# Chapter 4 Digital Transmission

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Chapter 4 Digital Transmission. 4-1 DIGITAL-TO-DIGITAL CONVERSION. line coding , block coding , and scrambling . Line coding is always needed; block coding and scrambling may or may not be needed. Figure 4.2 Signal element versus data element.
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Chapter 4Digital Transmission4-1 DIGITAL-TO-DIGITAL CONVERSIONline coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed.4.#Figure 4.2 Signal element versus data elementr = number of data elements / number of signal elementsBaseline wanderingBaseline: running average of the received signal powerDC ComponentsConstant digital signal creates low frequenciesSelf-synchronizationReceiver Setting the clock matching the sender’s Figure 4.4 Line coding schemesHigh=0, Low=1
• No change at begin=0, Change at begin=1
• H-to-L=0, L-to-H=1
• Change at begin=0, No change at begin=1
• Bipolar schemes: AMI (Alternate Mark Inversion) and pseudoternaryMultilevel Schemes
• In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln
• m: the length of the binary pattern
• B: binary data
• n: the length of the signal pattern
• L: number of levels in the signaling
• Figure 4.13 Multitransition: MLT-3 schemeTable 4.1 Summary of line coding schemesBlock Coding
• Redundancy is needed to ensure synchronization and to provide error detecting
• Block coding is normally referred to as mB/nB coding
• it replaces each m-bit group with an n-bit group
• m < n
• Table 4.2 4B/5B mapping codesScrambling
• It modifies the bipolar AMI encoding (no DC component, but having the problem of synchronization)
• It does not increase the number of bits
• It provides synchronization
• It uses some specific form of bits to replace a sequence of 0s
• 4-2 ANALOG-TO-DIGITAL CONVERSIONThe tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulationanddelta modulation.Figure 4.21 Components of PCM encoderAccording to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal.What can we get from this:1. we can sample a signal only if the signal is band-limited2. the sampling rate must be at least 2 times the highest frequency, not the bandwidthFigure 4.26 Quantization and encoding of a sampled signalContribution of the quantization error to SNRdbSNRdb= 6.02nb + 1.76 dBnb: bits per sample (related to the number of level L)What is the SNRdB in the example of Figure 4.26?SolutionWe have eight levels and 3 bits per sample, so SNRdB = 6.02 x 3 + 1.76 = 19.82 dBIncreasing the number of levels increases the SNR.The minimum bandwidth of the digital signal is nb times greater than the bandwidth of the analog signal.Bmin= nb x BanalogWe have a low-pass analog signal of 4 kHz. If we send the analog signal, we need a channel with a minimum bandwidth of 4 kHz. If we digitize the signal and send 8 bits per sample, we need a channel with a minimum bandwidth of 8 × 4 kHz = 32 kHz.DM (delta modulation) finds the change from the previous sampleNext bit is 1, if amplitude of the analog signal is largerNext bit is 0, if amplitude of the analog signal is smallerFigure 4.31 Data transmission and modesChapter 5Analog TransmissionFigure 5.1 Digital-to-analog conversionFigure 5.2 Types of digital-to-analog conversion1. Data element vs. signal element2. Bit rate is the number of bits per second. 2. Baud rate is the number of signal elements per second. 3. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.S = N x 1/r baud r = log2LFigure 5.3 Binary amplitude shift keyingB = (1+d) x S = (1+d) x N x 1/rFigure 5.6 Binary frequency shift keyingFigure 5.9 Binary phase shift keyingFigure 5.12 Concept of a constellation diagramFigure 5.13 Three constellation diagramsQAM – Quadrature Amplitude Modulation
• Modulation technique used in the cable/video networking world
• Instead of a single signal change representing only 1 bps – multiple bits can be represented by a single signal change
• Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes)
• Figure 5.14 Constellation diagrams for some QAMsFigure 5.15 Types of analog-to-analog modulationFigure 5.16 Amplitude modulationThe total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.Figure 5.18 Frequency modulationFigure 5.20 Phase modulationThe total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal:BPM = 2(1 + β)B.Chapter 6Bandwidth Utilization:Multiplexing and SpreadingFigure 6.1 Dividing a link into channelsFigure 6.2 Categories of multiplexingFigure 6.4 FDM processFDM is an analog multiplexing technique that combines analog signals.Figure 6.5 FDM demultiplexing exampleFigure 6.7 Example 6.2Figure 6.10 Wavelength-division multiplexingWDM is an analog multiplexing technique to combine optical signals.Figure 6.12 TDM
• TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.
• Two types: synchronous and statistical
• Figure 6.13 Synchronous time-division multiplexing
• In synchronous TDM, each input connection has an allotment in the output even if it is not sending data.
• In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.
• Figure 6.17 Example 6.9SolutionFigure 6.17 shows the output for four arbitrary inputs. The link carries 50,000 frames per second. The frame duration is therefore 1/50,000 s or 20 μs. The frame rate is 50,000 frames per second, and each frame carries 8 bits; the bit rate is 50,000 × 8 = 400,000 bits or 400 kbps. The bit duration is 1/400,000 s, or 2.5 μs. Figure 6.18 Empty slotsSynchronous TDM is not always efficientFigure 6.19 Multilevel multiplexingFigure 6.20 Multiple-slot multiplexingFigure 6.21 Pulse stuffingFigure 6.22 Framing bitsFigure 6.26 TDM slot comparisonFigure 6.27 Spread spectrumBss >> B1 Wrap message in a protective envelope for a more secure transmission. 2 the expanding must be done independently3 two types: frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS)Figure 6.28 Frequency hopping spread spectrum (FHSS)Figure 6.29 Frequency selection in FHSSFigure 6.32 DSSSDirect sequence spread spectrumReplace each data bit with n bits using a spreading code
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