More on Modulation Module B Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

Modulation Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line Analog Modulated Signal Binary Data Modem Telephone 33.6 kbps Modem Telephone Server A Client A PSTN

Modulation There are several forms of modulation Amplitude modulation (in Chapter 3) Frequency modulation Phase modulation Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation

Amplitude Modulation (AM) Amplitude is the intensity of the signal Loud or soft Amplitude (power)

Amplitude Modulation (AM) (from Chapter 3) Amplitude (low) Low Amplitude (0) Amplitude (high) High Amplitude (1) Amplitude Modulation (1011)

Waves Frequency of a wave The number of complete cycles per second Called Hertz kHz, MHz, GHz, THz Frequency (Hz) Cycles in One Second

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

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

Figure B-2: Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011)

Phase Modulation (PM) Human hearing is largely insensitive to phase So harder to understand than FM and AM But equipment is very sensitive to phase changes PM is used in all recent forms of modulation for telephone modems

Figure B-3: QAM Quadrature Amplitude Modulation (QAM) Uses two carrier waves: sine and cosine (out of phase), both amplitude-modulated

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 High/High 1111 Cosine (Quadrature) Wave