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Chapter 7 Computer Networks, the Internet, and the World Wide Web 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 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 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 Communication Links Switched, dial-up telephone line A circuit is temporarily established between the caller and callee • Circuit lasts for the duration of the call. Analog medium Requires modem at both ends to transmit information produced by a computer • Computer produces digital information Figure 7.1 Two Forms of Information Representation Figure 7.2 Modulation of a Carrier to Encode Binary Information Communication Links (continued) Dial-up phone links Initial transmission rates of 300 bps Later in 1980s increased to 1,200-9,600 bps Transmission rate now: 56,000 bps (56 Kbps) • Too slow for web pages, MP3, streaming video Still used to access networks in remote areas. Broadband Transmission rate: Exceeding 256,000 bps (256 Kbps) Is “always on”, so do not have to wait for connection Communication Links (continued) Options for broadband communications Broadband refers to transmission rates over 256K bps. Home use • Digital subscriber line (DSL) • Cable modem Commercial and office environment • Ethernet • Fast Ethernet • Gigabit Ethernet Communication Links (continued) Digital DSL is a “permanently on” connection Transmits digital signals at different frequencies, so no interference with voice. Line is often asymmetric with download & upload bandwidths. Cable subscriber line (DSL) modem Uses links that deliver TV signals to homes. Speeds roughly comparable to DSL. Signal is “always on”. Communication Links (continued) Commercial and office environment Ethernet – mid 1970’s • Most common with 10Mbps • Often available in dorms and new homes Fast Ethernet developed in • 100 Mbps – 1990’s • Coaxial cable, fiber-optic cable, or regular twisted-pair copper wire. Gigabit Ethernet project • In 1998, achieved 1000 Mbps • In 2003 achieved 10 billion bps Transmit entire contents of 1,700 books, each 300 pages long in a single second Figure 7.3 Transmission Time of an Image at Different Transmission Speeds Gigabit Networks Applications Transmitting real-time video images without flicker or delay Exchanging 3D medical images Transmitting weather satellite data Supporting researchers working on Human Genome project. 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 Wireless data communication Wireless local access network Transmits few hundred feet Connected to standard wired network Found in home, coffee shops, offices Wi-Fi (Wireless Fidility) connects wireless computer to internet within 150-300 ft of access point (called hot spots). Wireless wide-area access network Computer transmits message to remote base station, which may be miles away Base station typically a large antenna on top of a tower. Some are only line-of-sight, so not more than 10-50 miles apart. 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 Figure 7.4 Some Common LAN Topologies Common LAN Topologies Buses If two or more nodes use the link at the same time, messages collide and are unreadable Nodes must take turns using net. Rings Messages circulate in either clockwise or counterclockwise direction until at destination Star Central node routes information directly to any other node. Local Area Networks (continued) Ethernet Most widely used LAN technology Uses the bus topology Node places message including destination address on bus. This message is received by all other nodes All nodes check address to see if message is for them. • Nodes who are not addressed discard message Local Area Networks (continued) Two ways to construct an Ethernet LAN Shared cable Hubs: The most widely used technology Shared cable features Wire is strung through the building. Users connect into cable at nearest point. Repeater amplifies and forwards signal • If two Ethernets are connected using a repeater, they function as a single network. Bridge (or switch) joining two networks has knowledge about nodes on each • It forwards messages to other network only if needed. Figure 7.5 An Ethernet LAN Implemented Using Shared Cables Hubs for LANs In this construction of a LAN, a box called a hub is used in place of shared cable Hubs are boxes with a number of ports. Hubs are placed in a wiring closet. Each node in Ethernet LAN is connected to a port Hubs connects these ports using a shared cable inside hub Figure 7.6 An Ethernet LAN Implemented Using a Hub 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, packetswitched technology to deliver messages Figure 7.7 Typical Structure of a Wide Area Network 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 Routers are used to connect networks of both similar and dissimilar types. • A router can connect an LAN to a WAN. • A bridge can only connect two networks of identical type. Figure 7.8(a) Structure of a Typical Company Network Overall Structure of the Internet (continued) Internet Service Provider (ISP) Normally a business Provides a pathway from a specific network to other networks, or from an individual’s computer to other networks This access occurs through a WAN owned by the ISP ISPs are hierarchical Interconnect to each other in multiple layers to provide greater geographical coverage A regional or national ISP may connect to an international IPS called a tier-1 network or Internet backbone. T1 & T3 Lines A dedicated phone connection supporting data rates of 1.544 Mbits per second. Consists of 24 individual channels, each of which supports 64Kbits per second. Each 64Kbit/second channel can be configured to carry voice or data traffic. T-1 lines are a popular leased line option for businesses connecting to the Internet and for Internet Service Providers (ISPs) connecting to the Internet backbone. The Internet backbone itself consists of faster T-3 connections. Reference: Webopedia Figure 7.8(b) Structure of a Network Using an ISP Figure 7.8(c) Hierarchy of Internet Service Providers 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 2005 • 317 million nodes (hosts) • Hundreds of thousands of separate networks located in over 225 countries Communication Protocols A protocol A mutually agreed upon set of rules, conventions, and agreements for the efficient and orderly exchange of information Internet is operated by the Internet Society, a nonprofit society consisting of over 100 worldwide organizations, foundations, businesses, etc. TCP/IP The Internet protocol hierarchy Governs the operation of the Internet Named after 2 of the most successful protocols Consists of five layers (see next slide) Figure 7.10 The Five-Layer TCP/IP Internet Protocol Hierarchy Physical Layer Protocols govern the exchange of binary digits across a physical communication channel Specify such things as What signal is used to indicate a bit on the line How long will “bit on the line” signal last Will signal be digital or analog What voltage levels are used to represent 0 & 1 Goal: Create a bit pipe between two computers Data Link Layer Protocols carry out Error detection and correction Framing • Which bits in incoming stream belong together • Identifying start and end of message Creates an error-free message pipe Composed of two stages Layer 2a: Medium access control Layer 2b: Logical link control 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 Layer 2a Medium access control protocols Automatic Repeat Request (ARQ) Algorithm Part of Logical Link Protocols - layer 2b Assures message travels from A to B correctly 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 Stop and Wait ARQ Sender Sends the packet, then waits to hear from receiver. Sends the next packet Receiver Sends acknowledgement Sends negative acknowledgement Resends the packet again 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. Sources of Errors and Prevention Source of Error What causes it How to prevent it More important mostly on analog 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) 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) – If the same No errors in transmission – If different Error(s) in transmission Error Detection Techniques Parity checks Longitudinal Redundancy Checking (LRC) Polynomial checking Checksum Cyclic Redundancy Check (CRC) 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 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 receiver 01101011 parity 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 Using LRC for Error Detection Example: Send the message “DATA” using ODD parity and LRC Parity bit Letter ASCII 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 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: 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%) Network Layer Delivers a message from the site where it was created to its ultimate destination Critical responsibilities Create a universal addressing scheme for all network nodes Deliver messages between any two nodes in the network 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 Universal addressing scheme used IP (Internet Protocol) layer Example: • One author’s host name is macalester.edu • The IP address for host is (in decimal) 141.140.1.5 • Corresponds to following 32 bit address 10001101 10001100 0000000 100000101 • Clearly, the symbolic name is easier to remember. Network Layer (cont.) Domain Name System (DNS) It is the job of the DNS to convert symbolic names to a sequence of 32 binary bits DNS is a massive distributed database If local name server does not recognize host name, it is forwarded to a remote name servers until one locates its name. Routing Algorithms There are normally multiple routes a message can take to its destination. A routing algorithm called “Dijkstra’s shortest path” algorithm is used, based on time required to send a message along various paths. For large networks with 107 nodes or more, algorithm may require 1014 or more calculations. Network Layer (cont) Keeping these routing tables up to date is an enormous task. Data changes regularly, so they have to be recalculated frequently. When a network node or sections of network fails, must route around the failed nodes. Network Layer also handles other things Network management Broadcasting Locating mobile nodes 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 Uses ARQ algorithm on sending & receiving messages. Messages sent in packets and have to be reassembled in order at destination Standard applications (e.g., POP3 or IMAP for email) have fixed port numbers for all computers. Figure 7.15 Logical View of a TCP Connection Application Layer Implements the end-user services provided by a network There are many application protocols HTTP SMTP POP3 IMAP FTP Figure 7.16 Some Popular Application Protocols on the Internet 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 • Example: http://www.macalester.edu/about/history.html Network Services and Benefits Services offered by computer networks Electronic mail (email) • Send message when you want & read at receiver’s convenience • From U.S., arrives anywhere in world in about a minute. Bulletin boards Internet Services (cont) Services offered by c. networks (cont) News groups Chat rooms Resource sharing • Physical resources • Logical resources Network Services and Benefits (continued) Services offered by computer networks Client-server computing Information sharing (databases) Information utility • reference documents online Electronic commerce (e-commerce) A Brief History of the Internet and the World Wide Web: The Internet August 1962: First proposal for building a computer network Written by J. C. R. Licklider of MIT ARPANET Built by the Advanced Research Projects Agency (ARPA) in Dept of Defense in the late1960s Packet switching viewed as more secure than phone lines. First two nodes were at Stanford Research Institute (SRI) and UCLA, in 1969 • Univ. of Calif. at Santa Barbara & Univ. Utah added in 1969 Grew quickly during the early 1970s The Internet (continued) ARPANET (cont) Electronic Email developed in 1972 Huge success of ARPANET lead to creation of other research and commercial networks In 1973, Kahn at ARPA and Cerf at Stanford started work to allow different networks to communicate A common address scheme The TCP/IP protocols that provided a common language Additional “Killer Applications” in early 1980’s Telnet – Log on remotely to another computer FTP – File Transfer Protocol The Internet (continued) Most people with Internet access in early 1980’s were physicist, engineers, & computer scientist ARPANET connected only 235 computers in 1982, one of which was KSU Mathematical Science Dept. NSFNet: A national “network of networks” built by the National Science Foundation (NSF) in 1984 Goal was to bring internet services to all academic and professional groups. Included universities, libraries, museums, medical centers, etc. The Internet (continued) Many countries quickly followed lead of NSFNet and connected their professional groups to the internet. October 24, 1995: Formal acceptance of the term Internet by the U.S. Govt. Internet service providers start offering Internet access once provided by the ARPANET and NSFNet Figure 7.20 State of Networking in the Late 1980s The World Wide Web Development completed in May 1991 Designed and built by Tim Berners-Lee at CERN (European High Energy Physics Lab) in Geneva Switzerland. Purpose was to allow scientists to more easily exchange information such as research articles, experimental data Would be a big improvement of earlier methods of email messaging and FTP. Figure 7.21 Hypertext Documents The World Wide Web (cont) Components Hypertext • A collection of documents interconnected by pointers called links Hypertext links called a URL (Uniform Resource Locator) • The worldwide identification of a Web page located on a specific host computer In 1993 CERN announced their web technology would be freely available. Realized impact it could have on research throughout world. The World Wide Web (cont) A graphic browser (MOSIAC) was also released in late 1993 Allowed public to make use of new technology Forerunner of Netscape Navigator (1994) and Microsoft Internet Explorer (1995) The WWW became the killer app of the 21 century 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 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) Summary (continued) 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