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

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CS101 Introduction to Computing
Lecture 30
Internet 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 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

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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
• eMail
• 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
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

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

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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

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User’s
Computer
Browser displays index.html
X

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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?

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

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

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

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

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

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

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

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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’s
Computer

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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

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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

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

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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

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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

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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