CS101- Introduction to Computing- Lecture 30


Published on

Virtual University
Course CS101- Introduction to Computing
Lecture No 30
Internet Services

Published in: Education, Technology
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

CS101- Introduction to Computing- Lecture 30

  1. 1. 1 CS101 Introduction to Computing Lecture 30 Internet Services
  2. 2. 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. 3. 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. 4. 4 The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all of the humankind.
  5. 5. 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. 6. 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. 7. 7 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
  8. 8. 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. 9. 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. 10. 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. 11. 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. 12. 12 1990's 1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced Their GUI makes WWW & Internet more appealing to the general public
  13. 13. 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. 14. 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. 15. 15 www.vu.edu.pk IP addressDNS address
  16. 16. 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: – (IP address of the VU Web server)
  17. 17. 17 ??clientclient serverserver
  18. 18. 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. 19. 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: www.vu.edu.pk www.google.com
  20. 20. 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. 21. 21 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
  22. 22. 22 Internet Services There are many, but we will look at only the following: • FTP • Telnet • Web • eMail • Instant messaging • VoIP
  23. 23. 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. 24. 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. 25. 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. 26. 26 http://www.vu.edu.pk/cs/index.html Protocol Identifier Server Address Directory & File Name
  27. 27. 27 How does the Web work?
  28. 28. 28 User’s Computer User launches the browser on his/her computer Browser
  29. 29. 29 User’s Computer User types in the URL into the browser
  30. 30. 30 User’s Computer The browser breaks down the URL http://www.vu.edu.pk/cs/index.html http Protocol Identifier www.vu.edu.pk Server’s Name cs/index.html Directory & File Name
  31. 31. 31 User’s Computer DNS Server Browser sends server’s name to the DNS server Domain Name IP Address
  32. 32. 32 User’s Computer Web Server Internet Browser establishes a connection with the server
  33. 33. 33 User’s Computer Web Server Browser sends a ‘GET’ request for cs/index.html
  34. 34. 34 User’s Computer Web Server Server sends the requested file to the browser
  35. 35. 35 User’s Computer Browser displays index.html X
  36. 36. 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. 37. 37 How does an eMail system work?
  38. 38. 38 But first, the components: • eMail client • SMTP server • POP3 server
  39. 39. 39 eMail Clients • Programs used for writing, sending, receiving, and displaying eMail messages • Examples: Outlook, Communicator, Hotmail, YahooMail
  40. 40. 40 SMTP: Simple Mail Transfer Protocol A protocol used to send and receive eMail messages over a TCP/IP network
  41. 41. 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. 42. 42 Sender’s Computer The message is prepared using the eMail client eMail Client
  43. 43. 43 Sender’s Computer SMTP Server The eMail client sends it to the SMTP server
  44. 44. 44 Sender’s Computer SMTP Server POP3 Server If the receiver is local, it goes to the POP3 server
  45. 45. 45 Sender’s Computer POP3 Server The receiver picks it at his/her convenience Receiver's Computer SMTP Server
  46. 46. 46 Sender’s Computer SMTP Server SMTP Server Internet Otherwise, it is sent to receiver's SMTP server
  47. 47. 47 Sender’s Computer SMTP Server POP3 Server SMTP Server Which forwards it to the local POP3 server
  48. 48. 48 Sender’s Computer SMTP Server POP3 Server SMTP Server The receiver picks it at his/her convenience Receiver's Computer
  49. 49. 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. 50. 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
  51. 51. 51 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
  52. 52. 52 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
  53. 53. 53 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
  54. 54. 54 How instant messaging works?
  55. 55. 55 User launches the IM client My Computer IM Client Internet
  56. 56. 56 IM client finds the IM server & logs in My Computer IM Server
  57. 57. 57 It sends communication info (IP address, etc) to the IM server My Computer IM Server Temporary File
  58. 58. 58 IM server finds user’s contacts & sends him/her the communication info for the ones online My Computer IM Server
  59. 59. 59 IM server also tells the contacts that the user is online; sends his/her communication info to them My Computer IM Server Contact’s Computer
  60. 60. 60 My Computer IM Server Contact’s Computer 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
  61. 61. 61 My Computer IM Server Contact A’s Computer As new contact’s come online, IM server informs them about the user being online & vice versa Contact B’s Computer
  62. 62. 62 My Computer IM Server Contact A’s Computer Multiple, simultaneous conversations are possible Contact B’s Computer
  63. 63. 63 My Computer IM Server Contact A’s Computer When the user logs-off, his/her IM client informs the IM server Contact B’s Computer
  64. 64. 64 My Computer IM Server Contact A’s Computer IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status Contact B’s Computer
  65. 65. 65 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
  66. 66. 66 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?
  67. 67. 67 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
  68. 68. 68 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)
  69. 69. 69 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
  70. 70. 70 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
  71. 71. 71 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