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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 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.

4.#


Figure 4.2 Signal element versus data element

r = number of data elements / number of signal elements


Baseline wandering

Baseline: running average of the received signal power

DC Components

Constant digital signal creates low frequencies

Self-synchronization

Receiver 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



Multilevel Schemes pseudoternary

  • 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 pseudoternaryMultitransition: MLT-3 scheme


Table 4.1 pseudoternarySummary of line coding schemes


Block Coding pseudoternary

  • 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 pseudoternary4B/5B mapping codes


Scrambling pseudoternary

  • 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 CONVERSION pseudoternary

The 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 pseudoternaryComponents 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 at least 2 times the highest frequency contained in the signal.Quantization and encoding of a sampled signal


Contribution of the at least 2 times the highest frequency contained in the signal.quantization error to SNRdb

SNRdb= 6.02nb + 1.76 dB

nb: bits per sample (related to the number of level L)

What is the SNRdB in the example of Figure 4.26?

Solution

We 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.


The minimum bandwidth of the digital signal is n at least 2 times the highest frequency contained in the signal.b times greater than the bandwidth of the analog signal.

Bmin= nb x Banalog

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.


DM at least 2 times the highest frequency contained in the signal. (delta modulation) finds the change from the previous sample

Next bit is 1, if amplitude of the analog signal is larger

Next bit is 0, if amplitude of the analog signal is smaller


Figure 4.31 at least 2 times the highest frequency contained in the signal.Data transmission and modes


Chapter 5 at least 2 times the highest frequency contained in the signal.

Analog Transmission


Figure 5.1 at least 2 times the highest frequency contained in the signal.Digital-to-analog conversion


Figure 5.2 at least 2 times the highest frequency contained in the signal.Types of digital-to-analog conversion


1. Data element vs. signal element at least 2 times the highest frequency contained in the signal.

2. 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 at least 2 times the highest frequency contained in the signal.Binary amplitude shift keying

B = (1+d) x S = (1+d) x N x 1/r


Figure 5.6 at least 2 times the highest frequency contained in the signal.Binary frequency shift keying


Figure 5.9 at least 2 times the highest frequency contained in the signal.Binary phase shift keying


Figure 5.12 at least 2 times the highest frequency contained in the signal.Concept of a constellation diagram


Figure 5.13 at least 2 times the highest frequency contained in the signal.Three constellation diagrams


Qam quadrature amplitude modulation
QAM – Quadrature Amplitude Modulation at least 2 times the highest frequency contained in the signal.

  • 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 at least 2 times the highest frequency contained in the signal.Constellation diagrams for some QAMs


Figure 5.15 at least 2 times the highest frequency contained in the signal.Types of analog-to-analog modulation


Figure 5.16 at least 2 times the highest frequency contained in the signal.Amplitude modulation

The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.


Figure 5.18 at least 2 times the highest frequency contained in the signal.Frequency modulation


Figure 5.20 at least 2 times the highest frequency contained in the signal.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 at least 2 times the highest frequency contained in the signal.

Bandwidth Utilization:

Multiplexing and Spreading


Figure 6.1 at least 2 times the highest frequency contained in the signal.Dividing a link into channels


Figure 6.2 at least 2 times the highest frequency contained in the signal.Categories of multiplexing


Figure 6.4 at least 2 times the highest frequency contained in the signal.FDM process

FDM is an analog multiplexing technique that combines analog signals.


Figure 6.5 at least 2 times the highest frequency contained in the signal.FDM demultiplexing example


Figure 6.7 at least 2 times the highest frequency contained in the signal.Example 6.2


Figure 6.10 at least 2 times the highest frequency contained in the signal.Wavelength-division multiplexing

WDM is an analog multiplexing technique to combine optical signals.


Figure 6.12 at least 2 times the highest frequency contained in the signal.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 at least 2 times the highest frequency contained in the signal.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 at least 2 times the highest frequency contained in the signal.Example 6.9

Solution

Figure 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 at least 2 times the highest frequency contained in the signal.Empty slots

Synchronous TDM is not always efficient


Figure 6.19 at least 2 times the highest frequency contained in the signal.Multilevel multiplexing


Figure 6.20 at least 2 times the highest frequency contained in the signal.Multiple-slot multiplexing


Figure 6.21 at least 2 times the highest frequency contained in the signal.Pulse stuffing


Figure 6.22 at least 2 times the highest frequency contained in the signal.Framing bits


Figure 6.26 at least 2 times the highest frequency contained in the signal.TDM slot comparison


Figure 6.27 at least 2 times the highest frequency contained in the signal.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 at least 2 times the highest frequency contained in the signal.Frequency hopping spread spectrum (FHSS)


Figure 6.29 at least 2 times the highest frequency contained in the signal.Frequency selection in FHSS


Figure 6.32 at least 2 times the highest frequency contained in the signal.DSSS

Direct sequence spread spectrum

Replace each data bit with n bits using a spreading code


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