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Lecture 2

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Lecture 2

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  1. Lecture 2 Fundamentals of Data and Signals

  2. Objectives • In this lecture, we explore the following telecommunications concepts: • Different between data and signal • Four combination of analog and digital applications • Signals (to p4) (to p8) (to p3)

  3. Signals • Properties • Analog • Digital • Conversion between Analog and Digital • Analog data transmitted using analog signals • Digital data transmitted using digital signals • Digital data transmitted using analog signals • Analog data transmitted using digital signals • Where are digital data came from? (to p10) (to p27) (to p32) (to p34) (to p35) (to p48) (to p56)

  4. Data and signal • Data • are entities that convey meaning • Examples: computer files, music on CD, results from a blood gas analysis machine) • Signals • are the electric or electromagnetic encoding of data (telephone conversation, web page download) • Examples: Computer networks and data/voice communication systems transmit signals • Data and signals can be analog or digital • Analog Vs Digital (to p5) (to p2)

  5. Analog vs. Digital • Analog • is a continuous waveform, with examples such as (naturally occurring) music and voice • It is harder to separate noise from an analog signal than it is to separate noise from a digital signal (imagine the following waveform is a symphony with noise embedded) • Digital • is a discrete or non-continuous waveform with examples such as computer 1s and 0s • Noise in digital signal • You can still discern a high voltage from a low voltage • Too much noise – you cannot discern a high voltage from a low voltage (to p6) (to p7) (to p4)

  6. Analog signal (to p5)

  7. Digital Signal (to p5)

  8. Four combinations (to p9) Types of each application

  9. Table 2-1 Four combinations of data and signals We will talk about each of these in today’s lecture (to p2)

  10. Properties of analog signals • Four main properties: • 1. Frequency • 2. Bandwidth • 3. Amplitude • 4. Signal phase (to p11) (to p17) (to p19) (to p24) (to p3)

  11. Frequency • 1. Frequency youtube1, youtube2 , • the electronic signal is commonly diagram as a sine wave (see Figure 5-12) • unit of measurement is the Hertz, hz • human ear can hear between 20 hz to 15,000 hz • telephone voice ranges 300 to 3,000 hz • other feq ranges see Figures 5-13 to 5-15 (to p12) (to p13)

  12. FIGURE 5-12 Sine waves of differing frequencies. (to p11)

  13. FIGURE 5-13 The frequency ranges of some common sounds. (to p14) Other applications of frequency ranges

  14. FIGURE 5-14 The frequency spectrum showing the common names applied to certain frequency ranges. (to p15) More details= views

  15. FIGURE 5-15 A more detailed view of the frequency spectrum relevant to telecommunications. (to p10)

  16. FIGURE 5-18 Analog wave with constant frequency and varying amplitude. (to p19)

  17. Bandwidth • 2. Bandwidth YouTube • different between upper ad lower frequency range • for tel signal is 2700 hg (ie. 3000-300 hz) • voice circuits in tel design for 0 to 4000 hz • 0 to 300 hz, and 3000 to 4000 hz are referred to guard channel/band, which use as a buffer so that no interface between different signals • see Figure 5-17 (to p18) (to p10)

  18. FIGURE 5-17 Bandwidth of a voice channel. (to p10)

  19. Amplitude • 3. Amplitude YouTube • signal for measure loudness of signal • measurement unit is decibel (dB) • if dB is too strong for one line, caused crossed talk • if dB is too weak, caused attenuation. (to p16) (to p20) (to p21) (to p10)

  20. FIGURE 5-19 The relative power of a signal measured in decibels. (to p19)

  21. FIGURE 5-20 A signal loses strength as the distance it travels increases. This loss of strength is called attenuation. (to p22) Alternative example

  22. Note: textbook shows the calculation but we will not pick up in this subject (to p19)

  23. Signal phase • 4. Signal phase • measure the shift of sine ware • Phase changes often occur on common angles, such as 45, 90, 135, etc. • only important to data comm, and not detected by voice comm. (why?) • will discuss more in later chapter (to p24) (to p25) (to p10)

  24. FIGURE 5-21 Example of a phase shift. (to p23)

  25. (to p23)

  26. Digitization A series of bits, which representing characters, can be presented as a form or positive or negative phases in a comm system eg. “1” represent by a positive pulse “0” represent by a negative pulse (to p27)

  27. This alternative representation of data through pulse is called digital signal • Question: • How could we detect signal and represent them in digital format? (to p28)

  28. Representation of digital signals • 3 most common forms of digital signals: • 1. Unipolar • “1” bit is represented by positive voltage and “0” bit by no voltage • 2. Bipolar Non-return-to-zero (NRZ) • “1” bit representing by a positive voltage and “0” bit by a negative voltage • 3. Bipolar Return-to-zero • similar to NRZ except pulses between two different bits are shorter • (see Figure 8-5) (to p29) (to p31)

  29. FIGURE 8-5 Digital signals. (to p30) explanations (to p28)

  30. Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages • Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0 • Fundamental difference exists between NRZ-L and NRZI • With NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1, • With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1 (to p29)

  31. Representation of digital signals(cont). • Usages: • 1) Not popular adopted • 2) & 3) a distinction useful in trouble shooting when problems occur (to p3)

  32. Analog data transmitted using analog signals • In order to transmit analog data, you can modulate the data onto a set of analog signals • Broadcast radio and television are two very common examples of this (to p33)

  33. (to p3)

  34. Digital data transmitted using digital signals • This is same as the way we described the digital properties, ie: • Polar, non-polar, return/none-return-zeor, etc (to p3)

  35. Digital data transmitted using analog signals • Analog has four properties, and we convert the following three important ones: • Three Functional roles: • a) Frequency modulation • b) Amplitude modulation • c) Phase modulation (to p36) (to p40) (to p41) (to p3)

  36. Frequency modulation • a) Frequency modulation • When a 1 bit is sent, it represents a different or certain attributes (amplitude, frequency and phase), and 0 bit represents the change of different attributes • At the end, modem/device will sense the different and generate oscillator (that is wave) • These waves are converted to a digital signal at the end of a modem • special type of freq mod is know as frequency shift key (FSK) (to p37)

  37. An example of modem • Example: Bell type 103 modem • The original modem transmits 0 bits (or spaces) at 1070 Hz and 1 bits at 1270 Hz. The answer modem uses 2025 Hz for spaces and 2225 Hz for marks refer to Figures 8-10 and 8-11 (to p38) (to p39) Frequencies Bandwidth (to p35)

  38. FIGURE 8-10 Frequencies used by a Bell Type 103 modem. (to p37)

  39. FIGURE 8-11 Frequency modulation in a Bell Type 103 modem. (to p37)

  40. B) Amplitude modulation • One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation) (to p35)

  41. C) Phase modulation • is performed by shifting a sine wave 180 degree whenever a digit bit stream changes from 0 to 1 • see Figure 8-13 • generally, it’s known as phase shift keying (PSK) • another type is called differential phase shift keying (DPSK), taken place when the phase is shifted each time an one bit is transmitted; otherwise the phase remains as the same • see Figures 8-14 and 8-15 (to p42) (to p46) (to p44) (to p45)

  42. FIGURE 8-13 Phase shifts. (to p43) Or

  43. (to p41)

  44. FIGURE 8-14 Phase shift keying (PSK). Alternate when hit the different digital code, say from 1 to 0 (to p41)

  45. FIGURE 8-15 Differential phase shift keying (DPSK). Alternate when there is a same code in a sequence (to p41)

  46. Phase modulation • Depend upon number of possible shifts, a binary signal could be represented by a dibits, tribits or quadbits Example: Phase ShiftDibits 0 degree 00 90 degree 01 180 degree 10 270 degree 11 (to p47)

  47. Phase modulation Quadrature amplitude modulation (to p35)

  48. Analog data transmitted using digital signals • To convert analog data into a digital signal, there are two techniques: • Pulse code modulation (the more common) • Delta modulation (to p49) (to p55) (to p3)

  49. Pulse code modulation • The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values • The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution • When the binary values are later converted to an analog signal, a waveform similar to the original results (to p50) (to p51)

  50. (to p49)