Electronics Paper - II
Digital Communication and Networking
UNIT I
INTRODUCTION TO ELECTRONIC COMMUNICATION
What is Communication?
The word Communication has originates from the Latin word “Communes” .
Which means something Common to be share.
Communication is a process of exchanging Information , Ideas , thoughts etc by
Writing, Speech or Visual means, so that information communicated is completely
understood by everyone.
Communication in electronics
It is a process of transmitting, processing and receiving signal between two
or more location using electrical signals.
The basic components of an Electronic communication system include a
transmitter, a communication medium or channel, a receiver and noise.
Elements of Communication System
Basic Elements of Communication System
1) Source / Sender
2) Message Signal
3) Transmitter
4) Channel
5) Noise
6) Receiver
7) User
1. Source/ sender : The person who want to share the message with others. It
generates the message which to be transferred.
2. Message signal : It contains the information which sender wants to
communicate. Message signal may contains Audio, Video, text message,
Different types of files ( such as PDF, Word, PPT, Excel etc. )
3. Transmitter :
Transmitter is an electronic circuit which collects the incoming message signal
and modify it into a suitable form so that it can travel and received properly at
the receiving point.
Transmitter is a combination of Modulator, amplifier and Antenna.
Amplifier amplifies the incoming input signal to increase it’s strength.
Modulator circuit modifies the characteristic of incoming message signal such
as amplitude, frequency and phase.
Because of the process of amplification and modulation the strength of the
input signal increases so that it can cover a long distance.
Antenna converts electrical signal into electromagnetic waves to transmit the
signal into space.
4.Channel :-
Channel is the medium through which the signal can be transferred.
There are two types of medium wired and wireless .
The wired medium includes copper wire, coaxial cable, fiber optic cable etc.
The wireless medium includes Mobile tower, satellites etc.
5. Noise :
It is random, unwanted and unpredictable signal that will interfere with the
original Signal and distort it.
There are different noise sources which are internal and external to the channel.
6. Receiver :
Receiver is an electronic circuit which receive the transmitted information and
convert it into a form which is understandable by the humans. In other words it
extracts the original information from the transmitted signal. It performs function
like amplification, demodulation and filter.
7. User : It is usually a person or people who receive the information.
Modulation and Demodulation
Modulation is a process of superimposing low frequency signal
into high frequency signal.
It is defined as a process in which some characteristic of
carrier signal such as amplitude, frequency or phase changes
according to the instantaneous value of modulating signal.
Signals in the Modulation Process
Following are the three types of signals in the modulation
process.
1) Message or Modulating Signal
The signal which contains a message to be transmitted, is
called as a message signal. It is a baseband signal, which has
to undergo the process of modulation, to get transmitted.
Hence, it is also called as the modulating signal.
2) Carrier Signal
The high frequency signal which has a certain phase, frequency,
and amplitude but contains no information, is called a carrier
signal. It is just used to carry the signal to the receiver after
modulation.
3) Modulated Signal
The resultant signal after the process of modulation, is called as
the modulated signal. This signal is a combination of the
modulating signal and the carrier signal.
Need of Modulation
Antenna size gets reduced.
No signal mixing occurs.
Communication range increases.
Multiplexing of signals occur.
Adjustments in the bandwidth is allowed.
Reception quality improves.
PCM
Pulse code modulation (PCM) is a technique of digitally
representing analog signals.
It takes samples of the amplitude of the analog signal and
convert it to binary data.
In PCM analog signal is sampled at discrete time intervals
and then these samples are converted into n bit serial binary
code for Transmission.
PCM involves three steps
1. Sampling 2. Quantization 3. Encoding
Sampling
The first step in PCM is sampling.
Sampling is a process of measuring the amplitude of a continuous-
time signal at discrete instants, converting the continuous signal into
a discrete signal.
In sampling process analog signal is sampled at regular interval of
time and Sampling rate based on Nyquist Theorem.
According to the Nyquist theorem, the sampling rate must be at least
2 times the highest frequency contained in the signal.
It is also known as the minimum sampling rate and given by:
Fs =2*fh
PAM
Quantization
The process of converting an infinite number of analog
sampled value into finite number of values is called as
quantization.
In quantization, an analog sample value is converted into a
specifically defined set of quantization values.
Quantization approximates the analog sample values with
the nearest quantization values.
With quantization the total voltage range is divided into a
smaller number of sub ranges .
For ex. Consider a sine wave with an amplitude varying
between +3V and -3V. It has every value possible between
the range.
The following table shows a three bit sign magnitude code
with eight possible combination.
The most significant bit
represent sign bit and another
two bit represents magnitude.
The magnitude of a quantum is
called the resolution which is
equal to voltage of minimum
steps size.
In above table resolution is 1V.
The better is resolution the more accurate the quantized
value will resembles the original analog information.
Better resolution can be achieved by using PCM code with
more bits.
3. Encoding In this process quantized value is assigned a
n bit binary code.
Figure shows block diagram of PCM modulator and Demodulator.
Modulator consist of
1. Band Pass Filter 2. Sampler 3. ADC
4. Parallel to serial converter.
Serial Communication
In serial communication the data bits are transmitted serially over a common communication
link one after the other.
It does not allow simultaneous transmission of data because only a single channel is utilized.
Thereby allowing sequential transfer rather than simultaneous transfer.
It is highly suitable for long distance signal transmission as only a single wire or bus is used.
It can be connected between two points that are separated at a large distance with respect to
each other.
Since single data bit is transmitted per clock pulse thus the transmission of data is a quiet
time taking process.
Parallel Communication
In parallel communication the various data bits are simultaneously transmitted using multiple
communication links between sender and receiver.
Here, despite using a single channel between sender and receiver, various links are used and each bit
of data is transmitted separately over all the communication link.
In parallel communication for transmission of 8-bit of data, 8 separate communication links are
utilized. And so rather following a sequential data transmission, simultaneous transmission of data is
allowed.
This leads to a faster communication between the sender and receiver. But for connecting multiple
lines between sender and receiver multiple connecting unit are to be present between a pair of sender
and receiver.
Hence parallel communication is not suitable for long distance transmission, because connecting
multiple lines to large distances is very difficult and expensive.
Serial Communication Parallel Communication
Types of Serial Transmission
There are two types of serial communication
1) Asynchronous 2) Synchronous
2) Asynchronous : In Asynchronous Transmission, data is sent in form of byte or character.
3) In this transmission start bits and stop bits are added with data byte.
4) It does not require synchronization.
5) Start bit is added before a byte and Stop bit is added after a byte. Hence for each byte two extra bits are sent.
6) In this process transmitted data is encoded with start and stop bits, specifying the beginning and end of each
character.
6) As shown in the figure these additional bits provide the timing or synchronization for the connection by
indicating when a complete character has been sent or received; thus, timing for each character begins with
the start bit and ends with the stop bit.
7) The gaps appear between character transmissions, the asynchronous line is said to be in a mark state. A
mark is a binary 1 (or negative voltage) that is sent during periods of inactivity on the line .
8) The start bit, which precedes the data character, is always a space bit (binary 0) and the stop bit, which
signals the end of a character, is always a mark bit (binary 1).
9) With asynchronous transmission, a large text document is organized into long strings of letters (or
characters) that make up the words within the sentences and paragraphs. These characters are sent over the
communication link one at a time and reassembled at the remote location.
10) In asynchronous transmission, ASCII character would actually be transmitted using 10 bits. For
example, "0100 0001" would become "1 0100 0001 0". The extra one (or zero, depending on parity bit) at
the start and end of the transmission tells the receiver first that a character is coming and secondly that the
character has ended.
Synchronous Data transfer
Synchronous data transmission is a data transfer method in which a continuous stream of data signals is
accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the
receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets)
spaced by fixed time intervals.
Synchronous transmission modes are used when large amounts of data must be transferred very quickly from
one location to the other. The speed of the synchronous connection is attained by transferring data in large
blocks instead of individual characters.
Synchronous transmission synchronizes transmission speeds at both the receiving and sending end of the
transmission using clock signals built into each component.
A continual stream of data is then sent between the two nodes.
The data blocks are grouped and spaced in regular intervals and are preceded by special characters
called syn or synchronous idle characters.
In synchronous transmission the transmission of a large text document is done.
Before the document is transferred across the synchronous line, it is first broken into blocks of sentences
or paragraphs.
The blocks are then sent over the communication link to the remote site.
The following is a list of characteristics specific to synchronous communication
There are no gaps between characters being transmitted.
Timing is supplied by modems or other devices at each end of the connection.
Special syn characters precede the data being transmitted.
The syn characters are used between blocks of data for timing purposes
Comparison between Synchronous and Asynchronous Data transfer
Parameters for Comparison Synchronous Transmission Asynchronous Transmission
Clock pulse Transmitter and receiver shares a A common clock pulse is not shared
common clock pulse. by transmitter and receiver.
Speed of transmission Fast Comparatively slow.
Form of data transmission Data is sent in the form of frames Data is transmitted in the form of
or blocks. byte or character.
Time interval Constant Variable
Cost Expensive Comparatively less expensive.
Efficiency More efficient Less efficient
Need of external clock Exist Do not exist
Need of start and stop bit Not exist Exist
Circuit Complex Comparitively less complex.
Transmission Modes
Transmission mode means transferring of data between two devices. Communication occurs between individual
devices that are interconnected. There are three types of transmission mode:-
1) Simplex Mode 2) Half-Duplex Mode 3) Full-Duplex Mode
Simplex Mode
In Simplex mode, the communication is unidirectional, as on a one-way street. Only one of the
two devices on a link can transmit, the other can only receive.
Example: TV Transmission, Radio Transmission
Half-Duplex Mode
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device
is sending, the other can only receive, and vice versa. The half-duplex mode is used in cases where there is
no need for communication in both direction at the same time.
Example: Walkie- talkie in which message is sent one at a time and messages are sent in both the directions.
Full-Duplex Mode
In full-duplex mode, both stations can transmit and receive simultaneously. Full-duplex mode is
used when communication in both direction is required all the time.
Example: Telephone Network in which there is communication between two persons by a telephone
line, through which both can talk and listen at the same time.
