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CEN 4500 Data Communications

CEN 4500 Data Communications. Chapter 2: The Physical Layer. Instructor: S. Masoud Sadjadi http://www.cs.fiu.edu/~sadjadi/Teaching/ sadjadi At cs Dot fiu Dot edu. Recap: Physical Layer. Physical layer is the lowest layer in the hierarchy of the hybrid reference model

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CEN 4500 Data Communications

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  1. CEN 4500 Data Communications Chapter 2: The Physical Layer Instructor: S. Masoud Sadjadi http://www.cs.fiu.edu/~sadjadi/Teaching/ sadjadi At cs Dot fiu Dot edu

  2. Recap: Physical Layer • Physical layer is the lowest layer in the hierarchy of the hybrid reference model • The purpose of physical layer is to transport a raw bit stream from one machine to another. CEN 4500, S. Masoud Sadjadi

  3. Agenda • Theoretical Basis • Transmission Media • Guided Transmission Media • Wireless Transmission • Communication Satellites • Examples of Communication Systems • The Public Switched Telephone Network • The Mobile Telephone System • The Cable Television System • Summary CEN 4500, S. Masoud Sadjadi

  4. The Theoretical Basis for Data Comm. • Theoretical Basis • Information can be transmitted on wires by varying some physical properties such as voltage or current. • We represent the value of voltage or current as a function of time (f(t))to model the behavior of signal. • Fourier Analysis • Bandwidth-Limited Signals • Maximum Data Rate of a Channel CEN 4500, S. Masoud Sadjadi

  5. Fourier Analysis • Any reasonably behaved periodic function can be constructed as the sum of a (possibly infinite) number of sines and cosines. • You can see how it works by trying it at: http://www.jhu.edu/~signals/phasorlecture2/indexphasorlect2.htm • g(t)=1/2 c + n=1- ansin(2nft) + n=1- bncos(2nft) • A data signal that has finite duration (which all of them do) can be handled by just imagining that it repeats the entire pattern over and over forever. • Let’s see how we can use Fourier analysis to transmit the ASCII character “b” encoded in an 8-bit byte. (“b” is 98 or 01100010) CEN 4500, S. Masoud Sadjadi

  6. (a) A binary signal and its root-mean-square Fourier amplitudes. (b) The signal resulted from a channel that allows only the first harmonic to pass. (c) The resulted signalfrom a channel passing the first two harmonics. Bandwidth-Limited Signals CEN 4500, S. Masoud Sadjadi

  7. (d) The resulted signalfrom a channel passing the first four harmonics. (e) The resulted signalfrom a channel passing the first eight harmonics. Bandwidth-Limited Signals (2) CEN 4500, S. Masoud Sadjadi

  8. Bandwidth-Limited Signals (3) • Attenuation: No transmission facility can transmit signals without loosing some power in the process. • Distortion: Unfortunately, all transmission facilities diminish different frequencies by different amount, thus introducing distortion. • Bandwidth: The range of frequencies transmitted without being strongly attenuated. • The cutoff in practice is often from 0 to the frequency at which half the power gets through. CEN 4500, S. Masoud Sadjadi

  9. Bandwidth-Limited Signals (4) • Bandwidth • is a physical property of a transmission medium • depends on construction, thickness, and length of the medium. • Also, a filter might be introduced to limit the amount of bandwidth available to a customer • A telephone wire may have a bandwidth of 1MHz for short distances, but telephone companies add a filter restricting each customer to about 3100 Hz CEN 4500, S. Masoud Sadjadi

  10. Bandwidth-Limited Signals (5) • Data rate and harmonics (terms) • The time required to transmit “b”, 8 bits, on a b bits/sec line is 8/b sec. • So, the frequency of the first harmonic (term in Fourier transform) is b/8 Hz. • Ordinary telephone line • Often called a voice-grade line • Has an artificially-introduced cutoff frequency just about 3000 Hz • The number of the highest harmonic for 8-bit data that can pass is 3000/(b/8) or 24000/b. CEN 4500, S. Masoud Sadjadi

  11. Bandwidth-Limited Signals (6) • Making accurate reception of original bit stream is tricky, when going over 4800 bps • Limiting the bandwidth limits the data rate. We need sophisticated coding schemes. Relation between data rate and harmonics. CEN 4500, S. Masoud Sadjadi

  12. Maximum Data Rate of a Channel • Even perfect channel has a finite transmission capacity. • An arbitrary signal that has been run through a low-pass filter of bandwidth H can be completely reconstructed by making only 2H (exact) samples per second. • Sampling faster than 2H times per second is pointless, because the higher frequency components that such samples can recover have already been filtered out. CEN 4500, S. Masoud Sadjadi

  13. Maximum Data Rate of a Channel (2) • Nyquist’s Theorem States (Noiseless Channel): maximum data rate = 2H log2V bits/sec • For example, a noiseless 3-kHz channel cannot transmit binary (i.e., two level) signals at a rate exceeding 6kbps. • Shannon’s Results (Channel with thermal noise): maximum data rate = H log2(1 + S/N) bits/sec • For example, a channel of 3-kHz bandwidth with a signal to thermal noise ratio of 30 dB (typical in analog part of the telephone system) can never transmit much more than 30kpbs, no matter how many or how few signal levels are used and how often the samples are taken. • The minimum of the above two should be considered. CEN 4500, S. Masoud Sadjadi

  14. Agenda • Theoretical Basis • Transmission Media • Guided Transmission Media • Wireless Transmission • Communication Satellites • Examples of Communication Systems • The Public Switched Telephone Network • The Mobile Telephone System • The Cable Television System • Summary CEN 4500, S. Masoud Sadjadi

  15. Guided Transmission Data • Magnetic Media • Magnetic tape or Removable media (DVD) • Excellent bandwidth, but poor delay! • Twisted Pair • One of the oldest and still most common • Can be used for both analog and digital signal • Coaxial Cable • Better shielding, so it can span longer distances at higher speed. • Fiber Optics • Computer industry, a gain of 20 per decade! • Communication industry, a gain of 125 per decade! CEN 4500, S. Masoud Sadjadi

  16. Twisted Pair (a) Category 3 UTP, 16 MHz. (b) Category 5 UTP, 100 MHz. CEN 4500, S. Masoud Sadjadi

  17. Coaxial Cable • Better shielding than twisted pair. • Thick (75-ohm) and thin (50-ohm) • Modern cables have up to 1GHz bandwidth • Largely has been replaced by fiber optics on long-haul routes A coaxial cable. CEN 4500, S. Masoud Sadjadi

  18. Fiber Optics • Each ray is said to have a different mode • Current single-mode fibers can transmit data at 50 Gbps for 100 km without amplification. (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. CEN 4500, S. Masoud Sadjadi

  19. Transmission of Light through Fiber • Attenuation of light through fiber in the infrared region. f = c /2 CEN 4500, S. Masoud Sadjadi

  20. Fiber Cables • (a) Side view of a single fiber. • (b) End view of a sheath with three fibers. CEN 4500, S. Masoud Sadjadi

  21. Fiber Cables (2) • A comparison of semiconductor diodes and LEDs as light sources. CEN 4500, S. Masoud Sadjadi

  22. Fiber Optic Networks A fiber optic ring with active repeaters. CEN 4500, S. Masoud Sadjadi

  23. Fiber Optic Networks (2) • A passive star connection in a fiber optics network. CEN 4500, S. Masoud Sadjadi

  24. Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • AM and FM • Microwave Transmission • Microwave Oven • Infrared and Millimeter Waves • Remote control • Lightwave Transmission • Laser beam CEN 4500, S. Masoud Sadjadi

  25. The Electromagnetic Spectrum • The electromagnetic spectrum and its uses for communication. f = c  300,000 km/sec CEN 4500, S. Masoud Sadjadi

  26. 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. CEN 4500, S. Masoud Sadjadi

  27. Politics of the Electromagnetic Spectrum • Microwave: Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. • The ISM bands in the United States. • Industrial, Scientific, Medical for unlicensed usage CEN 4500, S. Masoud Sadjadi

  28. Lightwave Transmission • Laser beams cannot penetrate rain or thick fog, but they normally work well on sunny days. • Convection currents can interfere with laser communication systems. A bidirectional system with two lasers CEN 4500, S. Masoud Sadjadi

  29. Communication Satellites • Geostationary Satellites • GEO • Medium-Earth Orbit Satellites • MEO • Low-Earth Orbit Satellites • LEO • Satellites versus Fiber CEN 4500, S. Masoud Sadjadi

  30. 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. CEN 4500, S. Masoud Sadjadi

  31. Communication Satellites (2) • Orbit slots are not the only bone of contention. • Frequencies are too! • The downlink transmission interferes with existing microwave users • The principal satellite bands. CEN 4500, S. Masoud Sadjadi

  32. Communication Satellites (3) Very Small Aperture Terminals (VSATs) using a hub. CEN 4500, S. Masoud Sadjadi

  33. Low-Earth Orbit Satellites Iridium • Iridium is element 77, and originally there were supposed to be 77 satellites, but it was reduced to 66 (which is Dysprosium). • (a) The Iridium satellites from six necklaces around the earth. • (b) Each satellite have a maximum of 48 cells, so 1628 moving cells cover the earth. CEN 4500, S. Masoud Sadjadi

  34. Globalstar • Based on 48 LEO satellites, but different switching scheme than that of Iridium • (a) Relaying in space. • (b) Relaying on the ground. CEN 4500, S. Masoud Sadjadi

  35. Agenda • Theoretical Basis • Transmission Media • Guided Transmission Media • Wireless Transmission • Communication Satellites • Examples of Communication Systems • The Public Switched Telephone Network • The Mobile Telephone System • The Cable Television System • Summary CEN 4500, S. Masoud Sadjadi

  36. Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL and Wireless • Trunks and Multiplexing • Switching CEN 4500, S. Masoud Sadjadi

  37. Structure of the Telephone System • (a) Fully-interconnected network. • (b) Centralized switch. • (c) Two-level hierarchy. CEN 4500, S. Masoud Sadjadi

  38. Structure of the Telephone System • A typical circuit route for a medium-distance call. CEN 4500, S. Masoud Sadjadi

  39. Major Components of the Tel. Sys. • 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 CEN 4500, S. Masoud Sadjadi

  40. The Local Loop • Computer to computer call using both analog and digital transmissions. • Modem: A device for transmitting usually digital data over telephone wires by modulating the data into an audio signal to send it and demodulating an audio signal into data to receive it. • Codec: device that converts analog signals to digital form for transmission and converts signals traveling in the opposite direction from digital to analog form. Derived from coder-decoder. CEN 4500, S. Masoud Sadjadi

  41. (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation Modems CEN 4500, S. Masoud Sadjadi

  42. Modems (2) (a) QPSK: Quadrature Phase Shift Keying. (b) QAM-16: Quadrature Amplitude Modulation -16. (c) QAM-64: Quadrature Amplitude Modulation - 64. CEN 4500, S. Masoud Sadjadi

  43. Modems (3) (a) V.32 for 9600 bps. (b) V.32 bis for 14,400 bps. Note: baud is the number of samples per second and might be different from number of bits per second (bps). (b) (a) CEN 4500, S. Masoud Sadjadi

  44. Digital Subscriber Lines • Bandwidth versus distance over category 3 UTP for DSL. CEN 4500, S. Masoud Sadjadi

  45. Digital Subscriber Lines (2) • Operation of ADSL using discrete multitone modulation. CEN 4500, S. Masoud Sadjadi

  46. Digital Subscriber Lines (3) • Splitter: An analog filter that separates the 0-4000Hz band used by POTS from the data. • Network Interface Device (NID): marks the end of the telephone companies property. • DSLAM: DSL Access Multiplexer. A typical ADSL equipment configuration. CEN 4500, S. Masoud Sadjadi

  47. Wireless Local Loops • Architecture of an LMDS system. • LDMS: Local Multipoint Distribution Service CEN 4500, S. Masoud Sadjadi

  48. Agenda • Theoretical Basis • Transmission Media • Guided Transmission Media • Wireless Transmission • Communication Satellites • Examples of Communication Systems • The Public Switched Telephone Network • Trunks and Multiplexing • The Mobile Telephone System • The Cable Television System • Summary CEN 4500, S. Masoud Sadjadi

  49. Frequency Division Multiplexing • The frequency spectrum is divided into frequency bands, with each user having exclusive possession of some bands. (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel. CEN 4500, S. Masoud Sadjadi

  50. Wavelength Division Multiplexing • A variation of FDM used for fiber optic channels. Basically FDM at high frequency. CEN 4500, S. Masoud Sadjadi

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