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CS101 Introduction to Computing

Lecture 27Computer 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 acomputer 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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ComputerC

ComputerD

ComputerEComputer

A

ComputerB

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

5. 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 Networksaccording 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 Networksaccording 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

ComputerA

ComputerB

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P2P

• Inexpensive

• Limited connectivity

• Quite often used for connecting two LANs to form a WAN

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Star

ComputerA

ComputerD

ComputerC

ComputerB

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

AComputer

C

ComputerD

ComputerB

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

ComputerA

ComputerD

ComputerC

ComputerB

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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

ComputerB

ComputerA

ComputerC

Hub

ComputerE

ComputerD

ComputerF

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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PrivateNetworkInternet

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