Examples: Outlook, Communicator, Hotmail, YahooMail. 40

1
CS101 Introduction to ComputingLecture
30Internet Services
2
During the last lecture (Introduction to the
Internet)

• We looked at the role Internet plays in todays
  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

• Internet Network of Networks

• A large number of networks, interconnected
  physically
• Capable of communicating and sharing data with
  each other
• From the users point view, Internet a
  collection of interconnected networks looks
  like a single, unified network

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

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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

12

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

13
Todays 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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203.215.177.33www.vu.edu.pk
IP address
DNS 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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server
?
client
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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

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

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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

22
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
  developers 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 users 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

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How does the Web work?
28
User launches the browser on his/her computer
Users Computer
Browser
29
User types in the URL into the browser
Users Computer
30
The browser breaks down the URL
Users Computer
http//www.vu.edu.pk/cs/index.html
cs/index.html Directory File Name
www.vu.edu.pk Servers Name
http Protocol Identifier
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Browser sends servers name to the DNS server
Domain Name
Users Computer
DNS Server
IP Address
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Browser establishes a connection with the server
Users Computer
Internet
Web Server
33
Browser sends a GET request for cs/index.html
Users Computer
Web Server
34
Server sends the requested file to the browser
Users Computer
Web Server
35
Browser displays index.html
Users Computer
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

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

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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
Senders Computer
eMail Client
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The eMail client sends it to the SMTP server
Senders Computer
SMTP Server
44
If the receiver is local, it goes to the POP3
server
Senders Computer
POP3 Server
SMTP Server
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The receiver picks it at his/her convenience
Senders Computer
POP3 Server
SMTP Server
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Otherwise, it is sent to receiver's SMTP server
Senders Computer
SMTP Server
Internet
SMTP Server
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Which forwards it to the local POP3 server
Senders Computer
SMTP Server
POP3 Server
SMTP Server
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The receiver picks it at his/her convenience
Senders Computer
SMTP Server
POP3 Server
SMTP Server
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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

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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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 contacts 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
IM Client
Internet
My Computer
56
IM client finds the IM server logs in
My Computer
IM Server
57
It sends communication info (IP address, etc) to
the IM server
Temporary File
My Computer
IM Server
58
IM server finds users 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
60
Now the users the contacts IM clients are
ready to communicate directly (P2P)
Contacts Computer
My Computer
IM Server
The IM server doesnt play any part in this P2P
communication
61
As new contacts come online, IM server informs
them about the user being online vice versa
Contact As Computer
My Computer
IM Server
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Multiple, simultaneous conversations are possible
Contact As Computer
My Computer
IM Server
Contact Bs Computer
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When the user logs-off, his/her IM client informs
the IM server
Contact As Computer
My Computer
IM Server
Contact Bs Computer
64
IM server erases the temporary file and informs
the users contacts about his/her offline
status
Contact As Computer
My Computer
IM Server
Contact Bs Computer
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Key Point

• Once the IM server provides the communication
  info to the user and his/her contacts IM client,
  the two are able to communicate with each other
  without the IM servers 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 doesnt my IM client look for the users
  contacts IM client without the IM servers 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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Todays 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
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