DIGITAL COMMUNICATION
Introduction
Digital technology is a branch of Electronics and
communication. It makes use of digital signals, which are presented in discrete
steps. A continuous signal is an analog signal. The output of a data
source is transmitted from one point to another in a digital communication
system. A data system can transmit rectangular pulses at speeds ranging from
100 to 500 Kbits per second.
A typical
application of this system are:
• Computer to computer communication
• Computer interrogation (either for data storing or data
manipulation and calculation)
• Programming
• Data collection
• Telemetry and alarm system
• Financial credit information
• Transfer travel and accommodation booking services
Digital communication may be classified into two
categories. They are.
• A
web-based system (Online System)
Data is transferred directly to or from a computer in an online system.
The online system can either be real-time or non-real-time. A real-time
system demands an immediate reaction. The data transmission speed in an on-real
system is significantly slowed.
• Offline
System
Data is sent to or from an intermediate storage location, such as a card,
a paper tape punch, magnetic tape, or a disc, in an off-line system.
Based on transmission, digital communication can be
classified into three categories:
1. Simplex
Simplex refers to a one-way channel
connection. here the communication is possible in only one direction.
Example: Radio, TV, etc.
2. Semi-Duplex
A semi-duplex connection is
required for either direction transmission. Communication can be possible only
one way at a specified time.
3. Full-Duplex
A full-duplexer allows data to be
transmitted in both ways at the same time.
BASIC
ELEMENTS OF DIGITAL COMMUNICATION SYSTEM
A digital information
source generates a limited number of messages. A nice example of a digital
source is a typewriter. A source's ability to emit a finite number of
characters (messages) is restricted.
A digital
communication system is a system that sends data from a digital source to a
digital sink. A digital waveform is a time function with just a discrete set of
possible values. Only two values exist in the digital waveform since it is a
binary waveform.
Voltages and
currents with digital signals are common in electronic digital communication
systems.
If an analog
signal must be transmitted, it is first converted to a digital signal using an
Analog to Digital Converter before being sent over the channels. Because an
analog signal has a continuous range of values that is a function of time.
For example,
a sine wave of 1000 Hz might be used to represent a binary 1 in a digital line
transmission, whereas a sine wave of 500 Hz could be used to represent a binary
0. As a result, analog waveforms produced from an analog source are used to
convey digital data.
Block diagram of Digital Communication System
Digital signals are the coded representation of information that is transmitted in digital communication.
Figure:
Block diagram of a Digital Communication System
The figure
shows a block schematic of the essential parts of a digital communication
system.
The block
diagram description is:
INFORMATION SOURCE
The message signal to be sent is produced by the information
source. In the case of a digital source, the information source generates a
discrete and random message signal that does not change over time.
When using an analog source, the data is analog and changes
over time. By sampling and quantizing the analog signal, an Analog to Digital
Converter may transform it into a digital signal.
A message is created by a source
of information, such as a human voice, a television image, teletype data, or
the temperature and pressure of the atmosphere.
✓
Because the message in these cases is not electrical, a transducer is needed to
transform it into an electrical waveform known as the message signal.
✓A
baseband signal is another term for the waveform. The term "baseband"
refers to the frequency range that the message signal generated at the source
falls inside.
✓
An analog or digital message signal can be used.
✓
The amplitudes and times of analog signals fluctuate constantly across their
intervals. A voice signal, a television signal, and the location of the signal
are all examples of signal types.
✓
Both the amplitude and tome of a digital signal are discrete. Digital signals
include computer data and telegraph transmissions.
✓
As illustrated in the block diagram, an analog signal may always be transformed
into a digital signal by combining three fundamental operations: sampling,
quantizing and encoding.
✓
Only sample values of the analog signal at regularly spaced discrete-time
instants are maintained during the sampling procedure.
✓
Each sample value is approximated by the nearest discrete level in a limited
collection of levels throughout the quantizing processes. The selected level is
represented by a code word with a predetermined number of code components
throughout the encoding procedure.
SOURCE ENCODER
The source encoder receives the symbols supplied by the
information source. These symbols aren't capable of being directly
communicated. The source encoder is responsible for converting them into
digital format.
The code words for the symbols are assigned by the source
encoder. A unique code word exists for each different symbol. They choose a
unique value in the data set to give a series of codes to a specific piece of
data. Pulse code modulators, delta modulators, vector quantizers, and other
sources encoders are examples of common source encoders.
The encode function in source coding converts the digital
signal created at the source output into other digital signals. It is a
one-to-one mapping that is used to eliminate or minimize redundancy. As a
result of this, we can deliver an effective service.
CHANNEL ENCODER
The channel
encoder transforms a binary sequence into a transmittable message or
information signal. Noise and interference may be introduced into the signal
transmission process. Channel encoding is used to avoid these mistakes.
The input
sequence is supplemented by the channel encoder by some superfluous binary
bits. With some well-defined logic, these superfluous bits are inserted.
✓ The goal of channel coding is for the encoder to map
the incoming digital signal into a channel input and for the decoder to map the
channel output into an output digital signal, allowing for reliable
communication across a noisy channel.
✓
This is accomplished by adding redundancy in a predetermined manner in the
channel encoder and exploiting it in the decoder to precisely recreate the
original encoder input.
✓
We reduce redundancy in source coding, but we incorporate controlled redundancy
in channel coding to mitigate the channel noise impact.
✓
We can do source coding on its own, channel coding on its own, or both.
Naturally, in the latter situation, the source encoding is done as indicated in
fig.
✓
As illustrated in fig. 1, we create in reverse order in the receiver: channel
decoding comes first, followed by channel encoding in the transmitter.
✓
In the receiver, we create in reverse order channel decoding first, followed by
source decoding for any combination, resulting in a gain in system performance
at the expense of higher circuit complexity.
CHANNEL
The communication channel is a physical medium that is used
to transfer signals from a transmitter to a receiver located at a distance. Any
digital communication system's information backbone is formed by it. Optical
Fiber Cables can also provide a higher data rate.
This channel in a wireless system is made up of the atmosphere.
To complete the link, a multi-hop system may include coaxial cables, fiber
optic cables, and microwave links. Satellite channels make global communication
relatively simple.
DIGITAL
DEMODULATOR
The digital
demodulator transforms the communication channel's input modulated signal to a
binary bit sequence.
The digital
modulator converts the input binary sequence into an analog signal waveform
that can be sent via a communication channel without being distorted.
The following
modulators are used to generate equivalent modulated analog signals appropriate
for transmission across a band width-restricted analog transmission line of
short-haul or long haul systems, using digital on-off signals as keys.
1 Amplitude
Shift Keying (ASK)
2 Frequency
Shift Keying (FSK)
3 Phase Shift
Keying (PSK)
CHANNEL
DECODER
The channel
decoder reconstructs an error-free, precise bit sequence while reducing the
impacts of noise and distortion in the channel.
SOURCE
DECODER
The source
decoder reverses the source encoder's action. It transforms the channel
encoder's binary output to a symbol sequence. Decoders with variable and fixed
lengths are also conceivable. Memory is used by certain decoders to store the
code words.
Advantages of digital communication
• Digital circuits that are relatively affordable can be
employed.
• Data encryption is used to protect privacy.
• The dynamic range (the difference between the highest and
lowest value) can be increased.
• Data from Voice, Video, and Data sources may all be
combined and sent via a single digital transmission system.
• Unlike analog systems, noise does not build from repeater
to repeater in long-distance networks.
• Errors detected are minor, even when the received signal
contains a lot of noise, i.e. the S/N Ratio in Digital Systems is high. •
Errors are frequently rectified by using error correction coding.
Disadvantages of digital communication
• More bandwidth is necessary for digital systems than for
analog systems and synchronization is required.