practical fibre optics for engineers & technicians
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Practical Fibre Optics for Engineers & Technicians
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Data Communications
• Information is transmitted between two points in the form of data.
• Binary digital data uses a code constructed of ‘0’s and ‘1’s– In copper systems represented as
a high and low voltage levels.– In fibre systems represented as
the presence or not of a light pulse.
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TRANSMITTERS, RECEIVERS ANDCOMMUNICATIONS CHANNELS
• A communications process requires – A source– A transmitter– A communications channel– A receiver– A destination
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Communications Process
Transmitter ReceiverCommunicationsChannelSource Destination
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Important Concepts• Interface standards
– Electrical signal characteristics– Mechanical characteristics– Functional characteristics
• Coding – The code symbols used for transmission– Example : ASCII Code
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Communication Channels• Analog
– Varying amplitude, phase and frequency
• Digital– High - low voltages levels; on - off signals.
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Analog Signals
Velocity ofsignal movementthough channel
VaryingAmplitude
0
-
+Wavelength
Time
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Digital Signals
Velocity of signalthough channel
AMPLITUDE
0
-
+
T = Pulse Period
Time
OFFSPACE
ONMARK
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• SIGNAL ATTENUATION• Natural resistive cable properties absorb the
electrical energy and turn it into heat• Limits the length of the communications channel• Digital signals have fast rising edges which
represent high frequency components. Signal attenuation increases with increasing frequency.
• Use repeater, amplifiers and equalisers• Natural resistive properties of glass absorb
electromagnetic energy
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Signal Attenuation
1 1 1 10 000 TransmittedSignal
Signal atDistance d
Signal atDistance 2d
Time
Time
Time
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Signal Repeaters
Transmitter Repeater Receiver
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• CHANNEL BANDWIDTH• The difference between the highest and the lowest
frequencies that can pass though a channel.• Where the highest and lowest frequencies have
dropped to half power i.e. 3 dB drop in power.• Digital signals are constructed of many frequencies
but their transmission is limited by the channel analog bandwidth.
• The higher the bandwidth the higher the frequency that can be transmitted.
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Channel Bandwidth
3dBBandwidth
0dB
-3dB
BANDWIDTHMinimumFrequency
MaximumFrequency
FREQUENCY
AMPLITUDE
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Effect Of Channel Bandwidth on a Digital Signal
1 0 100OriginalDigitalData
1200 HzBandwidthSignal
4000 HzBandwidthSignal
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Digital Regenerator
Line Driver Digital Repeater Line Receiver
Channel Channel
DegradedSignal
Received
RegeneratedSignal
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TRANSMISSION MODES
• Simplex
• Half duplex
• Full duplex
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Simplex
Transmitter ReceiverData Flow
In One Direction Only
Station A Station B
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Half Duplex
Transmitter
Transmitter
Receiver
Receiver
Data Flow
Alternate Flow
Station A Station B
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Full Duplex
Transmitter
Transmitter
Receiver
SimultaneousData Flow
Station A Station B
Receiver
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LIGHT• Is represented by :
– Electromagnetic waves– Photons (particles)
• Travels at a speed of 3 x 108
m/s
• Reflect, refract and diffract off optical surfaces.
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DO YOU WANT TO KNOW MORE?
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