  # CS601 - Data Communication - Lecture Handout 11

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# Signals

## Need For Signals

• One of the major concerns of Physical layer is moving information in the form of electromagnetic signals across a TX medium
• Information can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user (human or m/c)
• Generally, the info usable to a person or application is not in a form that can be transmitted over a network
• For Example, you cannot roll up a photograph, insert it into the wire and transmit it across the city
• You can transmit however an encoded description of the photograph
• The binary digits must be converted into a form that TX. Medium can accept
• TX. Media work 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

• 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

• Data can be Analog or Digital
• Example of ANALOG Data is Human voice
• When somebody speaks, a continuous wave is created in the air.
• This can be captured by a Microphone and converted to an Analog Signal
• An example of DIGITAL data is Data stored in the memory of a computer in the form of 1s and 0s. It is usually converted to a digital signal when it is transferred from one position to the other inside or outside the computer
• Signals can be Analog or Digital

## ANALOG Signal

• It is a continuous waveform 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 1s and 0s
• The transition of a digital signal from value to value is instantaneous like a light being switched ON and OFF • We illustrate signals usually by plotting them on a pair of perpendicular axis
• Vertical axis represent the value or the strength of the signal
• Horizontal axes represent the passage of time
• The curve representing the Analog signal is smooth and continuous, passing through an infinite number
• The vertical lines of the digital signal shows the sudden jump the signal makes from value to value. The flat highs and the lows represent that those values are fixed
• In short, Analog signal varies continuously w.r.t Time whereas Digital signal varies instantaneous ## 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
• The completion of one full pattern is called a CYCLE
• Period: Time required (in Seconds) to complete one full cycle, represented by ‘T’ ## Aperiodic Signals

• An Aperiodic or Non-Periodic signal is the one that changes constantly without exhibiting a pattern or cycle that repeats over time • Fourier Transform
It has been proved by a technique called FOURIER TRANSFORM that any Aperiodic signal can be decomposed into an infinite number of Periodic Signals

## ANALOG SIGNALS

• Analog signals can be classified as Simple or Composite
• Simple Analog Signal(Sine Wave)
• Cannot be decomposed into simpler signal
• Composite Analog Signal
• Composed of multiple sine waves

## Sine Waves

• Sine Waves are the most fundamental form of Periodic Analog Signals
• The curve oscillates over the course of a cycle smoothly and consistently
• Each cycle consists of a single arc above the time axis followed by a single arc below it
• Sine Waves can be fully described by three characteristics:
• Amplitude
• Period/Frequency
• Phase

• Amplitude
• 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 the sine wave is equal to the highest value it reaches on the vertical axis
• Amplitude measured in Volts, Amperes or Watts ## Period & Frequency

• Period: Amount of time ( in seconds) a signal need to complete one cycle
• Frequency: Number of cycles completed in one second
• Unit of Period: Period is expressed in seconds • Communication industry uses 5 units to measure period
• Frequency is measured in hertz, There are 5 units used in Hertz

Seconds____Hertz
Milliseconds____Kilohertz
Microseconds____Megahertz
Nanoseconds____Gigahertz
Picoseconds____Terahertz

## Summary

• Signals
• Analog and Digital
• Analog and Digital Data & Signals
• Periodic & Aperiodic Signals
• Sine Waves and its Characteristics