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Why fiber?
Speed- operates at high speed upto gigabits
Bandwidth- large carrying capacity
Resistance- to EM noise
Provide electrical isolation
Distance- Signals can be transmitted further without need of repeaters
Maintenance cost- less
Requirements of digital fiber link
No. of digital signal – 8
No. of fiber cable – 1
Signal frequency- 0-20KHz
I/p-O/p voltage level- 0 & 24 V for (0 & 1 digital logic)
Transmission Range- 300m
Here, we have selected HFBR-1521 transmitter and HFBR-2521 receiver.Specifications
Low Cost
Data rate: upto 5 MBd
High Noise Immunity
Transmitters Incorporate a 660 nm Red LED for Easy Visibility
Propagation delay: 140 ns (max.)
Compatible with Standard TTL Circuitry
Fiber selected – HCS
core of silica(diameter: 200 µm)
cladding of special plastic(diameter: 230 µm)
Wavelength- 680 nm
Attenuation- 6db/km
Features
Medium bandwidth
transmission rates of less than 100 MBit/s
suitable for distances of up to 2 km
Voltage converter and isolation- optocoupler(4N35)
Shift register-
PISO – SN74LS674 (16 bit)
SIPO- SN74LS673 (16 bit)
Line code
The waveform pattern of voltage or current used to represent the 1’s and 0’s of a digital data on a transmission page link is called line encoding
The common types of line encoding are unipolar, polar, bipolar and Manchester encoding
Criteria for selection of line code
Minimize transmission hardware
Facilitate synchronization
Ease error detection and correction
Minimize spectral content
Eliminate a dc component
Less bandwidth
No long string of zeros
Comparison of line codes
The line code chosen for the fiber optic page link design is return to zero .
For removing long string of zeros scrambling is used.
Scrambling
Scrambler is a technique that manipulates a data stream before transmitting.
Widely used in satellite, radio relay  and fiber optics communications, PSTN modems
scrambler replaces sequences into other sequences without removing undesirable sequences, and as a result it changes the probability of occurrence of vexatious sequences.
Purpose of scrambling
It facilitates the work of a Clock recovery
It eliminates the dependence of a signal's power spectrum upon the actual transmitted data, making it more dispersed to meet maximum power spectral density requirements
eliminating long sequences consisting of '0' or '1' only
Multiplicative scrambling
Multiplicative scramblers are called so because they perform a multiplication of the input signal by the scrambler's transfer function in Z-space.
They are discrete linear time-invariant systems.
A multiplicative scrambler is recursive and a multiplicative descrambler is non-recursive.
multiplicative scramblers do not need the frame synchronization, that is why they are also called self-synchronizing.
Future Scope
Final algorithm of scrambling and descrambling will be prepared.
Based on the conceptual design and availability of the components final circuit will be designed and developed.
Conclusion
Various line coding techniques are explored based on the page link requirements and we have selected RZ coding with scrambling for the design.
Various types fiber cables are explored and HCS fiber cable(200 µm, 680 nm) for the page link is selected