More on Propagation Module B Copyright 2001 Prentice Hall

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 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 (1101) Modem Analog Signal

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

Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011) 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 High Amplitude Low Amplitude 90 Degrees Out of Phase, High Amplitude In Phase 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