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