More on Propagation Module B Copyright 2001 Prentice

More on Propagation • Modulation – Modems translate between digital devices and analog transmission lines; We will look at the processes used to modulate digital signals • Multiplexing – An important way to reduce costs is to multiplex (mix) several signals onto a transmission line • Trunk Lines – Trunk lines link the switches of carriers

Modulation

Modulation • Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line • There are several forms of modulation – Amplitude modulation – Frequency modulation – Phase modulation – Complex modulation

The Modulation Problem • Modem accepts a digital signal from the computer – Really, binary--ones and zeros – Two voltage levels • Modem converts into waves (analog) Digital Signal Modem (1101) Analog Signal

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

Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011) Wavelength 0 1 1 1

Wavelength • Physical distance between similar points in adjacent cycles – Not independent of frequency – Frequency * wavelength = speed of propagation – In a harp, for instance, long strings have low sounds Wavelength (meters)

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

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

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

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

Complex Modulation • Modern Modems Mix Phase and Amplitude In Phase High Amplitude 90 Degrees Out of Phase, High Amplitude Low Amplitude 180 Degrees Out of Phase

Complex Modulation • Baud rate: number of clock cycles per second – Usually 2, 400 to 3, 200 baud for telephone modems • Bits sent per clock cycle depends on number of possible states – 2 bits/clock cycle= number of possible states – In our example 8 possible states, so 2 n=8 – n must be 3 • Bit rate = baud rate * bits/clock cycle – In our example, bits/state = 3 – So if baud rate is 2, 400, bit rate would be 9, 600