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

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

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

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

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’

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

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

Seconds____Hertz
Milliseconds____Kilohertz
Microseconds____Megahertz
Nanoseconds____Gigahertz
Picoseconds____Terahertz

Section 4.1, 4.2, 4.3“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan