Computer Networks PHY

Transcription

1 Computer Networks The Physical Layer 1 PHY Transmitting information on wires. How is information represented? Digital systems. Analog systems. 2

2 What is a signal? What is a system? Signals and Systems 3 Signals and Systems (cont d) Signal: electro-magnetic wave carrying information. Time varying function produced by physical device (voltage, current, etc.). System: device (or collection thereof) or process (algorithm) having signals as input and output. 4

3 Signals and Systems (cont d) 5 Signals and Systems (cont d) Periodic signals: f(t+t) = f(t) Period = T (seconds) Frequency = 1/ Period cycles / sec. = Hertz (Hz) 6

4 Analog Technology Analog devices maintain exact physical analog of information. E.g., microphone: the voltage v(t) at the output of the mic is proportional to the sound pressure v(t) 7 Digital Technology It uses numbers to record and process information Inside a computer, all information is represented by numbers. Analog-to-digital conversion: ADC Digital-to-analog conversion: DAC ADC DAC 8

5 Digital Technology All signals (including multimedia) can be encoded in digital form. Digital information does not get distorted while being stored, copied or communicated. 9 Digital Communication Technology Early example: the telegraph (Morse code). Uses dots and dashes to transmit letters. It is digital even though uses electrical signals. The telephone has become digital. CDs and DVDs. Digital communication networks form the Internet. The user is unaware that the signal is encoded in digital form. 10

6 Two Levels are Sufficient Computers encode information using only two levels: 0 and 1. A bit is a digit that can only assume the values 0 and 1 (it is a binary digit). A word is a set of bits Example: ASCII standard for encoding text A = ; B = ; A byte is a word with 8 bits. 11 Definitions 1 KB = 1 kilobyte = 1,000 bytes = 8,000 bits 1 MB = 1 megabyte = 1,000 KB 1 GB = 1 gigabyte = 1,000 MB 1 TB = 1 terabyte = 1,000 GB 1 Kb = 1 kilobit = 1,000 bits 1 Mb = 1 megabit = 1,000 Kb 1 Gb = 1 gigabit = 1,000 Mb 1 Tb = 1 terabit = 1,000 Gb 12

7 Digitization Digitization is the process that allows us to convert analog to digital (implemented by ADC). Analog signals: x(t) Defined on continuum (e.g. time). Can take on any real value. Digital signals: q(n) Sequence of numbers (samples) defined by a discrete set (e.g., integers). 13 Digitization - Example Analog signal x(t) Digitized signal q(n) x(t) q(n)

8 Some Definitions Interval of time between two samples: Sampling Interval (T). Sampling frequency F=1/T. E.g.: if the sampling interval is 0.1 seconds, then the sampling frequency is 1/0.1=10. Measured in samples/second or Hertz. Each sample is defined using a word of B bits. E.g.: we may use 8 bits (1 byte) per sample. 15 Bit-rate Bit-rate = numbers of bits per second we need to transmit For each second we transmit F=1/T samples. Each sample is defined with a word of B bits. Bit-rate = F*B. Example: if F is 10 samples/s and B=8, then the bit rate is 80 bits/s. 16

9 Example of Digitization Bit-rate=BF=16 bits/second B=4 bits/sample F=4 samples/second Time (seconds) 17 Bit-rate - Example 1 What is the bit-rate of digitized audio? Sampling rate: F= 44.1 KHz Quantization with B=16 bits Bit-rate = BF= Kb/s Example: 1 minute of uncompressed stereo music takes more than 10 MB! 18

10 Bit-rate - Example 2 What is the bit-rate of digitized speech? Sampling rate: F = 8 KHz Quantization with B = 16 bits Bit-rate = BF = 128 Kb/s 19 Data Transmission Analog and digital transmission. Example of analog data: voice and video. Example of digital data: character strings Use of codes to represent characters as sequence of bits (e.g., ASCII). Historically, communication infrastructure for analog transmission. Digital data needed to be converted: modems (modulator-demodulator). 20

11 Digital Transmission Current trend: digital transmission. Cost efficient: advances in digital circuitry. (VLSI). Advantages: Data integrity: better noise immunity. Security: easier to integrate encryption algorithms. Channel utilization: higher degree of multiplexing (time-division mux ing). 21 Guided Transmission Data Magnetic Media Twisted Pair Coaxial Cable Fiber Optics 22

12 Examples? Advantages? Disadvantages? Magnetic Media 23 Twisted Pair Oldest but still very common. Telephone system. Cheap and effective for long ranges. Bundles of twisted pairs. Can transmit both analog and digital signals. Bandwidth depends on thickness of wire and distance traveled. Mb/s for few kilometers. 24

13 Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP. 25 Twisted Pair 26

14 Coaxial Cable Better performance than twisted pair, i.e., higher bandwidth and longer distances. Good noise immunity. But Bandwidths close to 1GHz. Used widely in telephone networks for longer distances; but gradually being replaced by fiber. Used for CATV! 27 Coaxial Cable 28

15 Fiber Optics Optical transmission. Optical transmission system: light source, medium, and detector. Pulse of light = 1. No light = 0. Transmission medium: ultra thin fiber of glass. Detector: generates electrical pulse when perceives light. 29 Transmitting Light (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. 30

16 Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. 31 Fiber Optic Networks A fiber optic ring. 32

17 Fiber Optic Networks (2) A passive star connection in a fiber optics network. 33 Fiber versus Copper Wire Fiber can handle much higher bandwidths. Low attenuation: 50km without repeater. Unaffected by power surges/outages, and interference. Fiber is thin and lightweight: easy to deploy and add new capacity. Difficult to tap. But 34

18 Fiber versus Copper (cont d) Fiber can be damaged easily. Optical transmission is unidirectional, so need 2 fibers or 2 frequencies for 2-way communication. Fiber and fiber interfaces is more expensive. 35 Public Switched Telephone System Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching 36

19 Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. 37 Structure of the Telephone System (2) A typical circuit route for a medium-distance call. 38

20 Major Components of the Telephone System Local loops: Connection from subscriber to end office. Trunks Outgoing lines connecting offices. Toll office: Connects end offices. Switching offices Where calls are moved from one trunk to another. 39 PSTN 40

21 Local Loop Last mile. End office-subscriber connection. Analog, twisted pair. Traditionally, voice but it has been changing: data transmission. To transmit data, conversion digital to analog: modem. At phone office, data usually converted back to digital for long-distance transmission over trunks. 41 Transmission Impairments Problems that happen with signal as it propagates. Attenuation: loss of energy as signal propagates. Different frequencies suffer different attenuation. Different Fourier components attenuated by different amount. Distortion: different Fourier components shifted in time. Noise: unwanted energy from other sources. E.g., thermal noise: unavoidable random motion of electrons in wire. 42

22 Modulation Signal with wide range of frequencies is undesirable. Square waves exhibit wide frequency range. To avoid that, AC signaling is used. Sine wave carrier to carry information. Modulation: Information is encoded in the carrier by varying either amplitude, frequency, or phase. 43 Modulation: Examples Binary signal Amplitude modulation Frequency modulation Phase modulation 44

23 Modem Modulator-demodulator. Modulates digital signal at the source and demodulates received signal at the destination. How to transmit faster? Nyquist says that capacity is achieved at 2*H*log 2 V. So there is no point sampling faster than 2*H. But, can try to send more bits per sample. 45 Baud Rate Baud rate = symbols/sec. Data rate = bits/sec. If 2 voltage levels are used, then 1 symbol=1bit. Baud rate = bit rate. But, if can encode more than 1 bit in a symbol E.g., if voltages 0, 1, 2, and 3, every symbol consists of 2 bits. Thus, 2400 baud line corresponds to 4800 bps. The same thing for 4 different frequencies: QPSK. 46

24 Bandwidth, Baud- and Bit Rates Bandwidth: physical property of medium. Range of frequencies transmitted with adequate quality. Measured in Hz. Baud rate is number of samples/sec or symbols/sec. Modulation technique determines number of bits/symbol: symbols/sec * bits/symbol. Modern modems transmit several bits/symbol frequently combining multiple modulation schemes. 47 Full Duplex, Half Duplex, Simplex Full duplex: traffic in both directions simultaneously. Half duplex: traffic in both directions but 1 direction at a time. Simplex: traffic allowed only one way. Examples? 48

25 What s next? Modems were getting faster, e.g., 56Kbps. But, demand for faster access was growing! CATV and satellite as competitors. Phone company s response: DSL. Broadband access. ADSL: asymmetric digital subscriber line. When you subscribe to DSL service, you are connected to the local office without the filter to frequencies below 300Hz and above 3400Hz. Physical limitation still exists and depends on thickness, length, etc. 49 Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL. 50

26 Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation. Available 1.1MHz local loop spectrum divided into 256 channels (4.3KHz each). 51 ADSL Typically, 32 channels for upstream and the rest for downstream traffic. Usually, 512 Kbps downstream and 64 Kbps upstream (standard) and 1 Mbps downstream and 256 Kbps upstream (premium). Within each channel, modulation scheme is used (sampling at 4000 baud). 52

27 Typical ADSL Setup A typical ADSL equipment configuration. 53 Wireless Local Loop Last mile is wireless. Why? Historically: local telcos had monopoly for local telephone service. In the mid 1990 s market open to competition, e.g., long distance carriers. Cheaper alternative to stringing cables to customers is using a wireless local loop. Mobile telephony? Fixed wireless. 54

28 Wireless Local Loops Architecture of an LMDS system. Tower with multiple highly directional antennae; but small range (2-5Km). 55 Trunking and Multiplexing 56

29 Trunking Deployment of high-bandwidth pipes. Current and future demand. Switching offices higher in the PSTN hierarchy. Multiplexing: ability to send a number of conversations simultaneously over the same pipe. Multiplexing schemes: Frequency Division Multiplexing (FDM). Time Division Multiplexing (TDM). 57 The Multiplexing Problem frequency Shared channel (how to divide resource among multiple recipients?) Analogy: a highway shared by many users time 58

30 Frequency-Division Multiplexing frequency user 1 user 2 user 3 user 4 guard-band Analogy: a highway has multiple lanes time 59 Time-Division Multiplexing frequency user 1 user 2 user 3 user 4 user 1 user 2 guard-band Requirement: precise time coordination time 60

31 Frequency-Time-Division frequency time-slot (usually of the same size) time 61 Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (c) The multiplexed channel. 62

32 FDM versus TDM FDM requires analog circuitry. TDM can be done entirely using digital electronics. But TDM can only be used for digital data. Analog signals from local loops need to be digitized (at the local office). At end office, all individual local loops arrived, are digitzed, and multiplexed. 63 TDM Multiplexing 64

33 PCM Pulse Code 65 PCM Pulse Code Modulation: Digitization of voice channels. Sampling frequency If voice signal peaks at 4KHz, what s the sampling frequency? Nyquist: 8000 samples/sec, or 125 microsec/sample. Each sample is 8 bits (7 for data and 1 for control). Data rate: 7*8000 = 56Kbps of data and 8Kbps of signaling (per channel). No world-wide standard for PCM. In the US and Japan: T1 (technically DS1). 66

34 T1 The T1 carrier (1.544 Mbps). T1: 24 multiplexed voice channels: Mbps. 67 T2 and Beyond Multiplexing T1 streams into higher carriers. 68

35 SONET/SDH SONET and SDH multiplex rates. SONET: Synchronous Optical NETwork. SDH: Sync Digital Hierarchy. Optical TDM for fiber transmission 69 Switching 70

36 Circuit- and Packet Switching (a) Circuit switching. (b) Packet switching. 71 Switching Circuit- Message- Packet Switching 72

37 Packet Switching 73 Wireless Transmission 74

38 Wireless Transmission Electron movement: electromagnetic waves that propagate through space. T R 75 Propagation Maximum speed: speed of light, c, 3*10 8 m/s. In vacuum, all EM waves travel at the same speed c. Otherwise, propagation speed is function of frequency (c = λ * f), where f is frequency (Hz) and λ is wavelength (m). 76

39 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. 77 Radio Transmission ~1Km (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. E.g., AM radio uses MF. (b) In the HF and VHF bands, they bounce off the ionosphere. E.g., Hams and military. 78

40 Microwave Transmission Above 100MHz. Waves travel in straight lines. Directionality. Better quality. Space Division Multiple Access. But, antennas need to be aligned, do not go through buildings, multi-path fading, etc. Before fiber, microwave transmission dominated long-distance telephone transmission. 79 Politics of the Electromagnetic Spectrum Need agreements to regulate access. International and national. Local governments allocate spectrum for radio (AM and FM), TV, mobile phones, emergency services, etc. In the US, FCC. World-wide, ITU-R tries to coordinate allocation so devices work everywhere. Separate frequency band that is unregulated. ISM: Industrial, Scientific, and Medical. Household devices, wireless phones, remote controls, etc. 80

41 Spread Spectrum Narrow frequency band -> good reception (power, bandwidth). But in some cases, wide band is used, aka, spread spectrum. Modulate signal to increase bandwidth of signal to be transmitted. 2 variations: Frequency Hopping (FH). Transmitter hops frequencies Direct Sequence (DS). Use spreading code to convert each bit of the original signal into multiple bits. 81 Infrared Transmission Short range (e.g., remote controls). Directional, cheap. But, do not pass through obstacles. 82

42 Lightwave Transmission Unguided optical transmission. E.g., laser communication between two buildings for LAN interconnection. High bandwidth, low cost. Unidirectionality. Weather is a major problem (e.g., rain, convection currents). 83 Communication Satellites Weather balloons. The moon. Artificial satellites: Geostationary. Medium-Earth Orbit. Low-Earth Orbit. 84

43 Satellite Communications SAT ground stations 85 Satellite Communications Satellite-based antenna(e) in stable orbit above earth. Two or more (earth) stations communicate via one or more satellites serving as relay(s) in space. Uplink: earth->satellite. Downlink: satellite->earth. Transponder: satellite electronics converting uplink signal to downlink. 86

44 Orbits Shape: circular, elliptical. Plane: equatorial, polar. Altitude: geostationary (GEO), medium earth (MEO), low earth (LEO). 87 Communication Satellites 88

45 GEOs High-flying satellites. Orbit at 35,863 Km above earth and rotates in equatorial plane. Many GEO satellites up there! 89 GEO: Plus s and minus s Plus s: Stationarity: no frequency changes due to movement. Tracking by earth stations simplified. At that altitude, provides good coverage of the earth. Minus s: Weakening of signal. Polar regions poorly served. Delay! Spectral waste for point-to-point communications. 90

46 Principal Satellite Bands. Downlink frequencies interfere with microwave.. Internationally-agreed frequency bands. 91 LEO Satellites Circular or slightly eliptical orbit under 2,000 Km. Orbit period: 1.5 to 2 hours. Coverage diameter: 8,000 Km. RTT propagation delay < 20ms (compared to > 300ms for GEOs). Subject to large frequency changes and gradual orbit deterioration. 92

47 LEO Constellations Advantages over GEOs: Lower delay, stronger signal, more localized coverage. But, for broad coverage, many satellites needed. Example: Iridium (66 satellites). 93 LEOs SAT constellation SAT SAT ground stations 94

48 Low-Earth Orbit Satellites Iridium (a) (b) (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth. 95 In Summary GEOs Long delay ms. LEOs Relatively low delay ms. Large variations in delay - multiple hops/route changes, relative motion of satellites, queuing. 96

49 Satellite Data Rates Satellite has transponders, each ranging from Mbps. T1: 1.54 Mbps. T2: Mbps. T3: Mbps. T4: Mbps. 97 The Mobile Telephone System First-Generation Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data 98

50 The Cell Concept (a) Frequencies not reused in adjacent cells. (b) To add more users, smaller cells. 99 Mobile Phone System Structure Hierarchy. Base station. Mobile Switching Center (MSC). MSCs connected through PSTN. 100

51 Handoffs As mobile phones move, they switch cells, and thus base stations. Soft versus hard handoffs. Two base stations while handoff is in progress. Hard handoff. Roaming. 101 Cable Television 102

52 Community Antenna Television An early cable television system. 103 Internet over Cable Cable television 104

53 DSL The fixed telephone system. 105 ADSL versus Internet over Cable Both uses fiber in the backbone. ADSL uses twisted pair and IoC uses coax on the edge. Coax has higher capacity but shared with TV. IoC s capacity is unpredicatble as it depends on how many users/traffic. 106