Page 196 - DCAP406_DCAP_207_Computer Networks

Unit 12: Application Layer

7. Now, there are several possible actions: Notes
The destination machine has no more messages to transmit, it will end the connection
with a QUIT command. This command is answered with a 221 Service closing
transmission channel reply.
The destination machine has no more messages to transmit, but it is ready to receive
messages (if any) from the other side. It will issue the TURN command. The two
SMTPs now switch their role of sender/receiver and the client machine that was
previously the destination machine now transmits messages by starting with step 3
above.

The client machine wants to transmit another message and simply follows step 3 to
transmit a new MAIL command.

Task List the advantages and disadvantages of SMTP.

12.3 World Wide Web

You must be very much familiar with the term World Wide Web, which is also known as web or
WWW or W3 and has established itself as the most popular part of the Internet by far. It is an
incredible mines of information, once you start searching anything ranging from documents to
pictures to software, it almost appears limitless. It provides you documents, sound files, view
images, animation, and video, speak and hear voice, and view programs that run on practically
on any software in the world. Therefore, it facilitates the rich and diverse communication by
enabling you to access and interact with text, graphics, animation, photos, audio and video. It
has now become so simple for you to understand how the web works and what it is. Its
implementation is based on client server system and employs your personal computer as client,
web browser software, a connection to an Internet service provider, servers, routers and switches
to direct the flow of information. You may be aware of all terms used in the formation of a web
except web browser.
A browser is software, which your computer uses to view WWW documents and access the
Internet. The browser program residing in your computer facilitates you with the advantages of
text formatting, hypertext links, images, sounds, motion, and other features. Internet Explorer
and Netscape are some of the widely used browsers. Browsers have sub programs called plug-
ins to handle the documents you find on the Web. It may also other plug-ins stored elsewhere in
your computer.

Web is very simple to use. Whenever you wish to visit any website, say your institute’s website,
you simply enter the address or URL of the website in your web browser to forward your
request to the web server of the institute to provide you the intended web-page. The institute’s
web server then sends your request on the Internet to find the intended website. Once it is
obtained, the web server returns the same to your computer where the browser loaded with
different plug-ins interprets the data, displaying it on your computer screen. The intended
webpage, which is now available on your desktop, may have click able links. On clicking on the
same, you may visit other pages. In this manner, the information scattered across the globe can
be linked together.
It now becomes essential to explain as to how the different webpages with different text format
and standards could be linked to a particular web page. The binding forces that hold the Web

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