Communication is defined as transfer of
information, such as thoughts and messages between two entities. The invention
of telegraph, radio, telephone, and television made possible instantaneous
communication over long distances. In the context of computers and information
technology (IT), the data are represented by binary digit or bit has only two
values 0s and 1s. In fact anything the computer deals with is 0s and 1s only.
Due to this it is called discrete or digital. In the digital world messages,
thoughts, numbers. etc can be represented in different streams of 0s and
1s.
Data communications concerns itself with the
transmission (sending and receiving) of information between two locations by
means of electrical signals. The two types of electrical signals are analog and
digital. Data communication is the name given to the communication where
exchange of information takes place in the form of 0s and 1s over some kind of
media such as wire or wireless. The subject-Data Communications deals with the
technology, tools, products and equipment to make this happen. In communications and information
processing, encoding is
the process by which information from a source is
converted into symbols to be communicated. Decoding is
the reverse process, converting these code symbols back into information
understandable by a receiver.
Entire data communication system revolves
around three fundamental concepts.
- Destiny: The system should
transmit the message to the correct intended destination. The destination
can be another user or another computer.
- Reliability: The system should
deliver the data to the destiny faithfully. Any unwanted signals (noise)
added along with the original data may play havoc!
- Fast: The system should transmit the data as
fast as possible within the technological constraints. In case of audio
and video data they must be received in the same order as they are
produced without adding any significant delays.
Data
Communication model
The figure shows in the
block diagram of a typical communication model. The communication model has
five sub systems viz., user, transmitter, communication channel, receiver and
destiny.
(a)
The block diagram of a data communication model
(b)
A typical dial-up network
Figure shows a typical
dial-up network setup. The data communication equipment (DCE) at the
transmitting end converts the digital signals into audio tones (modulation) so
that the voice grade telephone lines can be used as guided media during
transmission. At the far end the receiving audio tones, they are converted back
to digital signals (Demodulation) by the data communication equipment (DCE) and
fed to the far end data terminal equipment (DTE).
- User:
There will be a source that generates the message and a transducer that
converts the message into an electrical signal. The source can be a person
in front of a microphone or a computer itself sending a file. The user
terminal is known as data terminal equipment (DTE).
- Transmitter:
Can be a radio frequency modulator combining the signal coming out of the
data equipment terminal. Here the radio frequency is acting as the carrier
for the data signal. Or in case of direct digital transmission the
transmitter can be
- Communication
channel: Can be guided media (twisted pair, coaxial cable, fiber optics)
or unguided media (air, water etc). In both the cases communication is in
the form of electromagnetic waves. With guided media the electromagnetic
waves are guided along a physical path. Unguided media also called
wireless the transmitting electromagnetic waves are not guided along with
a physical path. They are radiated through air/vacuum/water., etc.
- Receiver: The
receiver amplifies the received signals removes any unwanted signals
(noise) introduced by the communication channel during propagation of the
signal and feeds to the destiny.
- Destiny:
The user at the other end finally receives the message through the data terminal
equipment stationed at the other side.
Types
of communication
Based on the requirements,
the communications can be of different types:
- Point-to-multipoint
communication: In this type of communication, there is one sender and
multiple recipients. For example, in voice conferencing, one person will
be talking but many others can listen. The message from the sender has to
be multicast to many others.
- Broadcasting:
In a broadcasting system, there is a central location from which
information is sent to many recipients, as in the case of audio or video
broadcasting. In a broadcasting system, the listeners are passive, and
there is no reverse communication path.
- Simplex
communication: In simplex communication, communication is possible only in
one direction. There is one sender and one receiver; the sender and
receiver cannot change roles.
- Half-duplex
communication: Half-duplex
communication is possible in both directions between two entities
(computers or persons), but one at a time. A walkie-talkie uses this
approach. The person who wants to talk presses a talk button on his
handset to start talking, and the other person’s handset will be in
receiving mode. When the sender finishes,
he terminates it with an over message. The other person can press
the talk button and start talking. These types of systems require limited
channel bandwidth, so they are low cost systems.
- Full-duplex
communication: In a full-duplex communication system, the two parties the
caller and the called can communicate simultaneously, as in a telephone
system. However, note that the communication system allows simultaneous
transmission of data,but when two persons talk simultaneously, there is no
effective communication! The ability of the communication system to
transport data in both directions defines the system as full duplex.
Digital
signals cannot be sent in a full-duplex system with a single channel. If two signals are sent at once, the
result will be unreadable because the voltages will add or cancel,
resulting in a garbled string of bits.
Full-duplex transmission of digital signals can be accomplished
with two channels, one for each direction.
Each station has its receiving apparatus listening on one channel,
and its sending capability on the other. One channel can support half-duplex
with the understanding that only one station can send at one time. In order to support half-duplex each
station must possess both receiving and transmitting equipment. Thus each station can transmit and receive. Note that collisions and corrupted data
occur if both stations attempt to transmit at once. We leave the theory of detecting and
preventing simultaneous transmission in half-duplex to discussions of data
flow control
Depending on the type of
information transmitted, we have voice communication, data communication, fax
communication, and video communication systems. When various types of
information are clubbed together, we talk of multimedia communications. Even a
few years ago, different information media such as voice, data, video, etc.
were transmitted separately by using their own respective methods of
transmission. With the advent of digital communication and “convergence
technologies,” this distinction is slowly disappearing, and multimedia
communication is becoming the order of the day.
|
|
Simplex A to B only |
|
|
Half-Duplex A to B or B to A
 |
Full-Duplex A to B and B
to A
Figure: Simplex, Half-Duplex
and Full-Duplex.
Modes
of transmission
When we talk of data
communication we are primarily concerned with serial transmission although
other type of transmission does exist. In serial transmission the data is
transmitted bit by bit as a stream of 0s and 1s. Protocols are implemented for
these types of transmissions so that the communication takes place in a
well-defined manner. Protocols are mutually agreed set of rules and are
necessary because the format of transmission should be understood by the
receiver
The following key factors have to be observed
regarding serial transmission:
- Timing problem: There should be some mechanism to know when the bit has arrived and at what rate the next bit is going to arrive at the serial input terminal of the receiver. We will see this can be accomplished in two ways.
- Error detection: Provision should be made (during transmission itself) to verify the integrity of the received data. Like parity, checksum bits.
- Error correction: Ability to correct the data in case of corrupted data reception.
Communication
Protocols
You may be
wondering how computers send and receive data across communication links. The answer is data communication software. It
is this software that enables us to communicate with other systems .The data
communication software instructs computer systems and devices as to how exactly
data is to be transferred from one place to another. The procedure of data transformation in the
form of software is commonly known as protocol. The data transmission software
or protocols perform the following functions for the efficient and error free
transmission of data.
1. Data sequencing: A long message to be transmitted is broken into
smaller packets of fixed size for error free data transmission.
2. Data Routing: It is the process of finding the most efficient route
between source and destination before sending the data.
3. Flow control: All machines
are not equally efficient in terms of speed. Hence the flow control regulates
the process of sending data between fast sender and slow receiver.
4. Error
Control: Error detecting and
recovering is the one of the main functions of communication software. It ensures that data are transmitted without
any error. Many communication channels are
subject to channel noise,
and thus errors may be introduced during transmission from the source to a
receiver. Error detection techniques allow detecting such errors, while error
correction enables reconstruction of the original data.
The general definitions of the terms are as follows:
- Error detection is the detection of errors
caused by noise or other impairments during transmission from the
transmitter to the receiver.
- Error correction is the detection of errors
and reconstruction of the original, error-free data.
Error correction may generally be realized in two different
ways:
- Automatic repeat request (ARQ) (sometimes also referred to
as backward error correction): This is an error control
technique whereby an error detection scheme is combined with requests for
retransmission of erroneous data. Every block of data received is checked
using the error detection code used, and if the check fails,
retransmission of the data is requested – this may be done repeatedly,
until the data can be verified.
- Forward error correction (FEC): The sender encodes the
data using an error-correcting code (ECC) prior to transmission. The
additional information (redundancy) added by the code is used
by the receiver to recover the original data. In general, the
reconstructed data is what is deemed the "most likely" original
data.
Digital
and Analog Transmission
Data is transmitted
from one point to another point by means of electrical signals that may be in
digital and analog form. So one should know the fundamental difference between
analog and digital signals. In analog signal the transmission power varies over
a continuous range with respect to sound, light and radio waves. On the other
hand, a digital signal may assume only discrete set of values within a given
range. (See figures given below) Examples are computer and computer related equipment.
Analog signal is measured in Volts and its frequency is in Hertz (Hz).A digital
signal is a sequence of voltage represented in binary form. When digital data
are to be sent over an analog form the digital signal must be converted to analog
form. So the technique by which a digital signal is converted to analog form is
known as modulation. And the reverse process, that is the conversion of analog
signal to its digital form, is known as demodulation. The device, which converts
digital signal into analog, and the reverse, is known as modem.
Figure: Analog Signal
Figure: Digital Signal
Asynchronous Transmission
In asynchronous
transmission data is transferred character by character and each character
(frame by frame i.e. each character is an asynchronous frame in asynchronous
transmission) and can be 5 to 8 bits long. The term “Asynchronous” means it is
asynchronous at frame level. The bits are still synchronized at bit level during
reception.
Figure: Asynchronous
data format
- In a steady
stream, interval between characters is uniform (length of stop element can
be 1,1.5 or 2 stop bits - as programmed earlier)
- In idle state,
receiver looks for transition 1 to 0 (start signal)
- Then samples
next five, seven or eight intervals (as programmed earlier) Timing only
needs maintaining within each frame (bit level).
- Looks for
parity (if programmed earlier).
- Then looks for
next 1 to 0 for next frame .
- Simple.
- Cheap. Minimum
hardware & software requirement to implement.
- Overhead of 2
or 3 bits per frame (~20%).
- Good for data
with large gaps in between each frame (keyboard, low speed data).
Synchronous
Transmission
In Synchronous transmission a block of data in
the form of bits stream is transferred without starts / stop bits. The block
can be of any arbitrary length. In order to establish synchronization with
remote computer the transmitter transmits synch pulses initially. When the
receiver locks to the transmitter’s clock frequency a block of data gets
transmitted. See figure.
The Characteristics
are as follows
- Block
of data transmitted without start or stop bits
- Initially
synch pulses are transmitted (Clocks must be synchronized).
- Can
use separate clock line (In that case synch pulses are not needed!)
- Good
over short distances
- Subject
to impairments
- Embed
clock signal in data (
- Carrier
frequency (analog) is used
- Need
to indicate start and end of block
- Use
preamble and post amble (to leave sufficient space between blocks)
- More
efficient (lower overhead) than asynchronous transmission.
Figure: The synchronous frame format
Multiplexing
By Multiplexing different message signals
can share a single transmission media (The media can be guided or unguided).
All they need is they should either differ in their frequency slot or
wavelength slot or in time slot.
Frequency
domain multiplexing (FDM)
In this each message signal is
modulated by different radio frequency signals called rf carriers. At the receiving
end filters are used to separate the individual message signals. Then they are
demodulated (removing the rf carrier) to retrieve back the original messages.
Figure: Frequency
domain multiplexing
The Radio /TV
broadcasting are the best examples for frequency domain multiplexing. Several
individual stations broadcast their programs in their own allotted frequency
band sharing the same unguided media. The receiver tunes his set according to
his choice. The cable TV network is another example of Frequency domain
multiplexing employing guided media.
Wavelength division multiplexing (WDM)
Wavelength division
multiplexing is a type of FDM scheme used in fiber optical communications where
various wavelengths of infrared light are combined over strands of fiber.
Optical communication with few exceptions are digital since light transmitters
and receivers are usually poorly suited for analog modulation.
Figure: A Typical
wavelength division multiplexer
Time
domain multiplexing (TDM)
A
type of multiplexing where two or more channels of information are transmitted
over the same media by allocating a different time interval ("slot"
or "slice") for the transmission of each channel. The channels take
turns to use the media. Some kind of periodic synchronizing signal or
distinguishing identifier is usually required so that the receiver can tell
which channel is which.
A typical practical setup combines a set of
low-bit-rate streams, each with a fixed and pre-defined bit rate, into a single
high-speed bit stream that can be transmitted over a single channel.
The main reason to use TDM is to take
advantage of existing transmission lines. It would be very expensive if each
low-bit-rate stream were assigned a costly physical channel (say, an entire
fiber optic line) that extended over a long distance.
Figure: Time division multiplexing.
Choose the following
1.
Telephone systems may be classified as:
a)
Duplex and asymmetrical. b) Duplex and asymmetrical.
c)
Duplex and symmetrical. d)
Simplex and asymmetrical.
2.
Asynchronous transmission may be defined as:
a) Communication where the receiver clock must be in exact
synchronism with that of the transmitter.
b) Communication where receiver will operate satisfactorily, even if
its clock frequency is appreciably different to that of the transmitter.
c) Communication where the receiver clock must be in approximate
synchronism with that of the transmitter.
d) None of these
3.
Synchronous transmission may be defined as:
a) Communication where the receiver will operate satisfactorily, even
if its clock frequency is appreciably different to that of the transmitter.
b) Communication where the receiver clock must be in approximate
synchronism with that of the transmitter.
c)
Communication
where the receiver clock is arranged to be in exact synchronism with that of
the receiver.
4.
__________ is
the process by which information from a source is
converted into symbols to be communicated.
a)
Encoding b)decoding c)encryption d)decryption
5.
___________ is
the reverse process, converting these code symbols back into information
understandable by a receiver.
a)
Encoding b)decoding c)encryption d)decryption
6.
____________at the transmitting end
converts the digital signals into audio tones
a)
Data
communication equipment b) Data transfer equipment
c) Data transmission equipment d) Data collection equipment
True
or false for the following statement
- The basic
requirements for working of a communication system are sender, medium and
receiver.
- In full duplex
the communication channel issued in both directions at the same time.
Answer
the following
- Define data
communication. How it is different from other types of communication?
- Draw the block
diagram of typical data communication model and explain its constituents.
- Enumerate
different types of communication. Site examples for each of them?
- Discuss in
detail about multiplexing.
- Why Protocols
are needed in data communication?
- What is
communication protocol?
- What is the
difference between asynchronous and synchronous transmission?
- In a city the
following traffic rules are implemented for reducing traffic block. In
road A the vehicles can go from East to West. In road B the vehicles can
pass only in one direction at a time. In road C, the vehicles are free to
move in both directions at the same time. How do you correlate the above
type of traffic system with modes of data communication?
- What is meant
by wave division multiplexing?
- What is meant
by time domain multiplexing?
- What is
frequency division multiplexing?
- What are the
advantages of Asynchronous transmission?
- What are the
advantages of Synchronous transmission?
- What is the
difference between digital and analog transmission?
- Explain
different ways of realizing error correction?
- Differentiate
error correction and error detection?
- What
are the functions of communication protocols for the efficient and error
free transmission of data?
- What are the fundamental concepts of data communication?
