The cost of services in a global packet-switched network is 2-3 times lower than the cost of services in a circuit-switched network, although the total traffic per unit time will be the same.

Networks with dedicated channels are used to organize backbone connections between large local networks. Contact byanalog dedicated lines are made using modems. Contact bydigital allocated channels is carried out using equipment using the principle of time division of channels (TDM). The interconnection of local networks using dedicated channels is carried out by routers and remote bridges. The main disadvantage is the high cost of services.

Circuit switched networks are based on technologyISDN and the use of analogue channels. NetISDN digital and free from the disadvantages of analog communication (long connection establishment time, low channel quality), but charging is still carried out not for the volume of transmitted traffic, but for the connection time.

Packet switched networks are the main means of any information, from television to facsimile. These networks includeX.25 , Frame Relay , ATM , TCP/IP . In global packet-switched networks (excluding TCP/IP), packet routing is used based on the creation of two types of channels -switched virtual circuits (SVC) and permanent virtual channels (PVC). There are two modes for promoting packages -standard and switching modebased on virtual channel number .

Standard The mode is used only to route only the first packet transmitted, which is necessary to establish a connection. It turns out that the first packet lays a virtual channel, setting up intermediate switches, and the remaining packets pass through the virtual channel in switching mode.

As an example, Appendices 5 and 6 consider the global Internet.

Organization of complex connections in global networks. In global networks, communication between LANs is carried out through bridges.

Bridges are software and hardware systems that connect LANs to each other, as well as LANs and remote PC workstations, allowing them to interact with each other to expand the capabilities of collecting and exchanging information.

A bridge is usually defined as a connection between two networks that use the same communication protocol, the same media type, and the same addressing structure.

There are two basic types of NETWARE bridges: n internal n external.

If the bridge is located on a file server, it is an internal bridge. If the bridge is located in the workstation - an external bridge. External bridges and their software are installed on a workstation that does not function as a file server. Therefore, an external bridge can transfer data more efficiently than an internal bridge.

There are dedicated and combined bridges.

A dedicated PC is a PC used as a bridge and cannot function as a workstation. Combined - can function as both a bridge and a workstation at the same time. The advantage is limited to the cost of purchasing an additional computer. The disadvantage is the lack of potential capabilities of the workstation located in it. When an application program on a PC hangs and causes the PC operating as a bridge to stop, the bridge program also stops operations.

This failure interrupts the sharing of data between networks, and also interrupts PC sessions that are connected through a bridge to the file server. Since the dedicated bridge is not used as a PC, no PP will cause such a failure and will not interrupt operation. When choosing a bridge, you need to weigh the cost of the equipment against the risk of bridge failure. A local bridge transmits data between networks that are located within the cable's distance limitations. Local bridges are used in the following cases: 1 to divide large networks into two or more subnetworks in order to increase performance and reduce the cost of communication lines. For example, in one organization, different departments share the same network. Because large networks are slower than small ones, that is, it is possible to allocate compactly located departments into small subnets.

Using a local Netware bridge, departments can continue to share data as if they were on the same network, while gaining the speed and flexibility of a small network. 2 using a local bridge you can expand the physical capabilities of the network. If a Netware network has the maximum number of nodes supported by its hardware addressing scheme and there is a need to add several more nodes, then a Netware bridge is used to expand such a network.

In this case, it is not necessary to include an additional file server in the network. 3 connecting networks into an Internet. In order for users on each network to access information from other networks, it is necessary to connect these networks, forming an Internet. Remote bridges are used when distance does not allow connecting networks via cable.

For example, connecting a network in Kostroma to a network in Novgorod will require the use of a remote bridge, since the cable length limit for the local bridge will be exceeded. A remote bridge uses an intermediate transmission medium, telephone lines, to connect to a remote network or remote PCs. When connecting a network to a remote network, a bridge must be installed at each end of the connection, and when connecting a network to a remote PC, a bridge is required only at the network. The choice of modems for organizing remote interaction should be determined by the characteristics and type of communication channels, as well as the requirements for the capabilities of the modems and their cost.

Note V - up to 2400 baud - tel. communication channels 1 baud 1 bit sec, used with low and medium-speed asynchronous modems asynchronous V - up to 19.2 baud - in dedicated lines, synchronous is usually a telephone line with a maximum speed of V 64 Kbit s, or a dial-up telephone. line with a data transfer rate of V 9600 bits. Netware remote bridges support two kinds of serial transmission methods: asynchronous and synchronous.

The main difference between a protected-mode bridge and a real-mode bridge is the amount of memory it can support. A protected bridge allows you to add memory, while a real bridge provides minimal memory. Bridge in protected mode.

The bridge software in protected mode supports a standard 1 MB of bridge memory and 640 KB of additional RAM. memory The software also supports the installation of memory cards with a total capacity of up to 8 MB. This amount of additional memory allows you to have a bridge on which additional tasks can be executed. Valua Added Processes - VAP in memory capacity up to 7 MB. If you plan to install more than one or two VAP processes, you should select a bridge in protected mode.

In this case, it is necessary to determine additional number of memory cards. The number of cards to be added depends on how many VAP processes you plan to run. If more than two VAP processes will be running, at least one card must be installed. Note. If you want to run 4 VAP processes, such as a print VAP and a queuing VAP, the bridge must run in secure mode. Before using the bridge in protected mode, you must make sure that your computer type is suitable for working in combined mode.

Bridge in real mode. The real-mode bridge software supports the standard 640 KB of main memory, in which case the bridge can run one or two additional VAP-oriented processes. Bridges in real mode can be either dedicated or combined. The computer network allows network users to use the network printing service in their work. PU network printing devices can be printers, plotters or any peripheral devices.

A PU is networked if it is connected externally to a PC workstation or a network, and can be used in the interests of various users or groups of network users from different parts of the network. The latest models of modern PUs have great functionality and high performance. They are quite expensive and their use in the form of local ones will be associated with large material costs. NETWARE printing service allows multiple users to use it more efficiently.

For example, one XEROX laser printer connected to the network will provide an opportunity to save money without purchasing others. When a non-networked station sends a print request to a printer connected to it, the request is immediately forwarded for execution. If the user works with network printers, then the information that he outputs from the PU will be sent first to the file or print server, and only then to the printer.

When the printer is ready to fulfill the next request, the print server selects a print job from the queue and sends it to the printer corresponding to this queue. A print server is part of a file server software component that selects print jobs from a queue and sends them to the printer. The print server can also be present on the network in the form of a specialized workstation, which is designed to serve the printing process on the network, or it can be combined with bridge software.

On a network, the process of network printing can also be carried out on printers connected to regular remote PCs. The NETWARE print server increases the printing capabilities of the network; it can serve up to 16 printers connected to various computers included in the network and can be installed - installing a software product on a PC on a file server, bridge or specialized PC. Print server software is usually combined with file server software and uses VAP processes that are loaded on the file server.

Print server VAP processes use up to 128 K of memory while working on a file server or bridge, including DOS when loading on a bridge. For each printer, another 10 K is added. When using a specialized print server on a PC, its operation requires 200 K of memory, plus 10 K for each connected printer. These numbers may vary depending on the workload of the print server. The remote printer requires 9 K of memory on its PC. This figure also includes the buffer volume required for the printer to operate. The remote printer will function when the file server is disabled if the print server is designed as a specialized PC or is installed on a bridge.

In the NETWARE system, the printing process is implemented as follows: the PC shell sends a file over the network to a file or print server, where, according to system planning, it is buffered and queued with the parameters of the print job. When users simultaneously send information to print, the request received first will be processed first.

All subsequent requests are queued and will be processed in that order unless they receive the highest priority. A print job is a set of characteristics that define how printing should be done. These include the mode, format, number of copies, and the specific printer that will do the job. Each user creates a print job and sends it to a file or print server, where it is already queued.

NETWARE version 2.15 allows one printer to serve multiple queues, and one queue can be serviced by multiple printers. For example, if there are multiple print requests, Printer0 and Printer1 may be assigned to a higher priority queue. You can also define which users are allowed to submit print jobs to each queue. Any print queue must be planned using special means.

You can map print queues to printers using commands entered from the file server console or from a prepared autoexec.sys file. 4.3.

End of work -

This topic belongs to the section:

Information technologies in economics. Fundamentals of Network Information Technologies

LANs are being intensively introduced into medicine, agriculture, education, science, etc. Local network - LAN - Local Area Network, this name.. Currently, information and computing systems are usually divided into 3.. TOP Technical and Office Protocol - technical automation protocol and administrative agency. MAR TOR..

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