Chapter 4 Digital Transmission. 4.#2 4-1 DIGITAL-TO-DIGITAL CONVERSION line coding, block coding,...
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Transcript of Chapter 4 Digital Transmission. 4.#2 4-1 DIGITAL-TO-DIGITAL CONVERSION line coding, block coding,...
Chapter 4
Digital Transmission
4.# 2
4-1 DIGITAL-TO-DIGITAL CONVERSION4-1 DIGITAL-TO-DIGITAL CONVERSION
line codingline coding, , block codingblock coding, and , and scramblingscrambling. Line . Line coding is always needed; block coding and scrambling coding is always needed; block coding and scrambling may or may not be needed.may or may not be needed.
Figure 4.2 Signal element versus data element
r = number of data elements / number of signal elements
Baseline wanderingBaseline: running average of the
received signal power
DC ComponentsConstant digital signal creates low
frequencies
Self-synchronizationReceiver Setting the clock matching the
sender’s
Figure 4.4 Line coding schemes
• High=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 pseudoternary
Multilevel 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 scheme
Table 4.1 Summary of line coding schemes
Block 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 codes
Scrambling
• 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 CONVERSION4-2 ANALOG-TO-DIGITAL CONVERSION
The tendency today is to change an analog signal to The tendency today is to change an analog signal to digital data. digital data.
In this section we describe two techniques, In this section we describe two techniques, pulse code modulationpulse code modulation andand delta modulationdelta modulation..
Figure 4.21 Components of PCM encoder
According 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-limited
2. the sampling rate must be at least 2 times the
highest frequency, not the bandwidth
Figure 4.26 Quantization and encoding of a sampled signal
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 dB
Increasing the number of levels increases the SNR.
Contribution of the quantization error to SNRdb
SNRdb= 6.02nb + 1.76 dBnb: bits per sample (related to the number of level L)
We 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.
The minimum bandwidth of the digital signal is nb times greater than the bandwidth of the analog signal.
Bmin= nb x Banalog
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 smaller
Figure 4.31 Data transmission and modes
Chapter 5
Analog Transmission
Figure 5.1 Digital-to-analog conversion
Figure 5.2 Types of digital-to-analog conversion
1. 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 = log2L
Figure 5.3 Binary amplitude shift keying
B = (1+d) x S = (1+d) x N x 1/r
Figure 5.6 Binary frequency shift keying
Figure 5.9 Binary phase shift keying
Figure 5.12 Concept of a constellation diagram
Figure 5.13 Three constellation diagrams
QAM – 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 QAMs
Figure 5.15 Types of analog-to-analog modulation
Figure 5.16 Amplitude modulation
The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.
Figure 5.18 Frequency modulation
Figure 5.20 Phase modulation
The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal:BPM = 2(1 + β)B.
Chapter 6
Bandwidth Utilization:Multiplexing and
Spreading
Figure 6.1 Dividing a link into channels
Figure 6.2 Categories of multiplexing
Figure 6.4 FDM process
FDM is an analog multiplexing technique that combines analog signals.
Figure 6.5 FDM demultiplexing example
Figure 6.7 Example 6.2
Figure 6.10 Wavelength-division multiplexing
WDM is an analog multiplexing technique to combine optical signals.
Figure 6.12 TDM
1. TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.
2. Two types: synchronous and statistical
Figure 6.13 Synchronous time-division multiplexing
1. In synchronous TDM, each input connection has an allotment in the output even if it is not sending data.
2. 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 6.9
SolutionFigure 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 slots
Synchronous TDM is not always efficient
Figure 6.19 Multilevel multiplexing
Figure 6.20 Multiple-slot multiplexing
Figure 6.21 Pulse stuffing
Figure 6.22 Framing bits
Figure 6.26 TDM slot comparison
Figure 6.27 Spread spectrum
Bss >> B
1 Wrap message in a protective envelope for a more secure transmission.
2 the expanding must be done independently
3 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 FHSS
Figure 6.32 DSSS
Direct sequence spread spectrum
Replace each data bit with n bits using a spreading code