UGANDA CHRISTIAN UNIVERSITY FACULTY OF SCIENCE AND TECHNOLOGY

1. H3 Computer Networks BSCS 2 Introduction to Data Transmissions Lecturer: Rebecca Asiimwe Phone Number:+256 712-997- 544 Email: rasiimwe@techology.ucu.ac.ug UGANDA CHRISTIAN UNIVERSITYFACULTY OF SCIENCE AND TECHNOLOGY Department of Information Technology

2. Lecture Milestones • 3. Introduction to Data Transmissions: • Concepts and Terminology • Frequency, Spectrum and Bandwidth •  Analog and digital transmissions • Analog and Digital data, Signals and Transmission •  Transmission impairments, • Attenuation • Delay Distortion • Noise •  Channel capacity • Nyquist Bandwidth • Shannon Capacity Formula BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

3. Introduction • Data transmission occurs between transmitter and receiver over some transmission medium. The data to be transmitted (can be voice, data, image or video) can all be represented by electromagnetic signals. • Depending on the transmission medium and the communications environment, either analog or digital signals can be used to convey information. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

4. Introduction • As we discuss this unit, we shall view signals as made up of a number of frequencies and also look at a key parameter that characterizes the signal-Bandwidth, which is the width of the range of frequencies that comprises the signal. • We shall also explore problems faced in designing a communications facility-Impairments. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

5. Note: • The successful transmission of data depends primarily on two factors: • The quality of the signal being transmitted and • The characteristics of the transmission medium. • The objective of this lecture and the next is to provide us with an intuitive feeling for the nature of these two factors. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

6. Concepts and Terminology • Transmitter • Receiver • Medium • Guided medium (waves are guided along a physical path) • e.g. twisted pair, optical fiber • Unguided medium (wireless) • e.g. air, water, vacuum BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

7. Concepts and Terminology • Direct link • Used to refer to a transmission path between two devices in which signals propagate directly from transmitter to receiver with no intermediate devices other than amplifiers or repeaters used to increase signal strength-this term can apply to both guided and unguided media • Point-to-point • A guided transmission medium is point-to-point if it provides a Direct link between two devices and those are the only two devices sharing the medium • Multi-point • More than two devices share the same medium or link BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

8. Concepts and Terminology • Simplex • Signals transmitted in only One direction (one station is the transmitter and the other the receiver) e.g. Television, Keyboard • Half duplex • Both stations can transmit (Either direction), but only one way at a time e.g. police radio • Full duplex • Both stations may transmit simultaneously (Both directions at the same time) e.g. telephone BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

9. Electromagnetic Signals • Employed to transmit voice, data, images, video, etc. • Can be analog or digital representation to convey information • Major characteristics: • Bandwidth – width of the range of frequencies – the greater the bandwidth, the greater its data-carrying capacity • Potential for error – digital data less prone to errors • Acceptable error rate BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

10. Frequency, Spectrum and Bandwidth • A signal for transmission is generated by the transmitter and transmitted over a medium. • The signal is a function of time and can also be expressed as a function of frequency, that is, the signal consists of components of different frequencies • The frequency domain view of a signal is more important to an understanding of data transmission than a time domain view, but both views will be introduced. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

11. Frequency, Spectrum and Bandwidth • Time domain concepts • Analog signal • Signal intensity varies in a smooth fashion over time (no breaks or discontinuities in the signal ) • Digital signal • Signal intensity maintains a constant level for some period of time and then changes to another constant level BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

12. Analog & Digital Signals BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

13. Periodic signal • Pattern repeated over time • Sine wave for analog signal • Square wave for digital signal Mathematically a signal s(t) is defined to be periodic if and only if s(t+T) = s(t) -∞<t<+∞ BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

14. Where the constant T is the period of the signal (T is the smallest value that satisfies the equation). • Otherwise the signal is aperiodic. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

15. PeriodicSignals BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

16. Sine Wave • The Sine Wave is the fundamental periodic signal. • A general sine wave can be represented by three parameters: 1. Peak Amplitude (A) • Maximum value or strength of signal over time • Measured in volts BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

17. 2. Frequency (f) • Rate of change of signal / Rate at which the signal repeats in (cycles per second) • Hertz (Hz) or cycles per second • Period = amount of time it takes for one repetition (T) • Therefore T=1/f 3. Phase () • Measure of the relative position in time within a single period of a signal. • The general sine wave can be written as: • s(t) = A sin(2 π ft + ϕ) BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

18. Varying Sine Wavess(t) = A sin(2ft +) BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

19. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

20. Wavelength (λ) • Distance occupied by one cycle • Assume that a signal is traveling with a velocity v, then the wavelength is related to the period as follows. • λ = ν T • Equivalentlyλf = ν • ν =c, the speed of light in free space which is approximately 3 x 108 m/s. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

21. Frequency Domain Concepts • An Electromagnetic signal is usually made up of many frequencies • Components of the signal are sine waves • Any signal is made up of component sine waves of various frequencies in which each component is a sinusoid. • For each signal there is a time domain function s(t) that specifies the amplitude of the signal at each instant in time. Similarly there is a frequency domain function s(f) that specifies the peak amplitude of the constituent frequencies of the signal BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

22. Addition of FrequencyComponents(T=1/f) BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

23. Spectrum & Bandwidth • Spectrum • range of frequencies contained in signal • Absolute bandwidth • width of spectrum • Effective bandwidth • Often just bandwidth • Narrow band of frequencies containing most of the energy (Band within which most of the signal energy is concentrated) BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

24. Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried • Although a given wave form may contain frequencies over a very broad range, any transmission system will be able to accommodate only a limited band of frequencies. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

25. Analog and Digital Data Transmission • Data • Entities that convey meaning or information • Signals • Electric or electromagnetic representations of data • Signaling • Physical propagation of the signal along a suitable medium • Transmission • Communication of data by propagation and processing of signals BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

26. Analog and Digital Signals • Means by which data are propagated • Analog • Continuously variable e.g. audio (voice / music) and video, most data collected by sensors like temperature and pressure • Various media • wire, fiber optic, space • Speech bandwidth 100Hz to 7kHz • Telephone bandwidth 300Hz to 3400Hz • Video bandwidth 4MHz • Digital • Binary -text BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

27. Advantages & Disadvantages of Digital • +Cheap • +Faster • +Less susceptible to noise interference • -Attenuation (suffers from attenuation more than analog signals do) • The figure in the next slide shows the effect of attenuation of digital signals. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

28. Attenuation of Digital Signals BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

29. Conversion of Voice Input into Analog Signal BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

30. All sound frequencies whose amplitude is measured in terms of loudness are converted into electromagnetic frequencies whose amplitude is measured in volts. • The telephone handset contains a simple mechanism for making such a conversion. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

31. Binary Digital Data • From computer terminals etc. • Two dc components • The higher the data rate of the signal, the greater is its required effective bandwidth. In relation to this; the greater the bandwidth of the Transmission system, the higher the data rate that can be transmitted over that system. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

32. Conversion of PC Input to Digital Signal BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

33. Data and Signals • Usually use digital signals for digital data and analog signals for analog data • Can use analog signal to carry digital data • Modem (Modulator/Demodulator) • Most common modems represent digital data in the voice spectrum and hence allow those data to be propagated over ordinary voice telephone lines. • Can use digital signal to carry analog data • Compact Disc audio • Codec (Coder-Decoder) BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

34. Analog Signals Carrying Analog and Digital Data BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

35. Digital Signals Carrying Analog and Digital Data BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

36. Analog Transmission • Analog signal transmitted without regard to content • May be analog or digital data • Attenuated over distance • Use amplifiers to boost signal • Amplifying also amplifies noise BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

37. Digital Transmission • Concerned with content • Integrity endangered by noise, attenuation etc. • Repeaters used • Repeater receives signal • Extracts bit pattern /recovers pattern • Retransmits new signal • Attenuation is overcome • Noise is not amplified BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

38. Advantages of Digital Transmission • Preferred method of data transmission is Digital because of these important reasons • Digital technology • The advent of large-scale integration (LSI) and very large-scale integration (VLSI) technology has caused a continuing drop in the cost and size of digital circuitry. Analog equipment has not shown a similar drop. • Data integrity • With the use of repeaters rather than amplifiers, the effects of noise and other signal impairments are not cumulative. Thus it is possible to transmit data longer distances and over lower quality lines by digital means while maintaining the integrity of the data. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

39. Capacity utilization • It has become economical to build high bandwidth links including satellite channels and optical fiber • High degree of multiplexing is needed to utilize such a capacity effectively and this is more easily and cheaply achieved with digital (time division) rather than analog (frequency division)-to be looked at in subsequent sections • Security & Privacy • Encryption techniques can be readily applied to digital data and to analog data that have been digitized • Integration • By treating both analog and digital data digitally, all signals have the same form and can be treated similarly. Thus economies of scale and convenience can be achieved by integrating voice, video, and digital data. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

40. Transmission Impairments • Signal received may differ from signal transmitted due to various transmission impairments. • Analog - impairments degrade signal quality • Digital - bit errors may be introduced; a binary bit 1 is transformed into a binary zero and vice versa. • Most significant impairments are: • Attenuation and attenuation distortion • Delay distortion • Noise BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

41. Attenuation • The strength of the signal falls off with distance over any transmission medium • Depends on medium • Attenuation introduces three considerations for the transmission engineer. • The received signal strength: • must be sufficient enough to be detected by the receiver • must be sufficiently higher than noise to be received without error • Attenuation is an increasing function of frequency BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

42. Attenuation • The first two considerations /problems are dealt with by attention to signal strength and the use of amplifiers or repeaters. • Beyond a certain distance, the attenuation becomes unacceptably great and repeaters and amplifiers are used to boost the signal at regular intervals. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

43. Attenuation • The third problem is noticeable for analog signals. Because attenuation varies as a function of frequency, the received signal is distorted, reducing intelligibility. • To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. This is commonly done for voice-grade telephone lines using loading coils that change the electronic properties of the line; the result is to smooth out attenuation effects. Another approach is to use amplifiers. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

44. Delay Distortion • Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency. • For a band limited signal, the velocity tends to be highest near the center frequency and fall off toward the edges of the band. Thus various frequency components of the signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

45. This effect is referred to as delay distortion because the received signal is distorted due to varying delays experienced at its constituent frequencies. • Critical for digital data • Consider that a sequence of bits is being transmitted, using either analog or digital signals. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

46. Because of delay distortion, some of the signal components of one bit position will spill over into other bit positions causing inter-symbol interference which is a major limitation to maximum bit rate over a transmission channel. • Equalization can be used to deal with this impairment. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

47. Noise • For any data transmission event, the received signal will consist of the transmitted signal modified by the various distortions imposed by the transmission system plus additional unwanted signals that are inserted somewhere between transmission and reception. • The latter unwanted signals are referred to as noise. It is noise that is the major limiting factor in communications system performance. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

48. Noise may be divided into four categories. • Thermal noise • Intermodulation Noise • Crosstalk • Impulse Noise BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

49. Noise • Thermal • Due to thermal agitation of electrons • It is present in all electronic devices and transmission media and is a function of temperature • It is Uniformly distributed across the bandwidths used in communication systems and hence is often referred to as white noise. • Cannot be eliminated since the signals received by satellite earth stations are weak (it is particularly significant for satellite communication) so places an upper bound on communication systems performance. BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug

50. The amount of thermal noise to be found in a bandwidth of 1Hz in any device is • No = kT (W/Hz) • Where: • No = noise power density in watts per 1Hz of bandwidth • k = Boltzmann’s constant = 1.38x10-23 J/K • T = temperature, in kelvins (absolute temperature), where the symbol K is used to represent 1 kelvin BSCS 2-Comp.Networks rasiimwe@technology.ucu.ac.ug