2. Analog and Digital Transmission
Data
Entities that convey meaning.
Signals
Electric or electromagnetic representations of data.
Transmission of Information
Communication of data by propagation and processing of signals.
Transmission of information from one component to another in the form
of signals. A system collecting information about the pressure of gas in a
pipe and transmit to control room. A system might take a television
picture from one side of world to other.
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3. Analog Transmission
• Analog transmission is a transmission method of conveying
information using a continuous signal which varies in amplitude,
phase, or some other property in proportion to that information.
• It could be the transfer of an analog source signal, using an
analog modulation method such as frequency modulation (FM)
or amplitude modulation (AM), or no modulation at all.
• The data is attenuated over long distance by boosting signal with
amplifiers which also amplifies noise.
• Transmission may be analog or digital (MODEM) data.
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4. Digital Transmission
• Digital transmission is the sending of information over a physical
communications media in the form of digital signals like pulse.
• Analog signals must therefore be digitized first before being
transmitted. Digital signals is transmitted with regard to content.
• Digital transmission also may transmit analog or digital data.
• Can use digital signals to carry analog data. E.g. Compact disc (CD),
Codec (Programs that encode digital data) etc.
• Less attenuation due to less noise.
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8. Analog Transmission or Communication
System
8
Fig. Analog Communication System
Destination
• The message signal to be transmitted is analog. This analog message can be obtained
from sources such as speech, video shooting etc.
• The message signal is then modulated on some carrier frequency by the modulator.
• Presently, all the AM, FM radio transmission and TV transmission is analog
communication.
• The analog communication needs lower bandwidth compared to digital
communication. But the effect of noise interference is more in case of analog
communication.
9. Digital Transmission or Communication
System
9
Fig. Block Diagram of Digital Communication System
10. Digital Transmission or Communication
System
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• Efficiently converting the source into a sequence of binary digits is a process, which is
called source encoding of data compression.
• Channel encoder adds some redundancy into binary information sequence that can be
used for handle noise and interference effects at the receiver.
• Digital modulator maps the binary information sequence into signal waveforms.
• Communication channel is used to send the signal from the transmitter to the receiver.
Physical channels: the atmosphere, wireless, optical compact disk.
• Digital demodulator receives transmitted signals contains the information which is
corrupted by noise.
• Channel decoder attempts the reconstruction the original information sequence from
knowledge of the code used by channel encoder.
• Source decoder attempts the reconstruction the original signal from the binary
information sequence using the knowledge of the source encoding methods.
• The difference between the original signal and the reconstructed signal is measured of
the distortion introduced by the digital communication system.
11. Digital Transmission or Communication System
Advantages of Digital Transmission
• Noise Immunity
• Multiplexing
• Easy to Store
• Resistant to additive Noise
• Used for Long Distance
• Transmission errors can be detected easily
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Disadvantages of Digital Transmission
• More Bandwidth requirement
• Extra Circuitry for encoding and Decoding
• Require Synchronization
12. Advantages of Digital Transmission
• Noise Immunity
Digital signals are inherently less susceptible than analog signals to
interference caused by noise because with digital signals it is not necessary to
evaluate precise amplitude, frequency or phase. Instead pulses are evaluated during
the precise time interval and simple determination is made whether the pulse is
above or below a prescribed reference.
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13. Advantages of Digital Transmission
• Multiplexing
Digital signals are better suited than analog signals for processing and
combining using a technique multiplexing.
• Easy to Store
It is simple to store digital signals than analog signals.
• Used for Long Distance
Digital regenerators sample noisy signals and then reproduce an entirely new
digital signal with same S/N ratio as the original transmitted signals. So digital
transmitted signals can be transported longer distance than analog signals.
• Transmission errors can be detected easily
The transmission errors can be detected and corrected more easily and
accurately than is possible with analog signals.
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14. Advantages of Digital Transmission
• Resistant to additive Noise
Digital transmission systems are more resistant to analog system to additive
noise because they use signal regeneration rather than signal amplification. Noise
produced in electronic circuit is additive, therefore S/N ratio becomes worse each
time an analog signal is amplified.
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15. Disadvantages of Digital Transmission
• More Bandwidth Requirement:
The transmission of digitally encoded original analog signal. BW
is one of the important aspects of any communication system because it
is costly and limited.
• Extra Circuitry for encoding and Decoding
Analog signals must be converted to digital pulses prior to
transmissions and converted back to their original analog form at
receiver, thus require additional circuitry for encoding and decoding.
• Require Synchronization
Digital transmissions require precise time synchronization
between the clocks in transmitter and receiver.
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16. Fiber Optics
Introduction
• Method of transmitting information from one place to another by
sending pulses of light through an optical fiber.
• Optical Fiber is a cylindrical waveguide system through which optical
wave can propagate.
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18. Fiber Optics
Working Principle: Total Internal Reflection(TIR)
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TIR only takes place when both of the following two conditions are met:
• A light ray is in the more dense medium and approaching the less
dense medium.
• The angle of incidence for the light ray is greater than the so-called
critical angle.
19. Acceptance Angle in Optical Fiber
The maximum angle within which light will be accepted by an element,
such as a detector or waveguide. Also called acceptance cone.
The system calculate the acceptance angle of an optical fiber using
following formula:
θm = sin-1((1/ηo) * (ηf
2-ηc
2)1/2)
Where, θm = Acceptance Angle, ηo = Exterior Refractive Index, ηf =
Fiber Refractive Index, ηc = Cladding Refractive Index
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22. Types of optical fiber
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1.Single mode fibers
2.Multi mode fibers
2.1 Step index
2.2 Graded Index
23. Types of optical fiber
1. Single mode fibers
-Used to transmit one signal per fiber used in
telephone and cable TV.
-Small cores (9 microns diameter)
-Transmit infrared from laser
2. Multi mode fibers: used to transmit many
signals per fiber used in computer networks,
larger cores(62.5 microns diameter) and
transmit infrared from LED
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24. Multimode Fibers
Sr. No. Step Index Graded Index
1 The refractive index of the core is uniform and step or
abrupt change in refractive index takes place at the
interface of core and cladding in step index fibers.
The refractive index of core is non-uniform, the
refractive index of core decreases parabolically
from the axis of the fiber to its surface.
2 The light rays propagate in zigzag manner inside the
core. The rays travel in the fiber as meridional rays
and they cross the fiber axis for every reflection.
The light rays, propagate in the form of skew rays
or helical rays. They will not cross the, fiber axis.
3 Step index fiber is of two types viz; mono mode fiber
and multi-mode fiber.
Graded index fiber is of only one type, that is multi-
mode fiber.
4 The refractive index of the core of step index fiber is
constant throughout the core.
The refractive index of the core of the graded index
fiber is maximum at center, core and then it
decreases towards core-Cladding interface.
5 The refractive index profile may be defined as: The refractive index profile may be defined
6 Modal dispersion affects signal quality in step index
fiber.
Graded index fiber, provides zero dispersion as the
velocity of modes is changed by changing R.I, such
that time taken by all modes is same.
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26. Advantages of Optical Fiber
• Extremely High Bandwidth:
-Fiber optic cables have a much greater bandwidth than metal cables.
-Optical fiber has a bandwidth capability of 400MHz/km or greater.
-Optical fiber can provide data transmission performance up to
10Gbps, 40Gbps and even 100Gbps with new hardware that is now available.
• Low Power Loss:
- An optical Fiber offers low power loss. This allows for longer
transmission distances.
- In comparison to copper; in a network, the longest recommended
copper distance is 100m while with fiber, it is 2000m.
- In order to increase the transmission distance, a repeater must be
installed in the middle of the path to re-generate the signal
- Optical fiber cable allows data to be sent far without as many
repeater devices that are required by other types of cabling.
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27. Advantages of Optical Fiber
• Interference:
- Fiber optic cables are immune to electromagnetic interference. -
It can also be run in electrically noisy environments without concern, as
electrical noise will not affect fiber.
- Optical fiber transmits light (photons) rather than electrons, it does
not radiate electromagnetic fields, nor is it susceptible to electromagnetic
fields.
• Size:
- In comparison to copper, a fiber optic cable has nearly 4.5 times as
much capacity as the wire cable has and a cross sectional area that is 30 times
less.
• Weight:
- Fiber optic cables are much thinner and lighter than metal wires.
- They also occupy less space with cables of the same information
capacity. Lighter weight makes fiber easier to install. 27
28. Advantages of Optical Fiber
• Secure Transmissions :
- optical fiber cabling provides an extremely secure transmission
medium.
- optical fiber is a dielectric, it does not present a spark hazard.
- fiber optic cabling does not radiate magnetic fields.
- the light (photons) is confined within the fiber which makes it
impossible to tap the signal without cutting into the fiber.
- fiber is the most secure medium available for carrying sensitive data.
• Flexibility :
- An optical fiber has greater tensile strength than copper or steel fibers
of the same diameter. It is flexible, bends easily and resists most corrosive
elements that attack copper cable.
• Cost :
- The raw materials for glass are plentiful, unlike copper. This means glass
can be made more cheaply than copper. 28
29. Disadvantages of Optical Fiber
• Requires skilled manpower for installation
• Difficult to repair and maintenance
• High installation cost
• Splicing (joining two optical fibers) is difficult
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30. Application of Optical Fibers
• In Communication- Compared to a conventional
system they offer better reliability, large information
transmission capacity, cost effective etc.
• In Medical Science – With the advent of fiber optics
the otherwise inaccessible parts of the body are now
visible to surgeon without actually cutting through the
body. Ex. Endoscopy.
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31. Application of Optical Fibers
•Military Applications- Optical Fibers are lighter
in transportation and more reliable in terms of
secrecy as compared to conventional systems.
•Entertainment – A coherent Optical Fiber bundle
offers better enlargement of the image displayed
on a TV or screen.
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