Download - 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
• 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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http://www.vu.edu.pk/cs/index.html
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
http://www.vu.edu.pk/cs/index.html
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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• 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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Using Instant Messaging (2)
• 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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Using Instant Messaging (3)
• 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
My Computer IM Server
Temporary File
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IM server finds user’s contacts & sends him/her the communication info for the ones online
My Computer IM Server
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IM server also tells the contacts that the user is online; sends his/her communication info to them
My Computer IM Server
Contact’sComputer
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My Computer IM Server
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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My Computer IM Server
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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My Computer IM Server
ContactA’s Computer
Multiple, simultaneous conversations are possible
ContactB’s Computer
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My Computer IM Server
ContactA’s Computer
When the user logs-off, his/her IM client informs the IM server
ContactB’s Computer
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My Computer IM Server
ContactA’s Computer
IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status
ContactB’s Computer
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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