Virtual University Course CS101- Introduction to Computing Lecture No 27 Computer Networks

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CS101 Introduction to Computing
Lecture 27
Computer Networks

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During the last lecture …
(Web Design for Usability)
• We looked at the role of usability in Web
design
• We identified some of the factors affecting
the usability of a Web page

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don’t make me think!

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think roadside billboard
rather than Dewan-e-Ghalib

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Designs should be
consistent &
predictable
(unified)

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What’s a Good Site?
• The one that achieves the result that it was
designed for
• Generally, that result can only be achieved by
giving the user what s/he wants, as quickly as
possible, without her/im expending much effort
• One definition of usability: Let the user have
what s/he wants, quickly, without much effort
• “Quickly” is important!

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Website Navigation
• The interface/controls that a Website
provides to the user for accessing
various parts of the Website
• It probably is the most important aspect
of the design of a Website

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Good designs
assist the user
in recovering
from errors

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Today’s Goals:
(Computer Networks)
• We will become able to appreciate the role of
networks in computing
• We will look at several different types of
networks
• We will familiarize ourselves with networking
topologies and protocols

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All of you have used
computer networks.
What is a
computer network?

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Computer Network
Multiple computers that are
connected together to share
information and other resources

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Examples of Computer Network Usage
• I can send an eMail message to a remote
computer using the SMTP protocol
• I can browse documents residing on a remote
computer using the HTTP protocol
• I can download or upload files to a remote
computer using the FTP protocol
• I can run a program on a remote computer
using the TELNET protocol

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Computer
C
Computer
D
Computer
E
Computer
A
Computer
B
Hub
Example of
a Computer
Network

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Components of Conventional Computer Networks
1. Computers
2. Network Interface Cards (NIC)
– I/O device that plugs into the computer
– Enables it to communicate over a network
3. Hub
– The network traffic controller

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Components of Conventional Computer Networks
4. Cables
– Are either electrical or optical
– Not required at all for wireless networks
4. Protocol
– Rules governing communications over the
network

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How Does a Conventional Network Work?
1. Suppose computer A wants to send a
message to D
2. Computer A sends the message to its NIC
3. The NIC translates the message into electrical
pulses suitable for the computer network in
use & transmits it to the hub through the cable

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How Does a Conventional Network Work?
4. The hub receives them and forwards them to
all computers connected to the it
5. The NICs of all computers connected to the
hub receive the forwarded electrical pulses
6. The NIC of computer D decides that the
message is for it, & translates the pulses
back to a form suitable for the computer

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Hub (1)
• A device that is used to connect several
computers to form a network
• A hub has several ports. The number generally
is 8, 12, 16, 24, 32, or 48

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Hub (2)
• Each computer in a network is connected to
one of those ports through a cable
• A computer wanting to send a message to one
of the others in the network sends a message
to the hub, which, in turn, broadcasts the
message to all others connected to it

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Packet (1)
• The smallest unit of data transmitted over a
computer network
• A message to be transferred over the network
is broken up into small packets by the sending
computer
• Each packet contains the following info:
– Sender's address
– Destination address
– Data
– Error-recovery info

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Packet (2)
• All packets travel independently
• When all packets are received by the
destination computer, it reassembles them to
form the original message

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Types of Computer Networks
according to the network access policy
• Private
• Public

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Private Networks (1)
• Organizations having many computers usually
connect them in the form of private networks
• Access to these network is restricted to
authorized computers only

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Private Networks (2)
• This allows computers from within the
organization to exchange info, but keeps the
info private and protected from outsiders
• All equipment on a private network is generally
for the exclusive use of that organization

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Public Networks
• All networks that are not private, are … public
• Example: Internet
• Communication equipment used in these
networks is generally being used by users
belonging to several (possibly thousands of)
organizations as well as those belonging to no
organization

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VPN: Virtual Private Network (1)
• From the user’s point-of-view, a VPN looks like
a secure, private network
• VPNs use public telecom infrastructure,
maintaining privacy through security procedures

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VPN: Virtual Private Network (2)
• VPNs provide secure network connections for
distance computers without using dedicated,
private channels to supply the connection
• Key benefit of VPNs over conventional PNs:
Lower cost

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Types of Computer Networks
according to the distance between nodes
• LAN: Local Area Network)
• WAN: Wide Area Network)

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LAN
• A network of computers located in the same
building or a handful of nearby buildings
• Examples:
– Computer network at your PVC
– Computer network of a University campus

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WAN
• A network in which computers are separated by
great distances, typically across cities or even
continents
• May consist of several interconnected LANs
• Example:
– The network connecting the ATM of a bank located
in various cities
– A network connecting the local and oversea offices
of a SW house
– Internet

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Connecting LANs to other Networks
Special-purpose devices are used to link
LANs to other networks
They may belong to one of the following
categories:
– Routers
– Bridges
– Gateways
– Modems

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Router
• A special-purpose computer that directs data
traffic when several paths are available
• A router examines the destination info in each
arriving packet and then routes it through the
most efficient path available
• The router either delivers the packet to the
destination computer across a local network or
forwards the packet to another router that is
closer to the final destination

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Bridge
• Used to form a connection between two
separate, but similar networks
• In a way, it creates an extended LAN by
passing information between two or more LANs

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Gateway
• A special-purpose computer that connects and
translates between networks that use different
communications protocols
• LAN’s may use a gateway (or router) to
connect to the Internet

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Modem (1)
• I/O device used for connecting two
computers over telephone lines
• modem = modulator + demodulator

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Modem (2)
• Modulator converts computer messages to
electrical pulses that are suitable for
transmission over the telephone lines
• Demodulator converts electrical pulses
received over telephone lines into
messages that are comprehensible for
computers

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Network Topologies
• The pattern in which computers are connected
to form a network
• Popular patterns:
– Point-to-point
– Star
– Bus
– Ring
• Networks are also formed by combining 2 or
more of these 4 basic patterns

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P2P
Computer
A
Computer
B

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P2P
• Inexpensive
• Limited connectivity
• Quite often used for connecting two LANs to
form a WAN

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Star
Computer
A
Computer
D
Computer
C
Computer
B
Server

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Star (1)
• A computer sends the address of the intended
receiver and the data to the server
• The server then sends the message to the
intended receiver
• This topology allows multiple messages to be
sent simultaneously

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Star (2)
• Costly, because it uses an additional computer
to direct the data
• Costly, because each node is individually wired
to the hub
• If the server goes down, so does the network
• If any of the nodes goes down, the rest of the
network is not affected

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BusComputer
A
Computer
C
Computer
D
Computer
B
Bus: A high
speed cable

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Bus (1)
• No server is required
• One computer sends data to another by
broadcasting the address of the receiver and
the data over the bus
• All the computers in the network look at the
address simultaneously, and the intended
recipient accepts the data

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Bus (2)
• A bus network, unlike ring or star networks,
allows data to be sent directly from one
computer to another
• However, only one computer at a time can
transmit data. The others must wait to until the
bus gets idle
• If any of the nodes goes down, the rest of the
network is not affected

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Ring
Computer
A
Computer
D
Computer
C
Computer
B

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Ring (1)
• No server is required
• A computer sends the message to its neighbor.
The neighbor examines the message to
determine if it is the intended recipient
• If the data are not intended for that particular
neighbor, it passes the message to the next
computer in the ring

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Ring (2)
• This process is repeated until the data arrive at
their intended recipient
• This topology allows multiple messages to be
carried, simultaneously
• Data transmission is slow since each message
is checked by each computer

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Ring (3)
• New nodes are difficult to add
• Messages propagate in one direction only
• The network fails if a single node fails

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Combination
Computer
B
Computer
A
Computer
C
Hub
Computer
E
Computer
D
Computer
F
Hub

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Networking Protocols
• Networks use protocols, or rules, to exchange
info through shared channels
• Protocols prevent collisions of packets caused
by simultaneous transmission between two or
more computers
• Several protocols are available for various
types of networks. Here we discuss two that
are popular for LANs: Ethernet; Token Ring

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Ethernet Protocol
• A computer using this protocol checks if a
shared connection is in use before transmitting
a message
• If not, the computer transmits data
• Two computers may sense an idle connection
and may send packets simultaneously. To
account for such situations, transmitting
computers continue to monitor the connection
and re-transmit if a packet collision occurs

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Token Ring Protocol
• This protocol passes a special message called
a token through the network
• A computer that receives the token is given
permission to send a packet of information
• If the computer has no packet to send, it
passes the token to the next computer

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Computer Networks
=
Computers
+
Communications

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Types of Communication Channels
1. Wire
2. Wireless
A key characteristic of these channels is bandwidth

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Bandwidth
• Capacity of a communication channel for
carrying data
• Measured in bits/s (bps), kb/s, Mb/s, Gb/s, Tb/s
• Optical fiber channels have the highest (1 Tb/s)
• Telephone lines the lowest (56 kb/s)

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Types of Communication Channels
Wire
–Copper
• Twisted-pair
• Coaxial cable
–Optical fiber
Wireless
–Line-of-sight
• Microwave
• Optical
–Non-line-of-sight
• Satellite
• Radio
• Cellular

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Wireless (Radio) LANs Are Becoming Popular
Key benefits:
– Set-up time
– Set-up cost
– Maintenance cost
– Cost Key challenges:
– Security & privacy
– Quality of service
– Cost

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Network Security (1)
• Keeping an eye on the security of private
networks (e.g. LANs) is relatively easy
• However, their connections to other networks
(e.g. the Internet) pose a security risk because
the one has no control over users on those
networks

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Network Security (2)
• Applications transferred from the Internet to the
LAN may contain computer viruses
• External, unauthorized users may gain access
to sensitive data
• A special type of gateway - a firewall – can
keep external users from accessing resources
on the LAN while letting LAN users access the
external info

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Firewall
• A system that that guards a private network,
enforcing an access/deny policy to all traffic
going to and coming from the Internet
• It keeps an eye on all the packets that go in
and out of the private network and blocks them
or allows them to continue to their destination
according to the policy

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Private
NetworkInternet
Firewall

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Firewall Policy: Example
One can configure a firewall to allow only
eMail to enter the private network, thus
shielding it from any malicious attacks
except for those via eMail

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In Today’s Lecture
• We looked at the role of networks in
computing
• We looked at several different types of
networks
• We familiarized ourselves with networking
topologies and protocols

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Next Lecture:
Introduction to the Internet
• To become able to appreciate the role of the
Internet in today’s computing
• To become familiar with the history and
evolution of the Internet