Lecture 07

Course Instructor : Sehrish Rafiq

Department Of Computer Science

University Of Peshawar

Data Communications & NetworkingLecture-07

Lecture overview Signals Digital signal Analog signal Periodic & Aperiodic signals Some network terminologies Types of analog signals Transmission Impairments

Duties of Physical Layer

Signals To be transmitted, data must be transformed to

electromagnetic signals.

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 Signal Analog signal

Digital & Analog Signals

Analog Signal 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.

Digital Signal A digital signal is discrete. It can have only a limited number of defined values,

often as simple as 1 and 0. 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:

Periodic Aperiodic(non-periodic )

Note:-In data communication, we commonly use periodic analog

signals

and aperiodic digital signals.

Periodic Signal 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.

Periodic Analog Signal

Period( T )

Periodic Digital Signal & analog signal

Aperiodic Signals An Aperiodic signal changes without exhibiting

a pattern or cycle that repeats over time. Aperiodic signal can be decomposed in to

infinite number of periodic signals.

Analog signal

Types of analog signals

Simple Analog signals(sine wave) Composite Signals

Simple analog signal / sine wave The sine wave is the most fundamental form of

a periodic analog signal. 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:

Amplitude Period Or frequency Phase

Simple analog signal / sine wave

Amplitude The amplitude of a signal is the value of the

signal at any point on the wave.

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 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 Period refers to the amount of time (in

seconds) a signal needs to complete one cycle .

Period and Frequency are inverse of each other.

T=1/f f=1/T

Frequency and period

Units of frequency and period

Phase The term phase describes the position of the

waveform relative to time zero. 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 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-domain.

A frequency domain-plot shows the relationship between amplitude and frequency.

Time and frequency domains

Composite Signals 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 domain graph.

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 support.

Frequency spectrum & Bandwidth

Digital signal

Digital 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).

Bit rate and bit interval

Through put 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 second.

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 speed.

Propagation Time = Distance/Propagation speed

Propagation Time

Wave length Wave length is the distance a simple signal

can travel in one period.

Transmission impairment

Attenuation 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 the medium.

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 signal.

Attenuation

Distortion Distortion means that a signal changes its form

or shape. Distortion occurs in a composite signal,made of

different frequencies. Each signal component has its own

propagation speed through the medium and therefore its own delay in arriving at the destination.

Noise

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.

Noise

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