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Chapter 2/1 (Physical Layer)

Chapter 2/1 (Physical Layer). Theoretical Basis for Data Communication Guided Transmission Media Wireless Transmission Communication Satellites PSTN - Public Switched Telephone System. The Theoretical Basis for Data Communication. Fourier Analysis Bandwidth-Limited Signals

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Chapter 2/1 (Physical Layer)

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  1. Chapter 2/1 (Physical Layer) • Theoretical Basis for Data Communication • Guided Transmission Media • Wireless Transmission • Communication Satellites • PSTN - Public Switched Telephone System EETS 7304

  2. The Theoretical Basis for Data Communication • Fourier Analysis • Bandwidth-Limited Signals • Maximum Data Rate of a Channel: Nyquist Theorem Shannon Capacity Theorem EETS 7304

  3. Fourier Transform: periodic signals g(t) where: From: Follows: EETS 7304

  4. Successive approximations to the original signal A binary signal and its root-mean-square Fourier amplitudes. EETS 7304

  5. Bandwidth-Limited Signals EETS 7304

  6. Successive approximations to the original signal EETS 7304

  7. Relation between data rate and harmonics If b is bit rate (bps) then bit duration is 1/b s Time to send 8 bits T = 8/b s. Therefore for b = 300 bps T = 8/300 = 0.02667 s = 26.67 ms. Since T is the period the first harmonic has frequency 1/T = 300/8 = 37.5 Hz. Within the bandwidth of 3000 Hz you can send 3000/37.5 = 80 harmonics. EETS 7304

  8. Fourier Transform: periodic signals A A A = = = = = = = - = = C ; C C ; C C 0 ; C C ; C C 0 ; - - - - 0 1 1 2 2 3 3 4 4 p p 2 3 where Example: Pulse train A t T T0 A/2 T0/T = 2 =>> n -5 -4 -3 -2 -1 0 1 2 3 4 5 6 for T0/T=2 => EETS 7304

  9. non-periodic signals Example: unit impulse function d(t) 1/(2d) t -d d EETS 7304

  10. t Delta impulse or =>> white spectrum. 1/2d 1 f -d d 0 white spectrum EETS 7304

  11. Nyquist (1924) analog to digital The minimum sampling rate to recover any bandlimited signal to H Hz is 2H (samples per second). Example: assume 2400 bps 10 harmonics bandwidth 3000 Hz. T = 3.33 ms. Minimum sampling rate is 6000 samples/sec, or every 1/6 = 0.167 ms or 20 samples per period. EETS 7304

  12. Telephone example • Subscriber loop has a bandwidth over 1 MHz. However, switching office filters every signal to 4000 Hz. Therefore, after entering a switch the analog voice is limited to 4000 Hz. Minimum sampling rate is then 8000 samples (pulses) per sec. • Pulse per sec = baud. • Since the value of every sample is represented by 8 bits a digital voice bit rate is: 8000 bauds * 8 bits/baud = 64000 bps = 64 kbps. • Inversely Nyquist theorem also says that maximum sample (pulse) rate going through the filter H that can be recovered is 2H. • In addition if max amplitude pulse is divided into V levels then the bit rate carried with 2H baud is 2H*log2V. • For example modem with 2400 bauds and V = 64 has bit rate 14,400 bps. EETS 7304

  13. Shannon (1948) theorems • Bit rate in a noisy channel. Shannon Capacity Theorem: maximum bit rate = H log2(1 + S/N), where H - channel bandwidth, S – signal power, N – noise power. • For instance: Voice-grade line channel H = 3000 Hz and S/N = 30 dB -> S/N = 1000, therefore, maximum bit rate = 3000 log2(1001) = 30000 bps. • Definition of dB = 10 log10(S/N). EETS 7304

  14. Guided Transmission Media • Twisted Pair • Coaxial Cable • Fiber Optics EETS 7304

  15. Twisted Pair (a) Category 3 UTP (Unshielded Twisted Pair), 16 Mbps. (b) Category 5 UTP, 100 Mbps. Category 6 and 7, 250 – 600 Mbps. EETS 7304

  16. Coaxial Cable 50 Ohm and 75 ohm 1 GHz EETS 7304

  17. Fiber Optics • 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. Multimode uses total reflection. Single mode 50 Gbps over 100 km. EETS 7304

  18. Transmission of Light through Fiber Attenuation = 10*log10(transmitted power/received power) per km of light through fiber in the infrared region. f = c/l -> f = 300,000,000/1.3*10^-6 = 2.31*10^14 = 231 THz. Bandwidth = 300,000,000*(1/1.22 – 1/1.37)*10^6 = 0.27*10^14 = 27000 GHz EETS 7304

  19. Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers. EETS 7304

  20. A comparison of semiconductor laser and LEDs as light sources. EETS 7304

  21. A fiber optic ring with active repeaters EETS 7304

  22. A passive star connection in a fiber optics network EETS 7304

  23. Fiber vs. copper • Repeaters: copper every 5 km vs. fiber 50 km. • 1000 twisted pairs 1 km long weights 8 Tones vs. 2 fibers with more capacity weight 100 kg. • Fiber doesn’t leak the light -> excellent security. • Fiber is much lighter to hang on the poles. • Fiber is dug about 1 m (3 ft) underground or replaces copper in ducts. • Fiber is new technology therefore more expensive parts. EETS 7304

  24. Wireless Transmission • The Electromagnetic Spectrum • Radio Transmission • Microwave Transmission Above 100 MHz for relaying 80 km. EETS 7304

  25. The Electromagnetic Spectrum EETS 7304

  26. Wireless bit rate With current technology it is possible to code about 8 bps/Hz of bandwidth. Examples: • Coax 750 MHz bandwidth -> 6 Gbps. • Fiber: f = c/l -> Df = cDl/l2 For l = 1.3 m band Dl= 0.17 m that gives the bandwidth Df = 40 THz to get about 300 Tbps. Two modes of transmission: • narrow band (Df/f << 1) notably frequency hopping spread spectrum (to avoid fading). • direct sequence spread spectrum. EETS 7304

  27. 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. Microwave transmission uses above 100 MHz. MCI (Microwave Communications Inc.) was a long distance carrier before merged with WorldCom. EETS 7304

  28. The ISM bands in the United States Radio spectrum is owned by governments or FCC in US. FCC allocates the spectrum while governments sell it to carriers on auctions. Some spectrum is left unsold for unlicensed use. Bluetooth and WiFi WiFi ISM (Industrial, Scientific, Medical) are not allocated but limited by distance like garage door openers, cordless phones, radio controlled toys etc.It is mandated by power < 1 W. EETS 7304

  29. Communication Satellites • Geostationary Satellites 1962 • Medium-Earth Orbit Satellites – GPS 24 GPS satellites orbiting at 18,000 km every 6 hours. • Low-Earth Orbit Satellites (Communications) EETS 7304

  30. P 24 h 6 h 1.5 h Communication Satellites Orbital period proportional to radius^(3/2). Latency = round-trip delay time Number of satellites needed for global coverage. EETS 7304

  31. ITU allocates the orbits as well as satellite bands EETS 7304

  32. Low-Earth Orbit Satellites Motorola Iridium launched 1997 • The Iridium: six necklaces by 11 = 66 satellites (750 km). • Each satellite covers 48 cells = 1628 moving cells. • Satellite phones didn’t have much success in competition with terrestrial and 5 b$ was sold for 25 m$ resumed service in 2001. EETS 7304

  33. Globalstar (a) Relaying in space (Iridium). (b) Relaying on the ground (Globalstar 48 LEOs). EETS 7304

  34. Public Switched Telephone System • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL and Wireless EETS 7304

  35. Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. EETS 7304

  36. Structure of the Telephone System (2) A typical circuit route for a medium-distance call. EETS 7304

  37. Major Components of the Telephone System • Local loops: analog twisted pairs going to houses and businesses: the weakest link. • Trunks: digital fiber optics connecting the switching offices. Three different way of multiplexing: frequency, time, and wavelength. • Switching offices: where calls are moved from one trunk to another. Types of switching: circuit switching vs. packet switching. EETS 7304

  38. The Politics of Telephones 1984 US was divided into 164 LATAs (Local Access and Transport Area equivalent to an area code). BOC has monopoly by its ILECs (Incumbent Local Exchange Carrier) within LATA. AT&T (IXC#1) and competitors (IXC#2) use IXCs (IntereXchange Carrier) for traffic between LATAs. Long Distance carrier also must build IXC POP switches at each LATA to connect it to IXC. BOCs are required to connect to each IXC POP. 1996 interference was allowed. EETS 7304

  39. Present day Internet POP – Point Of Presence are ISP (e.g. AOL) modems connected to Regional ISP network. Regional ISP network is connected to the backbone. Backbones are connected by NAP (Network Access Point) or by their own routers. Finally Server Farm (multiplicity of identical servers) are connected to the router. EETS 7304

  40. The Local Loop: Modems, ADSL, and Wireless ISP1 handles Internet call from the computer. Digital > modem>analog subscriber line>codec>digital trunk>codec>analog subscriber line>modem (bank) >ISP1 computer (POP). ISP2 handles it faster. EETS 7304

  41. TD RTS DTR RD CTS DSR RI CD GRND TD RTS DTR RD CTS DSR RI CD GRND DSR DTR RI RTS CTS CD TD RD DTE DTE DCE DCE Handshaking between two modems in RS-232C PC Modem Data Set Ready DSR DTR RI RTS CTS CD TD RD Data Terminal Ready Ring Indicator Request To Send Clear To Send Carrier Detect Transmit Data Receive Data RI RTS RTS Modem PC DB9 bit connector EETS 7304

  42. RS232 electrical signals ASCII Data (binary) 0 0 1 1 1 1 0 Start( “0”) + 7 data + parity + Stop (at least 1.5 “1”) +15 V Start “0” 0 0 1 1 1 1 0 line signals t parity Stop “1” Stop “1” -15 V Amplitude “0” +5/+15 V “1” -5/-15 V Speed pulse/sec = baud 1200/2400/ 4800/9600/ 19200 baud Control characters RTS – 0011110 RI - 0000111 EETS 7304

  43. (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation Modems use carriers between 1 and 2 kHz EETS 7304

  44. Modems (2) (a) QPSK. (b) QAM-16: V.32 for 9600/2400 = 4. (c) QAM-64: V32 bis for 14,400/2400 = 6. EETS 7304

  45. Modems (3) • 2400 *5 = 12000 bps (b) 2400 * 7 = 16800 bps EETS 7304

  46. Digital Subscriber Lines Maximum bit rate versus distance over category 3 UTP for DSL. EETS 7304

  47. Digital Subscriber Lines Operation of ADSL using discrete multitone modulation. 256 channels * 4312.5 Hz = 1.1040 MHz. It works as 250 different frequency modems. Ch – 0 voice; Ch 1-5 not used 32 Chs * 32 kbps = 1 Mbps upstream data + 1 ch for control 216 Chs * 32 kbps = 7 Mbps downstream data + 1 for control EETS 7304

  48. Digital Subscriber Lines NID – Network Interface Device. DSLAM – Digital Subscriber Line Access Multiplexer. EETS 7304

  49. Wireless Local Loops Connection to the CLEC (Competitive Local Exchange Carrier): LMDS (Local Multipoint Distribution System). FCC allocated bandwidth 198 MHz at 2.5 GHz taken from Instructional TV. EETS 7304

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