1. Using the Internet from Home: The Physical Layer Chapter 4 Copyright 2001 Prentice Hall Revision 2: July 2001

2. 2 Orientation n Using the Internet from Home – There are other applications – There are other ways to access the Internet n Chapter 3 – Upper layers: HTTP, TCP, IP, and PPP – Governed by messages n This Chapter (4) – Physical layer standards – Transmit one bit at a time – Direct connection host-router and router-router

3. 3 Analog Transmission n In analog transmission, the state of line can vary continuously and smoothly among an infinite number of states – States can be signal strengths, voltages, or other measurable conditions – Human voice is analog; telephone mouthpiece generates analogous electrical signal Time Strength

4. 4 Digital Transmission n Time is divided into fixed-length clock cycles – For modems, there are a few thousand clock cycles per second – For LANs, there are millions of clock cycles per second n The line is kept in one of only a few possible states (conditions) during each time period – cycle; this is why the signal must be kept constant n At the end of each time period, the line may change abruptly to another of these few states – Can also stay the same

5. 5 Digital Versus Binary Transmission n Digital transmission: a few states n Binary transmission: exactly two states (1 and 0) – Binary is a special case of digital DigitalBinary Two StatesFew States 0 1

6. 6 Digital Versus Binary Transmission n Sender and Receiver associate one or more bits with each state – Simplest case: High state = 1, Low state = 0 – If four states, might have the following: n Highest = 11 n Second highest = 10 n Next highest = 01 n Lowest = 00

7. 7 Number of States versus Number of Bits Represented per Clock Cycle n 2 Bits per clock cycle =Number of states – For 1 bit per clock cycle, – 2 states are required (One for 1, one for 0) – 2 1 =2 – Binary 1 0 000 1 Clock Cycle States

8. 8 Number of States versus Number of Bits Represented per Clock Cycle n 2 Bits/clock cycle =States/clock cycle – For 2 bits per clock cycle, 4 states are required (2 2 =4) – For 3 bits per clock cycle, 8 states are needed (2 3 =8) – For 4 bits per clock cycle, 16 states are needed (2 4 =16) 3 (11) 2 (10) 1 (01) 0 (00) 00 01 10 11 Clock Cycle States

9. 9 Number of States versus Number of Bits Represented per Clock Cycle n 2 Bits per clock cycle =States/clock cycle – With 4 states, send two bits per clock cycle (2 2 =4) – With 8 states, send 3 bits per clock cycle (2 3 =8) – With 16 states, send 4 bits per clock cycle (2 4 =16) 3 (11) 2 (10) 1 (01) 0 (00) 00 01 10 11 Clock Cycle States

10. 10 Bits and Baud n Baud Rate = Number of clock cycles/sec – In this example, 4 baud (not 4 bauds/second) – Note: Number of clock cycles, not actual line changes n Bit Rate = Number of bits/second – In this example, 8 bits/second 00 01 10 01 1 Second Possible Change Not Made

11. 11 Equations n For Each Clock Cycle – 2 Bits per clock cycle = Number of possible states (Eq. 1) n Overall – Bit rate = Baud Rate * Bits per clock cycle (Eq. 2) n Example – Baud rate of 10,000 with four possible states – Bits per clock cycle = 2 (2 2 =4) (Eq. 1) – Bit rate = 10,000 * 2 (Eq. 2) – Bit rate = 20,000 bps

12. 12 Transmission Speeds n Bit: A single 1 or 0 n Bits per second (bps) – Factors of 1,000 (not 1,024 as in memory) – kilobits per second (kbps)--Note lower case k – megabits per second (Mbps) – gigabits per second (Gbps) – terabits per second (Tbps) – petabits per second (Pbps) n Occasionally given in bytes per second (Bps) – Bits per second / 8 – Uncommon 100101001... New

13. 13 Wire Propagation Effects n Propagation Effects – Signal changes as it travels – If change is too great, receiver may not be able to recognize it Distance Original Signal Final Signal

14. 14 Wire Propagation Effects: Attenuation n Attenuation: Signal Gets Weaker as it Propagates – May become too weak for receiver to recognize Signal Strength Distance

15. 15 Wire Propagation Effects: Distortion n Distortion: Signal changes shape as it propagates – Adjacent bits may overlap – May make recognition impossible for receiver Distance

16. 16 Wire Propagation Effects: Noise n Noise: Thermal Energy in Wire Adds to Signal – Noise floor is average noise energy – Random energy, so brief noise spikes sometimes occur Signal Strength Time Noise Spike Noise Floor

17. 17 Wire Propagation Effects: Noise n Noise: Thermal Energy in Wire Adds to Signal – If noise spikes become as large as the signal, they are likely to cause errors, switching 1s and 0s or just distorting the signal so that it cannot be received Signal Strength Time Signal Noise Spike Error

18. 18 Wire Propagation Effects n Noise and Attenuation – As signal attenuates, gets closer to noise floor – Smaller spikes can harm the signal – So noise errors increase with distance, even if the average noise level is constant Signal Strength Distance Signal Noise Floor

19. 19 Wire Propagation Effects: SNR n Want a high Signal-to-Noise Ratio (SNR) – Signal strength divided by average noise strength – As SNR falls, errors increase Signal Strength Distance Signal Noise Floor SNR

20. 20 Wire Propagation Effects: Noise & Speed n Noise and Speed – As speed increases, each bit is briefer – Noise fluctuations do not average out as much – So noise errors increase as speed increases One Bit Noise Spike Average Noise During Bit Low Speed (Long Duration) One Bit Noise Spike Average Noise During Bit High Speed (Short Duration) OK Error

21. 21 Wire Propagation Effects: Interference n Interference – Energy from outside the wire (nearby motors, other wires, etc.) – Adds to signal, like noise – Often intermittent (comes and goes), so hard to diagnose – Often called electromagnetic interference (EMI) Signal Strength Signal Interference New

22. 22 Wire Propagation Effects: Cross-Talk Interference n Cross-Talk Interference – Often, there are multiple wires in a bundle – Each radiates some of its signal – Causes “cross-talk” interference in nearby wires

23. 23 Wire Propagation Effects:Cross Talk n Wire Usually is Twisted – Usually, several twists per inch – Interference adds to signal over half twist, subtracts over other half – Roughly cancels out – Simple but effective Single Twist Interference -+ Signal

24. 24 Wire Propagation Effects:Cross Talk n Terminal Cross-Talk Interference – Wire must be untwisted at ends to fit into connectors – So cross-talk interference is high at termination – Problems severe if untwist more than about 1.25 cm (1/2 inch) – Usually the biggest propagation effect Terminal Cross Talk

25. 25 Practical Issues in Propagation Effects n Distance limits in standards prevent serious propagation effects – For instance, usually 100 meters maximum for ordinary copper wire – If stay within limits, usually no serious problems n Problems usually occur at connectors – Crossed wires – Poor connections – Cross-talk interference New

26. 26 Wire Media: UTP n Unshielded Twisted Pair (UTP) – Ordinary copper wire – Twisted several times per inch to reduce interference – Pair of wires needed for a complete electrical signal – Unshielded: nothing but plastic coating n No protection from interference such as a wrap- around foil covering

27. 27 Wire Media: UTP n Unshielded Twisted Pair (UTP) – Business telephone wiring traditionally comes in 4-pair UTP wire bundles – Used in LAN wiring to use existing building wiring technology Jacket

28. 28 Wire Propagation: RJ-45 n RJ-45 connector terminates a UTP bundle – Slightly wider than RJ-11 residential telephone connector – Width needed for 8 wires RJ-45 Connector RJ-45 Jack

29. 29 Wire Media: UTP to the Desktop n UTP – Dominant for line from desktop to first hub or switch – Inexpensive to buy and install – Rugged: can take punishment of office work – Easily 100 Mbps, 1 Gbps with careful insulation UTP First Hub or Switch

30. 30 Wire Media: Optical Fiber n Optical Fiber – Glass core, surrounding glass cladding – Light source turned on/off for 1/0 – Total internal reflection at boundary – Almost no attenuation Light Source Cladding Core Reflection

31. 31 Wire Media: Optical Fiber n Limited by Distortion – Light entering at different angles travels different distances (different number of reflections) – Called different modes – Light from successive bits becomes mixed over long distances Light Source Mod B

32. 32 Wire Media: Optical Fiber n Multimode Fiber – Wide core makes easy to splice (50 or 62 microns) – Many angles for rays (modes) – Short propagation distance (usually 200 m to 500 m) Light Source Mod B

33. 33 Wire Media: Optical Fiber n Single Mode Fiber – Narrow core difficult to splice (5 or 8 microns) – Only one angle for rays (one mode), so (almost) no distortion – Longer propagation distance (usually up to 2 km for LAN fiber, longer for long-distance fiber) – Narrow core makes fiber fragile and difficult to splice Mod B

34. 34 Wire Media: Optical Fiber n Optical Fiber – High speeds over long distances n 200 m to 2 km – Costs more than UTP, but worth it on long runs – Good for all links between hubs and switches within and between buildings in a site network Optical Fiber

35. 35 Wire Media: UTP and Optical Fiber n The emerging pattern: UTP from first hub or switch to desk, Fiber everywhere else on site

36. 36 Wire Media: Coax n Coaxial Cable – Used in cable TV, VCRs – Central wire, external concentric cylinder – Outer conductor wrapped in PVC Screw-On Connector Inner Wire Outer Conductor Wrapped in PVC

37. 37 Wire Media: Coaxial Cable n Coaxial Cable – Installed widely today in old 10 Mbps Ethernet LANs – Not being used in new installations n Optical fiber more cost-effective for long links n UTP more cost-effective for desktop links

38. 38 PC 232 Serial Ports n Ports – Connectors at back of PC – Plus related internal electronics to send/receive n PC 232 Serial Port – Follows EIA/TIA 232 standards

39. 39 PC 232 Serial Ports: 9-Pin and 25-Pin Ports n 9 pins or 25 pins n Parallel ports have 25 holes Pins Holes 9-pin Serial Port 25-pin Serial Port 25-pin Parallel Port

40. 40 232 Serial Ports: Send on One Pin Each Way n 9-Pin 232 Serial Ports – PC sends on Pin 3 (modem receives) – PC receives on Pin 2 (modem sends) – Pin 5 is a signal ground defining zero volts PCModem

41. 41 232 Serial Ports: Send on One Pin Each Way n 9-Pin 232 Serial Ports – Other pins are control signals to tell other side when it may transmit – Or tell PC what modem is hearing on the line (ringing, modem carrier signal) PCModem

42. 42 Serial and Parallel Transmission n Serial: one wire, one bit per clock cycle* – Second (ground) wire needed for circuit is not shown 10 One Bit in Clock Cycle One Bit in Clock Cycle Two *For simplicity, we assume binary transmission (2 possible states/clock cycle)

43. 43 Serial and Parallel Transmission n Parallel – N bits per second on N wires – Parallel is faster than serial 1 1 0 1 1 0 0 1 1 0 1 1 0 0 00 Eight Bits In Clock Cycle One Eight Bits In Clock Cycle Two

44. 44 Serial and Parallel Transmission n Parallel Transmission – N bits per second on N wires – N=8 in this example, but this is not the only possibility – N can also be 4, 16, 32, etc. 1 1 0 1 1 0 0 1 1 0 1 1 0 0 00

45. 45 Serial and Parallel Transmission n Parallel Transmission is Only for Short Distances – Usually up to about 2 meters (6 feet) – Wire propagation speeds vary – Over long distances, bits from different clock cycles overlap 1 1 0 1 1 0 0 0 1 1 0 1 1 0 0 0

46. 46 PC 232 Serial Ports: Voltages n For sending data – One is -3 to -15 volts (Yes, one is low) – Zero is +3 to +15 volts (Yes, zero is high) – Binary (only two possible states) +15v -15v 0 1 0

47. 47 PC 232 Serial Ports n PC 232 serial ports are binary because there are only two states (voltage levels) n PC 232 serial ports are serial because data is sent on only one wire at a time n These are separate things – One does not require the other

48. 48 Duplex n Full-duplex transmission: both sides can transmit simultaneously – Even if only one sends, still full-duplex line – Even if neither is sending, still full-duplex line AB Time 1 Both can send Both do AB Time 2 Both can send Only A does AB Time 3 Both can send Neither does

49. 49 Duplex n Half-duplex transmission: only one can transmit at a time; must take turns – Still half duplex if neither transmits ABAB Time 1 Only one side Can send A does Time 2 Only one side Can send Neither does

50. 50 Duplex n Duplex is a Characteristic of the Transmission System, Not of Use at a Given Moment – In full duplex, both sides can transmit at once; in half duplex, only one side can transmit at a time – Still full duplex system if only one side or neither side actually is transmitting at a moment – Still half duplex if neither side actually is transmitting at a moment

51. 51 Radio Propagation n Broadcast signal – Not confined to a wire

52. 52 Radio Waves n When Electron Oscillates, Gives Off Radio Waves – Single electron gives a very weak signal – Many electrons in an antenna are forced to oscillate in unison to give a practical signal

53. 53 Radio Propagation Problems n Wires Propagation is Predictable – Signals go through a fixed path: the wire – Propagation problems can be easily anticipated – Problems can be addressed easily n Radio Propagation is Difficult – Signals begin propagating as a simple sphere – Inverse square law attenuation – If double distance, only ¼ signal strength – If triple distance only 1/9 signal strength New

54. 54 Radio Propagation Problems n Radio Propagation is Difficult – Signals can be blocked by dense objects – Creates shadow zones with no reception New Shadow Zone

55. 55 Radio Propagation Problems n Radio Propagation is Difficult – Signals are reflected – May arrive at a destination via multiple paths – Signals arriving by different paths can interfere with one another: called multipath interference – Can be constructive or destructive interference – Very different reception characteristics with in a few meters or centimeters New

56. 56 Radio Propagation: Waves n Waves Amplitude (strength) Wavelength (meters) Frequency in hertz (Hz) Cycles per Second One Second 7 Cycles 1 Hz = 1 cycle per second 1 4 3 2

57. 57 Radio Propagation: Frequency Spectrum n Frequency Spectrum – Frequencies vary (like strings in a harp) – Frequencies measured in hertz (Hz) – Frequency spectrum: all possible frequencies from 0 Hz to infinity 0 Hz

58. 58 Frequencies n Metric system – kHz (1,000 Hz) kilohertz; note lower-case k – MHz (1,000 kHz) megahertz – GHz (1,000 MHz) gigahertz – THz (1,000 GHz) terahertz

59. 59 Radio Propagation: Service Bands n Service Bands – Divide spectrum into bands for services – A band is a contiguous range of frequencies – FM radio, cellular telephone service bands etc. 0 Hz Cellular Telephone FM Radio AM Radio Service Bands

60. 60 Radio Propagation: Channels and Bandwidth n Service Bands are Further Divided into Channels – Like television channels – Bandwidth of a channel is highest frequency minus lowest frequency 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

61. 61 Radio Propagation: Channels and Bandwidth n Example – Highest frequency of a radio channel is 43 kHz – Lowest frequency of the radio channel is 38 kHz – Bandwidth of radio channel is 5 kHz (43-38 kHz) 0 Hz Channel 3 Channel 2 Channel 1 Service Band Channel Bandwidth

62. 62 Radio Propagation: Channels and Bandwidth n Shannon’s Equation – W is maximum possible (not actual) transmission speed in a channel – B is bandwidth of the channel: highest frequency minus lowest frequency – S/N is the signal-to-noise ratio W = B Log 2 (1 + S/N)

63. 63 Radio Transmission: Broadband n Speed and Bandwidth – The wider the channel bandwidth (B), the faster the maximum possible transmission speed (W) – W = B Log 2 (1+S/N) Maximum Possible Speed Bandwidth

64. 64 Telephony is Narrowband n Bandwidth in Telephone Channels is Narrow – Sounds below about 300 Hz cut off to reduce equipment hum within telephone system – Sounds above about 3,400 Hz cut off to reduce the bandwidth needed to send a telephone signal 20 kHz300 Hz 3.4 kHz 3.1 kHz Revised Discussion

65. 65 Telephony is Narrowband n Bandwidth in Telephone Channels is Narrow – Signal is placed within a 4 kHz channel n Gives “guardbands” – Compared to 20 kHz channels, allows 5x number of signals in radio transmission 20 kHz300 Hz 3.4 kHz 3.1 kHz 4 kHz Channel Revised Discussion

66. 66 Telephony is Narrowband n Narrow Channels Mean Low Speed – Through Shannon’s equation, maximum possible transmission speed in each telephone channel is only about 35 kbps. n This is narrowband transmission 20 kHz300 Hz 3.4 kHz 3.1 kHz 4 kHz Channel Revised Discussion

67. 67 Broadband n Two Uses of the Term “Broadband” n Technically, the signal is transmitted in a single channel AND the bandwidth of the channel is large – Therefore, maximum possible transmission speed is high n Popularly, if the signal is fast, the system is called “broadband” whether it uses channels at all