Chapter 7

1. Chapter 7 Computer Networks, the Internet, and the World Wide Web

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. 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. 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. 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

6. Figure 7.1 Two Forms of Information Representation

7. Figure 7.2 Modulation of a Carrier to Encode Binary Information

8. 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

9. 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

10. Communication Links (continued) • Digital subscriber line (DSL) • 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 modem • Uses links that deliver TV signals to homes. • Speeds roughly comparable to DSL. • Signal is “always on”.

11. 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

12. Figure 7.3 Transmission Time of an Image at Different Transmission Speeds

13. 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.

14. 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

15. 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.

16. 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

17. Figure 7.4 Some Common LAN Topologies

18. 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.

19. 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

20. 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.

21. Figure 7.5 An Ethernet LAN Implemented Using Shared Cables

22. 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

23. Figure 7.6 An Ethernet LAN Implemented Using a Hub

24. 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

25. Figure 7.7 Typical Structure of a Wide Area Network

26. 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.

27. Figure 7.8(a) Structure of a Typical Company Network

28. 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.

29. 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 Internetbackbone. • The Internet backbone itself consists of faster T-3 connections. • Reference: Webopedia

30. Figure 7.8(b) Structure of a Network Using an ISP

31. Figure 7.8(c) Hierarchy of Internet Service Providers

32. 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

33. 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)

34. Figure 7.10 The Five-Layer TCP/IP Internet Protocol Hierarchy

35. 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

36. 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

37. 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

38. Layer 2aMedium access control protocols

39. Automatic Repeat Request (ARQ) Algorithm Part of Logical Link Protocols - layer 2b Assures message travels from A to B correctly

40. 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

41. Stop and Wait ARQ Sender Receiver Sends the packet, then waits to hear from receiver. Sends acknowledgement Sends the next packet Sends negative acknowledgement Resends the packet again

42. 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.

43. Source of Error What causes it How 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 More important mostly on analog

44. 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 • If the same  No errors in transmission • If different  Error(s) in transmission Larger the size, better error detection (but lower efficiency)

45. Error Detection Techniques • Parity checks • Longitudinal Redundancy Checking (LRC) • Polynomial checking • Checksum • Cyclic Redundancy Check (CRC)

46. 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

47. sender receiver EVEN parity 01101010 number of all transmitted 1’s remains EVEN parity sender receiver ODD parity 01101011 number of all transmitted 1’s remains ODD parity Examples of Using Parity To be sent: Letter V in 7-bit ASCII: 0110101

48. 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

49. Using LRC for Error Detection 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 Parity bit 1 1 0 1 Letter D A T A BCC 1 1 0 1 1 1 1 1 Note that the BCC’s parity bit is also determined by parity

50. 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: