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The Physical Layer

The Physical Layer. The Theoretical Basis for Data Communication. Fourier analysis Niquist chriterium for bandwidth-limited channel Shannon maximum data rate of a noisy channel. Fourier Transform. Periodic signals with period T =2 π / w Non-periodic signals.

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The Physical Layer

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  1. The Physical Layer

  2. The Theoretical Basis for Data Communication • Fourier analysis • Niquist chriterium for bandwidth-limited channel • Shannon maximum data rate of a noisy channel

  3. Fourier Transform • Periodic signals with period T=2π/w • Non-periodic signals

  4. Bandwidth-Limited Signals A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

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

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

  7. Band-Limited Channel w 1/Ts 2/Ts -1/Ts • Fourier transform of atypical signal

  8. Niquist Theorem • If the signal bandwidth has width of W, then it can be reconstructed by taking 2W samples per second. • Maximum data rate is where V is the number of different symbols

  9. Niquist Chriterium

  10. Power Spectrum Density • Autocorrelation function of signal or noise • Power spectrum density

  11. Shannon Theorem • If the signal bandwidth has width of W, and S/N is the signal-to-noise ratio, then the maximum data rate is

  12. Filtering • Channel behaves as a filter • When the noise is white (uncorrelated) Gaussian optimum filter has transfer function H(w)=X*(w).

  13. Modulation schemes (a) QPSK. (b) QAM-16. (c) QAM-64.

  14. Guided Transmission • Twisted Pair • Coaxial Cable • Fiber Optics

  15. Twisted Pair (a) Category 3 UTP 16 MHz. (b) Category 5 UTP 100MHz.

  16. Issues of Twisted-Pair Transmission • Attenuation • Distortion • Cross-talk • Impulse noise

  17. Coaxial Cable A coaxial cable 1GHz.

  18. 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.

  19. Light Propagation • From Maxwell equations: • From boundary conditions:

  20. Light Propagation

  21. Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. Bands 25-30THz, and last two bands have attenuation less than 5%/km

  22. Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers, diameter 8-10μm.

  23. Transmission Devices • Light emitting diode (LED) • Semiconductor lasers • Mach-Zehnder external modulator • EDFA • Photodiode

  24. Optical Transmitters 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 • Relationship between wavelength and frequency: • 100MHz waves are about 3m long, 1000MHz waves are 0.3m long. • An object distracts those waves, whose length is smaller or equal to the object dimension.

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

  30. 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.

  31. Issues in Wireless Transmission • Radio signals are omnidirectional, and penetrate through objects. Throughput is low. • HF radio and microwave signals are directed. Suffer from multipath fading, and are reflected against the buildings. • Above 4GHz, signals are absorbed by the rain.

  32. Politics of the Electromagnetic Spectrum • Goverments allocate frequencies, through contests, lottery, auctions. • In ISM only the power is specified, used for household devices. Infrared is available. 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. Fog and rain are disruptive too.

  34. Communication Satellites • Geostationary Satellites • Several kWs. 40 transponders with 80MHz. TDMA. • Medium-Earth Orbit Satellites • 24 GPS satellites. • Low-Earth Orbit Satellites • Iridium project started by Motorola

  35. 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.

  36. Communication Satellites The principal satellite bands.

  37. Geostationary Satellites Very Small Aperature Terminals (VSATs) using a hub.

  38. Low-Earth Orbit SatellitesIridium (a) The Iridium satellites from six necklaces around the earth. (b) 66 satellites, 1628 moving cells cover the earth, 253000 channels.

  39. Iridium and Globalstar (a) Relaying in space (Iridium). (b) Relaying on the ground (Globstar).

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

  41. Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

  42. Structure of the Telephone System 10km Coax, micorwave, fiber A typical circuit route for a medium-distance call.

  43. Major Components of the Telephone System • Local loops • Twisted pairs going to houses and businesses • Trunks • Fiber optics connecting the switching offices • Switching offices • Where calls are moved from one trunk to another

  44. The Politics of Telephones 164 LATAs 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.

  45. 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.

  46. (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation Modems

  47. Modems (b) (a) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps.

  48. Higher Bit-rate Modems • 35kbps is the Shannon limit • Line from a local office to an ISP is digitalized. • V90 35kbps upstream, 56kbps downstream • V92 48kbps upstream, 56kbps downstream

  49. Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.

  50. Digital Subscriber Lines Operation of ADSL using discrete multitone modulation. Up to 8Mbps downstream, and up to 1Mbps upstream

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