More on Modulation Module B Panko and Panko Business Data Networks and Security, 9th Edition © 2013 Pearson Education, Inc. Publishing as Prentice Hall

Modulation Modulation converts a 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 © 2013 Pearson Education, Inc. Publishing as Prentice Hall

Carrier Wave There is a carrier wave. This carrier wave is modulated (changed) to carry information. © 2013 Pearson Education, Inc. Publishing as Prentice Hall

Modulation There are several forms of modulation. Amplitude modulation Frequency modulation Phase modulation Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.1: Amplitude Modulation (AM) Amplitude is the intensity of the signal. Loud or soft Amplitude (power) © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.1: Amplitude Modulation (AM) Amplitude (low) Low Amplitude (0) Amplitude (high) High Amplitude (1) 1 Amplitude Modulation (1011) 1 1 © 2013 Pearson Education, Inc. Publishing as Prentice Hall

Waves Frequency of a wave Frequency (Hz) Cycles in One Second The number of complete cycles per second Called Hertz kHz, MHz, GHz, THz Frequency (Hz) Cycles in One Second © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.2: Frequency Modulation (FM) Wavelength Low Frequency (0) Wavelength High Frequency (1) 1 Frequency Modulation (1011) 1 1 © 2013 Pearson Education, Inc. Publishing as Prentice Hall

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 © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.3: Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1) 1 Frequency Modulation (1011) 1 1 © 2013 Pearson Education, Inc. Publishing as Prentice Hall

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. © 2013 Pearson Education, Inc. Publishing as Prentice Hall

Phase Shift Keying Phase shift keying Binary phase shift keying (BPSK) Another term for phase modulation Binary phase shift keying (BPSK) Phase modulation with two phases Quadrature phase shift keying (QPSK) Phase modulation with four phases © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.4: QAM Quadrature Amplitude Modulation (QAM) Uses two carrier waves: sine and cosine (90o out of phase), both amplitude-modulated © 2013 Pearson Education, Inc. Publishing as Prentice Hall

B.4: 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 (24 = 16). Sine Wave Amplitude Cosine (Quadrature) Wave Amplitude © 2013 Pearson Education, Inc. Publishing as Prentice Hall

QAM QAM Alternatives Trade-off 16 QAM 32 QAM 64 QAM 128 QAM 256 QAM More states allow more bits to be sent per clock cycle although with diminishing returns. But more states means that states are closer together, so digital error resistance is reduced. © 2013 Pearson Education, Inc. Publishing as Prentice Hall

© 2013 Pearson Education, Inc. Publishing as Prentice Hall