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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 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 Invitation to Computer Science, C++ Version, Third Edition 2 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 Invitation to Computer Science, C++ Version, Third Edition 3 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 Invitation to Computer Science, C++ Version, Third Edition 4 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 Invitation to Computer Science, C++ Version, Third Edition Diagram by PJD, 2005 5 Invitation to Computer Science, C++ Version, Third Edition Diagram by PJD, 2005 6 Figure 7.1 Two Forms of Information Representation Invitation to Computer Science, C++ Version, Third Edition 7 Figure 7.2 Modulation of a Carrier to Encode Binary Information Invitation to Computer Science, C++ Version, Third Edition 8 Communication Links (continued) Dial-up phone links Transmission rate: 56,000 bps (56 Kbps) Broadband Transmission rate: exceeding 128,000 bps (128 Kbps) Invitation to Computer Science, C++ Version, Third Edition 9 Communication Links (continued) Options for broadband communications Home use Digital subscriber line (DSL) Broadband Tutorial Cable modem CableModem Tutorial Commercial and office environment Ethernet Fast Ethernet Gigabit Ethernet Invitation to Computer Science, C++ Version, Third Edition Ethernet Tutorial Links added by PJD, 2005 10 Figure 7.3 Transmission Time of an Image at Different Transmission Speeds Invitation to Computer Science, C++ Version, Third Edition 11 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 Invitation to Computer Science, C++ Version, Third Edition 12 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 Invitation to Computer Science, C++ Version, Third Edition 13 Figure 7.4 Some Common LAN Topologies Invitation to Computer Science, C++ Version, Third Edition 14 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 Invitation to Computer Science, C++ Version, Third Edition 15 KSU 12/1/2004 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 Figure 7.5: An Ethernet LAN Implemented Using Shared Cables Invitation to Computer Science, C++ Version, Third Edition Comment by PJD, 2005 16 Figure 7.6 An Ethernet LAN Implemented Using a Hub Invitation to Computer Science, C++ Version, Third Edition 17 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 Invitation to Computer Science, C++ Version, Third Edition 18 •UOT = Packet - approx 1000 bytes •Non-fixed routing •Fault Tolerant Text by PJD, 2005 Figure 7.7 Typical Structure of a Wide Area Network Invitation to Computer Science, C++ Version, Third Edition 19 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” Invitation to Computer Science, C++ Version, Third Edition 20 KSU 12/1/2004 A router connects various networks. It differs from a bridge. a bridge connects two identical types of networks. A router can transfer messages between Figure two distinct networks, each using a totally7.8(a) different communication technique. Structure of a Typical Company Invitation to Computer Science, C++ Version, Third Edition Network Comment by PJD, 2005 21 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 Invitation to Computer Science, C++ Version, Third Edition 22 Figure 7.8(b) Structure of a Network Using an ISP Invitation to Computer Science, C++ Version, Third Edition 23 Figure 7.8(c) Hierarchy of Internet Service Providers Invitation to Computer Science, C++ Version, Third Edition 24 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 Invitation to Computer Science, C++ Version, Third Edition 25 Communication Protocols The internet is operated by the Internet Society, a non-profit, nongovernmental, professional society. This group establishes and enforces network protocol standards. A protocol IAB – Internet Architecture Board IETF – Internet Engineering Task Force 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 Invitation to Computer Science, C++ Version, Third Edition Comment by PJD, 2005 26 Internet Maps http://www.cybergeography.org/atlas/more_isp_maps.html http://www.isoc.org/internet/infrastructure/maps.shtml Invitation to Computer Science, C++ Version, Third Edition 27 Figure 7.10 The Five-Layer TCP/IP Internet Protocol Hierarchy Invitation to Computer Science, C++ Version, Third Edition 28 Physical Layer Protocols govern the exchange of binary digits across a physical communication channel Goal: create a “bit pipe” between two computers Invitation to Computer Science, C++ Version, Third Edition Figure added by PJD, 2005 29 Data Link KSU Layer 12/1/2004 Medium access control - How to arbitrate ownership of a shared line? Protocols carry out - Central master control node? Error handling Framing - Contention based approach? - no central control (identify start and end message) - equal access forofall Creates an error-free “message pipe” Composed of two services Layer 2a: medium access control Layer 2b: logical link control Invitation to Computer Science, C++ Version, Third Edition 30 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 Invitation to Computer Science, C++ Version, Third Edition 31 Layer 2a Medium access control protocols Invitation to Computer Science, C++ Version, Third Edition 32 Automatic Repeat Request (ARQ) Algorithm Part of Logical Link Protocols - layer 2b Assures message travels from A to B correctly Invitation to Computer Science, C++ Version, Third Edition 33 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 Invitation to Computer Science, C++ Version, Third Edition 34 Stop and Wait ARQ Sender Sends the packet, then waits to hear from receiver. Receiver Sends acknowledgement Sends the next packet Sends negative acknowledgement Resends the packet again Invitation to Computer Science, C++ Version, Third Edition 35 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. Invitation to Computer Science, C++ Version, Third Edition 36 Sources of Errors and Prevention Source of Error What causes it How to prevent it More important 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 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) Intermodulation Noise Signals from several circuits combine Use repeaters or amplifiers 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 (Spikes) mostly on analog Invitation to Computer Science, C++ Version, Third Edition 37 Error Detection Sender calculates an Error Detection Value (EDV) and transmits it along with data Receiver recalculates EDV and checks it against the received EDV Mathematical calculations Mathematical calculations ? = Data to be transmitted EDV Larger the size, better error detection (but lower efficiency) Invitation to Computer Science, C++ Version, Third Edition – If the same No errors in transmission – If different Error(s) in transmission 38 Error Detection Techniques Parity checks Longitudinal Redundancy Checking (LRC) Polynomial checking Checksum Cyclic Redundancy Check (CRC) Invitation to Computer Science, C++ Version, Third Edition 39 Parity Checking One of the oldest and simplest A single bit added to each character Receiving end recalculates parity bit Even parity: number of 1’s remains even Odd parity: number of 1’s remains odd 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 Invitation to Computer Science, C++ Version, Third Edition 40 Examples of Using Parity To be sent: Letter V in 7-bit ASCII: 0110101 EVEN parity sender 01101010 number of all transmitted 1’s remains EVEN ODD parity receiver parity sender number of all transmitted 1’s remains ODD Invitation to Computer Science, C++ Version, Third Edition receiver 01101011 parity 41 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 Invitation to Computer Science, C++ Version, Third Edition 42 Using LRC for Error Detection Example: Send the message “DATA” using ODD parity and LRC Letter ASCII Parity bit D 10001001 A 10000011 T 10101000 A 10000011 BCC 1 1 0 1 1 1 1 1 Note that the BCC’s parity bit is also determined by parity Invitation to Computer Science, C++ Version, Third Edition 43 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: Invitation to Computer Science, C++ Version, Third Edition 44 Cyclic Redundancy Check (CRC) P/G=Q+R/G Quotient Message (whole (treated as number) one long binary A fixed number number) (determines the length of the R) Example: P = 58 G=8 Q=7 R =2 Remainder: –added to the message as EDV) –could be 8 bits, 16 bits, 24 bits, or 32 bits long – 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%) Invitation to Computer Science, C++ Version, Third Edition 45 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 Invitation to Computer Science, C++ Version, Third Edition 46 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 Invitation to Computer Science, C++ Version, Third Edition 47 Ethernet (IEEE 802.3) Frame Used by Virtual LANs; (if no vLAN, the field is omitted If used, first 2 bytes is set to: 24,832 (8100H) Repeating pattern of 1’s and 0’s (1010101010) (number of bytes in the message field) Invitation to Computer Science, C++ Version, Third Edition Used to hold sequence number, ACK/NAK, etc., (1 or 2 bytes) 00 01 10 11 Used to exchange control info (e.g., type of network layer protocol used) 48 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 Invitation to Computer Science, C++ Version, Third Edition 49 Figure 7.15 Logical View of a TCP Connection Invitation to Computer Science, C++ Version, Third Edition 50 Application Layer Implements the end-user services provided by a network There are many application protocols, including: HTTP SMTP POP3 IMAP FTP Invitation to Computer Science, C++ Version, Third Edition 51 Figure 7.16 Some Popular Application Protocols on the Internet Invitation to Computer Science, C++ Version, Third Edition 52 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 Invitation to Computer Science, C++ Version, Third Edition 53 Network Services and Benefits Services offered by computer networks Electronic mail (email) Bulletin boards News groups Chat rooms Resource sharing Physical resources Logical resources Invitation to Computer Science, C++ Version, Third Edition 54 Network Services and Benefits (continued) Services offered by computer networks Client-server computing Information sharing Information utility Electronic commerce (e-commerce) Invitation to Computer Science, C++ Version, Third Edition 55 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 Invitation to Computer Science, C++ Version, Third Edition 56 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 Invitation to Computer Science, C++ Version, Third Edition 57 Figure 7.20 State of Networking in the Late 1980s Invitation to Computer Science, C++ Version, Third Edition 58 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 Invitation to Computer Science, C++ Version, Third Edition 59 Figure 7.21 Hypertext Documents Invitation to Computer Science, C++ Version, Third Edition 60 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 Invitation to Computer Science, C++ Version, Third Edition 61 Summary Computer network: a set of independent computer systems connected by telecommunication links Options for transmitting data on a network: dialup telephone lines, DSL, cable modem, Ethernet, Fast Ethernet Types of networks: local area network (LAN) and wide area network (WAN) Invitation to Computer Science, C++ Version, Third Edition 62 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 Invitation to Computer Science, C++ Version, Third Edition 63