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The Physical Layer

The Physical Layer. Chapter 2. The Theoretical Basis for Data Communication. Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate of a Channel. Fourier Analysis. G(t) periodic function : G(t) = ½ c + S a n sin(2 p nft) + S b n cos(2 p nft) a n = 2/T G(t)sin(2 p nft) dt

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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 • Bandwidth-Limited Signals • Maximum Data Rate of a Channel

  3. Fourier Analysis G(t) periodic function: G(t) = ½ c + San sin(2pnft) + S bn cos(2pnft) an = 2/T G(t)sin(2pnft) dt bn = 2/T G(t)cos(2pnft) dt c = 2/T G(t) dt

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

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

  6. Bandwidth-Limited Signals (3) • Period = 1/Frequency • Harmonics: Term of the sine/cosine function • Bandwidth: Range of transmitted frequencies without a strong attenuation. It is a physical property of the transmission medium (depends on construction, thickness, length, …) • Transmission media are not perfect: Noise: thermal (motion of electrons) or induced (by equipments) noise, or crosstalk (effect of one wire on another) Attenuation: loss of energy (amplifiers help) Distortion: change in shape in composite signals (different harmonics arrive with different delays)

  7. Bandwidth-Limited Signals (4) Compare figure (b) No transmission Relation between data rate and harmonics.

  8. Maximum Data Rate of a Channel • Without noise (Nyquist, 1924): max. rate = 2B log(L) bits/sec where: B: bandwidth of the channel (after a low pass filter) L: Number of discrete levels (e.g. binary: L = 2)  Example: A noiseless binary 3 kHz channel cannot exceed the rate 6000 bps. • With noise (Shannon 1944): max. rate (or capacity) C = B log(1 + S/N) bits/sec where: B: bandwidth of the channel S/N: Signal to noise ratio  Example: Extremely noisy channel (N very high). C = B log (1 + 0) = 0 (as expected: no data can be sent through this channel)

  9. Maximum Data Rate of a Channel Amp • Decibel (dB): For measuring signal strength engineers use the concept of decibel: dB is negative: signal is attenuated dB is positive: signal is amplified dB = 10 log10(P1/P2) Example: P1/P2 = ½  10log10(0.5) = -3dB attenuation Reason for using dB: decibel can be added/subtracted easily: -3 dB 7 dB -3 dB 1 dB

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

  11. Twisted Pair (a) Category 3 UTP (unshielded twisted pair). (b) Category 5 UTP. • 100 Hz to 5 MHz • Two copper wires each surrounded by an insulating material • Suitable for voice and data communication

  12. Twisted Pair • Cables are twisted to minimize noise effects. • In the past two parallel cables have been used:

  13. Noise on Twisted Pair Lines

  14. Shielded Twisted Pair Cable (STP)  More expensive than UTP  Less susceptible to noise  Only in IBM installation

  15. Coaxial Cable A coaxial cable.

  16. Fiber Optics Refraction of light

  17. Fiber Optics Critical Angle Reflection

  18. Fiber Optics Propagation modes: Multimode fiber More distortion (improvement possible) Single mode fiber Less distortion (beams are more focused)

  19. Fiber Optics • Light detector: • Photodiode: translates light to an electrical pulse. • Response time: 1 nsec  max. data rate: 1Gbps. •  Limiting factor is still computing and not communication power! •  Why? Fiber technology can achieve up to 50000 Gbps !!! • Light sources: light-emitting-diode (LED), Injection laser diode (ILD)

  20. Fiber Optics + Noise resistant: fiber optics uses light and not current, therefore, it is resistant to external noise. + Less attenuation: The signal can travel for miles without regeneration. + Higher bandwidth: currently limiting factor is signal generation and reception technology. - Higher cost: Fiber-optic cable is expensive, because of its precise manufacturing. Also, laser sources are expensive. - More installation/maintenance efforts: Any cracking of the core alters the signal. Connections must be perfectly aligned and matched for core size and provide a completely light-tight seal. - More fragile: Glass fiber is more easily broken than wire .

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

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

  23. Wireless Transmission • Radio Transmission • Microwave Transmission • Satellite Communication

  24. Radio Communication Band The electromagnetic spectrum and its uses for communication.

  25. Propagation Types

  26. Propagation Types • Surface propagation: Waves travel through the lowest level of the atmosphere emanating in all directions. Distance depends on the amount of power in the signal. • Tropospheric propagation: Two modes: either from antenna to antenna (line-of-sight) or broadcast into the upper layers of troposphere (where reflection to earth takes place). Latter method allows greater distances. • Ionospheric propagation: Similar to last type. Waves hit the ionosphere and are reflected back . It allows for even greater distance with less power. • Line-of-Sight propagation: Very high signals are transmitted in straight lines directly from antenna to antenna (which should be tall enough and facing each other). More sensitive for reflection, since signals cannot be completely focused. • Space propagation: Satellites are used instead of atmospheric reflection. Basically like line-of-sight transmission with an intermediary (the satellite). Allows very long distances.

  27. Frequency Ranges an Their Uses VLF Surface propagation Sensitive to noise LF Surface propagation Sensitive to attenuation MF Tropospheric propagation Absorption by ionosphere possible (use of line-of-sight helps) HF Ionospheric propagation

  28. Frequency Ranges an Their Uses VHF Line-of-sight propagation UHF Line-of-sight propagation

  29. Frequency Ranges an Their Uses SHF Line-of-sight or space propagation (satellite microwaves or radar communication) EHF Space propagation (mainly for scientific use)

  30. Terrestrial Microwaves • Propagated using line-of-sight transmission. • Provide the basis for most existent telephone systems • Propagate in one direction only  2 frequencies are needed e.g. for a phone conversation • Repeaters are used to achieve longer distances: • Repeaters are installed on each antenna • Signal received on each antenna is converted and transmitted to the next antenna

  31. Satellite Communication • Similar to line-of-sight transmission. • Satellite acts as a supertall antenna. • No limitation by the curvature of the earth.  longer distances achievable • Expensive, but leasing time and frequencies on existent ones cheap

  32. Geosynchronous Satellites • Fixed satellites are useless: They face earth antennas only for short time  like a stopped clock is accurate twice a day! • To ensure constant communication the satellite must move at the same speed as the earth. • One geosynchronous satellite cannot cover the whole earth:  at least three equidistant satellites are needed

  33. Frequency Bands for Satellite Communication The principal satellite bands.

  34. Communication Satellites VSATs (Very Small Aperture Terminals) using a hub. • VSAT do not have enough power to communicate directly with each other. • A hub is used relay traffic between VSTAs

  35. Modulation • Modems: Convert a digital signal to an analog one and vice versa. • Forms of modulation: • Amplitude modulation • Frequency modulation • Phase modulation • Amplitude Modulation Amplitude > 0 means 1, amplitude = 0 means 0

  36. Modulation More frequent means 1, less frequent means 0 • Frequency Modulation

  37. Modulation Phase changes • Phase Modulation

  38. Bit Rate Versus Baud Rate • Bit rate: Number of bits transmitted during one second • Baud rate: Number of signal units required to represent those bits. • Bit rate equals the baud rate times the number of bits represented by each signal unit: Bit rate = Baud rate x Number of bits per signal unit • This means that the bit rate is always greater or equal the baud rate: Bit rate >= Baud rate • Analogy in transportation: • Bit: passenger • Baud: car • If 100 cars go from A to B carrying one person, then 100 person are transported ( Bit rate = baud rate) • If, however, each car carries 4 persons, then 400 persons are transported ( Bit rate = 4 x baud rate)

  39. Phase Modulation Phase Shift Keying (PSK) Bit rate = Baud rate = 5 1 0 0 1 1 1 baud 1 baud 1 baud 1 baud 1 baud 1 second 0 (0) 1 (p) Constellation diagram for 2-PSK

  40. 4-PSK Bit rate = 10 Baud rate = 5 00 11 01 10 10 1 baud 1 baud 1 baud 1 baud 1 baud 1 second 01 (p/2) 10 (p) 00 (0) Constellation diagram for 4-PSK 11 (3p/2)

  41. 8-PSK and higher Constellation diagram for 8-PSK 010 (p/2) 011 (p3/4) 001 (p/4) 000 (0) 100 (p) 101 (5p/4) 010 (7p/4) 110 (3p/2) 16-PSK, 32-PSK, … are also possible

  42. Quadrature Amplitude Modulation (QAM) 01 00 10 11 • Example8-QAM: 2 amplitudes and 4 phases 011 010 001 101 • Idea: Combine changes of amplitude with changes of phase. • Example4-QAM: 1 amplitude and 4 phases 000 100 110 111

  43. 8-QAM Bit rate = 24 Baud rate = 8 101 100 001 000 010 011 111 110 1 baud 1 baud 1 baud 1 baud 1 baud 1 baud 1 baud 1 baud 1 second • Whenever the amplitude or the phase changes, a new symbol is transmitted. • In general, number of amplitudes less than that of phases because amplitudes are more sensitive to noise.

  44. 16-QAM • There are three popular 16-QAM configurations: • 3 amplitudes, 12 phases (ITU-T recommendation) • 4 amplitudes, 8 phases (ISO recommendation) • 2 amplitudes, 8 phases • Constellation diagrams:

  45. Multiplexing vs. No Multiplexing

  46. Frequency Division Multiplexing (FDM) • For analog devices (e.g. phones) • Channels carry the signals with different frequencies but at the same time.

  47. FDM • Similar signals as input • Signals are modulated (e.g. AM, FM) using different carrier signals (f1, f2, f3) • Combined signal is send through the link

  48. FDM • Filters are used to decompose the multiplexed signal into its constituents • Individual signal are then passed to a demodulator to generate the original signal

  49. Wave Division Multiplexing • For fiber optic channels only. • Same idea as FDM, but more reliable than FDM. prisms

  50. Time Division Multiplexing (TDM) • For digital devices (computers). • Like FDM, bandwidth of transmission medium high enough to accommodate the different signals. • Conceptual view:  Channel sectioned by time rather by frequency

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