Upcoming SlideShare
×

# Lecture 07

1,080 views

Published on

Published in: Technology
0 Likes
Statistics
Notes
• Full Name
Comment goes here.

Are you sure you want to Yes No
• Be the first to comment

• Be the first to like this

Views
Total views
1,080
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
30
0
Likes
0
Embeds 0
No embeds

No notes for slide

### 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!!!