This document provides an overview of various internet services including FTP, Telnet, the World Wide Web, email, instant messaging, and VoIP. It begins with an introduction to internet addressing using IP addresses and domain names. It then explains how each service works at a high level, covering topics like how files are transferred using FTP, how browsing the web works, how email is sent and received, how instant messaging allows for real-time communication, and how voice calls can be made over the internet. The goal is to familiarize students with the key internet services and underlying addressing scheme.

1. 1
CS101 Introduction to Computing
Lecture 30
Internet Services

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

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

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
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
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)

17. 17
??clientclient
serverserver

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

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
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
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
Protocol
Identifier
Server
Address
Directory &
File Name

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How does the Web work?

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

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

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

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

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

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

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

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

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

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How does an eMail system work?

38. 38
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

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

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

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

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

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

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

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

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

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

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
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
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

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
How instant messaging works?

55. 55
User launches the IM client
My Computer
IM Client
Internet

56. 56
IM client finds the IM server & logs in
My Computer IM Server

57. 57
It sends communication info (IP address, etc) to
the IM server
My Computer IM Server
Temporary
File

58. 58
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’s
Computer

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
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
My Computer IM Server
Contact
A’s Computer
Multiple, simultaneous conversations are
possible
Contact
B’s Computer

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
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

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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

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
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
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

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
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