To be transmitted, data must be transformed to electromagnetic
Data can be either digital or analog.
Analog refers to something that is continuous– a set of specific
points of data and all possible points between. e.g. human voice
Digital refers to something that is discrete – a set of specific points
of data with no other points in between. E.g. digital data.
Types of Signals
Digital & Analog Signals
It is a continuous wave form that changes smoothly over time.
As the wave moves from value A to value B,it passes through and
includes an infinite number of values along its path.
A digital signal is discrete.
It can have only a limited number of defined values, often as simple as 1
The transition of a digital signal from value to value is instantaneous,
like a light being switched on and off.
Digital & Analog Signals
Vertical axis: value or strength of a signal
Horizontal axis: Passage of time
Periodic & Aperiodic Signals
Both analog and digital signals can be of two forms:
In data communication, we commonly use periodic analog signals
and aperiodic digital signals.
A signal is a Periodic signal if it completes a pattern within a
measurable time frame, called a period.
A period is defined as the amount of time required to complete
one full cycle.
The completion of one full pattern is called a cycle.
The duration of a period represented by T may be different for
each signal but it is constant for any given periodic signal.
Types of analog signals
Simple Analog signals(sine wave)
Simple analog signal / sine wave
The sine wave is the most fundamental form of a periodic analog
Visually a simple oscillating curve, its change over the course of a
cycle is smooth and consistent, a continuous, rolling flow.
Sine wave can be described by three characteristics:
Period Or frequency
The amplitude of a signal is the value of the signal at any point on
It is equal to the vertical distance from a given point on the wave
form to the horizontal axis.
The maximum amplitude of a sine wave is equal to the highest
value it reaches on the vertical axis.
Amplitude is measured in either, volts ,amperes or watts.
Frequency is the number of cycles/Periods in one second.
Frequency is the relationship of a signal to time.
Frequency is the measure of the rate of change.
Electromagnetic signals are oscillating wave forms that is
they fluctuate continuously and predictably above and below
a mean energy level.
The rate at which a sine wave moves from its lowest to
highest level is its frequency.
Rate of change of signal with respect to time.
The unit of frequency is Hertz.
High & low Frequency
If the value of a signal changes over a very short span of time, its
frequency is high.
If it changes over a long span of time, its frequency is low.
If a signal does not change and it maintains a constant voltage level
the entire time it is active the signal is having 0 Hz frequency.
If a signal changes instantaneously, then its frequency is infinite.
Period refers to the amount of time (in seconds) a signal needs to
complete one cycle .
Period and Frequency are inverse of each other.
The term phase describes the position of the waveform relative to
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
It actually indicates the status of the first cycle.
Phase is measured in degrees or radians.
Time and frequency domains
The time-domain plot shows changes in signal amplitude
with respect to time(it is an amplitude versus time plot).
Phase and frequency are not explicitly measured on a time-
A frequency domain-plot shows the relationship between
amplitude and frequency.
Many useful wave forms do not change in a single smooth
curve between a minimum and maximum amplitude.
They jump, slide, wobble, spike and dip.
As along as any irregularities are consistent, cycle after cycle,
a signal is still periodic and logically must be describable in
the same terms used for sine waves.
Any periodic signal, no matter how complex can be
decomposed in to a collection of sine waves,each having a
measurable amplitude,frequency and phase.
Frequency spectrum & Bandwidth
The frequency spectrum of a signal is the collection of all the
component frequencies it contains and is shown using a frequency
The frequency spectrum of a signal is the combination of all sine
wave signals that make up that signal.
The bandwidth of a signal is the width of the frequency spectrum.
Band width refers to the range of component frequencies and
frequency spectrum refers to the elements within that range.
The bandwidth is the difference between the highest and the
lowest frequency within the range of frequencies the signal
Bit rate and bit interval
The bit interval is the time required to send one single bit.
The bit rate is the number of bit intervals / bits per second.
The unit for bit rate is bits per second(bps).
The throughput is the measurement of how fast data can pass
through an entity(such as a point or a network).
If we consider this entity as a wall through which bits pass,
throughput is the number of bits that can pass this wall in one
Propagation Speed & Propagation Time
Propagation Speed measures the distance a signal or a bit can travel through a
medium in one second.
The propagation speed of electromagnetic signals depends on the medium and
on the frequency of the signal.
Propagation time measures the time required for a signal (or a bit) to travel
from one point of the transmission medium to another.
The propagation time is calculated by dividing the distance by the propagation
Propagation Time = Distance/Propagation speed
Attenuation means loss of energy.
When a signal simple or complex travels through a medium it
loses some of its energy so that it can overcome the resistance of
That’s why a wire carrying electrical signals gets warm.
This problem is called attenuation.
To compensate for this loss, amplifiers are used to amplify the
Distortion means that a signal changes its form or shape.
Distortion occurs in a composite signal,made of different
Each signal component has its own propagation speed through the
medium and therefore its own delay in arriving at the destination.
The external energy that corrupts a signal.
Several types of noise may corrupt the signal.
Thermal noise is the random motion of electrons in a wire that creates an extra
signal not originally sent by the transmitter.
Induced noise comes from sources such as motors and appliances. These devices
act as sending antenna and the transmission medium acts as a receiving antenna.
Crosstalk is the effect of one wire on the other.
Impulse noise is a spike(a signal with high energy in a very short period of time)
that comes from power lines, lightening and so on.