UNIT II

ANALOG AND DIGITAL

Both data and the signals that represent them can be either
analog or digital in form.

Analog and Digital Data

Data can be analog or digital. The term analog data refers to
information that is continuous; digital data refers to information that
has discrete states.

For example, an analog clock that has hour, minute, and second
hands gives information in a continuous form; the movements of the
hands are continuous. On the other hand, a digital clock that reports
the hours and the minutes will change suddenly from 8:05 to 8:06.

 Analog data, such as the sounds made by a human voice, take on

continuous values. When someone speaks, an analog wave is
created in the air. This can be captured by a microphone and
converted to an analog signal or sampled and converted to a
digital signal.
 Digital data take on discrete values. For example, data are stored
in computer memory in the form of Os and 1s. They can be
converted to a digital signal or modulated into an analog signal
for transmission across a medium.

ANALOG AND DIGITAL SIGNALS

Signals can be analog or digital. Analog signals can have an infinite

number of values in a range; digital signals can have only a limited
number of values

Difference between Analog Signals and Digital Signals

The table below highlights all the key differences between Analog and
digital transmissions.
Parameter Analog Signal Digital Signal

Analog signals are used to

A digital signal transmits
communicate information
Definition data in a discrete function
in a continuous function
of time.
of time.

Analog signals represent

Signal data and information Digital signals use discrete
values using a continuous range values 0 and 1.
of values.

Signal
The bandwidth is low. The bandwidth is high.
bandwidth

Analog signals are better The digital signals are

suited for transmitting appropriate for computer
Suitability audio, video, and other and digital electronic
data via communication processes such as data
channels. storage and other things.

Effect of Analog signals are easily Digital signals are more

electronic influenced by electrical reliable and resistant to
noise noise. noise than Analog ones.

Because analog signals

As digital signals are
are more susceptible to
Accuracy noise-free, they have high
noise, their accuracy is
accuracy.
reduced.

Analog transmissions Digital transmissions

Power
require more power to utilize less power than
consumption
transmit data. analog signals.

Circuit Resistors, capacitors, Transistors, logic gates,

components inductors, and other ICs, etc.
 components

Temperature, current,
Data storage in computer
voltage, voice, pressure,
memory is one of the
Examples and speed are all
examples of digital
examples of analog
signals.
signals.

Used in computers,
Used in landline phones,
keyboards, digital
Applications thermometers, radios,
watches, and other
and other devices.
electronics

Periodic and Non periodic Signals

Both analog and digital signals can take one of two forms: periodic
or non periodic (sometimes refer to as a periodic, because the prefix
a in Greek means "non"). A periodic signal completes a pattern within
a measurable time frame, called a period, and repeats that pattern
over subsequent identical periods. The completion of one full pattern
is called a cycle. A non periodic signal changes without exhibiting a
pattern or cycle that repeats over time. Both analog and digital
signals can be periodic or non periodic. In data communications, we
commonly use periodic analog signals (because they need less
bandwidth, as we will see in Chapter 5) and non periodic digital
signals (because they can represent variation in data, as we will see
in Chapter 6).

PERIODIC ANALOG SIGNALS

Periodic analog signals can be classified as simple or composite. A
simple periodic analog signal, a sine wave, cannot be decomposed
into simpler signals. A composite periodic analog signal is composed
of multiple sine waves.

Sine Wave
The sine wave is the most fundamental form of a periodic analog
signal. When we visualize it as a simple oscillating curve, its change
over the course of a cycle is smooth and consistent, a continuous,
rolling flow. Each cycle consists of a single arc above the time axis
followed by a single arc below it.

A sine wave can be represented by three parameters: the peak

amplitude, the frequency, and the phase. These three parameters
fully describe a sine wave.

 Peak Amplitude
The peak amplitude of a signal is the absolute value of its highest
intensity, proportional to the energy it carries. For electric signals,
peak amplitude is normally measured in volts.

 Period and Frequency

Period refers to the amount of time, in seconds, a signal needs to
complete 1 cycle. Frequency refers to the number of periods in I s.
Note that period and frequency are just one characteristic defined in
two ways. Period is the inverse of frequency, and frequency is the
inverse of period, as the following formulas show.
 f=1/ T and T=1/f

Period is formally expressed in seconds. Frequency is formally

expressed in Hertz (Hz), which is cycle per second. Units of period
and frequency are shown in
Unit Equivalent Unit Equivalent

seconds (s) 1s Hertz (Hz) 1 Hz

Milliseconds 10-3 s Kilohertz (kHz) 103 Hz
(ms)

Microseconds 10-6 s Megahertz 106 Hz

(Ils) (MHz)
Nanoseconds 10-9 s Gigahertz 109 Hz
(ns) (GHz)
Picoseconds 10-12 s Terahertz (THz) 1012
(ps)

 Phase
The term phase describes the position of the waveform relative to
time O. If we think of the wave as something that can be shifted
backward or forward along the time axis, phase describes the amount
of that shift. It indicates the status of the first cycle. Phase is
measured in degrees or radians [360° is 2n rad; 1° is 2n/360 rad, and
1 rad is 360/(2n)]. A phase shift of 360° corresponds to a shift of a
complete period; a phase shift of 180° corresponds to a shift of one-
half of a period; and a phase shift of 90° corresponds to a shift of
one-quarter of a period.
1. A sine wave with a phase of 0° is not shifted.
2. A sine wave with a phase of90° is shifted to the left by ¼ cycle.
However, note 4 that the signal does not really exist before time O.
3. A sine wave with a phase of 180° is shifted to the left by ½ cycle.
However, note that the signal does not really exist before time O.

Wavelength
Wavelength is another characteristic of a signal traveling through a
transmission medium. Wavelength binds the period or the frequency
of a simple sine wave to the propagation speed of the medium.

Time and Frequency Domains

A sine wave is comprehensively defined by its amplitude,
frequency, and phase. We have been showing a sine wave by using
what is called a time-domain plot. The time-domain plot shows
changes in signal amplitude with respect to time (it is an amplitude-
versus-time plot)
Phase is not explicitly shown on a time-domain plot. To show the
relationship between amplitude and frequency, we can use what is
called a frequency-domain plot. A frequency-domain plot is concerned
with only the peak value and the frequency.
 v

Composite Signals
Simple sine waves have many applications in daily life. We can
send a single sine wave to carry electric energy from one place to
another. A composite signal is made of many simple sine waves.
A single a frequency sine wave is not useful in data communications;
we need to send a composite signal, a signal made of many simple
sine waves.

A composite signal can be periodic or non periodic. A periodic

composite signal can be decomposed into a series of simple sine
waves with discrete frequencies that have integer values (1, 2, 3, and
so on). A non periodic composite signal can be decomposed into a
combination of an infinite number of simple sine waves with
continuous frequencies, frequencies that have real values.

If the composite signal is periodic, the decomposition gives a series

of signals with discrete frequencies; if the composite signal is non
periodic, the decomposition gives a combination of sine waves with
continuous frequencies.

Bandwidth
The range of frequencies contained in a composite signal is its
bandwidth. The bandwidth is normally a difference between two
 numbers. For example, if a composite signal contains frequencies
between 1000 and 5000, its bandwidth is 5000 - 1000, or4000.
The bandwidth of a composite signal is the difference between the
highest and the lowest frequencies contained in that signal.

DIGITAL SIGNALS
In addition to being represented by an analog signal, information can
also be represented by a digital signal. A digital signal can have more
than two levels. In this case, we can send more than 1 bit for each
level.

Bit Length
We discussed the concept of the wavelength for an analog signal: the
distance one cycle occupies on the transmission medium. We can
define something similar for a digital signal: the bit length. The bit
length is the distance one bit occupies on the transmission medium.
Bit length=propagation speed x bit duration

Bit Rate
Most digital signals are non periodic, and thus period and frequency
are not appropriate characteristics. Another term-bit rate (instead
ofjrequency)-is used to describe digital signals. The bit rate is the
number of bits sent in Is, expressed in bits per second (bps)

Digital Signal as a Composite Analog Signal

Based on Fourier analysis, a digital signal is a composite analog
signal. The bandwidth is infinite, as you may have guessed. We can
intuitively come up with this concept when we consider a digital
signal. A digital signal, in the time domain, comprises connected
vertical and horizontal line segments. A vertical line in the time
domain means a frequency of infinity (sudden change in time); a
horizontal line in the time domain means a frequency of zero (no
change in time). Going from a frequency of zero to a frequency of
infinity (and vice versa) implies all frequencies in between are part of
the domain. Fourier analysis can be used to decompose a digital
signal. If the digital signal is periodic, which is rare in data
communications, the decomposed signal has a frequency domain
representation with an infinite bandwidth and discrete frequencies. If
the digital signal is non periodic, the decomposed signal still has an
infinite bandwidth, but the frequencies are continuous

Transmission ofDigital Signals

We can transmit a digital signal by using one of two different
approaches: baseband transmission or broadband transmission (using
modulation).
Baseband Transmission
Baseband transmission means sending a digital signal over a channel
without changing the digital signal to an analog signal. Baseband
transmission requires that we have a low-pass channel, a channel with
a bandwidth that starts from zero. This is the case if we have a
dedicated medium with a bandwidth constituting only one channel

Broadband Transmission (Using Modulation)

Broadband transmission or modulation means changing the digital
signal to an analog signal for transmission. Modulation allows us to
use a bandpass channel-a channel with a bandwidth that does not
start from zero. This type of channel is more available than a low-pass
channel.

Transmission Impairment

Transmission impairment occurs when the received signal is different

from the transmitted signal. As we know, a signal can be transmitted
as Analog signal or it can be transmitted as a digital signal.
In Analog signals due to transmission impairment the resulting
received signal gets different amplitude or the shape. In the case of
digitally transmitted signals at the receiver side we get changes in bits
(0's or 1's).

Causes
There are various causes of transmission impairments −
 Noise
 Distortion
 Attenuation

Noise
Noise is the major factor for the transmission distortion as any
unwanted signal gets added to the transmitted signal by which the
resulting transmitted signal gets modified and at the receiver side it is
difficult to remove the unwanted noise signal. These noises are
various kinds like shot noise, impulse noise, thermal noise etc.
Noise is diagrammatically represented as follows −

Consider normal signal as shown below −

Consider the Noise signal as shown below −

At the receiver side the sender signal combine with noise and
represented as follows −

The above signals are reconstructed by sampling. Increased data

rate implies "shorter" bits with higher sensitivity to noise.

Sources of Noise
The different sources of Noise are as follows −
 Thermal noise
 Intermodulation noise
 Crosstalk
 Impulse noise

Thermal noise
A kind of noise where the irregular electron movement in wire
produces an additional signal. It is also called as white noise because
the frequency encompasses over a broad range of frequencies.
Intermodulation noise
Here the signal transmission channel can share more than one signal
and the intermodulation noise is generated.
For instance, consider two signals S1 and S2 generate signals of
frequencies (S1 + S2) and (S1 - S2) that may interfere with the signals
of the same frequencies sent by the sender. In any part of the
communication system, intermodulation noise is introduced because
of this situation.

Cross talk
Cross talk is an effect of one wire on another wire. One wire acts as a
sending antenna and the other (transmission medium) acts as the
receiving antenna.
For example − telephone systems, it is a common experience to hear
conversation of other people in the background. This is known as cross
talk.

Impulse noise
Impulse noise is irregular pulses or spikes generated by phenomena of
power lines, lightning spark due to loss of contact in electric circuits
and so on. It is a primary source of error in digital data.

Distortion
This kind of distortion is mainly appearing in case of composite
signals in which a composite signal has various frequency components
in it and each frequency component has some time constraint which
makes a complete signal.
But while transmitting this composite signal, if a certain delay happens
between the frequencies components, then there may be the chance
that the frequency component will reach the receiver end with a
different delay constraint from its original which leads to the change in
shape of the signal. The delay happens due to environmental
parameters or from the distance between transmitter and receiver
etc.
Distortion is diagrammatically represented as follows −

Attenuation
Attenuation is generally decreased in signal strength, by which the
received signal will be difficult to receive at the receiver end. This
attenuation happens due to the majority factor by environment as
environment imposes a lot of resistance and the signal strength
decreases as it tries to overcome the resistance imposed.

The above picture shows that the signal loses power at its travels time.
Attenuation is diagrammatically represented as follows −

PERFORMANCE
 The performance of a network pertains to the measure of service
quality of a network as perceived by the user. There are different
ways to measure the performance of a network, depending upon the
nature and design of the network. Finding the performance of a
network depends on both quality of the network and the quantity of
the network.
 Bandwidth
 Latency (Delay)
 Bandwidth – Delay Product
 Throughput
 Jitter

Bandwidth
One characteristic that measures network performance is
bandwidth. However, the term can be used in two different contexts
with two different measuring values: bandwidth in hertz and
bandwidth in bits per second.

Bandwidth in Hertz
We have discussed this concept. Bandwidth in hertz is the range of
frequencies contained in a composite signal or the range off
requencies a channel can pass. For example, we can say the
bandwidth of a subscriber telephone line is 4 kHz.

Bandwidth in Bits per Seconds

The term bandwidth can also refer to the number of bits per second
that a channel, a link, or even a network can transmit. For example,
one can say the bandwidth of a Fast Ethernet network (or the links in
this network) is a maximum of 100 Mbps. This means that this
network can send 100 Mbps.

Relationship
There is an explicit relationship between the bandwidth in hertz and
bandwidth in bits per seconds. Basically, an increase in bandwidth in
hertz means an increase in bandwidth in bits per second. The
relationship depends on whether we have baseband transmission or
transmission with modulation.

Throughput
 The throughput is a measure of how fast we can actually send data
through a network. Although, at first glance, bandwidth in bits per
second and throughput seem the same, they are different. A link may
have a bandwidth of B bps, but we can only send T bps through this
link with T always less than B. In other words, the bandwidth is a
potential measurement of a link; the throughput is an actual
measurement of how fast we can send data.
For example, we may have a link with a bandwidth of 1 Mbps, but
the devices connected to the end of the link may handle only 200
kbps. This means that we cannot send more than 200 kbps through
this link. Imagine a highway designed to transmit 1000 cars per
minute from one point to another. However, if there is congestion on
the road, this figure may be reduced to 100 cars per minute. The
bandwidth is 1000 cars per minute; the throughput is 100 cars per
minute.

Latency (Delay)
The latency or delay defines how long it takes for an entire
message to completely arrive at the destination from the time the first
bit is sent out from the source. We can say that latency is made of
four components: propagation time, transmission time, queuing time
and processing delay.

Latency=propagation time + transmission time +queuing time +

processing delay

Propagation Time
Propagation time measures the time required for a bit to travel from
the source to the destination. The propagation time is calculated by
dividing the distance by the propagation speed.
Propagation time = Distance /Propagation speed
The propagation speed of electromagnetic signals depends on the
medium and on the frequency of the signal. For example, in a
vacuum, light is propagated with a speed of 3 x 10^8 mfs. It is lower
in air; it is much lower in cable.

Transmission Time
In data communications we don't send just 1 bit, we send a message.
The first bit may take a time equal to the propagation time to reach its
destination; the last bit also may take the same amount of time.
However, there is a time between the first bit leaving the sender and
the last bit arriving at the receiver. The first bit leaves earlier and
arrives earlier; the last bit leaves later and arrives later. The time
required for transmission of a message depends on the size of the
message and the bandwidth ofthe channel.
Transmission time=Message size /Bandwidth

Queuing Time
The third component in latency is the queuing time, the time
needed for each intermediate or end device to hold the message
before it can be processed. The queuing time is not a fixed factor; it
changes with the load imposed on the network. When there is heavy
traffic on the network, the queuing time increases. An intermediate
device, such as a router, queues the arrived messages and processes
them one by one. If there are many messages, each message will
have to wait.

Bandwidth-Delay Product
Bandwidth and delay are two performance metrics of a link.
However, as we will see in this chapter and future chapters, what is
very important in data communications is the product of the two, the
bandwidth-delay product

Jitter
Another performance issue that is related to delay is jitter. We can
roughly say that jitter is a problem if different packets of data
encounter different delays and the application using the data at the
receiver site is time-sensitive (audio and video data, for example). If
the delay for the first packet is 20 ms, for the second is 45 ms, and for
the third is 40 ms, then the real-time application that uses the packets
endures jitter.

Multiplexing
Multiplexing is a technique used to combine and send the multiple
data streams over a single medium. The process of combining the
data streams is known as multiplexing and hardware used for
multiplexing is known as a multiplexer.
Multiplexing is achieved by using a device called Multiplexer (MUX)
that combines n input lines to generate a single output line.
Multiplexing follows many-to-one, i.e., n input lines and one output
line.
Demultiplexing is achieved by using a device called Demultiplexer
(DEMUX) available at the receiving end. DEMUX separates a signal
into its component signals (one input and n outputs). Therefore, we
can say that demultiplexing follows the one-to-many approach.

Why Multiplexing?
o The transmission medium is used to send the signal from sender
to receiver. The medium can only have one signal at a time.
o If there are multiple signals to share one medium, then the
medium must be divided in such a way that each signal is given some
portion of the available bandwidth. For example: If there are 10
signals and bandwidth of medium is100 units, then the 10 unit is
shared by each signal.
o When multiple signals share the common medium, there is a
possibility of collision. Multiplexing concept is used to avoid such
collision.
o Transmission services are very expensive.

History of Multiplexing
o Multiplexing technique is widely used in telecommunications in
which several telephone calls are carried through a single wire.
o Multiplexing originated in telegraphy in the early 1870s and is now
widely used in communication.
o George Owen Squier developed the telephone carrier
multiplexing in 1910.

Concept of Multiplexing
o The 'n' input lines are transmitted through a multiplexer and
multiplexer combines the signals to form a composite signal.
o The composite signal is passed through a Demultiplexer and
demultiplexer separates a signal to component signals and transfers
them to their respective destinations.

Advantages of Multiplexing:
o More than one signal can be sent over a single medium.
o The bandwidth of a medium can be utilized effectively.

Multiplexing Techniques
Multiplexing techniques can be classified as:

Frequency-division Multiplexing (FDM)

o It is an analog technique.
o Frequency Division Multiplexing is a technique in which the
available bandwidth of a single transmission medium is subdivided
into several channels.
o In the above diagram, a single transmission medium is subdivided
into several frequency channels, and each frequency channel is given
to different devices. Device 1 has a frequency channel of range from 1
to 5.
o The input signals are translated into frequency bands by using
modulation techniques, and they are combined by a multiplexer to
form a composite signal.
o The main aim of the FDM is to subdivide the available bandwidth
into different frequency channels and allocate them to different
devices.
o Using the modulation technique, the input signals are transmitted
into frequency bands and then combined to form a composite signal.
o The carriers which are used for modulating the signals are known
as sub-carriers. They are represented as f1,f2..fn.
o FDM is mainly used in radio broadcasts and TV networks.

Advantages of FDM:
o FDM is used for analog signals.
o FDM process is very simple and easy modulation.
o A Large number of signals can be sent through an FDM
simultaneously.
o It does not require any synchronization between sender and
receiver.
Disadvantages of FDM:
o FDM technique is used only when low-speed channels are
required.
o It suffers the problem of crosstalk.
o A Large number of modulators are required.
o It requires a high bandwidth channel.
Applications Of FDM:
o FDM is commonly used in TV networks.
o It is used in FM and AM broadcasting. Each FM radio station has
different frequencies, and they are multiplexed to form a composite
signal. The multiplexed signal is transmitted in the air.

Wavelength Division Multiplexing (WDM)

o Wavelength Division Multiplexing is same as FDM except that the

optical signals are transmitted through the fibre optic cable.
o WDM is used on fibre optics to increase the capacity of a single
fibre.
o It is used to utilize the high data rate capability of fibre optic cable.
o It is an analog multiplexing technique.
o Optical signals from different source are combined to form a wider
band of light with the help of multiplexer.
o At the receiving end, demultiplexer separates the signals to
transmit them to their respective destinations.
o Multiplexing and Demultiplexing can be achieved by using a prism.
o Prism can perform a role of multiplexer by combining the various
optical signals to form a composite signal, and the composite signal is
transmitted through a fibre optical cable.
o Prism also performs a reverse operation, i.e., demultiplexing the
signal.

Time Division Multiplexing

o It is a digital technique.
o In Frequency Division Multiplexing Technique, all signals operate
at the same time with different frequency, but in case of Time Division
 Multiplexing technique, all signals operate at the same frequency with
different time.
o In Time Division Multiplexing technique, the total time
available in the channel is distributed among different users.
Therefore, each user is allocated with different time interval known as
a Time slot at which data is to be transmitted by the sender.
o A user takes control of the channel for a fixed amount of time.
o In Time Division Multiplexing technique, data is not transmitted
simultaneously rather the data is transmitted one-by-one.
o In TDM, the signal is transmitted in the form of frames. Frames
contain a cycle of time slots in which each frame contains one or more
slots dedicated to each user.
o It can be used to multiplex both digital and analog signals but
mainly used to multiplex digital signals.
There are two types of TDM:
o Synchronous TDM
o Asynchronous TDM
Synchronous TDM
o A Synchronous TDM is a technique in which time slot is
preassigned to every device.
o In Synchronous TDM, each device is given some time slot
irrespective of the fact that the device contains the data or not.
o If the device does not have any data, then the slot will remain
empty.
o In Synchronous TDM, signals are sent in the form of frames. Time
slots are organized in the form of frames. If a device does not have
data for a particular time slot, then the empty slot will be transmitted.
o The most popular Synchronous TDM are T-1 multiplexing, ISDN
multiplexing, and SONET multiplexing.
o If there are n devices, then there are n slots.

Concept Of Synchronous TDM

In the above figure, the Synchronous TDM technique is implemented.
Each device is allocated with some time slot. The time slots are
transmitted irrespective of whether the sender has data to send or
not.

Disadvantages of Synchronous TDM:

o The capacity of the channel is not fully utilized as the empty slots
are also transmitted which is having no data. In the above figure, the
first frame is completely filled, but in the last two frames, some slots
are empty. Therefore, we can say that the capacity of the channel is
not utilized efficiently.
o The speed of the transmission medium should be greater than the
total speed of the input lines. An alternative approach to the
Synchronous TDM is Asynchronous Time Division Multiplexing.
Asynchronous TDM
o An asynchronous TDM is also known as Statistical TDM.
o An asynchronous TDM is a technique in which time slots are not
fixed as in the case of Synchronous TDM. Time slots are allocated to
only those devices which have the data to send. Therefore, we can say
that Asynchronous Time Division multiplexor transmits only the data
from active workstations.
o An asynchronous TDM technique dynamically allocates the time
slots to the devices.
o In Asynchronous TDM, total speed of the input lines can be greater
than the capacity of the channel.
o Asynchronous Time Division multiplexor accepts the incoming
data streams and creates a frame that contains only data with no
empty slots.
o In Asynchronous TDM, each slot contains an address part that
identifies the source of the data.

o The difference between Asynchronous TDM and Synchronous TDM

is that many slots in Synchronous TDM are unutilized, but in
 Asynchronous TDM, slots are fully utilized. This leads to the smaller
transmission time and efficient utilization of the capacity of the
channel.
o In Synchronous TDM, if there are n sending devices, then there are
n time slots. In Asynchronous TDM, if there are n sending devices,
then there are m time slots where m is less than n (m<n).
o The number of slots in a frame depends on the statistical analysis
of the number of input lines.
Concept of Asynchronous TDM

In the above diagram, there are 4 devices, but only two devices are
sending the data, i.e., A and C. Therefore, the data of A and C are only
transmitted through the transmission line.
Frame of above diagram can be represented as:

The above figure shows that the data part contains the address to
determine the source of the data.

Transmission media
o Transmission media is a communication channel that carries the
information from the sender to the receiver. Data is transmitted
through the electromagnetic signals.
o The main functionality of the transmission media is to carry the
information in the form of bits through LAN(Local Area Network).
o It is a physical path between transmitter and receiver in data
communication.
o In a copper-based network, the bits in the form of electrical
signals.
o In a fibre based network, the bits in the form of light pulses.
o In OSI(Open System Interconnection) phase, transmission media
supports the Layer 1. Therefore, it is considered to be as a Layer 1
component.
o The electrical signals can be sent through the copper wire, fibre
optics, atmosphere, water, and vacuum.
o The characteristics and quality of data transmission are
determined by the characteristics of medium and signal.
o Transmission media is of two types are wired media and wireless
media. In wired media, medium characteristics are more important
whereas, in wireless media, signal characteristics are more important.
o Different transmission media have different properties such as
bandwidth, delay, cost and ease of installation and maintenance.
o The transmission media is available in the lowest layer of the OSI
reference model, i.e., Physical layer.

Some factors need to be considered for designing the transmission

media:
o Bandwidth: All the factors are remaining constant, the greater
the bandwidth of a medium, the higher the data transmission rate of a
signal.
o Transmission impairment: When the received signal is not
identical to the transmitted one due to the transmission impairment.
The quality of the signals will get destroyed due to transmission
impairment.
o Interference: An interference is defined as the process of
disrupting a signal when it travels over a communication medium on
the addition of some unwanted signal.
Causes of Transmission Impairment:
o Attenuation: Attenuation means the loss of energy, i.e., the
strength of the signal decreases with increasing the distance which
causes the loss of energy.
o Distortion: Distortion occurs when there is a change in the shape
of the signal. This type of distortion is examined from different signals
having different frequencies. Each frequency component has its own
propagation speed, so they reach at a different time which leads to
the delay distortion.
o Noise: When data is travelled over a transmission medium, some
unwanted signal is added to it which creates the noise.

Classification Of Transmission Media:

Guided Media
It is defined as the physical medium through which the signals are
transmitted. It is also known as Bounded media.

Types of Guided media:

Twisted pair:
Twisted pair is a physical media made up of a pair of cables
twisted with each other. A twisted pair cable is cheap as compared to
other transmission media. Installation of the twisted pair cable is easy,
and it is a lightweight cable. The frequency range for twisted pair
cable is from 0 to 3.5KHz.
A twisted pair consists of two insulated copper wires arranged in a
regular spiral pattern.
The degree of reduction in noise interference is determined by the
number of turns per foot. Increasing the number of turns per foot
decreases noise interference.
Types of Twisted pair:

Unshielded Twisted Pair:

An unshielded twisted pair is widely used in telecommunication.
Following are the categories of the unshielded twisted pair cable:

o Category 1: Category 1 is used for telephone lines that have low-

speed data.
o Category 2: It can support upto 4Mbps.
o Category 3: It can support upto 16Mbps.
o Category 4: It can support upto 20Mbps. Therefore, it can be
used for long-distance communication.
o Category 5: It can support upto 200Mbps.
Advantages of Unshielded Twisted Pair:
o It is cheap.
o Installation of the unshielded twisted pair is easy.
o It can be used for high-speed LAN.
Disadvantage:
o This cable can only be used for shorter distances because of
attenuation.

Shielded Twisted Pair

A shielded twisted pair is a cable that contains the mesh surrounding
the wire that allows the higher transmission rate.
Characteristics Of Shielded Twisted Pair:
o The cost of the shielded twisted pair cable is not very high and not
very low.
o An installation of STP is easy.
o It has higher capacity as compared to unshielded twisted pair
cable.
o It has a higher attenuation.
o It is shielded that provides the higher data transmission rate.
Disadvantages
o It is more expensive as compared to UTP and coaxial cable.
o It has a higher attenuation rate.

Coaxial Cable
o Coaxial cable is very commonly used transmission media, for
example, TV wire is usually a coaxial cable.
o The name of the cable is coaxial as it contains two conductors
parallel to each other.
o It has a higher frequency as compared to Twisted pair cable.
o The inner conductor of the coaxial cable is made up of copper, and
the outer conductor is made up of copper mesh. The middle core is
made up of non-conductive cover that separates the inner conductor
from the outer conductor.
o The middle core is responsible for the data transferring whereas
the copper mesh prevents from the EMI(Electromagnetic
interference).

Coaxial cable is of two types:

1. Baseband transmission: It is defined as the process of
transmitting a single signal at high speed.
2. Broadband transmission: It is defined as the process of
transmitting multiple signals simultaneously.
Advantages of Coaxial cable:
o The data can be transmitted at high speed.
o It has better shielding as compared to twisted pair cable.
o It provides higher bandwidth.
Disadvantages Of Coaxial cable:
o It is more expensive as compared to twisted pair cable.
o If any fault occurs in the cable causes the failure in the entire
network.

Fibre Optic
o Fibre optic cable is a cable that uses electrical signals for
communication.
o Fibre optic is a cable that holds the optical fibres coated in plastic
that are used to send the data by pulses of light.
o The plastic coating protects the optical fibres from heat, cold,
electromagnetic interference from other types of wiring.
o Fibre optics provide faster data transmission than copper wires.
Diagrammatic representation of fibre optic cable:

Basic elements of Fibre optic cable:

o Core: The optical fibre consists of a narrow strand of glass or
plastic known as a core. A core is a light transmission area of the fibre.
The more the area of the core, the more light will be transmitted into
the fibre.
o Cladding: The concentric layer of glass is known as cladding. The
main functionality of the cladding is to provide the lower refractive
index at the core interface as to cause the reflection within the core so
that the light waves are transmitted through the fibre.
o Jacket: The protective coating consisting of plastic is known as a
jacket. The main purpose of a jacket is to preserve the fibre strength,
absorb shock and extra fibre protection.

Following are the advantages of fibre optic cable over copper:

o Greater Bandwidth: The fibre optic cable provides more
bandwidth as compared copper. Therefore, the fibre optic carries
more data as compared to copper cable.
o Faster speed: Fibre optic cable carries the data in the form of
light. This allows the fibre optic cable to carry the signals at a higher
speed.
o Longer distances: The fibre optic cable carries the data at a
longer distance as compared to copper cable.
o Better reliability: The fibre optic cable is more reliable than the
copper cable as it is immune to any temperature changes while it can
cause obstruct in the connectivity of copper cable.
o Thinner and Sturdier: Fibre optic cable is thinner and lighter in
weight so it can withstand more pull pressure than copper cable.

Switching techniques
In large networks, there can be multiple paths from sender to
receiver. The switching technique will decide the best route for data
transmission.
Switching technique is used to connect the systems for making one-to-
one communication.

Classification of Switching Techniques

Circuit Switching
o Circuit switching is a switching technique that establishes a
dedicated path between sender and receiver.
o In the Circuit Switching Technique, once the connection is
established then the dedicated path will remain to exist until the
connection is terminated.
o Circuit switching in a network operates in a similar way as the
telephone works.
o A complete end-to-end path must exist before the communication
takes place.
o In case of circuit switching technique, when any user wants to
send the data, voice, video, a request signal is sent to the receiver
then the receiver sends back the acknowledgment to ensure the
availability of the dedicated path. After receiving the
acknowledgment, dedicated path transfers the data.
o Circuit switching is used in public telephone network. It is used for
voice transmission.
o Fixed data can be transferred at a time in circuit switching
technology.

Communication through circuit switching has 3 phases:

o Circuit establishment
o Data transfer
o Circuit Disconnect

Circuit Switching can use either of the two technologies:

Space Division Switches:
o Space Division Switching is a circuit switching technology in which
a single transmission path is accomplished in a switch by using a
physically separate set of cross points.
o Space Division Switching can be achieved by using crossbar
switch. A crossbar switch is a metallic cross point or semiconductor
gate that can be enabled or disabled by a control unit.
o The Crossbar switch is made by using the semiconductor. For
example, Xilinx crossbar switch using FPGAs.
o Space Division Switching has high speed, high capacity, and non
blocking switches.
Space Division Switches can be categorized in two ways:
o Crossbar Switch
o Multistage Switch

Crossbar Switch
The Crossbar switch is a switch that has n input lines and n output
lines. The crossbar switch has n2 intersection points known
as crosspoints.
Disadvantage of Crossbar switch:
The number of crosspoints increases as the number of stations is
increased. Therefore, it becomes very expensive for a large switch.
The solution to this is to use a multistage switch.

Multistage Switch
o Multistage Switch is made by splitting the crossbar switch into the
smaller units and then interconnecting them.
o It reduces the number of crosspoints.
o If one path fails, then there will be an availability of another path.
Advantages of Circuit Switching:
o In the case of Circuit Switching technique, the communication
channel is dedicated.
o It has fixed bandwidth.
Disadvantages of Circuit Switching:
o Once the dedicated path is established, the only delay occurs in
the speed of data transmission.
o It takes a long time to establish a connection approx 10 seconds
during which no data can be transmitted.
o It is more expensive than other switching techniques as a
dedicated path is required for each connection.
o It is inefficient to use because once the path is established and no
data is transferred, then the capacity of the path is wasted.
o In this case, the connection is dedicated therefore no other data
can be transferred even if the channel is free.

Message Switching
o Message Switching is a switching technique in which a message is
transferred as a complete unit and routed through intermediate nodes
at which it is stored and forwarded.
o In Message Switching technique, there is no establishment of a
dedicated path between the sender and receiver.
o The destination address is appended to the message. Message
Switching provides a dynamic routing as the message is routed
through the intermediate nodes based on the information available in
the message.
o Message switches are programmed in such a way so that they can
provide the most efficient routes.
o Each and every node stores the entire message and then forward
it to the next node. This type of network is known as store and
forward network.
o Message switching treats each message as an independent entity.

Advantages of Message Switching

o Data channels are shared among the communicating devices that
improve the efficiency of using available bandwidth.
o Traffic congestion can be reduced because the message is
temporarily stored in the nodes.
o Message priority can be used to manage the network.
o The size of the message which is sent over the network can be
varied. Therefore, it supports the data of unlimited size.
Disadvantages of Message Switching
o The message switches must be equipped with sufficient storage to
enable them to store the messages until the message is forwarded.
o The Long delay can occur due to the storing and forwarding facility
provided by the message switching technique.

Packet Switching
o The packet switching is a switching technique in which the
message is sent in one go, but it is divided into smaller pieces, and
they are sent individually.
o The message splits into smaller pieces known as packets and
packets are given a unique number to identify their order at the
receiving end.
o Every packet contains some information in its headers such as
source address, destination address and sequence number.
o Packets will travel across the network, taking the shortest path as
possible.
o All the packets are reassembled at the receiving end in correct
order.
o If any packet is missing or corrupted, then the message will be
sent to resend the message.
o If the correct order of the packets is reached, then the
acknowledgment message will be sent.

Approaches of Packet Switching:

There are two approaches to Packet Switching:
Datagram Packet switching:
o It is a packet switching technology in which packet is known as a
datagram, is considered as an independent entity. Each packet
contains the information about the destination and switch uses this
information to forward the packet to the correct destination.
o The packets are reassembled at the receiving end in correct order.
o In Datagram Packet Switching technique, the path is not fixed.
o Intermediate nodes take the routing decisions to forward the
packets.
o Datagram Packet Switching is also known as connectionless
switching.
Virtual Circuit Switching
o Virtual Circuit Switching is also known as connection-oriented
switching.
o In the case of Virtual circuit switching, a preplanned route is
established before the messages are sent.
o Call request and call accept packets are used to establish the
connection between sender and receiver.
o In this case, the path is fixed for the duration of a logical
connection.
Let's understand the concept of virtual circuit switching
through a diagram:
o In the above diagram, A and B are the sender and receiver
respectively. 1 and 2 are the nodes.
o Call request and call accept packets are used to establish a
connection between the sender and receiver.
o When a route is established, data will be transferred.
o After transmission of data, an acknowledgment signal is sent by
the receiver that the message has been received.
o If the user wants to terminate the connection, a clear signal is sent
for the termination.
Advantages of Packet Switching:
o Cost-effective: In packet switching technique, switching devices
do not require massive secondary storage to store the packets, so
cost is minimized to some extent. Therefore, we can say that the
packet switching technique is a cost-effective technique.
o Reliable: If any node is busy, then the packets can be rerouted.
This ensures that the Packet Switching technique provides reliable
communication.
o Efficient: Packet Switching is an efficient technique. It does not
require any established path prior to the transmission, and many
users can use the same communication channel simultaneously,
hence makes use of available bandwidth very efficiently.
Disadvantages of Packet Switching:
o Packet Switching technique cannot be implemented in those
applications that require low delay and high-quality services.
o The protocols used in a packet switching technique are very
complex and requires high implementation cost.
o If the network is overloaded or corrupted, then it requires
retransmission of lost packets. It can also lead to the loss of critical
information if errors are nor recovered.
o Differences b/w Datagram approach and Virtual Circuit approach
Datagram approach Virtual Circuit approach

Node takes routing decisions to Node does not take any routing
forward the packets. decision.

Congestion cannot occur as all Congestion can occur when the node is
the packets travel in different busy, and it does not allow other
directions. packets to pass through.

It is more flexible as all the It is not very flexible.

packets are treated as an
independent entity.