1. Dr. Clincy Professor of CS
Chapter 2 and 3 Handout #2
Dr. Clincy
Professor of CS
Dr. Clincy
Lecture

2. Data Vs Signal
Fully explain the difference between signal and data before getting into the details
(Digital Transmission)
DATA SIGNAL
D D A
(Analog Transmission)
A D
Dr. Clincy
Lecture

3. Data vs Signals
You probably have a good idea about digital and analog signals
What about analog and digital data ??
Analog data examples
Voice
Images
Digital data examples
Text
Digitized voice or images
Dr. Clincy
Lecture

4. Periodic Signal Characteristics
If the signal’s pattern repeats over and over, we called these signals Periodic Signals
Periodic Signals can be either Analog or Digital
Dr. Clincy
Lecture

5. Analog Periodic Signal Case
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)
Dr. Clincy
Lecture

6. Varying S(t) = A sin(2pft + f)
Dr. Clincy
Lecture
Note: 45 degrees because p is 180 degrees

7. What is Wavelength ?
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 ?
Dr. Clincy
Lecture

8. Electromagnetic Signals
Electromagnetic signal can be expressed as a function of time or frequency Function of frequency (more important)
Frequency-Domain Plot – peak amplitudes with respect to frequency
Time-Domain Plot – amplitude changes with respect to time
Different signals
Dr. Clincy
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9. Electromagnetic Signals - Frequency
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)
Dr. Clincy
Lecture

10. 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

11. Electromagnetic Spectrum for Transmission Media
Tell them how to study this chart
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12. Digital Signaling represented by square waves or pulses
Refers to transmission of electromagnetic pulses that represents 1’s and 0’s
1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
= cycles per second
Dr. Clincy
Lecture

13. 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 = sec = .02 msec
Dr. Clincy
Lecture

14. Digital Signal # bits per level = log2 of (#oflevels)
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)
Dr. Clincy
Lecture

15. 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)
Dr. Clincy
Lecture

16. 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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Lecture

17. 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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18. Impairments
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19. 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

20. 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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21. 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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22. 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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23. 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
Actual ratio
Dr. Clincy
Lecture