Lecture 07
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This document summarizes a lecture on data communications and networking. It discusses different types of signals including digital signals, which are discrete, and analog signals, which are continuous. Periodic signals repeat in a consistent pattern, while aperiodic signals do not. Simple analog signals like sine waves are characterized by amplitude, frequency, and phase. Composite signals contain multiple frequencies. Digital signals have a bit rate and bit interval. The document also covers network terminology, transmission impairments like attenuation and distortion, and noise.
Lecture 07
- 1. Course Instructor : Sehrish Rafiq Department Of Computer Science University Of Peshawar Data Communications & Networking Lecture-07
- 2. Lecture overview Signals Digital signal Analog signal Periodic & Aperiodic signals Some network terminologies Types of analog signals Transmission Impairments
- 3. Duties of Physical Layer
- 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. 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. Digital & Analog Signals Vertical axis: value or strength of a signal Horizontal axis: Passage of time
- 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. 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.
- 10. Periodic Digital Signal & analog signal
- 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. Analog signal
- 13. Types of analog signals Simple Analog signals(sine wave) Composite Signals
- 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. Simple analog signal / sine wave
- 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. 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. 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. 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
- 21. Units of frequency and period
- 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. 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. Time and frequency domains
- 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. 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. Frequency spectrum & Bandwidth
- 28. Digital signal
- 29. Digital signal
- 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. Bit rate and bit interval
- 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. 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. Propagation Time
- 35. Wave length Wave length is the distance a simple signal can travel in one period.
- 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. Attenuation
- 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. 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. Noise
- 42. Thanks!!!