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Chapter 4

Chapter 4. Digital Transmission. 4.1 Line Coding. Some Characteristics Line Coding Schemes Some Other Schemes. Services. E.g. in the tel. systems electric signals lose their strength over the metallic wire, and used amplifiers create distortion and phase changes.

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Chapter 4

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  1. Chapter 4 DigitalTransmission expanded by Jozef Goetz

  2. 4.1 Line Coding Some Characteristics Line Coding Schemes Some Other Schemes expanded by Jozef Goetz

  3. Services expanded by Jozef Goetz

  4. E.g. in the tel. systems electric signals lose their strength over the • metallic wire, and used amplifiers create distortion and phase changes. • In the digital signal it is not easy to distort a 0 to 1 or a 1 to 0. expanded by Jozef Goetz

  5. Channel Capacity • Channel capacity = max data rate • Data rate = rate of data in bps • Bandwidth - of transmitted signal in Hz • Noise - averaged over the transmission path • Error rate - rate at which errors occur • 0 sent and 1 received or vice versa expanded by Jozef Goetz

  6. expanded by Jozef Goetz

  7. Figure 4.1Line coding Line coding – process of converting, data, a sequence of bits, to a digital signal expanded by Jozef Goetz

  8. DEFINITIONS Data- entities that convey meaning, or information. Signals - are electric or electromagneticrepresentations of data. Transmission - the communication of data by the propagation and processing of signals. expanded by Jozef Goetz

  9. INTRODUCTION Both analog and digitalinformation can be encoded as either analog or digitalsignals. The particular encoding that is chosen depends on [1]the specificrequirements to be met, and [2]the media and communicationsfacilitiesavailable. • Digital Data, Digital Signals (DD) • Digital Data, Analog Signals (DA) • Analog Data, Digital Signals (AD) • Analog Data, Analog Signals (AA) expanded by Jozef Goetz

  10. DIGITAL DATA, DIGITAL SIGNALS A digital data is a sequence of discrete, discontinuous voltage pulses. Each pulse is a signalelement. Binary data are transmitted by encoding each data bit into signalelements. In the simplest case, there is one-to-onecorrespondence between bits and signalelements. expanded by Jozef Goetz

  11. DIGITAL DATA, DIGITAL SIGNALS The simplest form of digital encoding of digital data is to assign one voltage level to binary 1and another to binary 0. • More complexencodingschemes are used to improve performance by • altering the spectrum of the signal and • providing error detection and • synchronization capability. expanded by Jozef Goetz

  12. DIGITAL DATA, DIGITAL SIGNALS Encodingscheme is the mapping from databits to signal elements. There are many ways. If the signal elements all have the samealgebraicsign (that is, all positive or all negative) then the signal is UNIPOLAR. In the BIPOLAR signal, one logic level is represented by a positive voltage level, and the other by a negative voltage. expanded by Jozef Goetz

  13. Figure 4.2Signal level versus data level signal levelsrefers to - # of valuesallowed in a particular signal as # of signal levels data levelsrefers to - # of values (symbols)used to represent data an error on p.86, replace by b. Three signal levels, two data levels ( binary 1 or 0) expanded by Jozef Goetz

  14. pulse rate and bit rate • Bit Rate = Pulse Rate x log2 L • Pulse Rate - # of pulses per sec • a pulse is the min amount of time required to transmit a symbol drawn from a fixed alphabet.(a representation of a subset of digits or letters) • abinary alphabet, that is, an alphabet of two characters, typically denoted "0" and "1". • Bit Rate - # of bits per sec • L - # of data levels of the signal expanded by Jozef Goetz

  15. Example 1 A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows: Pulse Rate = 1/ 10-3= 1000 pulses/s Bit Rate = Pulse Rate x log2 L = 1000 x log2 2 = 1000 bps expanded by Jozef Goetz

  16. Example 2 A signal has four data levels (e.g. transmit 2 bits 00, 01, 10, 11 per pulse) with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows: Note: The four data levels correspond to a symbol composed of 2 bits. Pulse Rate = = 1000 pulses/s Bit Rate = PulseRate x log2 L = 1000 x log2 4 = 2000 bps expanded by Jozef Goetz

  17. Figure 4.3DC component • DC component – a residual direct-current component (0 frequency) • is undesirable and useless • can create errors in the output • takes extra energy • doesn’t pass via a transformer expanded by Jozef Goetz

  18. Figure 4.4Lack of synchronization – if bit intervals don’t correspond on both sites • Lack of synchronization – if bit intervals don’t correspond on both sites • Lack of synchronization – if the receive clock is faster (or slower), • the receiver might interpret the signal differently than the sender intended expanded by Jozef Goetz

  19. Example 3 In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps? Solution At 1 Kbps: 1000 bits sent 1001 bits received1 extra bps At 1 Mbps: 1,000,000 bits sent 1,001,000 bits received1000 extra bps expanded by Jozef Goetz

  20. Figure 4.5Line coding schemes expanded by Jozef Goetz

  21. Figure 4.6Unipolar encoding Unipolar encoding uses only one voltage level. • inexpensive • has a dc component • a lack of sync expanded by Jozef Goetz

  22. Figure 4.7Types of polar encoding Polar encoding uses two voltage levels (positive and negative). • dc component – the average voltage on the line is reduced expanded by Jozef Goetz

  23. Figure 4.8DIGITAL SIGNAL ENCODING FORMATS NRZ-L (Nonreturn to Zero) The most common, and easiest, way to transmit digital signals is to use two different voltage levels for the two binary digits. Binary 1 - negative voltage Binary 0 - positive voltage expanded by Jozef Goetz

  24. Figure 4.8DIGITAL SIGNAL ENCODING FORMATS NRZ-L (NonReturn to Zero - Level) • In NRZ-L the level of the signal is dependent upon the state of the bit. • receiver R is relying on its clock when is sent a long stream of 0s or 1s Binary 1 - negative voltage Binary 0 - positive voltage expanded by Jozef Goetz

  25. Figure 4.8DIGITAL SIGNAL ENCODING FORMATS NRZ-I (NonReturn to Zero, Invert on ones) In NRZ-I the signal is inverted if a binary 1 is encountered, so sync is provided If the current bit is binary 0,then the current bit is encoded with the same signal as the preceding bit. expanded by Jozef Goetz

  26. DIGITAL SIGNAL ENCODING FORMATS NRZ-L & NRZI Advantages Disadvantages • NRZ codes are easy to engineer. • Make efficient use of bandwidth. • Commonly used for digital magneticrecording. • The presence of DC component. • NRZ-L: lack of synchronization capability • any drift between the timing of transmitter and receiver will result in loss of synchronization between the two expanded by Jozef Goetz

  27. Figure 4.9RZ (Return to Zero) encoding • RZ encoding has 3 values to build up sync per each bit. • the signal changes between and during each bit • 1 represented by positive to zero (in the 2nd part of the bit) • 0 represented by negative to zero (in the 2nd part of the bit) • so it occupies more bandwidth. It is the most effective so far. A good encoded digital signalmust contain a provision for synchronization. expanded by Jozef Goetz

  28. Figure 4.10Manchester encoding uses an inversion • In the Manchester code, there is a transition at the middle of each bit period. • The midbit transition serves as a clockingmechanism and also as data. A low-to-high transition represents a 1, A high-to-low transition represents a 0. expanded by Jozef Goetz

  29. Figure 4.10Manchester encoding In Manchester encoding, the transition at the middle of the bit is usedfor both synchronization and bitrepresentation. The midbit transition is used only to provide clocking. expanded by Jozef Goetz

  30. Figure 4.11Differential Manchester encoding • The encoding of a 0 is represented by the presence of a transitionat thebeginningof a bit period, • 2 signal changes • 1 is represented by the absence of a transition at the beginningof a bit period • 1 signal change expanded by Jozef Goetz

  31. Figure 4.11Differential Manchester encoding • In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. • The bit representation is defined by the inversion or noninversionat the beginning of the bit. expanded by Jozef Goetz

  32. Figure 4.12Bipolar AMI (Alternative Mark Inversion) encoding In bipolar encoding, we use three levels: positive, zero, and negative. • Binary 1’s are represented by alternatingpositive and negative voltages. • This alternation occurs even when the 1 bits are not consecutive. expanded by Jozef Goetz

  33. Figure 4.132B1Q – 1 Binary 1 Quaternary uses four signal levels. Each pulse represents 2 bits. Four signal levels: 2 bits per level expanded by Jozef Goetz

  34. Figure 4.14MLT-3 signal – Multiline Transition, three level similar to NRZ-I * In NRZ-I the signal is inverted if a binary 1 is encountered and no transition at the beginning of a 0 bit butuses 3 levels – the signal transition from one level to the next at the beginning of a 1 bit transition because the next bit is 1 expanded by Jozef Goetz

  35. DIGITAL SIGNAL ENCODING FORMATS Pseudoternary (3 levels) A binary 1 is represented by no line signal A binary 0 is represented by positive or negative voltage. The binary 0 pulses must alternate in polarity. expanded by Jozef Goetz

  36. DIGITAL SIGNAL ENCODING FORMATS Pseudoternary (3 levels) Binary 1 - negative voltage, Binary 0 - positive voltage inverted if a binary 1 is encountered Binary 1’s are represented by alternatingpositive and negative voltages. A binary 1 is represented by no line signal A binary 0 is represented by positive or negative voltage. The binary pulses must alternate in polarity. The binary 0 pulses must alternate in polarity. A low-to-high transition represents a 1, A high-to-low transition represents a 0. the inversion (bit 0)or noninversion (bit 1) at the beginning of the bit expanded by Jozef Goetz

  37. 4.2 Block Coding To improve the performance of line coding, block coding was introduced. We need some kind of redundancy to ensure sync and error detection. expanded by Jozef Goetz

  38. Figure 4.15Block coding Division: of data to m bit chunks (m=4) Substitution: of data stream to n coded bit (n= 5) Line Coding for the channel/medium: used simple line code b/c the block coding procedure provides 2 desirable features: sync and error detection expanded by Jozef Goetz

  39. Figure 4.16Substitution in block coding • if one or more of the bits in the block is changed in such a way • that the one of the unused code is received, • the receiver R can easy detect theerror expanded by Jozef Goetz

  40. Table 4.1 4B/5B = 4Binary/5Binary encoding • select only the subset • of 32 5-bit blocks • in such a way that • no more than one leading 0 and • no more than two trailing 0s expanded by Jozef Goetz

  41. Table 4.1 Control characters don’t follow the 4B/5B rules of coding • Control characters • enable the physical • layer to be controlled: • e.g. • Idle • Start • End • Set • Reset • Halt expanded by Jozef Goetz

  42. Figure 4.17Example of 8B/6T = 8Binary/6Ternary Encoding is designated to substitute an 8-bit group with a six-symbol code, which each symbol is ternary, having 3 signal levels (-1, 0, +1). expanded by Jozef Goetz

  43. 4.3 Sampling • Pulse Amplitude Modulation - PAM • Pulse Code Modulation - PCM • SamplingRate: Nyquist Theorem • How Many Bits per Sample? • Bit Rate expanded by Jozef Goetz

  44. ANALOG DATA, DIGITAL SIGNALS • The process o transforminganalogdata into digitalsignals (less prone to noise and distortion) is known as digitization. • In the tel. systems electric signals lose their strength over the metallic wire, • and used amplifiers create distortion and phase changes. • In the digital signal it is not easy to distort a 0 to 1 or a 1 to 0. • To sendanalog data digitally we need to change it through process called sampling i.e. measuring the amplitude of the signal at equal intervals. expanded by Jozef Goetz

  45. ANALOG-TO-DIGITAL CONVERSION SAMPLING PROCESS expanded by Jozef Goetz

  46. Figure 4.18Pulse Amplitude ModulationPAM • PAM takes an analog signal, • samples it, and • generates a series of pulses based on the results of the sampling. • PAM uses a technique called sample and hold. • at a given moment signal is read, then held briefly expanded by Jozef Goetz

  47. Figure 4.18Pulse Amplitude ModulationPAM • Pulse amplitude modulation has some applications, but it is not used by itself in data communication • b/c even though it translates the original waveform to a series of pulses, • * these pulses are still of any amplitude (still an analog, not digital signal). • 1. However, it is the first step in another very popular conversion method called pulse code modulation. expanded by Jozef Goetz

  48. Figure 4.19Quantized PAM signal 2. Quantization is a method of assigning integral values in a specific range to sampled instances. Note: Be aware of a picture error: all 1st digits of all peak amplitudes should be removed 3. Binary coding: assigning sign and magnitude quantized samples expanded by Jozef Goetz

  49. Figure 4.21Pulse Code Modulation - PCM 4. Transformation: Binary digits are then transformed to a digital signals by using one of the line coding techniques, here is a unipolar signal • Pulse Code Modulation is made up of 4 processes • PAM • quantization • binary encoding • line coding expanded by Jozef Goetz

  50. Figure 4.22From analog signal to PCM digital code: steps 1-4 expanded by Jozef Goetz

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