Download Lecture 17

Chapter 2 and 3
Handout #2
Dr. Clincy
Professor of CS
Dr. Clincy
Lecture
Slide 1
Analog Periodic Signal Case
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Amplitude (A): signal value, measured in volts
Frequency (f): repetition rate, cycles per second or Hertz
Period (T): amount of time it takes for one repetition, T=1/f
Phase (f): relative position in time, measured in degrees
General sine wave is written as
S(t) = A sin(2pft + f)
Lecture
Slide 2
Varying S(t) = A sin(2pft + f)
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Lecture
Note: 45 degrees because p is 180 degrees
Slide 3
What is Wavelength ?
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The distance an electromagnetic wave can travel in the amount of time it
takes to oscillate through a complete cycle
Wavelength (w) = signal velocity x period or propagation speed x period
Recall: period = 1 / frequency
Another perspective of Wavelength: how
far did this signal travel AS the signal goes
through a FULL cycle ?
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Slide 4
Electromagnetic Signals
Electromagnetic signal can be expressed as a function of
time or frequency
Function of frequency (more important)
FrequencyDomain Plot
– peak
amplitudes
with respect
to frequency
TimeDomain Plot
– amplitude
changes with
respect to
time
Different signals
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Lecture
Slide 5
Electromagnetic Signals - Frequency
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Electromagnetic signal can be expressed
as a function of time or frequency
Function of frequency (more
important)
– Spectrum (range of frequencies)
– Bandwidth (width of the spectrum)
When we talk about spectrum, we mean
the range of frequencies the
electromagnetic signal takes on
In the example, the signal has a
Frequency range of f to 3f
Therefore, a electromagnetic signal can
be a collection (addition) of periodic analog
Signals (Composite Signal)
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Lecture
Slide 6
Composite Periodic Signal
According to FOURIER ANALYSIS, a composite signal is a combination of
sine waves with different amplitudes, frequencies and phases.
Could
converged to a
square wave
3rd harmonic
9th harmonic
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Lecture
Slide 7
Electromagnetic Spectrum for
Transmission Media
Tell them how to study this chart
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Lecture
Slide 8
Digital Signaling
amplitude (volts)
• represented by square waves or pulses
• Refers to transmission of electromagnetic pulses that
represents 1’s and 0’s
1 cycle
time
(sec)
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frequency (hertz)
= cycles
per second
Lecture
Slide 9
Digital Signal Rate
• Each bit’s signal has a certain duration
• Example, given a data rate of 50 kbps (or 50,000 bps)
• Each would have a 0.02 microseconds duration
• Duration (or bit length) = 1/50000 = .00002 sec = .02 msec
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Lecture
Slide 10
Digital Signal
Sending 1 bit per level
Sending 2 bits per level
How many levels needed to send 5 bits at a time ????
# bits per level = log2 of (#oflevels)
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Lecture
Slide 11
Baseband Transmission
• In sending the digital signal over channel without changing the
digital signal to an analog signal
• Use low-pass channel – meaning the bandwidth can be as low as
zero
• Typical: 2 computers directly connected
In baseband transmission, the required bandwidth is proportional to the bit
rate; if we need to send bits faster, we need more bandwidth (the frequency
will need to increase)
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Lecture
Slide 12
Broadband Transmission
• Broadband transmission or modulation means
changing the digital signal to an analog signal for
transmission
• Modulation allows us to use a bandpass channel – a
channel where the bandwidth doesn’t start at zero
• Bandpass channels are more available than low-pass
channels
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Slide 13
Modulation of a digital signal for transmission on a
Bandpass channel (Broadband transmission)
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Slide 14
Channel Capacity
• As we know, impairments limits the actual data rate
realized
• The actual rate realized at which data can be
transmitted over a given path, under given conditions
is called Channel Capacity
• Four concepts
– Data rate – the rate, in bps, the data can be communicated
– Bandwidth – constrained by the Tx and transport medium –
expressed in cycles per second or Hertz
– Noise – average level of noise over the communication path
– Error rate – the rate in which erroneous bits are received
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Slide 15
Impairments
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Slide 16
Attenuation
Loss of energy – the signal can lose energy as it travels and try
to overcome the resistance of the medium
Decibel (dB) is a unit of measure that measures a signal’s lost
or gain of strength – can be expressed in power or voltage
dB = 10 log10 [P2/P1] = 20 log10 [V2/V1]
Samples of the power or voltage taken at times 1 and 2.
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Lecture
Slide 17
Distortion
Distortion is when the signal changes its form.
The each signal that makes up a composite signal could have different
propagation speeds across the SAME medium – because of this, the
different signals could have different delays (arriving at the receiver) –
this causes a distortion.
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Lecture
Slide 18
Noise
Thermal Noise - the uncontrollable or random motion of electrons in
the transport medium which creates an extra signal (not sent by the
transmitter)
Induced Noise – undesired devices acting as a transmitting antenna and
those signals being picked up
Cross Talk Noise – effect of one wire crossing another wire
Impulse Noise – spikes in energy (ie lightning)
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Slide 19
Signal to Noise Ratio
SNR = avg-signal-power/avg-noise-power
High SNR – good (less corruption)
Low SNR – bad (more noise than good power)
SNR is described in Decibels (dB)
SNRdB = 10 log10 SNR
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Lecture
Slide 20
Shannon Equation
• Shannon’s equation is used to determine the actual
capacity of a channel given noise exist
• C = B log2 (1 + SNR)
– B = Bandwidth
– C= Channel Capacity
– SNR = Signal-to-noise ratio
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Lecture
Actual ratio
Slide 21