Transcript Lecture 1 Introduction to the Computers

Lecture 13
Internet & Internet Services
CSCS100 - Spring 2008 – Forman Christian College
Asher Imtiaz
Wajeeha Akram
*Several of these slides have been adapted and modified from VU CS101 slides
(Dr. Altaf A. Khan) and Peter Norton’s supplementary material.
Today’s Goal
• Internet Architecture
• Addressing scheme used on the Internet
• To look at several services provided by
the Internet
•
•
•
•
Web
eMail
Instant messaging
VoIP
What Does the Internet Enable?
• Enables users located at far-way locations
to easily share information with others
located all over the world
• Enables users to easily and inexpensively
communicate with others located all over
the world
• 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 worldwide resource,
accessible to all of the
humankind.
“Cool” internet appliances
Web-enabled toaster +
weather forecaster
IP picture frame
http://www.ceiva.com/
World’s smallest web server
http://www-ccs.cs.umass.edu/~shri/iPic.html
Internet phones
Internet Users Worldwide
673M in 2002
1B+ in 2005
(48% wireless)
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
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
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
Internet ---- Web
Internet ---- Intranet
Internet Networking Protocols
Communications on the Internet is
controlled by a set of two protocols:
TCP and IP
TCP/IP (1)
Transmission Control Protocol/Internet Protocol
• Networking protocol used by all computers
and networks on the Internet
• Originally developed by the US DoD for
Unix, but now available for most other OSes
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
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
216.122.128.109
DNS address
IP address
www.fccollege.edu.pk
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:
• 216.122.128.109 (IP address of the FCC Web
server)
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
Domain Names
• IP addresses are fine for computers, but
difficult to recognize and remember for
humans
• A domain name is a meaningful, easy-toremember ‘label’ for an IP address
• Examples:
216.122.128.109
216.239.33.101
www.fccollege.edu.pk
www.google.com
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
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
TCP/IP Settings in a Computer
Internet Services
There are many, but we will
look at only the following:
•
•
•
•
Web
eMail
Instant messaging
VoIP
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.fccollege.edu.pk/cs/index.html
• Before, going any further, let us dissect this
URL
http://www.fcc.edu.pk/cs/index.html
Protocol
Identifier
Server
Address
Directory &
File Name
How does the Web work?
User launches the browser on his/her computer
User’s
Computer
Browser
User types in the URL into the browser
User’s
Computer
The browser breaks down the URL
User’s
Computer
http://www.fcc.edu.pk/cs/index.html
www.fcc.edu.pk
Server’s Name
http
Protocol
Identifier
cs/index.html
Directory &
File Name
Browser sends server’s name to the DNS server
User’s
Computer
Domain Name
DNS
Server
IP Address
Browser establishes a connection with the server
User’s
Computer
Internet
Web
Server
Browser sends a ‘GET’ request for cs/index.html
User’s
Computer
Web
Server
Server sends the requested file to the browser
User’s
Computer
Web
Server
Browser displays index.html
User’s
Computer
X
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
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
Instant Messaging
• The IM services available on the Internet
(e.g. ICQ, GoogleTalk, MSN Messenger,
Yahoo! Messenger and Skype) 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
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
• The selected contact will receive that
message almost immediately after you
press ‘Enter’
Using Instant Messaging (2)
• 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
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 Internetconnected computer
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
Who runs the Internet?
Who owns it?