Transcript More on Propagation

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
• 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
– kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
Frequency Modulation (FM)
Low
Frequency
(0)
Wavelength
Wavelength
High
Frequency
(1)
0
Frequency
Modulation
(1011)
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)
Amplitude (low)
Amplitude (high)
High
Amplitude
(1)
Amplitude
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
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 2n=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