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The Physical Layer

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  1. The Physical Layer Chapter 2

  2. The Theoretical Basis for Data Communication • Fourier Analysis • Any reasonably behaved periodic function can be written as Fourier series. • Bandwidth-Limited Signals • How fast a signal can be transmitted depends on the bandwidth, general meaning of how much information can be carried in a given time period (usually a second) over a communication link, measured mostly by frequency range. • Maximum Data Rate of a Channel

  3. Theory of Data Communications • The signal (for example, measured in volts) can be viewed as • a function of time, g(t), or • a function of frequency, G(f). • Time-Domain • Let g(t) denote the voltage on a wire at time t. • A signal, g(t), is periodic with period T if g(t+T)=g(t) for all t. • A signal is discrete if it only takes on a finite number of values. • The fundamental frequency is the inverse of the period, f = 1/T, and is measured in cycles per second (Hz).

  4. Frequency-Domain Analysis • Any "reasonably-behaved" periodic function, g(t), can be written as a Fourier Series - that is broken up into components with different frequencies. • The time, T, required to transmit a character depends on: • the encoding method • the signalling speed or baud rate; that is, how many times per second the signal changes its value (voltage). • Baud rate is not necessarily the same as bit rate. For example, if the values 0, 1, 2, 3, 4, 5, 6, 7 are used in a signal, then each signal value can represent 3 bits. That is 1 baud = 3 bps.

  5. Bandwidth-Limited Signals • A binary signal (‘b’ = 01100010) and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

  6. Bandwidth-Limited Signals (d) – (e) Successive approximations to the original signal.

  7. Frequency-Domain Analysis • Below we will only consider 2 voltage levels, so the bit rate is the same as the baud rate. • Let b = bit rate (measured in bits per second (bps)). • Then, it takes 8/b seconds to send 8 bits (one character). • So, T = 8/b, and the fundamental frequency is b/8 Hz. • A voice grade line is an ordinary telephone line and has an artificial cutoff frequency, fc, of about 3000Hz. So, the number of the highest harmonic that can be passed through is 3000/(b/8) = 24000/b. Note the highest harmonic has a frequency that is a multiple of the fundamental frequency (b/8) and the highest harmonic can have a frequency no more than 3000Hz.

  8. Bandwidth-Limited Signals Relation between data rate and harmonics.

  9. Maximum Data Rate of a Channel • Noiseless channel: Nyquist’s Theorem – If the signal has V discrete levels over a transmission medium of bandwidth H , the maximum data rate = 2H log2V bits/sec • Example: a noiseless 3-kHz channel cannot transmit binary signals at a rate exceeding 6000 bps (= 2 x 3000 log2 2). • Noisy Channel: Shannon’s Theorem maximum data rate = H log2(1 + S/N) bits/sec H: bandwdith, S: signal power, N: noise power • S/N (Signal-to-noise ratio), usually measured as 10 log10S/N in db = decibels, is called thermal noise ratio.

  10. Physical Interfaces • Physical layer is responsible for the generation, transmission, and receipt of binary data • Generation and Receipt • Conversion of data between binary and analog • E.g. wire: voltage is applied • +V means a 1 • -V means a 0 • 0V means no data

  11. Physical Interfaces +V t -V 0 1 1 0 1 0 0 1

  12. Physical Interfaces • Errors in physical layer: • Attenuation (reduced signal) • Distortion (wrong signal) • Influences to error: • Type of Media • Bit Rate • Distance • Finally, binary values are passed to Data Link

  13. Guided Transmission Data • Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics

  14. Guided Transmission Data • Magnetic Media: magnetic tape or removable media • Consider an industry standard Ultrium tape • It can hold 200 gigabytes. • A box 60x60x60 cm can hold about 1000 of these tapes. Total capacity is 200 terabytes or 1600 terabits (1.6 petabits). • The box can be sent to anywhere in US in 24 hours. The effect bandwidth is 1600 terabits/86,400 sec, or 19 Gbps. • If it is sent within an hour drive, the bandwidth is increased to over 400 Gbps. No computer network can even approach this.

  15. Twisted Pair Countervail the magnetic field • Properties • A twisted pair consists of two insulated copper wires. • Why twisted? • Used in telephone and local area networking • Run several kilometers • The bandwidth depends on the thickness of the wire and distance travelled. • Common types: UTP (Unshielded Twisted Pair) • Category 3: bandwidth of 16 MHz • Category 5: more twists per centimeter, which results in less crosstalk and better-quality signal over longer distance, bandwidth of 100 MHz • Category 6 and 7: 250 MHz and 600 MHz

  16. Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP.

  17. Coaxial Cable • 50-ohm cable for digital transmission • 75-ohm cable for analog transmission and cable television • 1 GHz • Local area networking and CATV

  18. Coaxial Cable A coaxial cable.

  19. Fiber Optics • Glass is used instead of copper wires • Light is transmitted instead of electrical current • Components: • Light source • Transmission medium • Detector: convert light plus and electronic signal • Single-mode fiber • Different rays bouncing around at different angle are said to be a multimode fiber. • If the fiber’s diameter is reduced to a few wavelength of light, the light can propagate in a straight line without bouncing, yielding a single-mode fiber. • 50 Gbps for 100 km

  20. 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.

  21. Transmission of Light through Fiber • Three bands are used: 0.85, 1.3, 1.55 μm Attenuation of light through fiber in the infrared region.

  22. Fiber Optics • Ways to connect fibers: • Terminate in connectors and plugged into fiber sockets: 10 ~ 20% light lose • Spliced mechanically: 10% light lose • fused • Comparison of fiber optics and copper wire • Advantages: • Higher bit-rates, immune to interference, hard to tap • Disadvantages: • Less familiar technology, unidirectional, easily damaged, expensive interfaces

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

  24. Fiber Cables • Light sources: LED (Light Emitting Diodes) and semiconductor lasers. • The receiving end consists of a photodiode. A comparison of semiconductor diodes and LEDs as light sources.

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

  26. Fiber Optic Networks A passive star connection in a fiber optics network.

  27. Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission • Infrared and Millimeter Waves • Lightwave Transmission

  28. Wireless Transmission • λf = c where λ is the wavelength, f is the frequency, and c is the speed of light, 3 x 108 m/s • Two basic modulation techniques used in spread spectrum signal transmission: • Frequency hopping: The transmitter hops from frequency to frequency. • Direct sequence: The signal is spread over a wide frequency band with specific coding for each channel. • The stream of information to be transmitted is divided into small pieces, each of which is allocated across to a frequency channel across the spectrum. • A data signal at the point of transmission is combined with a higher data-rate bit sequence (also known as a chipping code).

  29. Electromagnetic spectrum • LF (Low Frequency, 105 Hz): maritime • MF (Medium Frequency, 106 Hz): AM radio • HF (High Frequency, 107 Hz): radio • VHF (Very High Frequency, 108 Hz): FM radio, TV • UHF (Ultra High Frequency: 109 Hz): TV, terrestrial microwave • SHF (Super High Frequency:1010 Hz): Satellite, microwave • EHF (Extremely High Frequency, 1011 Hz) • THF (Tremendously High Frequency, 1012 Hz) • Higher frequency: IHF?, AHF?, PHF? (Incredibly, Astonishingly, Prodigiously).

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

  31. 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.

  32. Politics of the Electromagnetic Spectrum • Allocate spectrum policies • Beauty contest requires each carrier to explain why its proposal serves the public interest best. • Lottery • Auction • Open band: Frequencies are not allocated but restrained in a short range. The ISM bands in the United States.

  33. Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

  34. Communication Satellites • Geostationary Satellites (GEO) • Medium-Earth Orbit Satellites (MEO) • Low-Earth Orbit Satellites (LEO) • Satellites versus Fiber

  35. Communication Satellites • Geostationary Satellites (GEO) • VSAT (Very Small Aperture Terminals): 1-meter antennas, DirecPC • Low-Earth Orbit Satellites (LEO) • Iridium: 66 satellites • Globalstar: 48 satellites • Teledesic: 30 satellites

  36. 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.

  37. Communication Satellites The principal satellite bands.

  38. Communication Satellites VSATs using a hub.

  39. Low-Earth Orbit SatellitesIridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth.

  40. Globalstar (a) Relaying in space: Iridium (b) Relaying on the ground: Globalstar

  41. Satellites versus Fiber • A single fiber has more bandwidth but is not available to most users. • Satellites are possible for mobile communication. • Satellites are cheaper for Broadcasting. • Satellites can be deployed in places with hostile terrain or a poorly developed terrestrial infrastructure such as Indonesia. • Satellites can be deployed in areas where obtaining the right for laying fiber is difficult. • Satellites is possible for rapid military communication deployment.

  42. Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones (FYI) • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching

  43. Structure of the Telephone System • The PSTN (Public Switched Telephone Network) is the world's collection of interconnected voice-oriented public telephone networks. It's also referred to as the POTS (Plain Old Telephone Service). (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

  44. Structure of the Telephone System A typical circuit route for a medium-distance call.

  45. 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

  46. The Politics of Telephones • LATA (Local Access and Transport Area) is a geographic area covered by one or more local telephone companies, which are legally referred to as local exchange carriers (LECs). • LEC (Local Exchange Carrier) is a public telephone company in the U.S. that provides local service. Some of the largest LECs are the Bell operating companies (BOCs). • IXC (IntereXchange Carrier) is a company handling inter-LATA traffic such as AT&T, MCI, and Sprint. • A POP (Point of Presence) is a switching office built to handle calls from a LATA.

  47. 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.

  48. The Local Loop: Modems, ADSL, and Wireless • Transmission lines suffer from three major problems: • Attenuation • Delay distortion • Noise • The square waves used in digital signals have a wide frequency spectrum (usually, high frequency) and thus are subject to strong attenuation and delay distortion.

  49. Modems The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

  50. Modems • The modulation is introduced to solve this problem. • Amplitude: two different amplitudes are used to represent 0 and 1. • Frequency: different tones are used. • Phase: the wave is systematically shifted (45, 135, 225, or 315º). • A modem (modulator-demodulator) is a device that modulates outgoing digital signals to analog signals.