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CS101 Introduction to Computing Lecture 30 Internet Services

Explore the history and evolution of the Internet, key characteristics, addressing schemes, and services such as FTP, Telnet, Web, email, instant messaging, and VoIP.

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CS101 Introduction to Computing Lecture 30 Internet Services

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  1. CS101 Introduction to ComputingLecture 30Internet Services

  2. 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

  3. 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

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

  5. 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

  6. Key Characteristics (2) Universal Access Same functionality to everyone Growth Rate The fastest growing technology ever Freedom of Speech Promotes freedom of speech The Digital Advantage Is digital: can correct errors

  7. 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 Internet: Network of Networks

  8. 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

  9. 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

  10. 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

  11. 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

  12. 1990's 1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced Their GUI makes WWW & Internet more appealing to the general public

  13. 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

  14. 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

  15. 203.215.177.33www.vu.edu.pk DNS address IP address

  16. 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)

  17. server ? client

  18. 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

  19. 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

  20. 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

  21. DNS: Domain Name System (2) • Tables of DNs & IP addresses are distributed throughout the Internet on numerous servers • There is a DNS serverat 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

  22. Internet Services There are many, but we will look at only the following: • FTP • Telnet • Web • eMail • Instant messaging • VoIP

  23. 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

  24. 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

  25. 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

  26. Protocol Identifier Server Address Directory & File Name http://www.vu.edu.pk/cs/index.html

  27. How does the Web work?

  28. User launches the browser on his/her computer User’s Computer Browser

  29. User types in the URL into the browser User’s Computer

  30. The browser breaks down the URL User’s Computer http://www.vu.edu.pk/cs/index.html cs/index.html Directory & File Name www.vu.edu.pk Server’s Name http Protocol Identifier

  31. Browser sends server’s name to the DNS server Domain Name User’s Computer DNS Server IP Address

  32. Browser establishes a connection with the server User’s Computer Internet Web Server

  33. Browser sends a ‘GET’ request for cs/index.html User’s Computer Web Server

  34. Server sends the requested file to the browser User’s Computer Web Server

  35. Browser displays index.html User’s Computer X

  36. 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

  37. How does an eMail system work?

  38. But first, the components: • eMail client • SMTP server • POP3 server

  39. eMail Clients • Programs used for writing, sending, receiving, and displaying eMail messages • Examples: Outlook, Communicator, Hotmail, YahooMail

  40. SMTP: Simple Mail Transfer Protocol A protocol used to send and receive eMail messages over a TCP/IP network

  41. 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

  42. The message is prepared using the eMail client Sender’s Computer eMail Client

  43. The eMail client sends it to the SMTP server Sender’s Computer SMTP Server

  44. If the receiver is local, it goes to the POP3 server Sender’s Computer POP3 Server SMTP Server

  45. Receiver's Computer The receiver picks it at his/her convenience Sender’s Computer POP3 Server SMTP Server

  46. Otherwise, it is sent to receiver's SMTP server Sender’s Computer SMTP Server Internet SMTP Server

  47. Which forwards it to the local POP3 server Sender’s Computer SMTP Server POP3 Server SMTP Server

  48. Receiver's Computer The receiver picks it at his/her convenience Sender’s Computer SMTP Server POP3 Server SMTP Server

  49. 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

  50. 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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