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Analog Transmission

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Analog Transmission

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Analog Transmission

  1. 1. ANALOG TRANSMISSION
  2. 2. MODULATION OF DIGITAL DATA It is the process of changing one of the characteristics of an analog signal based on the information in a digital signal. e.g. To transmit digital data from one PC to another using a phone-line. Telephone line carries analog signal, so digital data should be converted using process of Modulation.
  3. 3. Most of us may be familiar with MODEM. MODEM stands for: MODULATOR/ DEMODULATOR MODEM uses Modulation process to convert digital data to a form suitable for transmission on a telephone line.
  4. 4. WHY MODULATION ? 1-For allowing multiple signals to share a single physical channel 2-Necessary for wireless communication where the antenna diameter must be at least equal to the wavelength of the carrier signal. This means, for a 3000 Hz signal through space, the antenna diameter must be at least 60 miles! 3-For a medium not suitable for digital transmission.
  5. 5. Modulation process uses two types of signals: (a) Information Signal: i- Analog or ii- Digital (b) Carrier Signal :A high frequency Sine wave One or more characteristics of Carrier are varied in accordance to
  6. 6. MODULATION PROCESS PRODUCES MODULATED SIGNAL, READY FOR AMPLIFIATION and TRANSMISSION
  7. 7. Sine Wave as a CARRIER • Think of a Sine Wave as a Carrier Signal, i.e. the signal onto which the information is loaded for sending to the end user • A Carrier Signal is used as the basis for sending e.m. signals between a transmitter and a receiver, independently of the frequency
  8. 8. Carrier signals • A Carrier Signal may be considered to travel at the speed of light, c, whether it is in free space or in a metal wire • Travels more slowly in most substances • The velocity, frequency, and wavelength of the carrier signal are uniquely connected by c = f λ Wavelength FrequencyVelocity of light
  9. 9. The receiver is tuned to the frequency of the carrier signal that is expected from the transmitter. See next slide
  10. 10. Basic Analog Modulation Techniques • Parameter to be varied Analog Modulation • Amplitude Amplitude Modulation AM • Frequency Frequency Modulation FM • Phase Phase Modulation PM • RADIO transmission uses AM and FM • TV broadcast uses FM
  11. 11. Basic Digital Modulation Techniques • Parameter to be varied Digital Modulation • Amplitude Amplitude Shift Keying • Frequency Frequency Shift Keying • Phase Phase Shift Keying
  12. 12. More on Digital Modulation • Amplitude Modulation (AM) also known as amplitude-shift keying. This method requires changing the amplitude of the carrier phase between 0 and 1 to encode the digital signal. • Frequency Modulation (FM) also known as frequency-shift keying. Must alter the frequency of the carrier to correspond to 0 or 1. • Phase Modulation (PM) also known as phase-shift keying. At each phase shift, the bit is flipped from 0 to 1 or vice versa.
  13. 13. Modulation Schematics
  14. 14. Coming slides further highlight the process of ASK FSK PSK
  15. 15. BAUD • Baud refers to the number of times a change of analog signal occurs in the circuit. • If one signal change carries per bit, then baud and bps are the same, e.g. baud = bps
  16. 16. Bit Rate vs. Baud Rate • bit: a unit of information • baud: a unit of signaling speed • Bit rate (or data rate): b – Number of bits transmitted per second • Baud rate (or symbol rate): s – number of symbols transmitted/sec • General formula: • Bit rate = Baud rate X Number of bits per symbol
  17. 17. Concept of Symbol • Symbol: Each modification of the carrier wave to encode information e.g. Sending one bit (of information) at a time – One bit encoded for each symbol (carrier wave – change)  1 bit per symbol e.g. Sending multiple bits simultaneously – Multiple bits encoded for each symbol (carrier wave change)  n bits per symbol, n > 1 – Need more complicated information coding schemes
  18. 18. Sending Multiple Bits per Symbol • Multiple bits per symbol might be encoded using: • amplitude, frequency, and phase modulation –e.g., in PSK: –Phase shifts of 0o , 90o , 180o , and 270o
  19. 19. Example: Two-bit ASK 4 symbols
  20. 20. EXAMPLE An analog signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the baud rate and bit rate. Baud Rate = No. of Signal units/sec = 1000 Bit rate = Baud rate X No. of bits per signal Unit = 1000 X 4 = 4000 bps
  21. 21. IMPORTANT NOTE If fewer (less) signal units are required to transmit more bits, less BANDWIDTH will be required on a medium.
  22. 22. BAND WIDTH IN ASK
  23. 23. BANDWIDTH IN ASK As shown in the previous slide If single bit is transmitted per signal unit, Bit Rate = Baud Rate Bandwidth = Baud Rate
  24. 24. BANDWIDTH IN FSK
  25. 25. BANDWIDTH IN FSK Fig shows the spectrum of FSK signal. Although one bit is transmitted per signal unit, but because of frequency shift if lowest frequency is fc0 and highest frequency is fc1, Bandwidth = Baud rate + fc1 – fc0
  26. 26. EXAMPLE In FSK transmission, if two carriers fc0 and fc1 are separated by 3000 Hz, find the required bandwidth at 2000 bps. Solution: fc1-fc0 = 3000 Hz Bit Rate = 2000 bps Baud Rate = Bit Rate = 2000 Bandwidth = fc1 – fc0 + Baud Rate Bandwidth = 3000 + 2000 = 5000 Hz
  27. 27. BANDWIDTH IN PSK
  28. 28. BANDWIDTH IN PSK As shown in the previous slide If single bit is transmitted per signal unit, Bit Rate = Baud Rate Bandwidth = Baud Rate
  29. 29. Constellation Diagrams or Phase State Diagram for 2PSK (2PSK because two phases and 1 bit per phase)
  30. 30. Example of 4-PSK: 4 Phases, 2 bits per phase
  31. 31. 4-PSK Constellation
  32. 32. Tribit using 8-PSK: 8 Phases 3 bits per phase
  33. 33. • Combined Modulation Techniques • Combining ASK and PSK on the same circuit. HOW? • Vary Amplitude A and Phase P of the Carrier simultaneously. • Let x variations in A • Let y variations in P • will give total variations of x times y • Thus corresponding number of bits per variation
  34. 34. QUADERATURE AMPLITUDE MODULATION Q A M The modulation technique to vary A and P simultaneously is called QAM IMPORTANT NOTE In QAM, number of variations in A is kept minimum because A is affected by noise. Number of variations in P is limited only by the interpretation of shifts in it. See next slide
  35. 35. QAM is a combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved.
  36. 36. QAM - Is a widely used family of encoding schemes. A common form: 16-QAM Uses: • 8 different phase shifts • 2 different amplitude levels, • 16 possible symbols • 4 bits/symbol
  37. 37. 16- QAM Constellation
  38. 38. –TCM – Trellis-Coded Modulation •An enhancement of QAM •Can transmit different number of bits on each symbol (6,7,8 or 10 bits per symbol)
  39. 39. Transmission Modes • Parallel mode –Uses several wires, each wire sending one bit at the same time as the others • A parallel printer cable sends 8 bits together • Computer’s processor and motherboard also use parallel busses (8 bits, 16 bits, 32 bits) to move data around
  40. 40. Serial Mode Sends bit by bit over a single wire Serial mode is slower than parallel mode
  41. 41. Parallel Transmission Example Used for short distances (up to 6 meters) (since bits sent in parallel mode tend to spread out over long distances) (8 separate copper wires)
  42. 42. Serial Transmission Example Can be used over longer distances (since bits stay in the order they were sent)
  43. 43. DTE and DCE • Data terminating equipments (DTEs) are noncommunciations-oriented components of a data communications environment. • Data communications devices (DCEs) are communications-oriented components of a networks, such as telephone switching equipment, media, modems, etc.
  44. 44. DTE and DTE
  45. 45. Modem • A modem is a DCE device. • Modems use amplitude, frequency, or phase shift to encode more that one bit per baud.
  46. 46. More on Modems • V-series of modem standards (by ITU-T) – V.22 • An early standard, now obsolete • Used FM, with 2400 symbols/sec  2400 bps bit rate – V.34 • Used 8.4 bits/symbol, with 3,428 symbols/sec  multiple data rates(up to 28.8 kbps) • Includes a handshaking sequence that tests the circuit and determines the optimum data rate
  47. 47. V.90 and V.92 Modems • Combines analog and digital transmission • Uses a technique based on PCM concept – Results in a max of 56 Kbps data rate • V.90 Standard – Based on V.34+ for Upstream transmissions (PC to Switch) – Max. upstream rate is 33.4 Kbps • V.92 Standard (most recent) – Uses PCM symbol recognition technique for both ways – Max. upstream rate is 48 kbps
  48. 48. 56k Modems • 56k modems, the fastest possible on voice grade lines, are based on the V.90 and V.92 standards. • Downstream transmissions (from phone switch to the user’s computer) use a technique based on recognizing the 8-bit digital symbol. • With the V.90 standard, upstream lower data rates. The max transmissions are still based on the V.34+ standard.
  49. 49. Digital Modulation may not mean what you think it means… • The transmitted signal is a continuous time signal (or analog signal) regardless of the modulation ‘analog’ or ‘digital’.

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