How To Understand The Theory Of Data Communication

Transcription

1 ICOM : Computer Networks Chapter 2: The Physical Layer By Dr. Yi Qian Department of Electronic and Computer Engineering Fall 2006

2 Outline The Theoretical Basis for Data Communication Guided transmission media Wireless transmission Communication satellites The public switched telephone network The Mobile telephone system Cable television 2

3 Fourier analysis The Theoretical Basis for Data Communication Power spectral density Bandwidth-limited signals Maximum data rate of a channel 3

4 Fourier Analysis Fourier series Periodic signal g(t) with period T, we can get its Fourier series: a_n, b_n, c g ( t ) a n = = 2 T 1 2 c T 0 + Fourier transform Discrete Fourier transform n = 1 a n g ( t ) sin( sin( 2π nft ) + 2π nft ) dt, b Time domain Frequency domain f(t) F(ω) => Fourier transform <= Inverse Fourier transform n n = 1 = b 2 T n cos( 2π nft ) T 0 g ( t ) cos( 2π nft ) dt, c = 2 T T 0 g ( t ) dt 4

5 Fourier Transform Periodic in one domain Discrete in another domain Aperiodic in one domain Continuous in another domain 5

6 Signal and System LINEAR Input signal x(t) System h(t) Output signal y(t) Impulse response h(t) Input signal X(n) System H(n) Output signal Y(n) Y(n)=X(n)H(n) 6

7 Decibel Decibel (db) The db value is calculated by taking the log of the ratio of the measured or calculated power (P2) with respect to a reference power (P1). db=10 x log_10 (P2/P1) 0 dbw = 1 watt 0 dbm = 1 milliwatt = watt * For amplitude, we shall use db=20 x log_10 (A2/A1) Example: P=V^2/R 7

8 Power Spectral Density (PSD) Suppose the Fourier transform of signal f(t) is F(ω), then the power spectral density of f(t) can be expressed as Φ(ω)= F(ω) ^2=F(ω)xF*(ω) The unit of power spectral density is watt/hz 8

9 Bandwidth Bandwidth can be observe from the power spectral density There are many definitions In general, bandwidth depends on the threshold power The maximum power is often used as the reference For example, suppose Φ(fc) is the maximum Φ(f1)= Φ(f2)=0.5 x Φ(fc) The figure shows the 3dB bandwidth 9

10 Ultra-Wideband According to FCC 20dBm/Mhz Emitted Signal Power GPS PCS Bluetooth, b Cordless Phones Microwave Ovens a -41 dbm/mhz UWB Spectrum Part 15 Limit Frequency (Ghz) 10.6 ( f f ) 2 H L fractional bandwidth = > f + f H L dB Bandwidth 10

11 Bandwidth-Limited Signals Bandwidth (of a system) The range of frequencies transmitted without being strongly attenuated From 0 to the frequency at which half the power gets through (for baseband, 3dB bandwidth) Root-mean-square (RMS) amplitude Sqrt(a_n^2+b_n^2) 11

12 Bandwidth-Limited Signals x(t) y(t) y(t) 12

13 Bandwidth-Limited Signals y(t) y(t) 13

14 Bandwidth-Limited Signals In the previous example Let the bit rate = b bits/sec, then we can derive T=8/b sec The frequency of the first harmonic is b/8 Hz Suppose the channel bandwidth is 3KHz, then the highest harmonic that can pass the channel is 3,000/(b/8)=24,000/b 14

15 Bandwidth-Limited Signals 15

16 Maximum Data Rate of A Channel Nyquist theorem A signal passes through a lowpass filter with bandwidth H The output signal can be completely reconstruct by making 2H samples per second Max data rate = 2H log_2 V bits/sec V is the number of discrete level of the signal Condition, noiseless system 16

17 Maximum Data Rate of A Channel In a noisy condition, the signal-to-noise ratio SNR is defined as S/N S: power of the signal N: power of the noise 10*log_10 (S/N) is often used Shannon capacity Max number of bits per sec= H log_2 (1+S/N) Example: H=3,000Hz, S/N=30dB Max data rate = 30,000 bps 17

18 Guided Transmission Data Magnetic Media Twisted Pair Coaxial Cable Fiber Optics 18

19 Performance Characteristics Bandwidth Delay Cost Ease of install and maintenance 19

20 Magnetic Media Tape, DVD Bandwidth can be huge at a low cost and reasonable delay Assume that express delivery is used This is the reason why the video on demand (VoD) services is not practical yet 20

21 Twisted Pair UTP: Unshielded Twisted Pair Category 3 UTP Category 5 UTP More twists per centimeter 21

22 Twisted Pair UTP: Unshielded Twisted Pair Usage Telephone line Ethernet Bandwidth UTP3: 16MHz UTP5: 100MHz UTP6: 250MHz UTP7: 600MHz 22

23 Coaxial Cable A coaxial cable. 23

24 Coaxial Cable Usage Telephone network: before fiber optics Cable TV Bandwidth 1GHz 24

25 Fiber Optics Usage From LAN to WAN Bandwidth Achievable: 50Tbps Practical: > 2Tb/s 25

26 Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. 26

27 Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. 27

28 Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. 28

29 Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources. 29

30 Fiber Optic Networks A fiber optic ring with active repeaters. 30

31 Fiber Optic Networks (2) A passive star connection in a fiber optics network. 31

32 Wireless Transmission The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission 32

33 History The movement of electrons can create a electromagnetic wave The electromagnetic wave was first predicted by James Clerk Maxwell in 1865 Henrich Hertz first observed the EM wave in

34 Definition Frequency: the number of oscillations per second of a wave Denoted as f and the unit of frequency is Hz (Hertz) Wavelength: The distance between two consecutive maxima (minima) Denoted as λ 34

35 EM Wave The EM wave can be transmitted and received by antenna Omni-directional Directional Smart EM wave travels in vacuum with the speed of light c=3x10^8 m/sec Note: the speed of EM wave in copper and optical fiber is about 2x10^8 m/sec Relationship f x λ=c (in vacuum) 35

36 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 36

37 The Electromagnetic Spectrum Band name (defined by ITU) VLF: very low frequency LF: low frequency MF: medium frequency HF: high frequency VHF: very high frequency UHF: ultra high frequency SHF: super high frequency EHF: extreme high frequency THF: tremendous high frequency 37

38 BAND f x λ=c (in vacuum) df/dλ = -c/λ^2 f = λ x c / λ^2 f: corresponding to bandwidth At high frequency band, it is possible to encode multiple bits per hertz Narrow band: f/f <<1 Wide band: Spread spectrum (CDMA system) Frequency hopping Direct sequence spread spectrum Ultra-wide band (UWB): f/f > 20% or f>500mhz According to FCC (Federal communications commission) FCC allow un-licensed band 3.1GHz~10.6GHz 10dB bandwidth 38

39 Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere. 39

40 Microwave Transmission If the frequency of a wave is larger than 100MHz, then the transmission is nearly straight lines Energy can be concentrated into a small beam Wave can hardly pass through building Usage Long distance telephone communications Mobile phone Television 40

41 Infrared and Millimeter Waves Transmission: directional Cost: low Cannot pass object Pros: less interference, security Cons: limited coverage Usage: Remote controller 41

42 Lightwave Transmission Convection currents can interfere with laser communication systems A bidirectional system with two lasers is illustrated 42

43 Lightwave Transmission Convection currents can interfere with laser communication systems 43

44 Politics of the Electromagnetic Spectrum International: ITU-R US: FCC The ISM bands in the United States ISM: Industrial, Scientific, Medical b Bluetooth a 44

45 Communication Satellites Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber 45

46 Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage. 46

47 Geostationary Satellite GEO: Geostationary-earth-orbit Altitude: 35,800km Motion of the satellite: none Obit slot assignment ITU Usage Telecomm Television Military 47

48 Frequency Band for Satellites The principal satellite bands. 48

49 VSAT: Very Small Aperture Terminals VSATs using a hub. 49

50 Medium-Earth Orbit Satellites MEO: Medium earth orbit Motion of the satellite: slow (in longitude) Example: 6 hours a circuit Usage No used in telecommunication 50

51 Low-Earth Orbit Satellites LEO: Low earth orbit Motion of the satellite: fast (in longitude) Cons Need more satellite to provide coverage Pros Low power Short delay Usage Iridium Globalstar 51

52 Iridium Number of satellites: 66 Altitude: 750km Orbits: 6 Each satellite can cover up to 48 cells Total cells: 1628 Each satellite has a capacity of 3840 channels Total channels: 253,440 Calls can be switched from one satellite to another 52

53 Iridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth. (a) (b) 53

54 Globalstar Number of satellites: 48 Connections cannot be switched between two satellites 54

55 Globalstar (a) Relaying in space. (b) Relaying on the ground. 55

56 Public Switched Telephone System Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching 56

57 Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. The total hierarchy is 5 in US 57

58 Structure of the Telephone System (2) A typical circuit route for a medium-distance call. Twisted pair 58

59 The Politics of Telephones The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it 59

60 The Politics of Telephones In 1984, the US government forced AT&T to break into AT&T long line and 23 BOCs (Bell Operating Company) BOCs were then grouped in to 7 RBOCs (regional BOCs) Deregulation: 1996 Local telephone companies, long-distance companies, mobile operators and cable TV companies can enter one another;s business 60

61 The Politics of Telephones Terms LATA: local access and transport areas (164 in US) LEC: local exchange carrier Example: BOCs ILEC: Incumbent LEC CLEC: Competitive LEC IXC: inter-exchange carrier AT&T long line, MCI, Sprint, 61

62 Major Components of the Telephone System Local loops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another 62

63 The Local Loop: Modems, ADSL, and Wireless The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and CODECs. D D Last Mile A D D A 63

64 Digital Modulation A binary signal Amplitude modulation Frequency modulation Frequency shift keying Phase modulation 64

65 Digital Modulation Baud: the number of samples per second Symbol rate Note: one symbol can be used to represent multiple bits Example: suppose baud =2,400 One bit (0 or 1) / symbol => bit rate=2,400 bps Two bits (0, 1, 2, 3) / symbol => bit rate=4,800 bps QPSK: Quadrature phase shift keying Digital modulation: mapping digital information to analog symbol 65

66 Digital Modulation Suppose we have two orthogonal signals f(t) and g(t) f(t)=0, t<0 or t>t g(t)=0, t< 0 or t>t 1/T \int f 2 (t) dt = 1 \int g 2 (t) dt = 1 \int f(t)xg(t) dt = 0 1/T f(t) T t g(t) T t Suppose we need to transmit two symbols a and b We can transmit af(t)+bg(t)=s(t) At the receiver side \int s(t) f(t) dt = a \int s(t) g(t) dt = b 66

67 Constellation QPSK QAM-16 QAM-64 QAM: Quadrature amplitude modulation 67

68 Constellation TCP: Trellis code modulation (introduce more bits for error correction) (a) (b) V.32 for 9600 bps V.32 bis for 14,400 bps 5 bits (4 information and 1 parity) 2^5=32b, symbol rate=2,400/sec 2^6=128 68

69 Constellation V.32 4 bits/symbol, 9,600bps V.32 bis 6 bits/symbol, 14,400bps V bits/symbol, 28,800bps V.34 bis 14 bits/symbol, 33,600bps 69

70 Modem Date rate can be dynamically adjust Many modem will compress data Source coding Direction Both directions, simultaneously: full duplex Both directions, one at a time: half duplex One direction: simplex 70

71 Speed Limitation Two loops: 35kbps (Shannon capacity) One loop: 70kbps (Shannon capacity) 56 kbps (4kHz sampling for voice signal, 7 bits available to users I US) V.90: 33.6kbps upstream, 56kbps downstream Upstream is the direction from user to ISP V.92: 48kbps up, 56kbps down 71

72 Digital Subscriber Lines (DSL) Bandwidth versus distanced over category 3 UTP for DSL. 72

73 Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation (DMT) The modulation scheme is also known as OFDM (orthogonal FDM) 73

74 Digital Subscriber Lines (3) A typical ADSL equipment configuration. Network interface device 74

75 Wireless Local Loops Architecture of an LMDS system Standardization: IEEE WMAN 75

76 Wireless Local Loops Difference between WLL and mobile phone WLL does not provide mobility Fixed wireless WLL can provide higher data rate WLL may utilize large antenna MMDS: Multi-channel multipoint distributed service Frequency band (by FCC) 2.1GHz and 2.5GHz, total bandwidth 198MHz LMDS: local multipoint distributed service Frequency band (by FCC): 28-31GHz, total bandwidth 1.3GHz 76

77 Multiplexing Multiplex To merge multiple low data rate links into one high data rate link Technique FDM WDM TDM 77

78 Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. 78

79 Wavelength Division Multiplexing Wavelength division multiplexing. 79

80 Time Division Multiplexing The T1 carrier (1.544 Mbps). 80

81 Time Division Multiplexing Multiplexing T1 streams into higher carriers. 81

82 SDH/SONET Two back-to-back SONET frames. 82

83 Time Division Multiplexing (5) SONET and SDH multiplex rates. 83

84 Switching Circuit switching Message switching Store-and-forward Packet switching 84

85 Circuit Switching (a) Circuit switching. (b) Packet switching. 85

86 Circuit Switching A path must be setup before the data transmission To establish this path, signaling messages must be transmitted in the network There is a connection setup time (delay) After the connection is established, the delay for data is the propagation delay 86

87 Message Switching Circuit switching Message Switching Packet Switching 87

88 Packet Switching A comparison of circuit switched and packet-switched networks. 88

89 The Mobile Telephone System First-Generation Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data 89

90 Advanced Mobile Phone System (AMPS) (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. 90

91 Channel Categories in AMPS Total number of duplex channels: channels: MHz (FDM 30KHz channel) 832 channels: MHz Four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data 91

92 Second-Generation Mobile Phones D-AMPS Digital Advanced Mobile Phone System GSM Global System for Mobile Communications CDMA PDC 92

93 D-AMPS Standard: IS-45, IS-136 Usage: USA, Japan Band (in addition to 850MHz band) Upstream: MHz Downstream: MHz Channel: 30kHz Compatible to AMPS Slots in a frame: 6 Digital voice Compression: vocoder 93

94 D-AMPS (a) A D-AMPS channel with three users. (b) A D-AMPS channel with six users. 94

95 GSM Standard ETSI: European Telecommunications Standards Institute An European standard organization in the telecommunication world Usage: world wide (most popular) Channel: 200kHz Slots in a frame: 8 95

96 GSM GSM uses 124 frequency channels, each of which uses an eight-slot TDM system 96

97 GSM Frame Structure A portion of the GSM framing structure. 97

98 CDMA Standard: IS-95 Qualcomm Usage: USA 98

99 CDMA (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C s signal 99

100 Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace , fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.) Options W-CDMA: USA Wideband CDMA UMTS: European Union Universal Mobile Telecommunication Systems TD-SCDMA: China 100

101 3G Mobile Phone Systems Not fully deployed Problems 2.5G can provide data communication GPRS: General Packet Radio Service An overlay model WLANs are widely deployed 3G can provide at most 2Mbps Relatively low Future: NOT CLEAR 4G 101

102 Cable Television Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable 102

103 Community Antenna Television An early cable television system. 103

104 Cable television Internet over Cable 104

105 Internet over Cable (2) The fixed telephone system. 105

106 Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access 106

107 Cable Modems Typical details of the upstream and downstream channels in North America. 107