Data and Computer Communications

1. Data and Computer Communications Chapter 3 – Data Transmission Eighth Edition by William Stallings Lecture slides by Lawrie Brown

2. Data Transmission • The successful transmission of data depends on two factors: • the quality of the signal being transmitted • the characteristics of the transmission medium

3. TransmissionTerminology-1 • data transmission occurs between a transmitter & receiver via some medium • guided medium • eg. twisted pair, coaxial cable, optical fiber • unguided / wireless medium • eg. air, water, vacuum

4. TransmissionTerminology-2 • direct link • no intermediate devices (other than amplifiers or repeaters) • apply to both guided and unguided media • point-to-point • direct link • only 2 devices share link • multi-point • more than two devices share the link

5. TransmissionTerminology-3 • simplex • one direction • eg. television • half duplex • either direction, but only one way at a time • eg. police radio • full duplex • both directions at the same time • eg. telephone

6. Electromagnetic Signal • Function of time • Time domain • Can also be expressed as a function of frequency • Signal consists of components of different frequencies • Frequency domain • For data transmission • Frequency domain >> Time domain

7. Frequency, Spectrum and Bandwidth • time domain concepts • analog signal • various in a smooth way over time • digital signal • maintains a constant level then changes to another constant level

8. Analogue & Digital Signals

9. Frequency, Spectrum and Bandwidth • time domain concepts • periodic signal • pattern repeated over time • s (t +T ) = s (t ) -¥< t < +¥ • Where T is the period of the signal • aperiodic signal • pattern not repeated over time

10. Periodic Signals

11. Sine Wave • peak amplitude (A) • maximum strength of signal • volts • frequency (f) • rate of change of signal • Hertz (Hz) or cycles per second • period = time for one repetition (T) • T = 1/f • phase () • Measure of the relative position in time within a single period of a signal

12. Sine Wave • General sine wave • s (t ) = A sin(2ft + ) • Note • 2 radians = 360° = 1 period Amplitude Frequency Phase

13. Varying Sine Wavess(t) = A sin(2ft +)

14. Wavelength () • is distance occupied by one cycle • between two points of corresponding phase in two consecutive cycles • assuming signal velocity v have  = vT • or equivalently f = v • especially when v=c • c = 3*108 ms-1 (speed of light in free space)

15. Frequency Domain Concepts • signal are made up of many frequencies • components are sine waves • Fourier analysis can shown that any signal is made up of component sine waves • where A0 is the DC component, An,Bn are the harmonics and f0 is the fundamental frequency. • can plot frequency domain functions

16. Frequency Domain Concepts • In practice, an electromagnetic signal will be made up of many frequencies • Note: • s(t) is composed of sine waves of frequencies fand 3f • Second frequency (3f) is an integer multiple of the first (f) • Total period of s(t) is equal to the period of the fundamental frequency

17. Frequency Domain Concepts + =

18. FrequencyDomainRepresentations • freq domain func of Fig 3.4c • freq domain func of single square pulse

19. Spectrum & Bandwidth • spectrum • range of frequencies contained in signal • Fig 3.4c, it extends from f to 3f in the • absolute bandwidth • width of spectrum (2f in Fig 3.4c) • effective bandwidth • often just bandwidth • narrow band of frequencies containing most energy • DC Component • component of zero frequency

20. Spectrum & Bandwidth • A bandwidth of a signal is a measure of how fast the signal varies. • A bandwidth of a channel is defined as the range of frequencies that is passed by a channel. • The bandwidth of a channel W measures the width of the window of frequencies that are passed by the channel.

21. Data Rate and Bandwidth • any transmission system has a limited band of frequencies • this limits the data rate that can be carried • square have infinite components and hence bandwidth • but most energy in first few components • limited bandwidth increases distortion • have a direct relationship between data rate & bandwidth • Explanation next !!

22. Data Rate and Bandwidth • Positive pulse represent binary 0 • Negative pulse represent binary 1. • represent the binary stream 0101. . . . • The duration of each pulse is 1/(2f ) • thus the data rate is 2f bits per second (bps). • What are the frequency components of this signal

23. Data Rate and Bandwidth

24. Data Rate and Bandwidth 1st, 3rd , and 5th,harmonics 1st, 3rd ,5th, and 7th harmonics Infinite number of harmonics

25. Data Rate and Bandwidth • it can be shown that the frequency components of the square wave with amplitudes A and –A can be expressed as follows: • Thus, this waveform has an infinite number of frequency components • hence an infinite bandwidth. • the kth frequency component, kf, is only 1/k, so most of the energy in this waveform is in the first few frequency components.

26. Data Rate and Bandwidth • transmit a sequence of alternating 1s and 0s as the square wave • What data rate can be achieved? • We look at three cases.

27. Data Rate and Bandwidth • Case I • digital transmission system (4 MHz) • approximate our square wave with three frequency components (f,3f,5f) • f = 1 MHz, the BW of the signal: • 5*106 – 1*106 = 4 MHz • T = 1/f = 1us => one bit occurs every 0.5 us • Data rate = 2*106 = 2 Mbps is achieved for 4 MHz BW

28. Data Rate and Bandwidth • Case II • approximate our square wave with three frequency components (f,3f,5f) • digital transmission system (8 MHz), f = 2 MHz • BW of the signal = 10*106 – 2*106 = 8 MHz • T = 1/f = 0.5us => one bit occurs every 0.25 us • Data rate = 4*106 = 4 Mbps is achieved for 8 MHz BW • doubling the bandwidth, we double the potential data rate.

29. Data Rate and Bandwidth • Case III • approximate our square wave with two frequency components (f,3f) • f = 2 MHz => bandwidth of 4 MHz • BW of the signal = • 6*106 – 2*106 = 4 MHz • T = 1/f = 0.5us => one bit occurs every 0.25 us • Data rate = 4*106 = 4Mbps is achieved for 4 MHz BW

30. Data Rate and Bandwidth • To summarize, • Case I: Bandwidth = 4 MHz; data rate = 2 Mbps • Case II: Bandwidth = 8 MHz; data rate = 4 Mbps • Case III: Bandwidth = 4 MHz; data rate = 4 Mbps • given bandwidth can support various data rates depending on the ability of the receiver to discern the difference between 0 and 1

31. Data Rate and Bandwidth • The greater the bandwidth, the higher the information-carrying capacity • Conclusions • Any digital waveform will have infinite bandwidth • BUT the transmission system will limit the bandwidth that can be transmitted • AND, for any given medium, the greater the bandwidth transmitted, the greater the cost • HOWEVER, limiting the bandwidth creates distortions

32. Data Rate and Bandwidth data rate of 2000 bits per second

33. Data Rate and Bandwidth With a bandwidth of 2500 Hz, or even 1700 Hz, the representation is quite good If the data rate of the digital signal is W bps, then a very good representation can be achieved with a bandwidth of 2W Hz

34. Analog and Digital Data Transmission • data • entities that convey meaning • signals & signalling • electric or electromagnetic representations of data, physically propagates along medium • transmission • communication of data by propagation and processing of signals

35. Examples of Analog and Digital Data • Analog • Video • the video image can be thought of as a time-varying analog signal. • Audio • Digital • Text • Integers

36. Acoustic Spectrum (Analog)

37. Video Interlaced Scanning • USA - 483 lines per frame, at 30 frames per sec

38. Analog Signals • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Examples of media: • Copper wire media • Twisted pair and coaxial cable • Fiber optic cable • Atmosphere or space propagation • Analog signals can propagate analog and digital data

39. Digital Signals • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation • Digital signals can propagate analog and digital data

40. Audio Signals (BW approximation) • freq range 20Hz-20kHz (speech 100Hz-7kHz) • easily converted into electromagnetic signals • varying volume converted to varying voltage • can limit frequency range for voice channel to 300-3400Hz

41. Video Signals (BW approximation) • USA - 483 lines per frame, at frames per sec • have 525 lines but 42 lost during vertical retrace • 525 lines x 30 scans = 15750 lines per sec • 63.5s per line • 11s for retrace, so 52.5 s per video line • max frequency if line alternates black (higher) and white (lower) voltage level. • subjective resolution is about 70% of 525-42, or about 338 lines • horizontal resolution is about 450 lines giving 225 cycles of wave in 52.5 s • max frequency of 4.2MHz

42. Digital Data (BW approximation) • as generated by computers etc. • has two dc components • bandwidth depends on data rate

43. Analog Signals

44. Digital Signals

45. Data and Signal Combinations • Digital data, digital signal • Equipment for encoding is less expensive than digital-to-analog equipment • Analog data, digital signal • Conversion permits use of modern digital transmission and switching equipment • Digital data, analog signal • Some transmission media will only propagate analog signals • Examples include optical fiber and satellite • Analog data, analog signal • Analog data easily converted to analog signal

46. Analog Transmission • Transmit analog signals without regard to content • Attenuation limits length of transmission link • Cascaded amplifiers boost signal’s energy for longer distances but cause distortion • Analog data can tolerate distortion • Introduces errors in digital data

47. Digital Transmission • Concerned with the content of the signal • Attenuation endangers integrity of data • Digital Signal • Repeaters achieve greater distance • Repeaters recover the signal and retransmit • Analog signal carrying digital data • Retransmission device recovers the digital data from analog signal • Generates new, clean analog signal

48. Advantages of Digital Transmission • Digital technology: – Low cost LSI/VLSI technology. • Data integrity: – Longer distances over lower quality lines. • Capacity utilization: – High bandwidth links economical. – High degree of multiplexing easier with digital techniques (time-division). • Security & Privacy: – Encryption is easier with digital symbols (done with mathematic operations). • Integration: – All signals are essentially digital. – Can treat analog and digital data similarly.

49. Advantages & Disadvantages of Digital Signals • cheaper • less susceptible to noise • but greater attenuation • digital now preferred choice

50. Transmission Impairments • Signal received may differ from signal transmitted. • Analog - degradation of signal quality. • Digital - bit errors. • Most significant impairments caused by – Attenuation and attenuation distortion. – Delay distortion. – Noise.