Communication
is the process of transfer of a message from one point to another.
Three basic
element of any communication system are:
1. A sender (source)
that creates and sends a message.
2. A medium
that carries the message.
3. A receiver (sink) that receives the message.
Data
communication: -
Data communication concerns the transmission
of digital data from one place to another over a network using various guided
and wireless transmission media.
Twisted pair cable or coaxial cable
and fiber optics are example of guided transmission media.
Wireless media use different ways to
transmit data from source to destination via satellite and tower. There are basically
two type of signals are used to transmit data.
1. Analog
signal
2. Digital
signal
Component
of data communication: -
Data
is the main component of data communication that is to be transferred from one
location to another. The data is send by sender in an encoded format over a
transmission medium to the receiver who decodes the data to its original form.
A communication system mainly consists of the following five components.
1. Message
2. Sender
3. Receiver
4. Medium
5. Protocol
Data
transmission: -
It
is the process of transferring information in the form of bits and bytes from
one place to another across the data communication transfer medium such as
copper wire and optical cable.
There
are basically two types of data transmission:
1. Serial transmission: - It is also known as sequential transmission.
In Serial transmission the bits are transmitted one after another in a sequence
over a single wire. Serial transmission of data is slow then parallel
transmission because only a single bit of information is transferred at any
given instant of time. Using this type of transmission a check bit can be
transferred easily. There are two way to transmit data in serial transmission.
a.
Asynchronous
transmission
b.
Synchronous
transmission
2. Parallel transmission: - In this
transmission multiple wire are used to transfer bits simultaneously. It is
faster than serial transmission because in parallel transmission one byte of
information can be transferred at a given instance of time. The parallel
transmission is used to transfer bits of information between internal buses of
a computer system.
Data transmission modes:-
There are three modes of data transmission
these are as follows: -
a. Simplex: -
A simplex communication system can transmit data
in one direction only. Devices connected to such circuit are either send only
or receive only. This mode is beneficial for application in which flow of
information is unidirectional. But in almost all data processing applications,
bidirectional communication is required because even when flow of data is
unidirectional, a return path is generally needed to send acknowledgement,
control or error signal.
b.
Half-duplex: - A Half-Duplex communication system can transmit data in both
direction, but in only one direction at a time. It can alternately send or
receive data. It requires two wires. It is most suitable for voice
communication using walkies-talkie in which one person speaks at a time.
c.
Full-Duplex: - Full-Duplex communication between two components means that both
can transmit and receive information between each other simultaneously.
Telephones are full duplex system so both parties on the phone can talk and
listen at the same time.
Analog
and digital signal: -
Analog signal has principle feature that they
are continuous where as digital signal consist of values measured at discrete intervals.
Digital
watches are called digital because they go from one value to the next without
displaying all the intermediate values. In contrast, watches with hands are
analog, because the hands moves continuously around the clock face. The hand
not only touches the numbers 1 and 2 ...., but also the infinite numbers of
point in between.
Computers
are digital machine because at their most basic level they can distinguish
between just two values 0 and 1 or off and on. There is no simple way to
represent all the values in between such as 0.25. All data that a computer
processes must be encoded digitally as a series of zeros and ones.
Modulation: -
For
transmission purpose, a high frequency carrier wave is used to carry the audio
signals. The process of changing some characteristics of a carrier wave in
accordance with the intensity of the signal is known as modulation. The resultant
wave is called the modulated wave. Therefore, modulation permits the
transmission to occur at high frequencies while it simultaneously all the
carrying of the audio signal.
Need of
modulation: -
Modulation
is necessary in communication system due to the following reasons:
Antenna
length:
-
In order to
transmit a wave effectively, the length of the transmitting antenna should be
approximately equal to the wavelength of the wave. So for transmitting audio
waves which wavelength is very high the length of antennas needs to be very
long which is impractical.
Operating
range:
-
The energy
of a wave depends upon its frequency. The grater the energy of the wave, the
greatest the energy possessed by it. Since the audio signal frequencies are
small they cannot be transmitted over the long distance.
Wireless
communication: -
At radio
frequency, radiation is not practical because the efficiency of radiation is
poor at low frequency. The
only particular solution for above problem is to modulate a high frequency
carrier wave with the audio signal and permit the transmission to occur at this
high frequency (carrier frequency)
Type of modulation: -
Modulation is a process of changing the amplitude or frequency or
phase of a carrier wave in accordance with the intensity of the signal.
Accordingly there are three types of modulations
Amplitude
modulation: - When the
amplitude of the high frequency carrier wave is changed in accordance with the
intensity of the signal it is called amplitude modulation. In amplitude
modulation, only the amplitude of the carrier wave is changed and the frequency
of the modulation wave remains the same.
a.
Noisy reception.
b.
Lake of audio quality.
c.
Small operating range.
Frequency
modulation: - When the
frequency of the carrier wave is changed in accordance with the intensity of
the signal. It is called frequency modulation. In FM only the frequency of the
carrier wave is changed. However the amplitude of the modulation wave remains
the same.
Advantages of frequency modulation:
a. It gives
noiseless reception.
b. Operating
range is high.
c. It gives
high fidelity reception.
d. The
efficiency of transmission is very high.
Phase modulation: - It is a change in a carrier phase angle. The
phase angle cannot change without affecting change in frequency. Therefore
phase modulation is in reality a second form of frequency modulation.
Modem: -
A special device called modem is used to
carry out the process of modulation and demodulation. It also stands for
modulator/demodulator. A modem is a device which translates data from binary
code (0’s and 1’s)/digital signal into analog that can be transmitted over
telephone network. Modulation is the conversion of data from binary to analog
and demodulation is the conversion of data from analog to binary. When a file
is sent from a pc to another pc, it is first converted into analog signal by
the modem then transmitted through telephone line and then back converted into
binary/digital when it arrives at the destination by the modem connected to
that computer.
When all the telephone line become digital
the modem will no longer by necessary.
Computer
network: -
A
computer network is a communication network where people communicate electronically
through computer. A communication network is any arrangement where a sender
transmits a message to a receiver over a channel consisting of sometime of
medium.
Types of
network
There
are many different kinds of networks. Networks are divided on their
geographical span.
1.
Local area
network (LAN): - LAN is a
computer network that spans a small area. Most LAN's are confined to a single
building or group of buildings.
Most LAN's connect workstation and personal computer. Each node
(individual computer) in a LAN has its own CPU which executes its own program
but it is also able to access data and devices (like printer, scanner, etc)
anywhere on the LAN. LAN's are capable of transmitting data at very fast rates
then over a limit on the number of computers that can be attached to a single
LAN.
2.
Wide Area
Network (WAN): - A WAN consists of two or more LAN's. Computer
connected to a WAN are often connected through public network, such as
telephone system. They can be connected through leased line or satellite. The
largest WAN in existence is the internet. Generally telecommunication networks
are WAN.WAN may use advanced technology such as Asynchronous transfer mode, frame
relay and synchronous optical network.
3. Metropolitan area network: - The MAN
generally expands throughout a city such as cable TV network. It can be in the
form of Ethernet, Token ring ATM or fiber distribution data interface (FDDI).
Metro Ethernet is a service which is provided by ISPS. This
service enables its users to expand their LAN. MAN can help an organization to
connect all of its offices in a city.
Backbone of MAN is high capacity and high speed fiber optics.
Network
topology
A
topology refers to the manner in which the cables are run to individual work
station on the network. A topology is concerned with the physical arrangement
of the network.
Types of
topology
Star
topology: - A star topology is designed with each node like
file server, workstation and peripherals connected directly to a central
network hub or concentrator. Data on a star network passes through the hub
before continuing its destination. The hub manages and control all functions on
the network Star network is frequently used in a LAN to connect several
microcomputer.
Advantages
A.
Easy to install
B.
No disturbance to the network when connecting or removing devices.
Disadvantages
A.
Requires more cable length.
B.
Expensive then linear bus topology
Ring
topology: - The ring topology network is a local area
network whose topology is a ring. It can be as simple as a circle or point to
point communication of computer at dispersed locations with no central host
computer. All the nodes are connected in a closed loop messages travel in a
ring with each node reading those messages addressed to it.
Advantage
A.
Grow of system has minimum impact on performance.
B.
All stations have equal access.
Disadvantage
A.
Most expensive topology
B.
Failure of one computer may impact other
C.
Complex
Linear
bus topology (LBT): - A LBT consists of a main run of cable with a terminator
at each end. All nodes are connected to a linear cable. Ethernet and local talk
networks use a LBT.
Advantage
A.
Easy to connect a computer or peripheral to a linear bus.
B.
Requires less cable then star topology.
Disadvantage
A.
Entire network shuts down if there is a break in the main cable.
B.
Terminators are required at both end of the main cable.
Tree
topology: - A tree topology combines characteristics of
linear bus and star typologies. It consists of group of stars configured
workstations connected to linear bus backbone cable/main cable.
Advantages
A.
Point to point wiring for individual segments.
Disadvantages
A.
If the main line breaks the entire segments goes down.
B.
More difficult to configures and wire then other typologies.
Hybrid
topology: - It is a combination of two or more topology in
such a way that the resulting network does not have the feature of any one of
the basic topology. Hybrid topology is generated whenever the two different
basic typologies are connected.
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