I've always been wondering, what 3G technology is all about. Prime Minister Manmohan Singh had first launched the 3G service in Delhi with MTNL.We also have so many mobile phone's with 3G technology, but still many of us do not know what is 3G Technology.

What is 3G?

3G is the next generation of wireless network technology that provides high speed bandwidth (high data transfer rates) to handheld devices. The high data transfer rates will allow 3G networks to offer multimedia services combining voice and data.

Additional features also include HSPA(High-Speed packet Access) data transmission capabilities able to deliver speeds up to 14.4Mbit/s on the downlink and 5.8Mbit/s on the uplink.

3G wireless networks have the bandwidth to provide converged voice and data services. 3G services will seamlessly combine superior voice quality telephony, high-speed mobile IP services, information technology, rich media, and offer diverse content.

What are the Advantages of 3G?

3G networks offer users advantages such as:

New radio spectrum to relieve overcrowding in existing systems.· More bandwidth, security, and reliability.

Interoperability between service providers.

Fixed and variable data rates.

Always-online devices. 3G will use IP connectivity, IP is packet based (not circuit based).

Rich multimedia services.

What are Some Disadvantages of 3G?

There are some issues in deploying 3G:

The cost of upgrading base stations and cellular infrastructure to 3G is very high.

Requires different handsets and there is the issue of handset availability.

Base stations need to be closer to each other (more cost).

Tremendous spectrum-license costs, network deployment costs, handset subsidies to subscribers, etc.

These are just few information about 3G service, but actually speaking there's a hell lot of things that we need to learn about. Just search through google, you will get millions of website's telling you what is 3G service actually means. I also read that BSNL is planing to launch 3G, hope its been launched all over India.

Please Folllow these steps to create email account on yahoo.com

1.Open Internet Explorer.

2.Now type www.yahoo.com in address bar.

3. Hit enter

Page will be open like this

4. Now click to Sign Up.

Registration form will be open like that

5. Now fill the all requirment carefully

 a. Fulfill about yourself ,  Name , Gendre , Birth date , Location & Postal code .

b. When you will click on yahoo ID and Email its will display more than 4 ID select 1 which you want to create and than type secret code             (Password) it will ask whenever you will open your account

c. Type alternate email address if you don,t have alternate email address than leave it . And select question than type answer of you question ( Its purpose of in case forgot your ID or Password If you will remember your question & answer you will recovered it.

d. Type captcha code

e. Check the box of term & condtion .

f. Press the tab "Create my account .

When your registration process will complete its will show the congratulation page

Print your congratulation page.

HTTP-HYPERTEXT TRANSFER PROTOCOL

The standard web transfer protocol is HTTP (Hyper Text Transfer Protocol). Each interaction consists of one ASCII request, followed by one MIME-like response. HTTP is constantly evolving. Several versions are in use and others are under development. The HTTP protocol consists of two distinct items: the set of requests from browsers to servers and the set of responses going back the other way.

All the newer versions of HTTP support two kinds of requests: simple requests and full requests. A simple request is just a single GET line naming the page desired, without the protocol version. The response is just the raw page, with no headers, no MIME, and no encoding.

For e.g.

GET /hypertext/WWW/TheProject.html

but without the HTTP/1.0. The page will be returned with no indication of its content type.

Full requests are indicated by the presence of the protocol version on the GET request line. Requests may consist of multiple lines, followed by a blank line to indicate the end of the request. The first line of a full request contains the command (of which GET is but one of the possibilities), the page desired, and the protocol/version. Subsequent lines contain headers.

Although HTTP was designed for use in the Web, it has been intentionally made more general than necessary with an eye to future object-oriented applications. For this reason, the first word on the full request line is simply the name of the method (command) to be executed on the web page (or general object). The built-in methods are listed below. The names are case sensitive, so GET is a legal method but get is not.

Figure (a). The built-in HTTP request methods.

The GET method requests the server to send the page (by which we mean object, the most general case), suitably encoded in MIME. However, if the GET request is followed by an If-Modified-Since header, the server only sends the data if it has been modified since the date supplied. Using this mechanism, a browser that is asked to display a cached page can conditionally ask for it from the server, giving the modification time associated with the page. If the cache page is still valid, the server just sends back a status line announcing that fact, thus eliminating the overhead of transferring the page again.

The HEAD method just asks for the message header, without the actual page. This method can be used to get a page’s time of last modification, to collect information for indexing purposes, or just to test a URL for validity. Conditional HEAD requests do not exist.

The PUT method is the reverse of GET: instead of reading the page, it writes the page. This method makes it possible to build a collection of web pages on a remote server. The body of the request contains the page. It may be encoded using MIME, in which case the lines following the PUT might include Content Type and authentication headers, to prove that the caller indeed has permission to perform the requested operation.

Somewhat similar to PUT is the POST method. It too bears a URL, but instead of replacing the existing data, the new data is “appended” to it in some generalized sense. Posting a message to a news group or adding a file to a bulletin board system are examples of appending in the context.

DELETE removes the page. As with PUT, authentication and permission play a major role here. There is no guarantee that DELETE succeeds, since even if the remote HTTP server is willing to delete the page, the underlying file may have a mode the forbids the HTTP server from modifying or removing it.

The LINK and UNLINK methods allow connections to be established between existing pages or other resources.

Every request gets a response consisting of a status line, and possible additional information (e.g., all or part of a web page). The status line can bear the code 200 (OK), or any one of a variety of error codes, for example 304 (not modified), 400 (bad request), or 403 (forbidden).

FILE TRANSFER PROTOCOL (FTP)

FTP is a common file transfer protocol. It is a protocol in the TCP/IP suite and is built on the client/server paradigm. A user, interacting with a local FTP program, connects to a remote site also running FTP. This can be done in a couple of ways. First is to simply enter the command

"ftp text-address"

Which will establish a connection to the specified remote computer. The second way is to enter

"ftp"

and wait for the prompt ftp>. Next the user enters

" ftp> open text-address"

to establish the connection. Sometimes connect is used instead of open. Once connected, the user is asked to enter a user identification followed by a password. On entering the appropriate identification and password the user then can peruse subdirectories, get directory lists, and get copies of files.

Many sites make files available to the general Internet community. This means a user can access them without having an account on that machine. When a user connects to the site he or she usually enters “anonymous” for the account name and either “guest” or his or her email address as the password. The latter is to track use of accesses. This application is often called anonymous FTP.

FTP allow a user to establish a remote connection. The difference with Telnet is that Telnet allows a legitimate login, whereas FTP primarily provides access to certain files and directories.

TELNET

One example of a network virtual terminal protocol is Telnet. It was designed for the ARPANET and is one of the protocols in the TCP/IP suite. Perhaps most people know Telnet as the application that allows remote logins. A virtual terminal is a data structure maintained by either the application software or a local terminal. Its contents represent the state of the terminal. For example, they may include the current cursor position, reverse video indicator, cursor shape, number of rows and columns, and color. Both the user and the application can reference this structure. The application writes to the virtual terminal without worrying about terminal-specific matters. Virtual terminal software does the required translation, and the data is displayed. When a user enters data, the process works in reverse. Virtual terminal protocols define the format of the data structure, software converts user input to a standard form, and the application then reads the standard screen.

To the user, a remote login appears to be no different than a login to a local computer. A user works at a PC (or is connection to another computer) that runs protocols to connect to a network. The protocols establish a connection over the network to a remote computer. The user and remote computer exchange commands and data using protocol such as TCP/IP.The user is working at a higher layer, however, so this is all transparent and appears much like a local login. The only difference may be slight delays between responses, especially if the remote computer is far away or network traffic is heavy.

Telnet works in a client/server mode. That is, a PC (or other computer) runs Telnet (client) locally and transmits data between the user and network protocols. It also can format and send specific commands. The remote computer (server) also runs its version of Telnet. It performs similar functions, exchanging data between network protocols and the operating system and interpreting user-transmitted commands.

A user typically uses Telnet in a couple of ways. One is to log in to a local computer, wait for a system prompt (“>” in our example), and enter the command

> Telnet text-address

The text address specifies the host computer to which the user wants to connect. Telnet then calls on the transport protocol to negotiate and establish a connection with the remote site. Once connected, the user must log in to the remote site by specifying the account number and password. Another way to use Telnet is to enter the command Telnet without a text address. The local system will respond with a Telnet prompt (Telnet >). If you are running on a graphical user interface (GUI), there is typically a Telnet icon you can access. Either way you can enter Telnet commands (or select them from a menu). For example, you can connect to the remote site by entering a connect or open command (depending on the local system) specifying the text address.

Once connected, Telnet works in the background completely transparent to the user. However, the user can escape from the remote login to give subsequent commands to Telnet. This is normally done by entering a control sequence such as Ctrl-]. This returns the Telnet prompt to the user but does not break the remote connection.

CHAT AND BULLETIN BOARDS

Chat is synchronous (happening in real time, like a phone conversation, unlike an e-mail exchange), line-by-line communication with another user over a network. Chat rooms are search for chats on subjects that interest you; if the room members are discussing the stated topic, you may meet some interesting person. Following are different types of chats:

1. With featured chats you can check out the chat rooms.
2. In the member chats, you’ll find the same sorts of categories as in the featured chats, but the chat rooms themselves are member created.
3. The private chat rooms are private. They are also useful for conversation with more than two other people. You can create a private chat room and invite your friends in to have them in your buddy list. You can also get into private chat rooms by guessing at names.
4. With channel chat you can connect to conference rooms, get into game rooms and can look for special some ones.

You can protect your right to quality chat. By double clicking the name of a rude chatter and, in the information about dialog box that comes up, check the Ignore Member button. Once a chatter is ignored, his comments won’t show up on your scrolling chat screen. You can also give chat preferences like getting notified when members arrive or leave. You can double-space incoming messages or alphabetize the member list. You can also enable chat room sounds.

Another way to go about chatting is to search the member directory for people who share your enthusiasm for say chess, live in the town you grew up etc. you can type location-specific and name-specific search words to narrow down the search. The advance search offers you the option of filling in everything about the person you seek.

You can chat in style by changing fonts, coloring letters, using bold, italics or underlines etc. you can also use shorthands in chatting. Little pictures can also be drawn through the keyboard.

Once you enter a chat room, you can create your member profile depending on what kind of attention you want to attract.

Bulletin boards are data banks that allow the free exchange of some software, files, or other information. Electronic bulletin boards are a way to meet other computer users, voice opinions, receive technical help, and download shareware etc. bulletin boards are often focused on a particular subject area. Boards are more intimate and personal and provide an easy way for people with similar interests to congregate and interact.

USENET

One of the most popular applications of computer networking is the worldwide system of newsgroups called net news. Often net news is referred to as USENET.

A newsgroup is a worldwide discussion forum on some specific topic. People interested in the subject can “subscribe” to the newsgroup. Subscribers can use a special kind of user agent, a news reader, to read all the articles (messages) posted to the newsgroup. People can also post articles to the newsgroup. Each article posted to a newsgroup is automatically delivered to all the subscribers, wherever they may be in the world. Delivery typically takes between a few seconds and a few hours, depending how far off the beaten path the sender and receiver are. In effect, a newsgroup is somewhat like a mailing list, but internally it is implemented differently. It can be thought of as a kind of high-level multicast

The number of newsgroups is so large that they are arranged in a hierarchy to make them manageable. Following figure shows the top levels of the ‘official’ hierarchies.

Figure  (b) . USENET hierarchies

Each of the categories listed is broken into subcategories, recursively. For example, rec.sport is about sports, rec.sport.basketball is about basketball, and rec.sport.basketball.women is about women’s basketball.

Numerous news readers exist. Like email readers, some are keyboard based; others are mouse based. In nearly all cases, when the new sreader is started, it checks a file to see which newsgroups the user subscribes to. It then displays a one-line summary of each as-yet-unread article in the first newsgroup and waits for the user to select one or more reading. The selected articles are then displayed one at a time. After being read, they can be discarded, saved, printed, and so on.

News readers also allow users to subscribe and unsubscribe to newsgroups. Changing a subscription simply means editing the local file listing which newsgroups the user is subscribed to.

News readers also handle posting. The user composes an article and then gives a command or clicks on an icon to send the article on its way. Within a day, it will reach almost everyone in the world subscribing to the newsgroup to which it was posted. It is possible to crosspost an article, that is, to send it to multiple newsgroups with a single command. It is also possible to restrict the geographic distribution of posting.

With USENET thousands of people who do not know each other can have worldwide discussions on a vast variety of topics. It is possible for someone with a problem to post it to the net. The next day, he may have a number of solutions for it.

To stop abusive postings called flamewar, an individual user can install a killfile, which specifies that articles with a certain subject or from a certain person are to be discarded upon arrival, prior to being displayed. Most news readers also allow an individual discussion thread to be killed, too. This is useful when a discussion looks like it is starting to get into an infinite loop.

If enough subscribers to a group get annoyed with newsgroup pollution, they can propose having the newsgroup be moderated. A moderated newsgroup is one in which only one person, the moderator, can post articles to the newsgroup. All postings to a moderated newsgroup are automatically sent to the moderator, who posts the good ones and discards the bad ones. Some topics have both a moderated newsgroup and an unmoderated one.

Since thousands of people subscribe to USENET for the first time every day, the same beginner's questions tend to be asked over and over. To reduce this traffic, many newsgroups have constructed a FAQ (Frequently Asked Questions) document that tries to answer all the questions that beginners have. Some of these are highly authoritative and run to over 100 pages. The maintainer typically posts them once or twice a month.

USENET is full of jargon such as BTW (By The Way), ROFL (Rolling On the Floor Laughing), and IMHO (In My Humble Opinion). Many people also use little ASCII symbols called smileys or emotions.

Although most people use their real names in postings, some people wish to remain totally anonymous, especially when posting to controversial news groups or when posting personal ads to news groups dealing with finding partners. This desire has led to the creation of anonymous remailers, which are servers that accept email messages (including postings) and change the From:, Sender:, and Reply-To: fields to make them point to the remailer instead of the sender. Some of the remailers assign a number to each user and forward email addressed to these numbers, so people can send email replies to anonymous postings like

As more and more people subscribe to USENET, there is a constant demand for new and more specialized news groups. Consequently, a procedure has been established for creating new ones. Suppose that somebody likes english movies and wants to talk to other english movie fans. He posts a message to news.groups naming the proposed group, say rec.movies.english, and giving the names of English movies.

Some of the Smaller news groups are implemented as mailing lists. To post an article to such a mailing list, one sends it to" the mailing list address, which causes copies to be sent to each address on the mailing list.

USENET is not generally implemented using mailing lists. Instead each site (campus, company, or Internet service provider) stores incoming mail in a single directory, say, news, with subdirectories for comp, sci, etc. These, in turn have subdirectories such as news/comp/os/minix. All incoming news is deposited in the appropriate directory. News readers just fetch the articles from there as they need them. This arrangement means that each site needs only one copy of each news article, no matter how many people subscribe to its newsgroup. After a .few days, articles time out and are removed from the disk.

To get on USENET, a site must have a newsfeed from another site on USENET. One can think of the set of all sites that get net news as the nodes of a directed graph. The transmission lines connecting pairs of nodes form the arcs of the graph. This graph is USENET. Note that being on the Internet is neither necessary nor sufficient for being on. USENET.

Periodically, each site that wants news can poll its newsfeed(s) asking if any new news has arrived since the previous contact. If so, that news is collected and stored in the appropriate subdirectory of news. In this manner, news diffuses around the network. It is equally possible for the newsfeed, rather than the receiver .to take the initiative and make contact when there is enough new news.

Not every site gets all newsgroups. There are several reasons here. First, the total newsfeed exceeds 500 MB per day and is growing rapidly. Storing it all would require a very large amount of disk space. Second, transmission time and cost are issues. Third, not every site is interested in every topic. Finally, some newsgroups are a bit too funky for the tastes of many system administrators, who then ban them, despite considerable local interest.

News articles have the same format as email messages, but with the addition of a few extra headers. This property makes them easy to transport and compatible with most of the existing email software.

A description of the news headers is:

The Path: header is the list of nodes the message traversed to get from the poster to the recipient. At each hop, the forwarding machine puts its name at the front of the list. This list gives a path back to the poster.

The Newsgroups: header tells which newsgroups the message belongs to. It may contain more than one newsgroup name. Any message crossposted to multiple newsgroups will contain all of their names.

Because multiple names are allowed here, the Followup-To: header is needed to tell people where to post comments and reactions to put all of the subsequent discussion in one newsgroup.

The Distribution: header tells how far to spread the posting. It may contain one or more state or country codes, the name of a specific site or network, or "world."

The Nntp-Posting-Host: header tells which machine actually posted the article, even if it was composed on a different machine.

The References: header indicates that this article is a response to an earlier article and gives the ID of that article. It is required on all follow-up articles and prohibited when starting a new discussion.

The Organization: header can be used to tell what company, university, or agency the poster is affiliated with. Articles that fill in this header often have a disclaimer at the end saying that if the article is goofy, it is not the organization's fault.

The Lines: header gives the length of the body.

The Subject: lines tie discussion threads together. Many news readers have a command to allow the user to see the next article on the current subject, rather than the next article that came in. Also, killfiles and kill commands use this header to know what to reject.

The Summary: is normally used to summarize the follow-up article. On follow-up articles, the Subject: header contains "Re: " followed by the original subject.

NNTP-NETWORK NEWS TRANSFER PROTOCOL

Now let us look at how articles diffuse around the network. The initial algorithm just flooded articles onto every line within USENET. While this worked for a while, eventually the volume of traffic made this scheme impractical, so something better had to be worked out.

Its replacement was a protocol called NNTP (Network News Transfer Protocol). NNTP is somewhat similar to SMTP, with a client issuing commands in ASCII and a server issuing responses as decimal numbers coded in ASCII. Most USENET machines now use NNTP.

NNTP was designed for two purposes. The first goal was to allow news articles to propagate from one machine to another over a reliable connection (e.g., TCP). The second goal was to allow users whose desktop computers cannot receive news to read news remotely.

Two general approaches are possible. In the first one, news pull, the client calls one of its newsfeeds and asks for new news. In the second one, news push, the newsfeed calls the client and announces that it has news. The NNTP commands support both of these approaches, as well as having people read news remotely.

To acquire recent articles, a client must first establish a TCP connection on one of its newsfeeds. Behind it is the NNTP daemon, which is either there all the time waiting for clients or is created as needed. After the connection has been established, the client and server communicate using a sequence of commands and responses. These commands and responses are used to ensure that the client gets all the articles it needs, but no duplicates. no matter how many newsfeeds it uses.

Figure(c)

The main ones used for moving articles between news daemons are listed in figure (c)

WEB BROWSERS

Most browsers have a point and click interface-the browser displays information on the computer’s screen and permit a user to navigate using the mouse. The information displayed includes both text and graphics. Furthermore, some of the information on the display is highlighted to indicate that an item is selectable. When the users places the cursor over a selectable item and clicks a mouse button, the browser displays new information that corresponds to the selected item.

Technically, the Web is a distributed hypermedia/hypertext system that supports interactive access. Here, information is stored as a set of documents. Besides the basic information, a document can contain pointers to other documents in the set. Each pointer is associated with a selectable item that allows a user to select the item and follow the pointer to a related document. Hypertext document contain only textual information, while hypermedia documents can contain additional representations of information, including digitized photographic images or graphics. There can be a non-distributed hypermedia system, in which information resides within a single computer. A non-distributed hypermedia system can guarantee that all links are valid and consistent.

In contrast, in a distributed hypermedia system, the Web distributes documents across a large set of computers. A system administrator can choose to add, remove, change, or rename a document on a computer without notifying other sites. Consequently, links among Web documents are not always consistent. For example, suppose document D₁ on computer C₁ contains a link to document D₂ on computer C₂. If the administrator responsible for computer C₂ chooses to remove document D₂, the link on C₁ becomes invalid.

Most browsers have numerous buttons and features to make it easier to navigate the web. Many have a button for going back to the previous page, a button for going forward to the next page (only operative after the user has gone back from it), and a button for going straight to the user’s own home page. Most browsers have a button or menu item to set a bookmark on a given page and another one to display the list of bookmarks, making it possible to revisit any of them with a single mouse click. Pages can also be saved to disk or printed. Numerous options are generally available for controlling the screen layout and setting various user preferences.

In addition to having ordinary text (not underlined) and hypertext (underlined), Web pages can also contain icons, line drawing, maps, and photographs. Each of these can (optionally) be linked to another page. Clicking on one of these elements causes the browser to fetch the linked page and display it, the same as clicking on text. With images such as photos and maps, which page is fetched next may depend on what part of the image was clicked on.

Not all pages are viewable in the conventional way. For example, some pages consist of audio tracks, video clips, or both. When hypertext pages are mixed with other media, the result is called hypermedia. Some browsers can display all kinds of hypermedia, but others cannot. Instead they check a configuration file to see how to handle the received data. Normally, the configuration file gives the name of a program, called an external viewer, or a helper application, to be run with the incoming page as input. If no viewer is configured, the browser usually asks the user to choose one. If no viewer exists, the user can tell the browser to save the incoming page to a disk file, or to discard it. Helper applications for producing speech are making it possible for even blind users to access the Web. Other helper applications contain interpreters for special web languages, making it possible to download and run programs from Web pages. This mechanism makes it possible to extend the functionality of web itself.

Many web pages contain large images, which take a long to load. For example, fetching an uncompressed 640 X 480 (VGA) image with 24 bits per pixel (922 KB) takes about 4 minutes over a 28.8- kbps modem line. Some browsers deal with the slow loading of images by first fetching and displaying the text, then getting the images. This strategy gives the user something to read while the images are coming in and also allows the user to kill the load if the page is not sufficiently interesting to warrant waiting. An alternative strategy is to provide an option to disable the automatic fetching and display of images.

Some page writers attempt to placate potentially bored users by displaying images in a special way. First the image quickly appears in a coarse resolution. Then the details are gradually filled in. For the user, seeing the whole image after a few seconds, albeit at low resolution, is often preferable to seeing it built up slowly from the top, scan line by scan line.

Some web pages contain forms that request the user to enter information. Typical applications of these forms are searching a database for a user-supplied item, ordering a product, or participating in a public opinion survey. Other web pages contain maps that allow users to click on them to zoom in or get information about some geographical area. Handling forms and active (clickable) maps requires more sophisticated processing than just fetching a known page.

Some browsers use the local disk to cache pages that they have fetched. Before a page is fetched, a check is made to see if it is in the local cache. If so, it is only necessary to check if the page if still up to date. If so, the page need not be loaded again. As a result, clicking on the BACK button to see the previous page is normally very fast.

To host a web browser, a machine must be directly on the Internet, or at least have a SLIP or PPP connection to a router or other machine that is directly on the Internet. This requirement exists because the way a browser fetches a page is to establish a TCP connection to the machine where the page is, and then send a message over the connection asking for the page. If it cannot establish a TCP connection to an arbitrary machine on the Internet, a browser will not work.

The Server Side

Figure (a) :  The parts of the Web model.

A URL has three parts:

• The name of the protocol(http)
• The name of the machine where the page is located (www.w3.org)
• The name of the file containing the page (hypertext/WWW/TheProject.html).

The steps that occur between the user’s click and the page being displayed are as follows:

• The browser determines the URL (by seeing what was selected).
• The browser asks DNS for the IP address of www.w3.org.
• DNS replies with 18.23.0.23
• The browser makes a TCP connection to port 80 on 18.23.0.23.
• It then sends a GET/hypertext/WWW/TheProject.html command.
• The www.w3.org server sends the file TheProject.html.
• The TCP connection is released.
• The browser displays all the text in TheProject.html.
• The browser fetches and displays all images in TheProject.Html.

Many browsers display which step they are currently executing in a status line at the bottom of the screen. In this way, when the performance is poor, the user can see if it is due to DNS not responding, the server not responding, or simply network congestion during page transmission.

For each in-line image (icon, drawing, photo etc.) on a page, the browser establishes a new TCP connection to the relevant server to fetch the image. If a page contains many icons, all on the same server, establishing, using, and releasing a new connection for each one is not wildly efficient, but it keeps the implementation simple.

Because HTTP is an ASCII protocol like SMTP, it is quite easy for a person at a terminal (as opposed to a browser) to directly talk to Web servers. All that is needed is a TCP connection to port 80 on the server.

No all servers speak HTTP. In particular, many older servers use the FTP, Gopher, or other protocols. Since a great deal of useful information is available on FTP and gopher servers, one of the design goals of the Web was to make this information available to Web users. One solution is to have the browser use these protocols when speaking to an FTP or Gopher server. Some of them, in fact, use this solution, but making browsers understand every possible protocol makes them unnecessarily large.

Instead, a different solution is often used: proxy servers.

A proxy server is a kind of gateway that speaks HTTP to the browser but FTP, Gopher, or some other protocol to the server. It accepts HTTP requests and translates them into, say, FTP requests, so the browser does not have to understand any protocol except HTTP. The proxy server can be a program running on the same machine as the browser, but it can also be on a free-standing machine somewhere in the network serving many browsers. Figure 4 shows the different between a browser that can speak FTP and one that uses a proxy.

Often users can configure their browsers with proxies for protocols that the browsers do not speak. In this way, the range of information sources to which the browser has access is increased.

In addition to acting as a go-between for unknown protocols, proxy servers have a number of other important functions, such as caching. A caching proxy server collects and keeps all the pages that pass through it. When a user asks for a page, the proxy server checks to see if it has the page. If so, it can check to see if the page is still current. In the event that the page is still current, it is passed to the user. Otherwise, a new copy is fetched.

Finally, an organization can put a proxy server inside its firewall to allow users to access the Web, but without giving them full Internet access. In this configuration, users  can talk to the proxy server, but it is the proxy server that contacts remote sites and fetches pages on behalf of its clients. This mechanism can be used, for example, by high schools, to block access to web sites the principal feels are inappropriate for tender young minds.

CREATING AND LOCATING INFORMATION ON THE WEB

The browser has a menu bar on top, where the user can quit, get help on using the program, and change certain display characteristics (font size, background color, etc.). Some local configuration may be required under one of the menu options. The browser may be purchased separately or may be provided by the Internet access provider.

A scroll bar allows the user to scroll through the document, forward or backward. Because there is no limit to how wide or small a hypertext/hypermedia document can be, scroll bars are often needed when the document is larger than the viewing window.

Usually, the first document on the screen is a home page. This is a special document that is intended to be viewed first. It contains an introduction of the information displayed and/or a master menu of the documents contained within this collected set of topics. Home pages are generally associated with a particular site, person, or named collection. Other interrelated documents are hyperlinked to other web pages.

Typically, clicking on the word (or link) with a mouse will cause another document to appear on the screen, which may hold more images and/or hyperlinks to other places. Some browsers represent text that is linked to other things by underlining or by using special colors. Images, also known as inline images, can be displayed within a page.

Users often create their own personal documents with collections of their favorite links or biographical information and make them publicly available. Usually called home pages (they are a virtual “home” for the user), they may also be called personal pages or hyplans (hypermedia plans).

In the display screen, there is also a set of navigation buttons. A user might go to many different screens by selecting multiple hyperlinks; these buttons provide a method for retracing the user’s steps and reviewing the documents that were previously explored.

The Back button brings the user to previously viewed documents. The forward button will bring the user to the page most recently viewed prior to taking the backward steps.

An open button allows the user to connect to other documents and networked resources by specifying the address of the desired document or resource. The user might be able to connect to a document stored locally on the machine currently being used or to one stored in another country. Such a document is normally transferred over the Internet in its entirety. Most browsers have a cache setting to allow faster access to these documents once they have been visited.

The print button allows the user to print out the document that is on the screen. The user may be given the choice of printing the document with images and formatting as seen on the screen or as a text-only document.

Typically, a person who is in charge of administrating a World Wide Web site is listed at the bottom of a home page. Any problems with the hyperlinks, images, documents, or questions about the site can be mailed to this Webmaster’s address.

Writing a web page in HTML

Web pages are written in a language called HTML (Hyper Text Markup Language). HTML allows users to produce web pages that include text, graphics, and pointers to other web pages.

The WWW distributes information and supports links to resources via Web pages. These documents can incorporate formatted text, color graphics, digitized sound, and digital video clips. Hypertext Markup language is the language used to make these pages become whatever the user intends them to be. HTML is used to display text, graphics, sounds, movies etc., over the internet on the WWW. WWW is an information system that links data from many different Internet services under one set of protocols. Web clients, called browsers or viewers, interpret Hypertext Markup Language documents delivered from Web servers. The WWW is a distributed, multimedia, hypertext system. It is distributed since information on the web can be located on any computer system connected to the Internet around the world. It is multimedia because the information it holds can be in the form of text, graphics, sound or even video. Hypertext means that the information is available using hypertext technique, which involves selecting highlighted phrases or images that, once selected, retrieve information related to the selected highlighted subject. The information being retrieved can be information located anywhere in the world. The normal way to provide information on the WWW is by writing documents in HTML.

HTML is designed to specify the logical organization of a document, with hypertext extensions. It achieves that goal by the use of instructions known as tags. HTML documents are in plain (ASCII) text format that contains embedded HTML tags. Document can be created in any text editor, including editors in a graphical environment (WYSIWYG). There are also many other tools including editors, designed specifically to assist in creating HTML documents. HTML defines the structural elements in a document (such as headers, citations, and addresses), layout information (bold and italics), and the use of inline graphics together with the ability to provide hypertext links. To view an HTML document, the user needs a browser. The browser interprets the instructional tags and presents the HTML document.

A proper Web page consist of a head and a body enclosed by and tags (formatting commands). The head is bracketed by the and tags and the body is bracketed by the and tags. The command inside the tags are called directives. Most HTML tags have this format, that is, to mark the beginning of something and to mark its end. Tags can be in either lowercase or uppercase. HTML parsers ignore extra spaces and carriage returns since they have to reformat the text to make it fit the current display area.

Tables can also be created in HTML whose entries could be clicked on to active hyperlinks. An HTML table consists of one or more rows, each consisting of one or more cells. Cells can contain a wide range of material, including text, figures, and even other tables. Cells can be merged, so, for example, a heading can span multiple columns.

HTML also provides forms for two-way traffic. Forms contain boxes or buttons that allow users to fill in information or make choices and then send the information back to the page’s owner.

CGI (Common Gateway Interface) is a standard for handling form’s data. Suppose that someone has a database (e.g. an index of web pages by keyword and topic) and wants to make it available to web users. The CGI way to make the database available is to write a script (a program) that interfaces (i.e. gateway) between the database and the web. This script is given a URL, by convention in the directory cgi-bin. HTTP servers know that when they have to invoke a method on a page located in cgi-bin, they are to interpret the file name as being an executable script or program and start it up. CGI scripts can also produce output and do many other things as well as accepting input from forms.

HTML makes it possible to describe how static web pages should appear, including tables and pictures. With the cgi-bin hack, it is also possible to have a limited amount of two-way interaction (forms, etc). However, rapid interaction with web pages written in HTML is not possible. To make it possible to have highly interactive web pages, Java language and interpreter is used. The main idea of using Java for interactive web pages is that a web page can point to a small Java program called an applet. When the browser reaches it, the applet is downloaded to the client machine and executed there in a secure way. Thus applets allow web pages to become interactive. For e.g., a game playing program (chess, tic-tac-toe etc.) written in Java can be downloaded along with its web page. Complex forms (e.g. spreadsheets) can be displayed, the users filling in items and seeing calculations made instantly. Applets also make it possible to add animation and sound to web pages.

URLs – UNIFORM RESOURCE LOCATORS

Web pages may contain pointers to other web pages. When the web was first created, it was immediately apparent that having one page point to another web page required mechanisms for naming and locating pages. In particular, there were three questions that had to be answered before a selected page could be displayed:

1. What is the page called?
2. Where is the page located?
3. How can the page be accessed?

If every page were somehow assigned a unique name, there would not be any ambiguity in identifying pages. Nevertheless, the problem would not be solved.

The solution chosen identifies pages in a way that solves all three problems at once. Each page is assigned a URL (Uniform Resource Locator) that effectively serves as the page’s worldwide name.

URLs have three parts:

• The protocol  (also called a scheme).
• The DNS name of the machine on which the page is located.
• A local name uniquely indicating the specific page (usually just a file name on the machine where it resides).

For example, a URL can be

http:/www.cs.ku.in/welcome.html

This URL consists of three parts:

• The protocol (http)
• The DNS name of the host (www.cs.ku.in)
• The file name (welcome.html)

with certain punctuation separating the pieces.

To make a piece of text clickable, the page writer must provide two items of information: the clickable text to be displayed and the URL of the page to go if the text is selected. When the text is selected, the browser looks up the host name using DNS. Now armed with the host’s IP address, the browser then establishes a TCP connection to the host. Over that connection, it sends the file name using the specified protocol and the page comes.

This URL scheme is open-ended in the sense that it is straightforward to have protocols other than HTTP. In fact, URLs for various other common protocols have been defined, and many browsers understand them. Slightly simplified forms of the more common ones are listed below:

The http protocol is the web’s native language, the one spoken by HTTP servers.The ftp protocol is used to access files by FTP, the Internet’s file transfer protocol. Numerous FTP servers all over the world allow people anywhere on the Internet to log in and download whatever files have been placed on the FTP server. The Web does not change this; it just makes obtaining files by FTP easier.

It is possible to access a local file as a Web page, either by using the file protocol, or more simply, by just naming it. This approach is similar to using FTP but does not require having a server.

The news protocol allows a web user to call up a news article as though it were a Web page. This means that a Web browser is simultaneously a newsreader. Many browsers have buttons or menu items to make reading USENET news even easier than using standard news readers.

Two formats are supported for the news protocol. The first format specifies a newsgroup and can be used to get a list of articles from a preconfigured news site. The second one requires the identifier of a specific news article to be given. The browser then fetches the given article from its preconfigured news site using the NNTP protocol.

The gopher protocol is used by the Gopher system. It is an information retrieval scheme, conceptually similar to the Web itself, but supporting only text and no images. When a user logs into a Gopher server, he is presented with a menu of files and directories, any of which can be linked to another Gopher menu anywhere in the world.

The last two protocols do not really have the flavor of fetching Web pages, and are not supported by all browsers. The mailto protocol allows users to send email from a Web browser. The way to do this is to click on the OPEN button and specify a URL consisting of mailto: followed by the recipient’s email address.

The telnet protocol is used to establish an on-line connection to a remote machine. It is used in the same way as the telnet program.

In short, the URLs have been designed to not only allow users to navigate the Web, but to deal with FTP, news, Gopher, email, and telnet as well, making all the specialized user interface programs for those other services unnecessary, and thus integrating nearly all Internet access into a single program, the Web browser.