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The physical layer

The physical layer. The Theoretical Basis for Data Communication. Fourier Analysis Any periodical signal can be decomposed as a sum of sinusoidal signals at frequencies which are multiple of the original frequency We call those the “harmonics” Bandwidth-Limited 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 • Any periodical signal can be decomposed as a sum of sinusoidal signals at frequencies which are multiple of the original frequency • We call those the “harmonics” • Bandwidth-Limited Signals • Not all harmonics pass through a channel • The result is a distortion in the shape of the signal • Maximum Data Rate of a Channel

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

  4. Bandwidth-Limited Signals (2) (d) – (e) Successive approximations to the original signal.

  5. Bandwidth-Limited Signals (3) Relation between data rate and harmonics.

  6. Guided Transmission Data • Magnetic Media • Write the data on a storage system (eg. tapes or hard drive), carry them over physically • Twisted Pair • Coaxial Cable • Fiber Optics

  7. Twisted Pair • Category 3 UTP (unshielded twisted pair) (b) Category 5 UTP • since about 1988 – more twists, less crosstalk, better signal over longer distances

  8. Coaxial Cable • More expensive than twisted pair • High bandwidth and excellent noise immunity

  9. 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.

  10. Single mode vs multi-mode • Multi-mode fiber: light reflected on various angles inside the fiber. • If the fiber is so narrow that it is only several wavelengths, the light can travel only in a single way, in a straight line, without bouncing. • The fiber acts like a wave guide • Called a single mode fiber • Smaller loss, more suitable for long distance transmission

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

  12. Fiber Cables -Core: 50 microns for multi-mode, 8-10 microns for single mode -Cladding: glass with a lower refraction index, to keep the light in the core -Connection: -connectors (plug in) – about 20% attenuation -mechanical splicing, tuned by an operator – 10% attenuation -fused (melted together) – almost no attenuation

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

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

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

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

  17. Narrow-band vs spread spectrum • Spectrum • About 8 bits / Hz (using all the tricks in the book) • Narrowband: • Δf / f << 1 • Spread spectrum • Frequency hopping spread spectrum • Several times / sec, military communications, good resistance to multipath fading • Direct sequence spread spectrum DSSS: 802.11b, CDMA telephony, GPS, Galileo, ZigBee • Ultra-wide band • any radio technology having bandwidth exceeding the lesser of 500 MHz or 20% of the arithmetic center frequency

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

  19. 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.

  20. Politics of the Electromagnetic Spectrum The ISM bands in the United States (Industrial, Scientifical, Medical: also known as unlicenced bands)

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

  22. Communication Satellites • Geostationary Satellites • Medium-Earth Orbit Satellites • Low-Earth Orbit Satellites • Satellites versus Fiber

  23. 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.

  24. Communication Satellites (2) The principal satellite bands.

  25. Communication Satellites (3) VSATs using a hub.

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

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

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

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

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

  31. 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

  32. 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. LATA: local access and transport areas LEC: local exchange carrier IXC: interexchange carrier This is the result of the 1984 breakup of the AT&T monopoly.

  33. 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.

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

  35. Modems (2) (a) QPSK. (b) QAM-16. (c) QAM-64.

  36. Modems (3) (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. (b) (a)

  37. Digital Subscriber Lines Bandwidth versus distance over category 3 UTP for DSL.

  38. Digital Subscriber Lines (2) Operation of ADSL using discrete multitone modulation.

  39. Digital Subscriber Lines (3) A typical ADSL equipment configuration.

  40. Wireless Local Loops Architecture of an LMDS system.

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

  42. Wavelength Division Multiplexing Wavelength division multiplexing.

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

  44. Time Division Multiplexing (2) Delta modulation.

  45. Time Division Multiplexing (3) Multiplexing T1 streams into higher carriers.

  46. Time Division Multiplexing (4) Two back-to-back SONET frames.

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

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

  49. Message Switching (a) Circuit switching (b) Message switching (c) Packet switching

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

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