1. Module 3.0: Data Transmission
Terminology and fundamental concepts
Harmonics
Bandwidth
Transmission impairments
Channel Capacity and Nyquist’s Theorem
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2. Terminology Medium Guided medium e.g. twisted pair, optical fiber
Unguided medium
e.g. air, water, vacuum
Simplex
One direction
e.g. Television
Half duplex
Either direction, but only one way at a time
e.g. police radio
Full duplex
Both directions at the same time
e.g. telephone
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3. Fundamental Concepts Time domain concepts Continuous signal (Analog)
Various in a smooth way over time
e.g. sound, video
Discrete signal (Digital)
Maintains a constant level then changes to another constant level
e.g., text, integers
Periodic signal
Pattern repeated over time
Aperiodic signal
Pattern not repeated over time
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4. Sine Wave (Harmonic) Components
Peak Amplitude (A)
maximum strength of signal
volts
Frequency (f)
Rate of change of signal
Hertz (Hz) or cycles per second
Period = time for one repetition (T)
T = 1/f
Phase ()
Relative position in time
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5. Wavelength Length of wave or Distance occupied by one cycle
Meaning the distance a simple signal can travel in one period
Distance between two points of corresponding phase in two consecutive cycles 
Assuming signal velocity v
 = vT
f = v
c = 3*108 m/s (speed of light in free space) or  = 3*108 meters.
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6. Bandwidth Bandwidth (in analog communications)
The total capacity of communications channel measured in hertz(Hz).
It is the difference between the highest and lowest frequencies capable of being carried over a channel.
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
Bandwidth (in digital communications and networking)
The theoretical capacity of communications channel expressed in bits per second (bps), which is called data rate.
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7. Propagation Delay
Propagation time measures the time required for a signal (or a bit) to travel from one point of the transmission medium to anoterh.
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8. Analog and Digital Data Transmission
Signals
Electric or electromagnetic or light representations of data. Means of propagating Data.
Analog Signal
Represent data with continuously varying sine wave.
Continuously variable. Various media, e.g. wire, fiber optic, space
Speech bandwidth 100Hz to 7kHz, Telephone bandwidth 300Hz to 3400Hz, Video bandwidth 4MHz
Digital Signal
Represent data with sequence of voltage pulses
Use two DC components and vary it over time. Component of zero frequency.
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9. Digital Signal
In RS-232, voltage that varies between -15 and -5 volts is interpreted as a binary 0, and voltage that varies between +5 and +15 volts is interpreted as a binary 1. Voltage that varies between -5 and +5 volts is ignored; that is, no interpretation is made.
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10. Analog Signal
A single frequency with fixed A and  has 0 bandwidth. It carries no information. However, when it is combined with other frequencies or its components become variable, the bandwidth gets changed.
Amplitude Change
Frequency Change
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11. Analog Signal
Phase Change
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12. Modem Modulation Techniques
Usually use digital signals for digital data and analog signals for analog data
Can use analog signal to carry digital data
Modem
Digital signals have a wide spectrum and thus are subject to strong attenuation and delay distortion. DC is good for short distances.
Analog signals are used instead with different modulation of components.
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13. Analog vs. Digital Transmission
Communication of data by propagation and processing of signals
Analog Transmission:
Analog signal transmitted without regard to content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Also amplifies noise
Digital Transmission:
Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater
receives signal
extracts bit pattern
retransmits
Attenuation is overcome
Noise is not amplified
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14. Transmission Impairments
Signal received may differ from signal transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
Attenuation and attenuation distortion
Delay distortion
Noise
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15. Attenuation Attenuation: Signal strength falls off with distance
Depends on medium
Received signal strength:
must be enough to be detected
must be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency
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16. Delay Distortion Delay Distortion: Only in guided media
Propagation velocity varies with frequency
Called also “Intersymbol Interference”. Due to delay distortions, some of the signal components of one bit position will spill over into other bit positions.
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17. Noise Additional signals inserted between transmitter and receiver
Thermal
Due to thermal agitation of electrons
Uniformly distributed
White noise
Crosstalk
A signal from one line is picked up by another
Impulse
Irregular pulses or spikes
Short duration
High amplitude
e.g. External electromagnetic interference such as lightning or cut in cable or fault in communication equipment.
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18. Channel Capacity Data rate = Capacity In bits per second
Rate at which data can be communicated
Nyquist’s Theorem
In 1920, Henry Nyquist discovered that sampling rate must be at least twice the highest frequency, in order to construct the original signal.
Called also Sampling Rate theorem.
See nice animation at
Also, maximum data rate (how much data you can represent in one signal) of any signal of frequency w is
MDR (noiseless) = 2 w log2 M
M is the values used per signaling state
Shannon’s formula (only thermal noise)
MDR = w log2 (1 + SNR)
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