Data Communication Principles

Prepared by: Kind Kishor
Tribhuvan University

Analog and digital data.
Analog and digital signal.
Transmission impairment.
Channel capacity.
Data transmission.
Data encoding.
Multiplexing.

 Analog data – representation variable takes on
continuous values in some interval.
e.g. voice, temperature, etc.
Digital data – representation variable takes on
discrete(a finite & countable number of) values in a given
interval.
e.g. text, digitized images, etc.

Electric or electromagnetic representation of data.
 The signal intensity varies in a smooth fashion over a time,
or no breaks in the signal.
 Has infinitely many levels of intensity over a period of time.
 The signal intensity maintains a constant level for some
period of time then changes to another constant level.
 Has only a limited number of defined values.

 periodic signal – completes a pattern within some
measurable time frame, called a period , and then repeats
that pattern over subsequent identical periods.
Aperiodic signal – changes without exhibiting a
pattern that repeats over time.
a. Periodic analog signal b. Periodic digital signal

Periodic analog signals are two types i.e. simple and composite analog
signal.
Simple Analog Signal – cannot be decomposed into simpler signals.
Composite Analog Signal – composed of multiple sine waves.
Sine wave – most fundamental form of periodic analog signal. It can be
represented by 3 parameters: peak amplitude, frequency, and phase.
Mathematically,
s(t) = A⋅sin(2πft +ϕ)
Where,
S=instantaneous amplitude
A=peak amplitude
F=frequency
Φ=phase

1. Peak amplitude (A ):- absolute value of signal’s highest intensity .
unit: volts [V]
2. Period and Frequency(f):-number of periods in one second where
period is amount of time to complete 1 cycle. unit: hertz [Hz]
3. phase (φ):-absolute position of the waveform relative to time 0. unit:
degrees [º] or radians [rad] where 360º = 2π rad
Bandwidth:
The range of frequency that a medium can pass i.e. Difference
between highest and lowest frequency that channel can
satisfactorily pass.

sequence of voltage pulses.
 each pulse represents a signal element.
Most digital signals are aperiodic.
Key digital-signals terms:
• bit interval – time required to send a single bit, unit: [sec]
• bit rate – number of bit intervals per second – unit: [bps]
A digital signal with two levels.

Property of a transmission medium which causes the signal to be
degraded, reduced in amplitude, distorted or contaminated.
The causes are:
–Attenuation: loss of energy as the signal propagates outwards.
–Distortion : the signal changes its form or shape i.e. phase.
–Noise
:

The maximum rate at which data can be transmitted over a given
communication path or channel under given conditions.
Data rate, bandwidth, noise, and error rate are involved in the
evolution of channel capacity.
Shannon Capacity Formula:
Introduced by Claude Shannon in 1994.
To determine highest data rate for a noise channel.
Unit-bps(bit per second).
Capacity=
Where,
B=bandwidth
SNR=signal-to-noise ratio(S/N)

•Communication of data through propagation and
processing of signals.
•Binary data can be transmitted either in parallel or
serial form.

1. Parallel Transmission:
‐ Use n wires to send n bits at
one time.
‐ Its advantage is speed whereas
it is expensive.
2. Serial Transmission:
‐ one bit follows another.
‐ It is cheap because it does not
require n communication
channel.

1. Asynchronous Transmission:- information is
received and translated by agreed upon patterns.
Patterns are based on grouping the bit stream into
bytes.
2. Synchronous Transmission:- the bit stream is
combined into longer ‘frames’ which may contain
multiple bytes. Timing becomes very important.
3. Isochronous Transmission:- A form of data
transmission that guarantees to provide a certain
minimum data rate, as required for time-
dependent data such as video or audio.

various techniques of impressing data or information on
an electrical, or electromagnetic signal that would
propagate through the physical medium making up the
communication link between the two devices.
Schemes of Data Encoding:

Digital signal:
Discrete, discontinuous voltage pulses.
Each pulse is a signal element.
Binary data encoded into signal elements.
Line Coding:
A process of converting digital data to digital signals.
Converts a sequence of bits to digital signal.

i. Signal element v/s Data element:- data element is
smallest entity that can represent a piece of
information whereas signal element is the shortest
unit of a digital signal.
ii. Data rate v/s Signal rate:- data rate(bit rate)is
number of data element(bit) sent in 1(s) whereas
signal rate(pulse rate) is number of signal element
sent in 1(s).
iii. DC Components:- frequencies around zero.
iv. Self-Synchronization:- correctly interpret the
signals received from the sender.

Unipolar: all signal levels are on one side of the time axis
either above or below.
Positive voltage defines bit 1 and zero voltage
defines bit 0.
Polar: signal levels are both side of the time axis.
Voltage level for 0 can be positive and for 1 can be
negative.
It can be further divided into four categories.

• Two different voltages for 0 and 1 bits
• Negative voltage for one value and positive for the
other, eg.
—Binary 0 : Positive
—Binary 1 : Negative
• Voltage constant during bit interval
—no transition i.e. no return to zero voltage
L

•Uses three values: positive, negative, and zero.
•The signal goes to 0 in the middle of each bit and
remains there until the beginning of the next bit.

oThe duration of bits is divided into two halves.
oThe voltage remains at one level during 1st half and moves
to another level in the second half.
oTransition occurs at the middle of each bit period
oTransition serves as clock and data
oLow to high represents binary 1
oHigh to low represents binary 0

›Combines the ideas of RZ and NRZ-I.
›There is always transition at the middle of the bit.
›The bit values are determined at the beginning of the
bit.
›There is a transition when next bit is 0.
›There is no transition when next bit is 1.

oTransmission of digital data with analog signals.
oModulation involves operation on one or more of
the three characteristics of a carrier signal i.e.
amplitude, frequency, or phase.
oWe will examine three basic modulation
Techniques:
—Amplitude Shift Keying (ASK)
—Frequency Shift Keying (FSK)
—Phase Shift Keying (PSK)

Values represented by different amplitudes of carrier.
Usually, one amplitude is zero
i.e. presence and absence of carrier is used.
Frequency and phase remain constant.

The frequency of the carrier signal is varied to
represent data.
Frequency of the modulated signal is constant for the
duration of one signal element, but changes for the
next signal element if the data element is changed.
Peak amplitude and phase remain constant for all
signal element.

oThe phase of the carrier signal is varied to represent
binary 0 or 1.
oTwo phases represent two binary digits.
oBoth peak amplitude and frequency remain constant.

set of techniques that allows the simultaneous transmission
of multiple signals across a single data link.
n lines share the bandwidth of one link.
Three are three basic multiplexing techniques:

‐An analog technique.
‐It can be applied when the bandwidth of a link is greater
than the combined bandwidths of the signals to be
transmitted.
‐ a scheme in which numerous signals are combined for
transmission on a single communications line or channel.
‐Each signal is assigned a different frequency (subchannel)
within the main channel.
When FDM is used in a communications network, each input
signal is sent and received at maximum speed at all times i.e.
bandwidth is greater.

–An analog technique.
– A method of combining multiple signals on laser beams
at various infrared (IR) wavelengths for transmission
along fiber optic media.
–A technique for using a fiber (or optical device)to carry
many separate and independent optical channels.
WDM is conceptually the same as FDM, except that the
multiplexing and de-multiplexing involve optical signals
transmitted through fiber-optic channels.

–A digital technique.
–a mechanism for dividing the bandwidth of a link into
time slots.
–TDM repeatedly transmits a fixed sequence of time slots
over a single transmission channel.
–The same link is used as in FDM; here, how-ever, the link
is shown sectioned by time rather than by frequency.

Data Communication Principles

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Data Communication Principles