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Introduction to Computing

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Introduction to Computing ­ CS101
VU
LECTURE 30
INTERNET SERVICE
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
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
The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all
of the humankind.
Key Characteristics
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
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
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
TCP/IP 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
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
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
1990's1993 - CERN releases WWW, developed by Tim Berners-Lee
It uses HTTP and hypertext, revolutionizing the way info is presented & accessed on Internet
1990's1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced
Their GUI makes WWW & Internet more appealing to the general public
Today's Goal: Internet Services
To look at several services provided by the Internet
­FTP
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User's
Computer
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Introduction to Computing ­ CS101
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­Telnet
­Web
­eMail
­Instant messaging
­VoIP
·
But first, we need to find out about the addressing scheme used on the Internet
30.1 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
IP address
DNS address
www.vu.edu.pk
203.215.177.33
IP Address
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)
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
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
216.239.33.101
www.google.com
30.2 DNS: Domain Name System
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
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
30.3 Internet Services
There are many, but we will look at only the following:
FTP
Telnet
Web
eMail
Instant messaging
VoIP
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
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Directory &
File Name
Introduction to Computing ­ CS101
VU
Typical use: Transferring Web content from the developer's PC to the Web server
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
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
/cs/index.html
http
:// www.vu.edu.pk
Server
Address
Protocol
Identifier
How does the web works
User launches the browser on his/her computer
Browser
User's
Computer
User types in the URL into the browser
The browser breaks down the URL into 3 parts :
Protocol Identifier
User's
Server Address
Computer
Directory & File Name
Browser sends server's name to the DNS server
Domain Name
DNS
User's
Server
Computer
Browser establishes a connection with the server
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Browser sends a `GET' request for cs/index.html
User's
Computer
Internet
Web
Server
Server sends the requested file to the browser
User's
Computer
Web
Browser displays index.html
Server
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
30.3 How does an eMail system work?
But first, the components:
eMail client
User's
SMTP server
Computer
POP3 server
eMail Clients
Programs used for writing, sending, receiving, and displaying eMail messages
Examples: Outlook, Communicator, Hotmail, YahooMail
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
The message is prepared using the eMail client
Sender's
eMail Client
Computer
The eMail client sends it to the SMTP server
SMTP
Sender's
Server
Computer
If the receiver is local, it goes to the POP3 server
Sender's
POP3
Computer
SMTP
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The receiver picks it at his/her convenience
Sender's
Receiver's
Computer
Computer
POP3
Server
SMTP
Server
Otherwise, it is sent to receiver's SMTP server
Sender's
Computer
SMTP
Server
SMTP
Internet
Server
Which forwards it to the local POP3 server
Sender's
SMTP
Computer
Server
POP3
SMTP
Server
Server
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The receiver picks it at his/her convenience
Sender's
SMTP
Computer
Serve
r
Receiver'
s
SMTP
Computer
Serve
r
POP3
The Trouble with email
Serve
r
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
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
30.4 Using Instant Messaging
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
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
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?
User launches the IM client
IM Client
Internet
My Computer
IM client finds the IM server & logs in
My Computer
IM
It sends communication info (IP address, etc) to the IM server
Server
IM
My
Server
Computer
Temporary File
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
Contact's
Computer
My Computer
IM Server
Now the user's & the contact's IM clients are ready to communicate directly (P2P)
Contact's
Computer
My Computer
The IM server doesn't play any
part in this P2P communication
IM Server
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As new contact's come online, IM server informs them about the user being online & vice versa
Contact
A's Computer
My
Computer
IM Server
Contact
B's
Computer
Multiple, simultaneous conversations are possible
Contact
A's Computer
My Computer
IM
Server
Contact
B's Computer
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When the user logs-off, his/her IM client informs the IM server
Contact
A's Computer
My Computer
IM
Server
Contact
B's Computer
IM server erases the temporary file and informs the user's contact's about his/her `offline' status
Contact
A's Computer
My Computer
Contact
B's
Computer
IM Server
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
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?
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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30.5 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)
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
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
Next Lecture:
Next Lesson (Lesson 31) - the third one in the four-Lesson productivity SW sequence - will be on
developing presentations
However, during Lesson 33, we will become familiar with the role that graphics and animations play in
computing
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Table of Contents:
  1. INTRODUCTION
  2. EVOLUTION OF COMPUTING
  3. World Wide Web, Web’s structure, genesis, its evolution
  4. Types of Computers, Components, Parts of Computers
  5. List of Parts of Computers
  6. Develop your Personal Web Page: HTML
  7. Microprocessor, Bus interface unit, Data & instruction cache memory, ALU
  8. Number systems, binary numbers, NOT, AND, OR and XOR logic operations
  9. structure of HTML tags, types of lists in web development
  10. COMPUTER SOFTWARE: Operating Systems, Device Drivers, Trialware
  11. Operating System: functions, components, types of operating systems
  12. Forms on Web pages, Components of Forms, building interactive Forms
  13. APPLICATION SOFTWARE: Scientific, engineering, graphics, Business, Productivity, Entertainment, Educational Software
  14. WORD PROCESSING: Common functions of word processors, desktop publishing
  15. Interactivity to Forms, JavaScript, server-side scripts
  16. ALGORITHMS
  17. ALGORITHMS: Pseudo code, Flowcharts
  18. JavaScript and client-side scripting, objects in JavaScript
  19. Low, High-Level, interpreted, compiled, structured & object-oriented programming languages
  20. Software Design and Development Methodologies
  21. DATA TYPES & OPERATORS
  22. SPREADSHEETS
  23. FLOW CONTROL & LOOPS
  24. DESIGN HEURISTICS. Rule of thumb learned through trial & error
  25. WEB DESIGN FOR USABILITY
  26. ARRAYS
  27. COMPUTER NETWORKS: types of networks, networking topologies and protocols
  28. THE INTERNET
  29. Variables: Local and Global Variables
  30. Internet Services: FTP, Telnet, Web, eMail, Instant messaging, VoIP
  31. DEVELOPING PRESENTATIONS: Effective Multimedia Presentations
  32. Event Handlers
  33. GRAPHICS & ANIMATION
  34. INTELLIGENT SYSTEMS: techniques for designing Artificial Intelligent Systems
  35. Mathematical Functions in JavaScript
  36. DATA MANAGEMENT
  37. DATABASE SOFTWARE: Data Security, Data Integrity, Integrity, Accessibility, DBMS
  38. String Manipulations:
  39. CYBER CRIME
  40. Social Implications of Computing
  41. IMAGES & ANIMATION
  42. THE COMPUTING PROFESSION
  43. THE FUTURE OF COMPUTING
  44. PROGRAMMING METHODOLOGY
  45. REVIEW & WRAP-UP of Introduction to Computing