CS101- Introduction to Computing- Lecture 30

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CS101 Introduction to Computing

Lecture 30Internet Services

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During the last lecture …(Introduction to the Internet)

• We looked at the role Internet plays in today’s computing

• We reviewed some of the history and evolution of the Internet

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Internet: The Enabler

• Enables attractively-priced workers located in Pakistan to provide services to overseas clients

• Enables users to easily share information with others located all over the world

• Enables users to easily, inexpensively communicate with others remote users

• Enables the users to operate and run programs on computers located all over the world

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The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all of the humankind.

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Key Characteristics (1)

Geographic Distribution

Global - reaches around the world

Robust Architecture

Adapts to damage and error

Speed

Data can travels at near ‘c’ on copper, fiber, airwaves

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Key Characteristics (2)

Universal AccessSame functionality to everyone

Growth RateThe fastest growing technology ever

Freedom of SpeechPromotes freedom of speech

The Digital AdvantageIs digital: can correct errors

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Internet: Network of Networks

• A large number of networks, interconnected physically

• Capable of communicating and sharing data with each other

• From the user’s point view, Internet – a collection of interconnected networks – looks like a single, unified network

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TCP/IP (2)Transmission Control Protocol/Internet Protocol

• TCP breaks down the message to be sent over the Internet into packets

• IP routes these packets through the Internet to get them to their destination

• When the packets reach the destination computer, TCP reassembles them into the original message

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1960's

1969 - DoD-ARPA creates an experimental network – ARPANET – as a test-bed for emerging networking technologies

ARPANET originally connected 4 universities & enabled scientists to share info & resources across long distances

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1980's

1983 - The TCP/IP protocols becomes the only set of protocols used on the ARPANET

This sets a standard for all networks, and generates the use of the term Internet as the net of nets

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1990's

1993 - CERN releases WWW, developed by Tim Berners-Lee

It uses HTTP and hypertext, revolutionizing the way info is presented & accessed on Internet

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1990's

1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced

Their GUI makes WWW & Internet more appealing to the general public

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Today’s Goal: Internet Services

• To look at several services provided by the Internet– FTP– Telnet– Web– eMail– Instant messaging– VoIP

But first, we need to find out about the addressing scheme used on the Internet

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Internet Addressing

• Regular post cannot be delivered unless we write a destination address on the envelope

• Same is true for the Internet

• Regular post can be delivered at the intended address even if the given address is not precise. That is not the case for Internet addressing

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203.215.177.33

www.vu.edu.pk

IP addressDNS address

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IP Address (1)

• A unique identifier for a computer on a TCP/IP network

• Format: four 8-bit numbers separated by periods. Each 8-bit number can be 0 to 255

• Example:– 203.215.177.33 (IP address of the VU Web server)

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??clientclient

serverserver

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IP Address (2)

• Networks using TCP/IP route messages based on the IP address of the destination

• Any IP addresses (as long as they are unique) can be assigned within a PN

• However, connecting a PN to the Internet requires using unique, registered IP addresses

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Domain Names

• IP addresses are fine for computers, but difficult to recognize and remember for humans

• A domain name is a meaningful, easy-to-remember ‘label’ for an IP address

• Examples:

203.215.177.33 www.vu.edu.pk

216.239.33.101 www.google.com

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DNS: Domain Name System (1)

• DNS is the way that Internet domain names are located & translated into IP addresses

• Maintaining a single, central table of domain name/IP address relationships is impractical– Billions of DNS-IP translations take place every day– The DNS-IP tables get updated continuously

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DNS: Domain Name System (2)

• Tables of DNs & IP addresses are distributed throughout the Internet on numerous servers

• There is a DNS server at most ISPs. It converts the domain names in our Internet requests to actual IP addresses

• In case it does not have a particular domain name in its table, it makes a request to another DNS server on the Internet

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Internet Services

There are many, but we will look at only the following:

• FTP

• Telnet

• Web

• eMail

• Instant messaging

• VoIP

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FTP: File Transfer Protocol

• Used to transfer files between computers on a TCP/IP network (e.g Internet)

• Simple commands allow the user to:– List, change, create folders on a remote computer

– Upload and download files

• Typical use: Transferring Web content from the developer’s PC to the Web server

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Telnet Protocol

• Using Telnet, a user can remotely log on to a computer (connected to the user’s through a TCP/IP network, e.g. Internet) & have control over it like a local user, including control over running various programs

• In contrast, FTP allows file operations only

• Typical use: Configuring and testing of a remote Web server

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The Web

• The greatest, shared resource of information created by humankind

• A user may access any item on the Web through a URL, e.g.

http://www.vu.edu.pk/cs/index.html

• Before, going any further, let us dissect this URL

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ProtocolIdentifier

ServerAddress

Directory & File Name

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How does the Web work?

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User’sComputer

User launches the browser on his/her computer

Browser

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User’sComputer

User types in the URL into the browser

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User’sComputer

The browser breaks down the URL


httpProtocolIdentifier

www.vu.edu.pkServer’s Name

cs/index.htmlDirectory &File Name

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User’sComputer

DNSServer

Browser sends server’s name to the DNS server

Domain Name

IP Address

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User’sComputer

WebServer

Internet

Browser establishes a connection with the server

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User’sComputer

WebServer

Browser sends a ‘GET’ request for cs/index.html

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User’sComputer

WebServer

Server sends the requested file to the browser

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User’sComputer

Browser displays index.html

X

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eMail

• Computer-to-computer messaging

• Inexpensive, and quite quick, but not instant!

• The most popular service on the Internet, even more than surfing, but soon to be overtaken by instant messaging

• Billions are sent every day

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How does an eMail system work?

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But first, the components:

• eMail client

• SMTP server

• POP3 server

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eMail Clients

• Programs used for writing, sending, receiving, and displaying eMail messages

• Examples: Outlook, Communicator, Hotmail, YahooMail

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SMTP: Simple Mail Transfer Protocol

A protocol used to send and receive eMail messages over a TCP/IP network

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POP3: Post Office Protocol

• A protocol used for receiving eMail messages

• A POP3 server maintains text files (one file per user account) containing all messages received by a user

• eMail client interacts with the POP3 server for discovering and downloading new eMail messages

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Sender’sComputer

The message is prepared using the eMail client

eMail Client

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Sender’sComputer

SMTPServer

The eMail client sends it to the SMTP server

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Sender’sComputer

SMTPServer

POP3Server

If the receiver is local, it goes to the POP3 server

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Sender’sComputer

POP3Server

The receiver picks it at his/her convenience

Receiver'sComputer

SMTPServer

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Sender’sComputer

SMTPServer

SMTPServer

Internet

Otherwise, it is sent to receiver's SMTP server

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Sender’sComputer

SMTPServer

POP3Server

SMTPServer

Which forwards it to the local POP3 server

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Sender’sComputer

SMTPServer

POP3Server

SMTPServer

The receiver picks it at his/her convenience

Receiver'sComputer

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The Trouble with eMail

• Slow response times

• No way of knowing if the person we are sending eMail to is there to read it

• The process of having a conversation through eMail by exchanging several short messages is too cumbersome

Instant messaging (IM) solves these problems

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Instant Messaging

• The IM services available on the Internet (e.g. ICQ, AIM, MSN Messenger, Yahoo! Messenger) allow us to maintain a list of people (contacts) that we interact with regularly

• We can send an instant messages to any of the contacts in our list as long as that contact is online

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Using Instant Messaging (1)

• Whenever a contact in our list comes online, the IM client informs us through an alert message and by playing a sound

• To send an instant message to a contact, just click on the contact in the IM client, and start typing the message

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• The selected contact will receive that message almost immediately after you press ‘Enter’

• When the contact’s IM client receives the message, it alerts the contact with a blinking message and by playing a sound

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• That contact then can type a response to the received message, and send it instantly

• Several such conversations can be carried out simultaneously, each occupying a separate IM windows

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How instant messaging works?

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User launches the IM client

My Computer

IM Client

Internet

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IM client finds the IM server & logs in

My Computer IM Server

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It sends communication info (IP address, etc) to the IM server


Temporary File

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IM server finds user’s contacts & sends him/her the communication info for the ones online


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IM server also tells the contacts that the user is online; sends his/her communication info to them


Contact’sComputer

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Contact’sComputer

Now the user’s & the contact’s IM clients are ready to communicate directly (P2P)

The IM server doesn’t play any part in this P2P communication

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ContactA’s Computer

As new contact’s come online, IM server informs them about the user being online & vice versa

ContactB’s Computer

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Multiple, simultaneous conversations are possible


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When the user logs-off, his/her IM client informs the IM server


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IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status


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Key Point

• Once the IM server provides the communication info to the user and his/her contact’s IM client, the two are able to communicate with each other without the IM server’s assistance

• This server-less connection is termed as a P2P connection

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Question

• Why do we require the server in the first place?

• Why doesn’t my IM client look for the user’s contact’s IM client without the IM server’s help?

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Answer

• Many users (including almost all home users) do not have permanent IP addresses. They are assigned temporary IP addresses by their ISP each time they connect to the Internet

• The server-based IM scheme removes the need of having permanent IP numbers

• It also gives IM users true mobility, allowing them the use of IM from any Internet-connected computer

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VoIP: Voice over IP

• Voice delivered from one device to another using the Internet Protocol

• Voice is first converted into a digital form, is broken down into packets, and then transmitted over a TCP/IP network (e.g. Internet)

• Four modes:– C2C– C2T– T2C– T2T (with a TCP/IP net somewhere in between)

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Pro

Much cheaper than traditional phone service

Con

Noticeably poor quality of voice as compared with land-line phone service, but not much

worse than cell phone service

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Today’s Goal: Internet Services

• We looked at several services provided by the Internet– FTP– Telnet– Web– eMail– Instant messaging– VoIP

• We also found out about the addressing scheme used on the Internet

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Next Lecture:

• Next lecture (Lecture 31) - the third one in the four-lecture productivity SW sequence - will be on developing presentations

• However, during lecture 33, we will become familiar with the role that graphics and animations play in computing

Education

CS101- Introduction to Computing- Lecture 30