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How is connection established?




Being situated beside, Bluetooth device can establish not only connection of a type a point - - to - point, when there are only two devices, but also the point to - many points, when one device works simultaneously with several others. At connection of one device with several by others, the device which serves some connections is called master, and connected devices –slave. To one master can be connected up to seven active slaves. There are can be a set inactive slave, which can not exchange data with master while all channels are engaged, except for active slave (that is, devices which actively exchange data), remain are synchronized with it (Such structure is called piconet).

There can be only one master in one piconet but everyone slave can be master for other devices simultaneously, and form piconet. Several piconets connected thus form scatternet. In scatternet different devices can not only to be at the same time master and slave simultaneously for various piconet, but also only slave for different piconets.

It is possible to familiarize with this structure in the submitted below figure more evidently.

Moreover, in case of necessity any slave can become master in piconet. Previous master thus becomes slave.

Thus, in scatternet can be united so much Bluetooth devices, how much it is necessary. The logic connections can be formed how it is required, and can change somehow, m case of necessity. A unique condition, various piconet included in one scatternet should have the different channels of communication, that is work on various frequencies and have various hopping channel. Hopping is a regular change of frequencies determined in parameters hopping sequence. Specification provides 10 variants hopping sequence, 5 with a cycle in 79 changes and 5 with a cycle in 23 changes. With any hopping sequence of frequency are replaced 1600 hops/ sec. Hopping used to prevent attenuation of a radio signal and interference.

As was already spoken, the automatic installation of connection between Bluetooth devices situated within the limits of reach is one of major Bluetooth features. Therefore, Bluetooth device starts its work in a unfamiliar environment from device discovery, in other words search others Bluetooth devices. For this purpose the inquiry is sent, and the answer to it depends not only on presence in radius of communication active Bluetooth devices, but also from a mode in which there are these devices. On this stage it is possible three basic modes.

 

Discoverable mode

 

The devices, being in this mode, always answer all inquiries, received by them.

Limited discoverable mode

 

In this mode there are devices which can answer inquiries only limited time, or should answer only at observance of the certain conditions.

 

Non-discoverable mode

 

The devices, taking place in this mode, as it is visible from the name of a mode, do not answer new inquiries.

But it is not all. Even if device finds out success, it can be in connectable mode or in non-connectable mode. In non-connectable mode the device does not allow to adjust some important parameters of connection, and, thus, though it can be revealed but to exchange the data with it, it will be impossible. If the device is in connectable mode, at this stage Bluetooth device agree among themselves about a used range of frequencies, size of pages, quantity both order of hops, and other physical parameters of connection

If the process of detection of devices has passed normally, new Bluetooth device receives a set of addresses accessible of Bluetooth devices, and it follows device name discovery, when the new device finds out names of all accessible Bluetooth devices from the list Everyone Bluetooth device should have the unique address (like as MAC-addresses at network payments), but at a level of the user not this address is usually used, and name of the device, which can be any, and it is no necessary to be global unique. The name of the Bluetooth device can be length up to 248 bytes, and use code page according to Unicode UTF-8 (at use UCS-2. the name can be short up to 82 symbols). The specification provides, that Bluetooth of the device are not obliged to accept more first 40 symbols of a name of another Bluetooth device. If Bluetooth the device has the limited size and limited computing power screen, quantity of symbols, which it will accept can be reduced up to 20.

Another major Bluetooth features is the automatic connection of Bluetooth devices to services given by others Bluetooth devices. Therefore, after there is a list of names and addresses, the search of accessible services given by accessible devices is carried out service discovery. The reception or granting any services are main courses of Bluetooth creation, therefore for search of possible services there is special protocol named is Service Discovery Protocol (SOP).

 

Protection technology

Bluetooth has the protection technology of the transmitted data which ha\e been built-in m protocol. Depending on carried out tasks, three modes of protection in which are stipulated can there is a device.

Security mode 1 (non secure), the device can not independently initiate protective procedures.

Security mode 2 (service level enforced security), the device does not initiate protective procedures yet is not established and the connection is not adjusted After the connection is established, the procedures of protection are obligatory, and are defined i determined) by a type and requirements of used services.

Security mode 3 (link level enforced security), the protective procedures are initiated during an establishment and tuning of connection. If the removed device can not fulfill the requirements of protection, the connection is not established.

Of course, that Security mode 3 and 2 can be used together, that is at first protected connection is established, and then it is protected according to the requirements and opportunities of a concrete service

Basis of Bluetooth safety system, used in Security mode 3, is the concept of a session key, or Bond. The session key generates during connection of two devices, and is used for identification and enciphering of the transmitted data. For a key generation most various components, both meanings, beforehand known for both devices, and physical addresses of devices can be used. Combining protection at a level of connection with protection at a level of the applications (where any of existing on today systems of protection of the data can be used absolute) it is possible to create the reliably enough protected connections. But from the point of view of protected connection's construction, obvious weakness of Bluetooth stay an opportunity of interception of the traffic, and for this purpose will not necessary to use specific equipment. However, this problem is not new, and now often it is necessary to use open networks, like the Internet, where the interception of the traffic is possible, care of the closed data.

Digital house

If competent use Bluetooth technologies, it can really turn human life. Already now at exhibitions the printers with Bluetooth are demonstrated which should not be connected to the computer, enough to bring it in a room to switch on in the socket, and it is possible to begin to print. Same correctly and in the return order, enough enter a room with notebook equipped Bluetooth in hands, and at once it is possible to use the printer, enter a local network for an exchange of files and documents, to use the Internet connection. RJ-45 is socket for notebook, which can be met in some planes and airports soon should depart in the past.Some illustrations with an exhibition Comdex Fall: Toshiba.

 

Multimedia notebook, projector and digital camera

Collection completely wireless PDA with The extended communication opportunities

 

Headphones, video camera, microphone, commutator as clock.

The systems hands tree with use Bluetooth become so simple in use, as far as it only is possible Enough that the mobile telephone with Bluetooth by the interface simply would be somewhere inside the automobile, and its connection to hands free to system will take place automatically. Standardization used in Bluetooth, will allow making wireless mice, keyboard, video-game machine and game pads which will suit any PC, and for what begin work with requisite device-it will enough to approach to computer, instead of to switch off it, and climp under a table with wires. Due to same standardization, it is possible in some years we get rid of heap of remote consoles on a sofa separate for the TV, separate for the video recorder, separate for music centre, separate for DVD - player, separate for the conditioner, and so on, and we shall operate all engineering in an apartment by means of mobile telephone, which besides and will be PDA. But it is not a limit. Recently is much spoken about the concept "of a digital house ". Some, well known and serious corporation literally were mad about it. The technology Bluetooth can lift this idea on new height. Imagine, you have trinket (bracelet, ring, everything), with Bluetooth by the device capable to accept \our vote and to pass it on the central computer in the house. The powerful server identifies a vote, distinguishes a sound command, and by menace Bluetooth will give a command for any periphery scattered in the house. Thus, the voice commands will be possible given for all things m the house, TVs, bulbs, curtains, conditioners, microwave oven, waste tank in a toilet, eventually! And thus there will be no necessity that this entire periphery would have sufficient computing power. There is enough of powerful computer in the house for vote recognition. Besides, will not come to care of that what a new refrigerator, for example, to connect and to adjust under your home network, it is enough only to be convinced, that the refrigerator has Bluetooth interlace Moreover, rule by the house it will be possible through the mobile telephone, practically from any point of globe. It is all certainly conveniently, but here as well as in any network can appear burglar. Already now it is possible to think up about ten measures, which strongly will complicate life of burglar, but as was spoken above, the counteraction of the reservation both shell proceeds, and end to this process will be never But as, all it can be conveniently, if we do not notice "little technical malfunctions''. It is in the future. And today, Bluetooth devices can be met basically only at exhibitions, or on sale, but for the prices, which can not promote their wide expansion. Besides, majority of Bluetooth devices demonstrated today are not universality, that it is far from being always can find out each other or to use given services, if beforehand is not stipulated that particularly THIS device should work with concrete THIS. But it is only beginning, and both technology, and software will continue to be improved. Sometime, can even be absolutely speed, that today becomes seems by a wild fantasy by a reality.

 

Text 5

THE LANGUAGE OF E-MAIL

E-mail is the simplest and most immediate function of the Internet for many people. Run through a list of questions that new e-mail users ask most and some snappy answers to them.

What is electronic mail? Electronic mail, or e-mail as it's normally shortened to, is just a message that is composed, sent and read electronically (hence the name). With regular mail you write out your message (letter, postcard, whatever) and drop it off at the post office. The postal service then delivers the message and the recipient reads it. E-mail operates basically the same-way except that everything happens electronically. You compose your message using e-mail software, send it over the lines that connect the In­ternet's networks and the recipient uses an e-mail program to read the message.

How does e-mail know how to get where it's going? Everybody who's connected to the Internet is assigned a unique e-mail address. In a way, this address is a lot like the address of your house or apartment because it tells everyone else your exact location on the Net. So anyone who wants to send you an e-mail message just tells the e-mail program the appropriate address and runs the Send command. The Internet takes over from there and makes sure the missive arrives safely.

What's this netiquette stuff I keep hearing about? The Net is a huge, unwieldy mass with nо "powers-that-be" that can dictate content or standards. This is, for the most part, a good thing because it means there's no censorship and no one can wield authority arbi­trarily. To prevent this organized chaos from descending into mere anarchy, however, a set of guidelines has been put together over the years. These guidelines are known collectively as netiquette (net­work etiquette) and they offer suggestions on the correct way to interact with the Internet's denizens. To give you a taste of netiquette, here are some highlights to consider.

• Keep your message brief and to the point and make sure you clear up any spelling slips or grammatical gaffes before shipping it out.

• Make sure the Subject lines of your message are detailed enough so they explain what your message is all about.

• Don't SHOUT by writing your missives entirely in upper case letters.

• Don't bother other people by sending them test messages. If you must test a program, send a message to yourself.

What's a flame? The vast majority of e-mail correspondence is civil and courteous, but with millions of participants all over the world, it's inevitable that some folks will rub each other the wrong way. When this happens, the combatants may exchange emotionally charged, caustic, often obscene messages called flames. When enough of these messages exchange hands, an out-and-out flame war develops. These usually burn themselves out after a while, and then the participants can get back to more interesting things.

Is e-mail secure? In a word, no. The Net's open architecture allows programmers to write interesting and useful new Internet services, but it also allows unscrupulous snoops to lurk where they don't belong. In particular, the e-mail system has two problems: it's not that hard for someone else to read your e-mail, and it's fairly easy to forge an e-mail address. If security is a must for you, then you'll want to create an industrial strength password for your home di­rectory, use encryption for your most sensitive messages, and use an anonymous remailer when you want to send something incog­nito.

Text 6

One of the main differences of the Internet from all rest, that is connected to computers, is that for successful work with it (him), generally speaking, it is not necessary of any books. Internet can serve both directory, and tutorial, and encyclopedia itself. However on the initial stage, as it seems, the advantage(benefit) of the book, distinctly and is accessible of a stating basis, can be rather significant. Such book I also have tried to write. But before you will get acquainted to it(her) more close, let's try to answer on the most important question, which, probably, you already set to yourselves: The Internet: that it, at last, such more or less scientific definition of the Internet, which the books and clauses on this subject are filled, certainly with, have the value. But the simplest explanation suffices for our purposes while: the Internet is a set of computers connected with each other by channels of communication (connection), plus a set of standard rules, on which they exchange by the information. Thus channels of communication (connection), perhaps, even are less important, than rule of data transfer on them, named in the Internet by the protocols. If even two computers cooperate on one of such protocols is already present Internet.

The invention and the perfection of modems - special devices allowing the computer to send the information on the phone, - has opened doors in the Internet to huge quantity (amount) of the people, at which is not present. Any special network equipment, and there is only personal computer and telephone socket nearby.

Both Internet and the modems for personal computers exist already rather long. However only at all for a long time - about 1990 - the Internet has typed (collected), at last critical weight of the users and resources necessary for network revolution, occurring on our eyes. The high-speed modems allowing the usual users of personal computers without restrictions to enjoy by all boons of the Internet have appeared even later. And the system World Wide Web (WWW, or "«World wide web"), invented in 1993 and former firstly only by one of many components of a network, has borrowed (occupied) a leading position and began to define (determine) the person of the Internet literally in last years - two. By and large, the world Internet boom now in the heat - and very much was lucky (carried) to you, that you were not late almost to the beginning of performance.

The Internet is first of all huge set of computers and programs. Among last you will find not only such, which are able well to decide (solve) your concrete tasks, but also is greater such, whose abilities it will be firstly complex (firstly difficult) even to present to you, probably. The Internet connection gives you a unique opportunity, what imperfect was your computer to have a look per the twenty first century and independently to accustom with such concepts, as «world cyber – social», «virtual money», «information environment Wednesday habitation». Item - word, with all of subjects, about what so love to write now journalists.

However it only part of the answer on a question «that such the Internet». The Internet today is not only huge quantity (amount) of computers, but also the improbable quantity (amount) of the people, for which network is an essentially new way of dialogue almost not having of analogues in the material world. The man - essence social, and dialogue with itself similar - one of his (its) first needs (requirements). Perhaps, till now still any technical invention (short of the telephone) does not make such revolution in this ancient as the world employment (occupation) - dialogue of the man with the man.

Certainly, what exactly will interest you in the Internet first of all - people or computers, - depends only on you. The ancient metaphor of the book as models of world, perhaps, deserves reconsideration - now on a role of such model computer approaches much better. When the speech goes about millions computers and their users all over the world, connected in a uniform network, the metaphor this already ceases be simply metaphor. That is why by exaggeration will not tell, that, leave in the Internet, you do (make) for yourselves accessible the whole world.

 

1. Basic protocols in Internet and search in them

Internet contains vast quantity (amount) of the information; on this in it (him) it is difficult to find that that is necessary if not to know where to search. The place of a presence (finding) (or address) each resource defines (determines) it(him) URL. In URL the type of the protocol indicating on what server contains the access is carried out: on WWW (on which specifies recording: http), Gopher, ftp, telnet, or WAIS, it depends on that what type of the information you will transfer. The network protocol - set of the precisely certain rules: how to request, to make out and to send on a network the given version of the information

Telnet

Millions computers are joined (attached) to a network Internet and many from them have interesting opportunities.

It appears, is quite feasible to reach these computers to look databases, available in them, and programs. The special program telnet allows transforming your computer into the client telnet for a data access and programs in numerous servers telnet. For example researcher regularly can work on several computers, which managers nominate to it (him) a special name and password.

In itself telnet - the communication facility is simple; neither any of the own interface, nor opportunities of search it (he) does not give.

Having contacted the help telnet with the removed machine and having entered in a field of search the entrance name (login) and password (password) you communicate with this machine and programs on further to it(her), and telnet cares only of maintenance of communication(connection) between you.

Certainly, many storehouses of the information, which were originally served by the special programs with access on telnet now it is possible to transfer (translate) on WWW, that not seldom and is done(made). However there were also such which on former give the services only on telnet (especially among the catalogues and information services of libraries).

Standard the link on telnet connection with some site Internet (i.e. URL- the address of this connection) looks so:

telnet: // < the address of the server >

FTP

Practically information is stored (kept) in the computer world as files. On this for itself заре Internet the special means for an exchange of files on a network - network protocol FTP (File Transfer Protocol) has appeared.

The quantity (amount) of files accessible on sites of all world on anonymous FTP is measured in astronomical figures and constantly grows. Besides Word Wide Web allows without any alteration existing FTP of archives, to supply with their descriptions of any degree of detail and convenient hypertext interface.

But how to learn (find out) where to address in searches of the necessary file?

Archie

The search on a name of a file on all anonymous FTP-sites of the world can be carried out with the help of system Archie. She(it) is developed by the several programmers from university Magellan in Canada and carries out indexation of stations FTP, showing files, accessible in everyone station. Archie informs several millions files taking place in more thousand stations FTP, and gives a surprisingly fast way of search of that place, where it is necessary to be directed for reception of a file, interesting for you. The search in a database of the server Archie is made with the help of keywords, which in this case represent simply names of files or fragments of names.

Now Archie is accessible through WWW. It WWW page with the form in which the keywords and options of search are specified.

Though the name of a file almost always has a certain attitude (relation) to his (its) contents, to use Archie for thematic search of the programs does not cost. This system will be useful, if you really know a name of the necessary file or part of a name. On the other hand if you search for the program, which has the standard reduced name, that is some probability that this reduction will be present and at a name of a file, so it is possible to try to find such file with the help Archie. The results of search represent the list of FTP-sites and complete names of files on these sites. The name of a file serves the link to this file, so the click on a name will allow at once receiving a file from the given site. And domes the address of FTP-archive allocated by a large greasy font will transfer you to the initial catalogue of file system on this archive.

Archie the list into the list into the list into the list into the reception.

URL-address indicating FTP-connection, owes, as a minimum to include a word “ftp” as the left part and address of the site in right:

ftp: // < the address of the server >.

WAIS

Almost all information resources Internet give an opportunity of automatic search, but only in one of them - in system WAIS (Wide Area Information Server, global information server) - the keyword search, is the basic method of access to the information.

The system WAIS represents the huge distributed (allocated) database, i.e. the separate parts of this database are located on different net points worldwide. The program - client for work with servers WAIS not only is able to communicate with them under the special protocol, but also stores (keeps) the list of all databases WAIS with their addresses and names, and sometimes and with the brief descriptions. There it is possible to find archives of news, every possible catalogues, directories, collections of scientific clauses and abstracts, indexes and much another. Practically any information, for which the database is the convenient form by performances which is interesting enough to a wide audience, can be submitted in WAIS.

The database WAIS is so great, that idle time of recordings - extremely unpromising way of access to the information. Therefore in WAIS the rather complex (difficult) method of keyword search and phrases is applied. The documents received as a result of search, are sorted by the WAIS-server by quantity (amount) of entry in them of keywords and on, as far as is complete in each of them all set of the ordered keywords is submitted. Such system is capable to give good results, is especial if to break process of search at some stages.

The first stage of search usually is the search special in a database under the name “directory-of-servers”, which contains small on volume of the description (abstracts) of all other bases WAIS. Thus it is better to specify not those keywords, which at the end interest you, and those general(common) concepts concerning your field of knowledge.

Having chosen on the first stage of one or several bases, it is possible to begin search actually of documents - that also it is possible to do (make) in some calls, getting acquainted after each search with results and changing the list of keywords, the necessary document will not be found yet. Besides WAIS has one useful property: if at the next stage of search you have found the document, which contents are especially close that you search, it(he) can be added to the list of the relevant documents. In result at the following stage WAIS will find the documents, not only containing a set of your keywords, but also similar on the dictionary structure on those texts, which you have specified as most suitable.

Though Netscape is capable to work with servers WAIS, for this purpose it (he) should know the address of the special proxy for the protocol WAIS, of which your provider can not appear. The special WAIS-clients working including under Windows, not only convenient for interactive search in databases, but also allows managing services any of the server – intermediary.

URL-address for WAIS looks as:

wais: // < the address of the server > / < the name of base >

Gopher

Each of the considered till now Internet protocols quite could exist in itself: telnet gives access to the programs, FTP - to files, WAIS - to databases. All these systems were created both developed almost independently from each other and put before itself the concrete purpose - to construct the mechanism of access to one certain kind of resources.

However as Internet of dews also became complicated, it (he) pushed the people to an idea, that the information on a nature is uniform and what not too reasonably to divide her (it) on different “kinds of resources” with unlike methods of access. By the end of 80-s' years the idea of a new universal remedy of work with the diverse information was carried in air.

The first system called to embody for life these dreams began Gopher, appeared on light 1991 at the American university of state Minnesota. Fundamental concept incorporated in its (her) basis was structure enclosed each other menu.

Having communicated Gopher- by the client with one of Gopher-servers, you first of all get in the root, main menu of this server. Having oriented in his (its) contents and passing further from one submenu in another, it is possible to engage actually in search of the information. The information in system Gopher is organized strictly hierarchically, “from the general to the particular”. On each step the server sends yours Gopher - client not only contents of the next menu, but also service information on, where there are objects appropriate to items it the menu. The client deduces on the screen only transfer of items, but when you choose one from them, it (he), having verified with the information, available at him (it), or will send on any of Gopher - servers search about distribution of the following of the menu, or will try to establish FTP- or telnet- connection with some site Internet.

Obviously from all information tools Internet Gopher most of all is similar on WWW - both these systems are called to serve the shell with diverse resources. However Gopher is deprived of those rich representational means, which do (make) WWW not only information catalogue, but also new mass media and even by a new genre of art. It is possible to tell, that Gopher is “subset” WWW - that very much simplifies integration itself Gopher as one of kinds of resources in WWW. Practically all that is told about work with Gopher - by the client fairly and for travel on system Gopher with the help of a browser WWW.

URL of one the menu consists of items Gopher - from addresses Gopher- of the server and listed those items of the menu, which will result you in initial item:

gopher: // < the address of the server > / < item of the menu >

WWW

In spite of the fact that per the first years of the existence Gopher has won the large popularity, the need in any became ripe more simple and in too time to maximum universal system, in which communication(connection) between resources would be more free and associative. Such system was developed in 1993 and is named World Wide Web (WWW). System WWW to be under construction on concept of a hypertext, or, is exacter than hypermedia. The hypertext is a text of which component are connected with each other and to other texts with the help of the links. The hypermedia is that that will turn out from a hypertext if to replace in his (its) definition a word “text” on “ any kinds of the information ”: the graphic images, video, sound etc.

World Wide Web “world wide web” means literally. Why she(it) so refers to as. As it is clear from the name, she (it) is global. You not always know in what part of light there is a computer, to which you are connected. You can begin to read page Web in Dallas, then to proceed (pass) in Australia, and all this on one pressing of the button of the mouse. WWW is similar to a web. It is a complex (difficult) information network connected hypermedia by communications (connections). WWW allows not refusing information resources already saved in Internet, other means, accessible with the help, - FTP, telnet and Gopher. It is more than that, the work with these resources through WWW is so convenient, that, say, FTP-clients, former at one time by separate class of the programs, now are used only by few.

And nevertheless main in World Wide Web is not convenience of access to FTP- of archive and Gopher-menu. The majority of servers of system WWW offers the information, which without WWW hardly in general when or would get in a network. Speed of creation and updating, the rich representational opportunities in a combination to ease of access and huge audience have made WWW by new mass media. The start WWW-servers and creation of WWW-pages already has turned of a style to new business - with all that is peculiar to modern business: by a payment for square centimeters of the advertising area.

On the other hand, the fast distribution of system so naturally uniting diverse resources, was promoted not in the last instance by its (her) origin not in the deepest layers of a business concern, and in scientific establishment - European laboratory of physics of particles, which employees did not begin to do (make) of secrets of the development and have not tried at all on it (her) to grow rich.

Fortunately, nature WWW as first of all means of search and organization of the information allows to hope, that this remarkable invention to not turn to the tool of only one commerce and advertising.

The servers and clients WWW communicate among themselves under the special protocol HTTP (Hyper Transfer Protocol, hypertext transfer protocol). URL for WWW looks as:

http: // < the address of the server >

 

To pass from one page Web to another - an interesting way of research World Wide Web, however you early or late need to find something concrete. But thus it to make it will be not possible. In Internet there are special tools of Web-search.

 

Tools of search in WWW

 

Tools of search of the information in Internet constructed on different principles and pursuing the different purpose, exist much. But all unites them that they settle down on the specially allocated network computers with powerful channels of communication (connection), serve every minute huge quantity (amount) of the visitors and require (demand) of the owners of significant expenses on support and updating. Nevertheless, almost all of them respond on searches of the users completely бескорыстно, and the sponsors and advertisers pay for this pleasure. On scales of influence on network community of retrieval systems, undoubtedly, are one of corner stones Internet.

The classification of retrieval systems is more convenient for building all on the basis of on the tax and processing of the information given to the users, - or else how many is automated in them, who types(collects) a database, in which the search is made: the people or computers.

Tools of search

Conditionally tools of search are subdivided into search means of a help type (directories) and retrieval systems in the pure state (search engines).

2.1 Thematic catalogues

The search tools of the first type name subject, or thematic catalogues more often. The company owning such catalogue, continuously conducts huge work, investigating, describing, cataloguing and displaying on half-glass contents of WWW-servers and other network resources scattered worldwide. Result her(it) titanium of efforts is the constantly updated hierarchical catalogue, at the top level the most general(common) categories, such as “business” are assembled, "”science", "”art" etc., and the elements of the lowermost level represent the links to separate WWW-pages and server together with the brief description of their contents.

Guarantees that such catalogue really covers all contents WWW, nobody will give, however possible(probable) not the completeness and selection of materials is with interest expiated there, that for the present not under force to any computer - intelligence of selection.

The subject catalogues give also opportunity of keyword search. However search this occurs not in contents of WWW-servers and in their brief descriptions stored (kept) in the catalogue.

The subject catalogues Internet can be counted literally on fingers, as their creation and support require (demand) huge expenses. To most known concern Yahoo, WWW Virtual Library, Galaxy and some other.

Yahoo

Is most popular in the population Internet the catalogue Yahoo. On the first page Yahoo, located to the address http://www.yahoo.com, you receive access to two basic method of work with the catalogue - keyword search and hierarchical tree of sections.

Having begun descent (release) on sections of the catalogue, you will see that each section contains the precisely same field for input of keywords and button Search, starting search.

Each section can include as transfer included in him (it) and actually links to pages concerning at once all section, with their brief descriptions.

Instead of travel you can at once get in the necessary place of the catalogue Yahoo with the help of search. Having entered one or several keywords divided (shared) by blanks, in a line of search and having pressed the button Search, you receive the list all in Yahoo that comprises the specified keywords. This list will be divided (shared) into two parts - “categories” and “sites”.

If total of the links returned as a result of search, exceeds 25, the list of the links will be broken on some parts.

 

Magellan

But not seldom happens so, that the list, given out by the machine, is very great and to see (overlook) it (him) simply not really. An output (exit) from this situation can become stricter selection of the information brought in the catalogue. One of most known such systems - catalogue Magellan to the address: http: // www.mckinley.com

This database contains the items of information on 80 thousand WWW-pages - that very much not much in comparison with those in millions, which exist in a network. However if Yahoo as the description of a resource uses one - two lines of the text, the employees of system Magellan on some of pages brought in their database, write the small reviews, and also estimate quality of these information resources on a five-mark scale. Till past bases of the reviews, Magellan owns as an own automatic index, for search in which it is necessary to throw the switch under a field of input in a rule (situation) entire database.

As a rule search represents one or several keywords divided (shared) by blanks.

Point

The service, similar by the principles, of firm Point (http: // www.pointcom.com) in general basic emphasis does (makes) not on search, and on work with the thematic catalogue.

The service Point is known in a network for that its (her) employees are constantly engaged оцениванием of network resources and conduct the lists of those sites, which they consider (count) belong to “ to the best five percents(interests) WWW ”.

Firm Point conducts a shared database all “of five-percentage WWW-pages, where about everyone it is possible to read the detailed license.

Virtual Library

The most old subject catalogue WWW is the catalogue Virtual Library:

http: // www.w3.org/hypertext/DataSources/bySubj ect/Overview.html

This system full enough covers a scientific layer WWW - servers of universities, laboratories and educational institutions.

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