CS101 Introduction to Computing

Lecture 27

Computer Networks

1

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 2

don’t make me

think

!

3

think

roadside billboard

rather than

Dewan-e-Ghalib

4

Designs should be

consistent

&

predictable

(unified)

5

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!

6

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 7

Good designs assist the user in recovering from errors

8

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 9

All of you have used computer networks.

What is a computer network

?

10

Computer Network

Multiple computers that are connected together to share information and other resources 11

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 12

Computer A Computer E

Example of a Computer Network

Computer D Hub Computer B Computer C 13

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 14

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 15

How Does a Conventional Network Work?

1. Suppose computer A wants to send message to D a 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 16

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 17

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 18

Hub (2)

• Each computer in a network one of those ports is connected to 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 19

Packet (1)

• The smallest unit of data computer network transmitted over a • 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 20

Packet (2)

• All packets travel independently • When all packets are received by the destination computer, it reassembles them to form the original message 21

Types of Computer Networks

according to the network access policy • Private • Public 22

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 23

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 24

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 25

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 26

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 27

Types of Computer Networks

according to the distance between nodes • LAN: Local Area Network) • WAN: Wide Area Network) 28

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 29

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 30

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 31

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 32

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 33

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 34

Modem (1)

• I/O device used for connecting two computers over telephone lines •

modem = modulator + demodulator

35

Modem (2)

• Modulator converts computer messages electrical pulses that are suitable for to transmission over the telephone lines • Demodulator converts electrical pulses received over telephone lines into messages that are comprehensible for computers 36

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 37 more of these 4 basic patterns

Computer A

P2P

Computer B 38

• Inexpensive • Limited connectivity

P2P

• Quite often used for connecting two LANs to form a WAN 39

Star

Computer A Computer D Server Computer B Computer C 40

Star (1)

• A computer sends the address receiver and the data of the intended to the server • The server then sends the message to the intended receiver • This topology allows multiple messages to be sent simultaneously 41

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 42

Computer A Computer C

Bus

Bus: A high speed cable Computer B Computer D 43

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 44

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 45

Computer A Computer D

Ring

Computer C Computer B 46

Ring (1)

• No server is required • A computer sends the message to its The neighbor examines neighbor. 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 47

Ring (2)

• This process is repeated their intended recipient until the data arrive at • This topology allows multiple messages to be carried, simultaneously • Data transmission is slow since each message is checked by each computer 48

Ring (3)

• New nodes are difficult to add • Messages propagate in one direction only • The network fails if a single node fails 49

Computer A Computer B Computer C Hub

Combination

Hub Computer D Computer E Computer

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 51

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 52

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 53

Computer Networks

=

Computers

+

Communications

54

Types of Communication Channels

1. Wire 2. Wireless A key characteristic of these channels is bandwidth 55

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) 56

Types of Communication Channels

Wire

– Copper • Twisted-pair • Coaxial cable

Wireless

– Line-of-sight • Microwave • Optical – Non-line-of-sight • Satellite • Radio • Cellular – Optical fiber 57

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 58

Network Security (1)

• Keeping an eye on the security of networks (e.g. LANs) is relatively private 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 59

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 60

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 61

Internet

Firewall

Private Network 62

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 63

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 64

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 65