Dcn a03-analog transmission
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  • 1. DATA COMMUNICATION AND NETWORKING ( 07CS51 )
  • 2. Digital - to - Analog Conversion input : digital data -> converted to -> output : bandpass analog signal what is done ? change one of the characteristics (amplitude / frequency / phase) of output analog signal how is it done ? based on the information in the input digital data Analog Transmission
  • 3. Digital - to - Analog Conversion Analog Transmission three mechanisms of modulating digital data into an analog signal by altering any of the three characteristics of analog signal: amplitude -> ASK : amplitude shift keying frequency -> FSK : frequency shift keying phase -> PSK : phase shift keying
  • 4. Digital - to - Analog Conversion Types Analog Transmission
  • 5. Digital - to - Analog Conversion Types : ASK, FSK, PSK Analog Transmission ASK FSK PSK
  • 6. Digital - to - Analog Conversion Data element vs. Signal element a data element is the smallest quantity, a bit , that can represent a piece of information a signal element (vehicle / carrier) carries … data elements (passengers) - can contain one or more bits the goal in digital communications is …. to send data elements what is the need ? -> to send data elements what can be sent ? -> signal elements Analog Transmission
  • 7. Digital - to - Analog Conversion Data element vs. Signal element bit rate : the number of data elements transmitted per second baud rate : the number of signal elements transmitted per second Analog Transmission
  • 8. Digital - to - Analog Conversion Data element vs. Signal element Analog Transmission r = no. of data elements ÷ no. of signal elements
  • 9. Digital - to - Analog Conversion Data element vs. Signal element define : r as the number of data elements carried by each signal element N = bit rate and S = baud rate Data (bit) rate vs. signal (baud) rate S = N x (1 ÷ r) in bauds r = log 2 L where L is the type of signal element in analog transmission, S ≤ r Analog Transmission
  • 10. Digital - to - Analog Conversion Data element vs. Signal element Example BF4e5.1 An analog signal carries 4 bits per signal element If 1000 signal elements are transmitted per second, find the bit rate Analog Transmission
  • 11. Digital - to - Analog Conversion Data element vs. Signal element Example BF4e5.1 r = 4 S = 1000 N = S x r = 4000 bps Analog Transmission
  • 12. Digital - to - Analog Conversion Data element vs. Signal element Example BF4e5.2 An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud How many data elements are carried by each signal element ? Analog Transmission
  • 13. Digital - to - Analog Conversion Data element vs. Signal element Example BF4e5.2 N = 8000 S = 1000 r = (N ÷ S) = 8 Analog Transmission
  • 14. Digital - to - Analog Conversion Carrier signal is a high frequency signal that acts as a base signal to the information signal receiving device is tuned to the frequency of the carrier signal (used by the sending device) digital information ( to be sent ) changes the carrier signal by modifying …….. amplitude / frequency / phase this process of modification is called modulation or shift keying Analog Transmission
  • 15. Digital - to - Analog Conversion Amplitude Shift Keying (ASK) amplitude of the carrier signal is varied to create signal elements frequency and phase remain constant Binary ASK (BASK) implemented using two levels also referred to as on-off-keying (OOK) Analog Transmission
  • 16. Digital - to - Analog Conversion Amplitude Shift Keying (ASK) : binary ASK modulation produces aperiodic composite signal, with continuous set of frequencies bandwidth is proportional to the signal ( baud ) rate Analog Transmission
  • 17. Digital - to - Analog Conversion Amplitude Shift Keying (ASK) : binary ASK Analog Transmission
  • 18. Digital - to - Analog Conversion Amplitude Shift Keying (ASK) : binary ASK define another factor, d , between 0 and 1, that depends on modulation and filtering process Bandwidth B = (1 + d ) x S bandwidth requirement varies between… S and 2S the middle of the bandwidth is where the frequency of carrier, f c , is located based on the available bandpass channel, f c can be chosen Analog Transmission
  • 19. Digital - to - Analog Conversion Amplitude Shift Keying (ASK) : binary ASK full-duplex transmission ; d = 1 two carrier frequencies : 225 and 275 KHz available bandwidth = (300 - 200) = 100 KHz B in each direction = 50 KHz S = 25 KHz and N = 25 Kbps in each direction Analog Transmission
  • 20. Digital - to - Analog Conversion Multi-level ASK (MASK) 4, 8,16 … amplitudes can be used for the signal data can be modulated using 2, 3, 4 … bits at a time in such cases, r = 2, r = 3, r = 4, …. Analog Transmission
  • 21.
    • Digital - to - Analog Conversion
    • Binary ASK : implementation
    • generate carrier using an oscillator
    • multiply the digital signal by the carrier signal
    Analog Transmission
  • 22. Digital - to - Analog Conversion Frequency Shift Keying (FSK) frequency of the carrier signal is varied to represent data frequency of the modulated signal is constant for the duration of one signal element and …. changes for the next signal element if the data element changes amplitude and phase remain constant for all signal elements Analog Transmission
  • 23. Digital - to - Analog Conversion Frequency Shift Keying (FSK) : binary FSK implemented using two carrier frequencies f 1 and f 2 , one each for data elements 0 and 1 both f 1 and f 2 are 2 Δ f apart Analog Transmission
  • 24. Digital - to - Analog Conversion Frequency Shift Keying (FSK) : binary FSK Analog Transmission 0 -> regular frequency ; 1 -> increased frequency
  • 25. Digital - to - Analog Conversion Frequency Shift Keying (BFSK) : bandwidth carrier signals are simple periodic sine waves but modulation creates aperiodic composite signal with continuous frequencies FSK can be considered as two ASK signals each with its own carrier frequency, f 1 or f 2 Bandwidth B = {(1 + d ) x S} + (2 Δ f) 2 Δ f should be at least equal to S for proper modulation and demodulation Analog Transmission
  • 26. Digital - to - Analog Conversion Binary FSK : implementation use of a voltage controlled oscillator (VCO) VCO changes its frequency according to input voltage 0 -> regular frequency ; 1 -> increased frequency Analog Transmission
  • 27. Digital - to - Analog Conversion Multilevel FSK : more than two frequencies are used f 1 to f 4 can be used to send 2 bits at a time f 1 to f 8 can be used to send 3 bits at a time Analog Transmission
  • 28. 01.10.11 04:00 End of Class - 8 15.09.2011 Data Communication & Networking
  • 29. Digital - to - Analog Conversion Phase Shift Keying (PSK) phase of the carrier signal is varied to represent two or more different signal elements amplitude and frequency remain constant Binary PSK (BPSK) implemented using two signal elements one with phase 0 o and other with 180 o Analog Transmission
  • 30. Digital - to - Analog Conversion Phase Shift Keying (PSK) : binary phase 0 o -> 1 bit ; phase 180 o -> 0 bit bandwidth requirement is the same as that of ASK Analog Transmission Advantages : (a) less susceptible to noise (b) requires only one carrier (less bandwidth) phase = 0 o phase =180 o
  • 31. Digital - to - Analog Conversion Phase Shift Keying (PSK) : binary Analog Transmission phase 0 o -> 0 bit ; phase 180 o -> 1 bit
  • 32. Digital - to - Analog Conversion Phase Shift Keying (PSK) : binary suppose the representation is : phase 0 o -> 0 bit ; phase 180 o -> 1 bit a constellation diagram or a phase-state diagram is another way of representing this modulation scheme Analog Transmission P = 0 P = 180
  • 33. Digital - to - Analog Conversion Binary PSK : implementation phase 0 o -> 1 bit ; phase 180 o -> 0 bit Analog Transmission the digital signal used here is polar NRZ - compare it with the unipolar used for BASK implementation
  • 34. Digital - to - Analog Conversion Quadrature PSK (QPSK): use of two bits at a time in each signal element -> decrease of baud rate -> reduction of required bandwidth uses two separate BPSK modulations : one in-phase and the other out-of-phase (quadrature) Analog Transmission
  • 35. Digital - to - Analog Conversion Quadrature PSK (QPSK): implementation Analog Transmission serial to parallel converter serial to parallel converter sends one bit to one modulator and the next bit to the other modulator
  • 36. Digital - to - Analog Conversion Quadrature PSK (QPSK): implementation Analog Transmission
  • 37. Digital - to - Analog Conversion Quadrature PSK (QPSK): Analog Transmission P = 90 P = 180 P = 180 P = 270 P = 0
  • 38. Digital - to - Analog Conversion Quadrature PSK (QPSK): constellation diagram helps defining the amplitude and phase of a signal element signal element type is represented as a dot the bit or combination of bits it carries is written next to the dot diagram has two axes X-axis -> related to the in-phase carrier Y-axis -> related to the quadrature carrier Analog Transmission
  • 39. Digital - to - Analog Conversion Quadrature PSK (QPSK): constellation diagram Analog Transmission 1 2 3 5 4 6 : peak amplitude of signal element : phase of signal element 5 6
  • 40. Digital - to - Analog Conversion Quadrature PSK (QPSK): constellation diagrams of three SKs Analog Transmission ASK BPSK QPSK uses only an in-phase carrier A = 1 P = 0 A = 1 P = 180 A = √2 P = +45
  • 41. Digital - to - Analog Conversion Quadrature PSK (QPSK): constellation diagram Analog Transmission -> pair of bits represented by each phase
  • 42. Digital - to - Analog Conversion 8-PSK constellation diagram Analog Transmission
  • 43. Digital - to - Analog Conversion Quadrature Amplitude Modulation (QAM) limitation of PSK : small differences in phase are difficult to detect -> limitation on bit rate QAM works on the basis of altering two characteristics of the carrier : amplitude and phase two carriers, one in-phase and another quadrature with two different levels are used Analog Transmission QAM is a combination of ASK and PSK
  • 44. Digital - to - Analog Conversion Quadrature Amplitude Modulation (QAM) Analog Transmission f c ∑  /2
  • 45. Digital - to - Analog Conversion Quadrature Amplitude Modulation (QAM) constellation diagrams (a) 4-QAM with four signal element types similar to ASK or OOK (b) 4-QAM similar to QPSK (c) 4-QAM with a signal with two positive levels (d) 16-QAM with 8 signal levels : 4 +ve & 4 -ve Analog Transmission
  • 46. Analog - to - Analog Conversion is a process of an analog signal modulating an analog signal the baseband signal or low frequency signal produced by each source needs to be shifted to a higher frequency band…………………… if the medium is bandpass in nature or only a bandpass channel is available Analog Transmission
  • 47. Analog - to - Analog Conversion Amplitude Modulation (AM) carrier signal is modulated such that …. its amplitude varies with the changing amplitudes of the modulating signal Analog Transmission
  • 48. Analog - to - Analog Conversion Frequency Modulation (FM) carrier signal is modulated such that …. its frequency varies with the changing amplitudes of the modulating signal Analog Transmission
  • 49. Analog - to - Analog Conversion Typical AM and FM frequency allocations AM : Range 530 KHz to 1700 KHz 612 KHz, 675 KHz, 765 KHz Bandwidth : 10 KHz FM : (all in MHZ) Range 88 to 108 90.4, 91.1, 91.9, 92.7, 93.5, 94.3, 98.3, 104.0, 107.2, 107.8 Bandwidth : 15 KHz Analog Transmission
  • 50. Analog - to - Analog Conversion Phase Modulation (PM) carrier signal is modulated such that …. its phase varies with the changing amplitudes of the modulating signal Analog Transmission