More on Modulation Module B Updated January 2009

More on Modulation Module B Updated January 2009 Raymond Panko’s Business Data Networks and Telecommunications, 7 th edition May only be used by adopters of the book © 2009 Pearson Education, Inc. Publishing as Prentice Hall

Modulation • Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line Binary Data Analog Modulated Signal Client A 33. 6 Telephone kbps Modem Telephone Modem Server A PSTN © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-2

Carrier Wave • There is a carrier wave • This carrier wave is modulated

Modulation • There are several forms of modulation – Amplitude modulation – Frequency modulation – Phase modulation – Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-4

Amplitude Modulation (AM) • Amplitude is the intensity of the signal – Loud or

Amplitude Modulation (AM) Low Amplitude (0) High Amplitude (1) Amplitude (low) Amplitude (high) 1

Waves • Frequency of a wave – The number of complete cycles per second – Called Hertz – k. Hz, MHz, GHz, THz Frequency (Hz) Cycles in One Second © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-7

Figure B-1: Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011)

Phase • Two signals can have the same frequency and amplitude but have different phases—be at different points in their cycles at a given moment Basic Signal 180 degrees out of phase © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-9

Figure B-2: Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1)

Phase Modulation (PM) • Human hearing is largely insensitive to phase – So harder to understand than AM or FM • But equipment is very sensitive to phase changes – PM is used in all recent forms of modulation for telephone modems and all forms of radio transmission © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-11

Figure B-3: QAM • Quadrature Amplitude Modulation (QAM) – Uses two carrier waves: sine and cosine (90 o out of phase), both amplitude-modulated © 2009 Pearson Education, Inc. Publishing as Prentice Hall B-12

Figure B. 3: QAM • Suppose each carrier wave has four possible amplitude levels – In each clock cycle, there are 16 combined possibilities – In each clock cycle, can send 4 bits (2^4=16) Sine Wave Cosine (Quadrature) Wave © 2009 Pearson Education, Inc. Publishing as Prentice Hall High/High 1111 B-13

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