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Introduction to Computing

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Introduction to Computing ­ CS101
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LESSON 27
COMPUTER NETWORKS
During the last Lesson ...
(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
Designs should be consistent & predictable (unified)
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!
Website Navigation
It probably is the most important aspect of the design of a Website
Good designs assist the user in recovering from errors
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
Computer Network
Multiple computers that are connected together to share information and other resources
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
Example of a Computer
Network
Computer
E
Computer
A
Computer
Hub
D
Computer
Computer
B
C
Components of Conventional Computer Networks
1.  Computers
2.  Network Interface Cards (NIC)
­I/O device that plugs into the computer
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Introduction to Computing ­ CS101
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­Enables it to communicate over a network
3.  Hub
­The network traffic controller
Components of Conventional Computer Networks
4.Cables
­Are either electrical or optical
­Not required at all for wireless networks
5.Protocol
­Rules governing communications over the network
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
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
Hub
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
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
Packet
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
All packets travel independently
When all packets are received by the destination computer, it reassembles them to form the original
message
Types of Computer Networks
according to the network access policy
Private
Public
27.1 Private Networks
Organizations having many computers usually connect them in the form of private networks
Access to these network is restricted to authorized computers only
Organizations having many computers usually connect them in the form of private networks
Access to these network is restricted to authorized computers only
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
27.2 Public Networks
All networks that are not private, are ... public
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Introduction to Computing ­ CS101
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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
27.3 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
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
Types of Computer Networks
according to the distance between nodes
LAN: Local Area Network)
WAN: Wide Area Network)
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
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
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
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
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
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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Introduction to Computing ­ CS101
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Modem
I/O device used for connecting two computers over telephone lines
modem = modulator + demodulator
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
27.4 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
P2P
Inexpensive
LimitedComnecttivity
con pu er
Computer
B
A
Quite often used for connecting two LANs to form a WAN
Star
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
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
Computer
D
Star
Computer
Computer
A
C
Server
Computer
B
Bus
No server is required
One computer sends data to another by broadcasting the address of the receiver and the data over the
bus
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Introduction to Computing ­ CS101
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All the computers in the network look at the address simultaneously, and the intended recipient accepts
the data
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
Bus
Computer
Computer
Computer
B
C
A
Computer
Bus: A high speed
D
Ring
cable
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
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
New nodes are difficult to add
Messages propagate in one direction only
Compu
The network fails if a single node fails
Compu
ter
Compu
Compu
ter
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Introduction to Computing ­ CS101
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Combination
Computer
Computer
D
A
Hub
Computer
Hub
Computer
B
E
Computer
C
Computer
F
27.5 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
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
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
Computer Networks = Computers + Communications
Types of Communication Channels
1 . Wire
2 . Wireless
A key characteristic of these channels is bandwidth
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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27.6 Types of Communication Channels
Wire
Wireless
­
­
Copper
Line-of-sight
 Twisted-
 Microwa
pair
ve
 Coaxial
 Optical
cable
­
Non-line-of-
­
Optical fiber
sight
 Satellite
 Radio
 Cellular
Wireless (Radio) LANs Are Becoming Popular
Key benefits:
Key challenges:
­   Security & privacy
­ Set-up time
­   Quality of service
­ Set-up cost
­   Cost
­ Maintenance cost
27.7 Network Security
Keepin­ an eoston the security of private networks (e.g. LANs) is relatively easy
g C ye
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
Network Security
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
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
Internet
Private
Firewall
Network
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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Introduction to Computing ­ CS101
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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
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
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Table of Contents:
  1. INTRODUCTION
  2. EVOLUTION OF COMPUTING
  3. World Wide Web, Webs structure, genesis, its evolution
  4. Types of Computers, Components, Parts of Computers
  5. List of Parts of Computers
  6. Develop your Personal Web Page: HTML
  7. Microprocessor, Bus interface unit, Data & instruction cache memory, ALU
  8. Number systems, binary numbers, NOT, AND, OR and XOR logic operations
  9. structure of HTML tags, types of lists in web development
  10. COMPUTER SOFTWARE: Operating Systems, Device Drivers, Trialware
  11. Operating System: functions, components, types of operating systems
  12. Forms on Web pages, Components of Forms, building interactive Forms
  13. APPLICATION SOFTWARE: Scientific, engineering, graphics, Business, Productivity, Entertainment, Educational Software
  14. WORD PROCESSING: Common functions of word processors, desktop publishing
  15. Interactivity to Forms, JavaScript, server-side scripts
  16. ALGORITHMS
  17. ALGORITHMS: Pseudo code, Flowcharts
  18. JavaScript and client-side scripting, objects in JavaScript
  19. Low, High-Level, interpreted, compiled, structured & object-oriented programming languages
  20. Software Design and Development Methodologies
  21. DATA TYPES & OPERATORS
  22. SPREADSHEETS
  23. FLOW CONTROL & LOOPS
  24. DESIGN HEURISTICS. Rule of thumb learned through trial & error
  25. WEB DESIGN FOR USABILITY
  26. ARRAYS
  27. COMPUTER NETWORKS: types of networks, networking topologies and protocols
  28. THE INTERNET
  29. Variables: Local and Global Variables
  30. Internet Services: FTP, Telnet, Web, eMail, Instant messaging, VoIP
  31. DEVELOPING PRESENTATIONS: Effective Multimedia Presentations
  32. Event Handlers
  33. GRAPHICS & ANIMATION
  34. INTELLIGENT SYSTEMS: techniques for designing Artificial Intelligent Systems
  35. Mathematical Functions in JavaScript
  36. DATA MANAGEMENT
  37. DATABASE SOFTWARE: Data Security, Data Integrity, Integrity, Accessibility, DBMS
  38. String Manipulations:
  39. CYBER CRIME
  40. Social Implications of Computing
  41. IMAGES & ANIMATION
  42. THE COMPUTING PROFESSION
  43. THE FUTURE OF COMPUTING
  44. PROGRAMMING METHODOLOGY
  45. REVIEW & WRAP-UP of Introduction to Computing