CS101 Introduction to Computing Lecture 27 Computer Networks

1. CS101 Introduction to ComputingLecture 27Computer Networks

2. 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

3. don’t make me think!

4. thinkroadside billboardrather thanDewan-e-Ghalib

5. Designs should be consistent & predictable(unified)

6. 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!

7. 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

8. Good designs assist the user in recovering from errors

9. 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

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

11. Computer Network Multiple computers that are connected together to shareinformation and other resources

12. 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

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

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

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

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

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

18. 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

19. 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

20. 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

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

22. Types of Computer Networksaccording to the network access policy • Private • Public

23. 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

24. 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

25. 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

26. 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

27. 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

28. Types of Computer Networksaccording to the distance between nodes • LAN: Local Area Network) • WAN: Wide Area Network)

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

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

36. Modem (2) • Modulator convertscomputer 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

37. 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

38. P2P Computer A Computer B

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

40. Computer D Star Computer A Server Computer C Computer B

41. 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

42. 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

43. Computer A Computer C Bus Bus: A high speed cable Computer B Computer D

44. 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 recipientaccepts the data

45. 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

46. Computer D Ring Computer A Computer C Computer B

47. 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

48. 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

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

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