Course Instructor : Sehrish Rafiq
Department Of Computer Science
University Of Peshawar
Data Communications & Networking
L...
Lecture overview
 Signals
 Digital signal
 Analog signal
 Periodic & Aperiodic signals
 Some network terminologies
 ...
Duties of Physical Layer
Signals
 To be transmitted, data must be transformed to electromagnetic
signals.
 Data can be either digital or analog.
...
Digital & Analog Signals
 Analog Signal
 It is a continuous wave form that changes smoothly over time.
 As the wave mov...
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 )
...
Periodic Signal
 A signal is a Periodic signal if it completes a pattern within a
measurable time frame, called a period....
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 ...
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 si...
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 d...
Frequency
 Frequency is the number of cycles/Periods in one second.
 Frequency is the relationship of a signal to time.
...
High & low Frequency
 If the value of a signal changes over a very short span of time, its
frequency is high.
 If it cha...
Period
 Period refers to the amount of time (in seconds) a signal needs to
complete one cycle .
 Period and Frequency ar...
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 somethin...
Time and frequency domains
 The time-domain plot shows changes in signal amplitude
with respect to time(it is an amplitud...
Time and frequency domains
Composite Signals
 Many useful wave forms do not change in a single smooth
curve between a minimum and maximum amplitude....
Frequency spectrum & Bandwidth
 The frequency spectrum of a signal is the collection of all the
component frequencies it ...
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 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)....
Propagation Speed & Propagation Time
 Propagation Speed measures the distance a signal or a bit can travel through a
medi...
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 o...
Attenuation
Distortion
 Distortion means that a signal changes its form or shape.
 Distortion occurs in a composite signal,made of d...
Noise
 The external energy that corrupts a signal.
 Several types of noise may corrupt the signal.
 Thermal noise is th...
Noise
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Lecture 07

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Lecture 07

  1. 1. Course Instructor : Sehrish Rafiq Department Of Computer Science University Of Peshawar Data Communications & Networking Lecture-07
  2. 2. Lecture overview  Signals  Digital signal  Analog signal  Periodic & Aperiodic signals  Some network terminologies  Types of analog signals  Transmission Impairments
  3. 3. Duties of Physical Layer
  4. 4. 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
  5. 5. 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.
  6. 6. Digital & Analog Signals Vertical axis: value or strength of a signal Horizontal axis: Passage of time
  7. 7. 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.
  8. 8. 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.
  9. 9. Periodic Analog Signal Period( T )
  10. 10. Periodic Digital Signal & analog signal
  11. 11. 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.
  12. 12. Analog signal
  13. 13. Types of analog signals  Simple Analog signals(sine wave)  Composite Signals
  14. 14. 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
  15. 15. Simple analog signal / sine wave
  16. 16. 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.
  17. 17. 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.
  18. 18. 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.
  19. 19. 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
  20. 20. Frequency and period
  21. 21. Units of frequency and period
  22. 22. 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.
  23. 23. 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.
  24. 24. Time and frequency domains
  25. 25. 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.
  26. 26. 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.
  27. 27. Frequency spectrum & Bandwidth
  28. 28. Digital signal
  29. 29. Digital signal
  30. 30. 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).
  31. 31. Bit rate and bit interval
  32. 32. 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.
  33. 33. 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
  34. 34. Propagation Time
  35. 35. Wave length  Wave length is the distance a simple signal can travel in one period.
  36. 36. Transmission impairment
  37. 37. 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.
  38. 38. Attenuation
  39. 39. 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.
  40. 40. 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.
  41. 41. Noise
  42. 42. Thanks!!!

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