Transcript Computer Network and Infrastructure

Computer Network and
Infrastructure
Data Transmission
Dr. E.C. Kulasekere
Computer Network and Infrastructure
Section Objectives
Understand the concepts and terminology
associated with signal transmission.
 Analog and digital data transmission
techniques.
 Transmission impairments.
 Channel capacity. Shannon and Nyquist
formulas.
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Data Transmission Terminology
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Transmitter and Receiver.
Medium of transmission: Guided (Copper wire) and
Unguided (Free space:wireless).
Transmission path:
Direct link: No intermediate devices
 Point-to-Point: Direct link with only two devices sharing
the link
 Multipoint: More than two devices share the link
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Data Transmission Terminology..
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Simplex
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Half duplex
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One direction (e.g. television)
Either direction, but only one way at a time (WT sets)
Full duplex
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Both directions at the same time
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Time Domain Concepts
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Continuous signal
 Varies
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in a smooth manner over time
Discrete signal
 Signal
amplitude is known for only discrete time
values.
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Periodic signal
 Pattern
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repeated over time
Aperiodic signal
 Pattern
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not repeated over time
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Analog/Digital Waveforms
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Periodic Waveforms
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Frequency Domain Concepts
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Peak Amplitude (A)
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maximum strength of signal (in volts !)
Frequency (f)
Rate of change of signal
 Hertz (Hz) or cycles per second
 Period = time for one repetition (T = 1 / f)
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Phase ()
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Relative position in time
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Data Rate and Bandwidth
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Any transmission system has a limited band of
frequencies
This limits the data rate that can be carried
Data: entities that convey meaning
Signals: electric or electromagnetic representations
of data
Transmission: communication of data by
propagation and processing of Electromagnetic
signals
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Analog and Digital Signals
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Both can be electromagnetic signals.
Analog will have continuous amplitude while the
digital will have pulsed amplitude.
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Analog and Digital Signals ..
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Digital signaling is cheaper than analog signaling
and less susceptible to noise impairments.
Disadvantage of digital signaling is they are more
prone to attenuation distortions.
The above discussion is concentrated on explaining
using analog signals in representing analog data
while digital signals are used to represent digital
data.
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Analog/Digital Data and Signals
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Usually the type of signal that is used for
transmission is dependent on what kind of medium
we have to transmit.
For some cases digital data with digital signaling
may not be suitable (similar for analog)
Can use analog signal to carry digital data
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Modem
Can use digital signal to carry analog data
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Compact Disc audio
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Analog Signals Carrying Digital
and Analog Data
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Digital Signals Carrying Digital
and Analog Data
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Transmission Impairments
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Signal received may differ from signal transmitted
Analog - degradation of signal quality (S/N ratio)
Digital - bit errors (BER – Bit Error Ratio)
Caused by
Attenuation
 Delay distortion
 Noise
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Transmission Impairments
Attenuation & Amplification
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Transmission Impairments
Delay Distortion
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Only in guided media
Propagation velocity varies with frequency
Hence signals are received at different times.
In digital signals, spill over occurs.
This distortion will impose a maximum bit rate over
transmission channels.
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Transmission Impairments
Noise
Additional signals inserted between transmitter and
receiver
Thermal (white) Noise
Intermodulation Noise
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Signals that are the sum and difference of original
frequencies sharing a medium
Crosstalk Noise
Impulse Noise
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Effect of Noise on a Digital
Signal
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The Four Concepts that
Determine the Channel Capacity
Data rate
In bits per second
 Rate at which data can be communicated
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Bandwidth
In cycles per second of Hertz
 Constrained by transmitter and medium
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Noise
Error Rate
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Nyquist Theory
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Shannon Theory
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Review Questions
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Differentiate between guided and unguided media
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With guided media, the electromagnetic waves are
guided along the physical path whereas unguided
media provide a means for transmitting
electromagnetic waves but do not guide them, for
example free space.
What is the relationship between a signal’s spectrum and
its bandwidth?
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The spectrum of a signal is the frequencies it contains
while the bandwidth of a signal is the width of the
spectrum.
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Review Problems
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Given an amplifier with an effective noise
temperature of 10,000K and a 10MHz bandwidth,
wt thermal noise level may be expected at its
output?
N
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N=10 log k + 10 log T + 10 log B
= –228.6 dBW + 10 log 104 + 10 log 107
= –228.6 + 40 + 70 = –118.6 dBW
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Review Problem
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A digital signaling system is required to operate at
9600bps. If a signal element is encodes a 4bit word,
what is the minimum required bandwidth of the
channel?
Using Nyquist's equation: C = 2B log2M
We have C = 9600 bps
log2M = 4, because a signal element encodes a 4-bit w
Therefore, C = 9600 = 2B ´ 4, and B = 1200 Hz
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