Powerpoint Templates Computer Communication and Networks Lecture # 08.

Powerpoint Templates

Computer Communication and Networks

Lecture # 08


Signals

Why We Need For Signals

Ϟ One of the major concerns of data communication is moving information in the form of electromagnetic signals across medium

Ϟ Information can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user


Signals

Why We Need For Signals

Ϟ For Example, you cannot roll up a photograph, insert it into the wire and transmit it across the city.

Ϟ You can transmit however an encoded description of the photograph.

Ϟ The binary digits must be converted into a form that transmission medium can accept.

Ϟ This is done by conducting energy along a physical path, so the data stream of 1s and 0s must be turned into energy in the form of EM signals


Analog and Digital

Both data and signals that represent them, can take either analog or digital form

ϞWhat is Analog ?

ϞWhat is Digital ?

Analog

Analog: Refers to something that is continuous in time"Continuous" A set of specific points of data and all possible points b/w them

Digital Digital: Refers to something that is discrete"Discrete" A set of specific points of data with no points in between


Analog and Digital Data

Data can be Analog or Digital

Analog Data

ϞExample of analog data is human voice, when somebody speaks, a continuous wave is created in the air.

ϞThis can be captured by a microphone and converted to an analog signal

Digital Data

ϞExample of digital data is data stored in the memory of a computer in the form of 1s and 0s.

ϞIt is usually converted to a digital signal when it is transferred from one position to the other inside or outside the computer


Analog and Digital Signals

Signals can be Analog or DigitalSignalDetectable transmitted energy that can be used to carry information

Analog Signal

ϞIt is a continuous waveform that changes smoothly over time.

ϞAs the wave moves from value ‘ A’ to value ‘B’, it passes through and includes an infinite number of values along its path

Digital Signal

ϞA digital signal is discrete. It can have only a limited number of defined values, often as simple as 1s and 0s.

ϞThe transition of a digital signal from value to value is instantaneous like a light being switched ON and OFF


Periodic and Aperiodic Signals

Signals (Analog or Digital) can be periodic or aperiodic Periodic Signals

A signal is called periodic if it completes a pattern within a measurable time frame called a period and then repeats that pattern over identical subsequent periods


Aperiodic Signals

An aperiodic or non-periodic signal is the one that changes constantly without exhibiting a pattern or cycle that repeats over time


Characteristics Of Signals

Signals can be described by three characteristics:

1. Amplitude2. Period/Frequency3. Phase

Amplitude ϞAmplitude of a signal is the value of the signal at any point on the wave.

ϞIt is equal to the vertical distance from a given point on the wave form to the horizontal axis.

ϞThe maximum amplitude of wave is equal to the highest value it reaches on the vertical axis.

ϞAmplitude measured in Volts, Amperes or Watts


Characteristics Of Signals

Period & Frequency ϞPeriod: Amount of time (in seconds) a signal need to complete one cycle.ϞFrequency: Number of cycles completed in one second

ϞFrequency is measured in hertz

Phase

ϞPhase describes the position of the waveform relative to time zero.

ϞIf 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.

ϞPhase is measured in Degrees or Radians.


Analog/Digital Conversions

Ϟ Analog data are a function of time and occupy a limited frequency spectrum; such data can be represented by an electromagnetic signal occupying the same spectrum.

Ϟ Digital data can be represented by digital signals, with a different voltage level for each of the two binary digits

But these are not the only possibilities

Ϟ Digital data can also be represented by analog signals by use of a modem (modulator/demodulator).

Ϟ Similarly, analog data can be represented by digital signals. The device that performs this function for voice data is a codec (coder-decoder)



•Data stored in the computer is in the form of 0’s and 1’s. To be carried

from one place to the other, data is usually converted to digital signals.

•This is called “Digital-to-Digital Conversion” or “Encoding digital data

into digital signals”

•Sometimes we need to convert analog data to the digital signal, for

example, conversion of telephone conversation to digital signal for a

no. of different reasons.

•This is called “Analog-to-Digital Conversion” or “Digitizing an Analog

Signal”



• We might want to send a digital signal coming out of computer

through a medium designed for analog signals, for example, to send

data from one place to the other using a telephone line.

• This is called “Digital-to-Analog Conversion” or “Modulating a digital

Signal”

• Often an analog signal is sent over long distances using analog

media, for example, voice or music from a radio station which is an

analog signal is transmitted through the air, however the frequency

of voice or music is not, suitable for this kind of transmission.

• The signal should be carried by a higher frequency signal. This is

called “Analog-to-Analog Conversion” or “Modulating an analog

Signal”


Data can be Data can be analoganalog or or digitaldigital. The term . The term analog dataanalog data refers to refers to information that is continuous; information that is continuous; digital datadigital data refers to information refers to information that has discrete states. Analog data take on continuous values. that has discrete states. Analog data take on continuous values. Digital data take on discrete values.Digital data take on discrete values.

Analog and Digital DataAnalog and Digital SignalsPeriodic and Nonperiodic Signals

Topics discussed in this section:Topics discussed in this section:

Signal:To be transmitted, data must be transformed to

electromagnetic signals.

In data communications, we commonly use periodic analog signals and nonperiodic

digital signals.


PERIODIC ANALOG SIGNALSPERIODIC ANALOG SIGNALS

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

Sine WaveWavelengthTime and Frequency DomainComposite SignalsBandwidth



Figure A sine wave


Figure Two signals with the same phase and frequency, but different amplitudes


Figure Two signals with the same amplitude and phase, but different frequencies


The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows:

Ms: 10 3 milliseconds

Example -1


Frequency is the rate of change with respect to time.

Change in a short span of timemeans high frequency.

Change over a long span of time means low frequency.

Frequency and period are the inverse of each other.


If a signal does not change at all, its frequency is zero.

If a signal changes instantaneously, its frequency is infinite.

Phase describes the position of the waveform relative to time 0.


Figure Three sine waves with the same amplitude and frequency, but different phases


Figure Wavelength and period


Figure The time-domain and frequency-domain plots of a sine wave


A complete sine wave in the time domain can be represented by one single spike in

the frequency domain.


Figure 9 shows a periodic composite signal with frequency f. This type of signal is not typical of those found in data communications. We can consider it to be three alarm systems, each with a different frequency. The analysis of this signal can give us a good understanding of how to decompose signals.

Example-2

Figure -9 A composite periodic signal


Figure Decomposition of a composite periodic signal in the time and frequency domains


Figure The time and frequency domains of a nonperiodic signal


The bandwidth of a composite signal is the difference between the

highest and the lowest frequencies contained in that signal.


Figure The bandwidth of periodic and nonperiodic composite signals


If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then

Example-3

The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz.


Figure The bandwidth for Example-4


DIGITAL SIGNALSDIGITAL SIGNALS

In addition to being represented by an analog signal, information can also be In addition to being represented by an analog signal, information can also be represented by a represented by a digital signaldigital signal. For example, a 1 can be encoded as a positive . For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.this case, we can send more than 1 bit for each level.

Bit RateBit LengthDigital Signal as a Composite Analog SignalApplication Layer



Figure Two digital signals: one with two signal levels and the other with four signal levels


Figure The time and frequency domains of periodic and nonperiodic digital signals


Figure Baseband transmission

A digital signal is a composite analog signal with an infinite bandwidth.


Figure Bandwidths of two low-pass channels


Figure Baseband transmission using a dedicated medium

Powerpoint Templates Computer Communication and Networks Lecture # 08.

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