1. Chapter 7: Computer Networks, the Internet, and the World Wide Web Invitation to Computer Science, C++ Version, Third Edition  2004 Course Technology Additions by Paul Durand, Kent State Univ, 2005

2. Invitation to Computer Science, C++ Version, Third Edition 2 Objectives In this chapter, you will learn about: Basic networking concepts Communication protocols Network services and benefits A brief history of the Internet and the World Wide Web

3. Invitation to Computer Science, C++ Version, Third Edition 3 Introduction Computer network  Computers connected together  Purpose: exchanging resources and information  Just about any kind of information can be sent Examples: television and radio signals, voice, graphics, handwriting, photographs, movies

4. Invitation to Computer Science, C++ Version, Third Edition 4 Basic Networking Concepts Computer network  Set of independent computer systems connected by telecommunication links  Purpose: sharing information and resources Nodes, hosts, or end systems  Individual computers on a network

5. Invitation to Computer Science, C++ Version, Third Edition 5 Communication Links Switched, dial-up telephone line  A circuit is temporarily established between the caller and callee  Analog medium  Requires modem at both ends to transmit information produced by a computer Computer produces digital information Diagram by PJD, 2005

6. Invitation to Computer Science, C++ Version, Third Edition 6 Diagram by PJD, 2005

7. Invitation to Computer Science, C++ Version, Third Edition 7 Figure 7.1 Two Forms of Information Representation

8. Invitation to Computer Science, C++ Version, Third Edition 8 Figure 7.2 Modulation of a Carrier to Encode Binary Information

9. Invitation to Computer Science, C++ Version, Third Edition 9 Communication Links (continued) Dial-up phone links  Transmission rate: 56,000 bps (56 Kbps) Broadband  Transmission rate: exceeding 128,000 bps (128 Kbps)

10. Invitation to Computer Science, C++ Version, Third Edition 10 Communication Links (continued) Options for broadband communications  Home use Digital subscriber line (DSL) Cable modem  Commercial and office environment Ethernet Fast Ethernet Gigabit Ethernet Broadband Tutorial CableModem Tutorial Links added by PJD, 2005 Ethernet Tutorial

11. Invitation to Computer Science, C++ Version, Third Edition 11 Figure 7.3 Transmission Time of an Image at Different Transmission Speeds

12. Invitation to Computer Science, C++ Version, Third Edition 12 Communication Links (continued) Wireless data communication  Uses radio, microwave, and infrared signals  Enables “mobile computing”  Types of wireless data communication Wireless local access network Wireless wide-area access network

13. Invitation to Computer Science, C++ Version, Third Edition 13 Local Area Networks Local area network (LAN)  Connects hardware devices that are in close proximity  The owner of the devices is also the owner of the means of communications  Common wired LAN topologies Bus Ring Star

14. Invitation to Computer Science, C++ Version, Third Edition 14 Figure 7.4 Some Common LAN Topologies

15. Invitation to Computer Science, C++ Version, Third Edition 15 Local Area Networks (continued) Ethernet  Most widely used LAN technology  Uses the bus topology  Two ways to construct an Ethernet LAN Shared cable Hubs: the most widely used technology

16. Invitation to Computer Science, C++ Version, Third Edition 16 Figure 7.5: An Ethernet LAN Implemented Using Shared Cables Comment by PJD, 2005 KSU12/1/2004 A bridge is also called a switch. - It is 'smarter' than a repeater. - It is aware of which nodes are on which side of the lan. - It determines whether a message should be passed on to another network

17. Invitation to Computer Science, C++ Version, Third Edition 17 Figure 7.6 An Ethernet LAN Implemented Using a Hub

18. Invitation to Computer Science, C++ Version, Third Edition 18 Wide Area Networks Wide area networks (WANs)  Connect devices that are across town, across the country, or across the ocean  Users must purchase telecommunications services from an external provider  Dedicated point-to-point lines  Most use a store-and-forward, packet-switched technology to deliver messages

19. Invitation to Computer Science, C++ Version, Third Edition 19 Figure 7.7 Typical Structure of a Wide Area Network UOT = Packet - approx 1000 bytes Non-fixed routing Fault Tolerant Text by PJD, 2005

20. Invitation to Computer Science, C++ Version, Third Edition 20 Overall Structure of the Internet All real-world networks, including the Internet, are a mix of LANs and WANs  Example: a company or a college One or more LANs connecting its local computers Individual LANs interconnected into a wide-area “company network”

21. Invitation to Computer Science, C++ Version, Third Edition 21 Figure 7.8(a) Structure of a Typical Company Network Comment by PJD, 2005 KSU12/1/2004 A router connects various networks. It differs from a bridge. a bridge connects two identical types of networks. A router can transfer messages between two distinct networks, each using a totally different communication technique.

22. Invitation to Computer Science, C++ Version, Third Edition 22 Overall Structure of the Internet (continued) Internet Service Provider (ISP)  A wide-area network  Provides a pathway from a specific network to other networks, or from an individual to other networks ISPs are hierarchical  Interconnect to each other in multiple layers to provide greater geographical coverage

23. Invitation to Computer Science, C++ Version, Third Edition 23 Figure 7.8(b) Structure of a Network Using an ISP

24. Invitation to Computer Science, C++ Version, Third Edition 24 Figure 7.8(c) Hierarchy of Internet Service Providers

25. Invitation to Computer Science, C++ Version, Third Edition 25 Overall Structure of the Internet (continued) Internet  A huge interconnected “network of networks”  Includes nodes, LANs, WANs, bridges, routers, and multiple levels of ISPs  Early 2003 170 million nodes (hosts) Hundreds of thousands of separate networks located in over 225 countries

26. Invitation to Computer Science, C++ Version, Third Edition 26 Communication Protocols A protocol  A mutually agreed upon set of rules, conventions, and agreements for the efficient and orderly exchange of information TCP/IP  The Internet protocol hierarchy  Governs the operation of the Internet  Five layers Comment by PJD, 2005 The internet is operated by the Internet Society, a non-profit, non- governmental, professional society. This group establishes and enforces network protocol standards. IAB – Internet Architecture Board IETF – Internet Engineering Task Force

27. Invitation to Computer Science, C++ Version, Third Edition 27 Internet Maps http://www.cybergeography.org/atlas/more_isp_maps.html http://www.isoc.org/internet/infrastructure/maps.shtml

28. Invitation to Computer Science, C++ Version, Third Edition 28 Figure 7.10 The Five-Layer TCP/IP Internet Protocol Hierarchy

29. Invitation to Computer Science, C++ Version, Third Edition 29 Physical Layer Protocols govern the exchange of binary digits across a physical communication channel Goal: create a “bit pipe” between two computers Figure added by PJD, 2005

30. Invitation to Computer Science, C++ Version, Third Edition 30 Data Link Layer Protocols carry out  Error handling  Framing (identify start and end of message) Creates an error-free “message pipe” Composed of two services  Layer 2a: medium access control  Layer 2b: logical link control KSU12/1/2004 Medium access control - How to arbitrate ownership of a shared line? - Central master control node? - Contention based approach? - no central control - equal access for all

31. Invitation to Computer Science, C++ Version, Third Edition 31 Data Link Layer (continued) Medium access control protocols  Determine how to arbitrate ownership of a shared line when multiple nodes want to send at the same time Logical link control protocols  Ensure that a message traveling across a channel from source to destination arrives correctly

32. Invitation to Computer Science, C++ Version, Third Edition 32 Layer 2a Medium access control protocols

33. Invitation to Computer Science, C++ Version, Third Edition 33 Automatic Repeat Request (ARQ) Algorithm Part of Logical Link Protocols - layer 2b Assures message travels from A to B correctly

34. Invitation to Computer Science, C++ Version, Third Edition 34 Automatic Repeat Request (ARQ) Process of requesting that a data transmission be resent Main ARQ protocols  Stop and Wait ARQ (A half duplex technique) Sender sends a message and waits for acknowledgment, then sends the next message Receiver receives the message and sends an acknowledgement, then waits for the next message  Continuous ARQ (A full duplex technique) Sender continues sending packets without waiting for the receiver to acknowledge Receiver continues receiving messages without acknowledging them right away

35. Invitation to Computer Science, C++ Version, Third Edition 35 Stop and Wait ARQ Sends the packet, then waits to hear from receiver. Sends acknowledgement Sends negative acknowledgement Resends the packet again Sends the next packet SenderReceiver

36. Invitation to Computer Science, C++ Version, Third Edition 36 Continuous ARQ Sender sends packets continuously without waiting for receiver to acknowledge Notice that acknowledgments now identify the packet being acknowledged. Receiver sends back a NAK for a specific packet to be resent.

37. Invitation to Computer Science, C++ Version, Third Edition 37 Source of Error What causes itHow to prevent it Line Outages Faulty equipment, Storms, Accidents (circuit fails) White Noise (hiss) (Gaussian Noise) Movement of electrons (thermal energy) Increase signal strength (increase SNR) Impulse Noise (Spikes) Sudden increases in electricity (e.g., lightning, power surges) Shield or move the wires Cross-talk Multiplexer guard bands are too small or wires too close together Increase the guard bands, or move or shield the wires Echo Poor connections (causing signal to be reflected back to the source) Fix the connections, or tune equipment Attenuation Gradual decrease in signal over distance (weakening of a signal) Use repeaters or amplifiers Intermodulation Noise Signals from several circuits combine Move or shield the wires Jitter Analog signals change (small changes in amp., freq., and phase) Tune equipment Harmonic Distortion Amplifier changes phase (does not correctly amplify its input signal) Tune equipment Sources of Errors and Prevention mostly on analog More important

38. Invitation to Computer Science, C++ Version, Third Edition 38 Error Detection Mathematical calculations ?=?= Mathematical calculations Data to be transmitted Sender calculates an Error Detection Value (EDV) and transmits it along with data Receiver recalculates EDV and checks it against the received EDV –If the same  No errors in transmission –If different  Error(s) in transmission EDV Larger the size, better error detection (but lower efficiency)

39. Invitation to Computer Science, C++ Version, Third Edition 39 Error Detection Techniques Parity checks Longitudinal Redundancy Checking (LRC) Polynomial checking  Checksum  Cyclic Redundancy Check (CRC)

40. Invitation to Computer Science, C++ Version, Third Edition 40 Parity Checking One of the oldest and simplest A single bit added to each character  Even parity: number of 1’s remains even  Odd parity: number of 1’s remains odd Receiving end recalculates parity bit  If one bit has been transmitted in error the received parity bit will differ from the recalculated one Simple, but doesn’t catch all errors  If two (or an even number of) bits have been transmitted in error at the same time, the parity check appears to be correct  Detects about 50% of errors

41. Invitation to Computer Science, C++ Version, Third Edition 41 Examples of Using Parity senderreceiver 01101010 EVEN parity parity number of all transmitted 1’s remains EVEN To be sent: Letter V in 7-bit ASCII: 0110101 senderreceiver 01101011 ODD parity parity number of all transmitted 1’s remains ODD

42. Invitation to Computer Science, C++ Version, Third Edition 42 LRC - Longitudinal Redundancy Checking Adds an additional character (instead of a bit)  Block Check Character (BCC) to each block of data  Determined like parity but, but counting longitudinally through the message (as well as vertically)  Calculations are based on the 1st bit, 2nd bit, etc. (of all characters) in the block 1st bit of BCC  number of 1’s in the 1st bit of characters 2nd bit of BCC  number of 1’s in the 2ndt bit of characters Major improvement over parity checking  98% error detection rate for burst errors ( > 10 bits)  Less capable of detecting single bit errors

43. Invitation to Computer Science, C++ Version, Third Edition 43 Letter D A T A Using LRC for Error Detection Note that the BCC’s parity bit is also determined by parity BCC1 1 0 1 1 1 1 1 Parity bit 1 0 1 Example: Send the message “DATA” using ODD parity and LRC ASCII 1 0 0 0 1 0 0 1 0 0 0 0 0 1 1 0 1 0 1 0 0 1 0 0 0 0 0 1

44. Invitation to Computer Science, C++ Version, Third Edition 44 Polynomial Checking Adds 1 or more characters to the end of message (based on a mathematical algorithm) Two types: Checksum and CRC Checksum  Calculated by adding decimal values of each character in the message,  Dividing the total by 255. and  Saving the remainder (1 byte value) and using it as the checksum  95% effective Cyclic Redundancy Check (CRC)  Computed by calculating the remainder to a division problem:

45. Invitation to Computer Science, C++ Version, Third Edition 45 P / G = Q + R / G Cyclic Redundancy Check (CRC) –Most powerful and most common –Detects 100% of errors (if number of errors <= size of R) –Otherwise: CRC-16 (99.998%) and CRC-32 (99.9999%) Message (treated as one long binary number) A fixed number (determines the length of the R) Remainder: –added to the message as EDV) –could be 8 bits, 16 bits, 24 bits, or 32 bits long Quotient (whole number) Example: P = 58 G = 8 Q = 7 R = 2

46. Invitation to Computer Science, C++ Version, Third Edition 46 Network Layer Delivers a message from the site where it was created to its ultimate destination Critical responsibilities  Creating a universal addressing scheme for all network nodes  Delivering messages between any two nodes in the network

47. Invitation to Computer Science, C++ Version, Third Edition 47 Network Layer (continued) Provides a true “network delivery service”  Messages are delivered between any two nodes in the network, regardless of where they are located IP (Internet Protocol) layer  Network layer in the Internet

48. Invitation to Computer Science, C++ Version, Third Edition 48 Ethernet (IEEE 802.3) Frame (number of bytes in the message field) Repeating pattern of 1’s and 0’s (1010101010) Used by Virtual LANs; (if no vLAN, the field is omitted If used, first 2 bytes is set to: 24,832 (8100H) Used to exchange control info (e.g., type of network layer protocol used) Used to hold sequence number, ACK/NAK, etc., (1 or 2 bytes) 00 01 10 11

49. Invitation to Computer Science, C++ Version, Third Edition 49 Transport Layer Provides a high-quality, error-free, order preserving end-to-end delivery service TCP (Transport Control Protocol)  Primary transport protocol on the Internet  Requires the source and destination programs to initially establish a connection

50. Invitation to Computer Science, C++ Version, Third Edition 50 Figure 7.15 Logical View of a TCP Connection

51. Invitation to Computer Science, C++ Version, Third Edition 51 Application Layer Implements the end-user services provided by a network There are many application protocols, including:  HTTP  SMTP  POP3  IMAP  FTP

52. Invitation to Computer Science, C++ Version, Third Edition 52 Figure 7.16 Some Popular Application Protocols on the Internet

53. Invitation to Computer Science, C++ Version, Third Edition 53 Application Layer (continued) Uniform Resource Locator (URL)  A symbolic string that identifies a Web page  Form protocol://host address/page  The most common Web page format is hypertext information Accessed using the HTTP protocol

54. Invitation to Computer Science, C++ Version, Third Edition 54 Network Services and Benefits Services offered by computer networks  Electronic mail (email)  Bulletin boards  News groups  Chat rooms  Resource sharing Physical resources Logical resources

55. Invitation to Computer Science, C++ Version, Third Edition 55 Network Services and Benefits (continued) Services offered by computer networks  Client-server computing  Information sharing  Information utility  Electronic commerce (e-commerce)

56. Invitation to Computer Science, C++ Version, Third Edition 56 A Brief History of the Internet and the World Wide Web: The Internet August 1962: first proposal for building a computer network  Made by J. C. R. Licklider of MIT ARPANET  Built by the Advanced Research Projects Agency (ARPA) in the 1960s  Grew quickly during the early 1970s

57. Invitation to Computer Science, C++ Version, Third Edition 57 The Internet (continued) NSFNet: A national network built by the National Science Foundation (NSF) October 24, 1995: Formal acceptance of the term “Internet” Internet service providers start offering Internet access once provided by the ARPANET and NSFNet

58. Invitation to Computer Science, C++ Version, Third Edition 58 Figure 7.20 State of Networking in the Late 1980s

59. Invitation to Computer Science, C++ Version, Third Edition 59 The World Wide Web Development completed in May 1991 Designed and built by Tim Berners-Lee Components  Hypertext A collection of documents interconnected by pointers called links  URL (Uniform Resource Locator) The worldwide identification of a Web page located on a specific host computer

60. Invitation to Computer Science, C++ Version, Third Edition 60 Figure 7.21 Hypertext Documents

61. Invitation to Computer Science, C++ Version, Third Edition 61 Summary of Level 3 Virtual environment  Created by system software  Easy to use and easy to understand  Provides services such as: Resource management Security Access control Efficient resource use Operating systems continue to evolve

62. Invitation to Computer Science, C++ Version, Third Edition 62 Summary Computer network: a set of independent computer systems connected by telecommunication links Options for transmitting data on a network: dial- up telephone lines, DSL, cable modem, Ethernet, Fast Ethernet Types of networks: local area network (LAN) and wide area network (WAN)

63. Invitation to Computer Science, C++ Version, Third Edition 63 Summary The Internet is a huge interconnected "network of networks" TCP/IP is the Internet protocol hierarchy, composed of five layers: physical, data link, network, transport, and application The World Wide Web is an information system based on the concept of hypertext