Digital Transmission & Analog Transmission

Digital Transmission&Analog Transmission

1. DIGITAL-TO-DIGITAL CONVERSION • Digital Data -> Digital Signal • Three techniques: • line coding (always needed) • block coding (working with NRZ-I) • Scrambling (working with AMI) 4.#

Figure 4.1 Line coding and decoding

Figure 4.2 Signal element versus data element r = number of data elements / number of signal elements

Data Rate Vs. Signal Rate • Data rate: the number of data elements (bits) sent in 1s (bps). It’s also called the bit rate • Signal rate: the number of signal elements sent in 1s (baud). It’s also called the pulse rate, the modulation rate, or the baud rate. • We wish to: • 1. increase the data rate (increase the speed of transmission) • 2. decrease the signal rate (decrease the bandwidth requirement) • Worst case, best case, and average case of r • S = c * N / r baud

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

Figure 4.5 Unipolar NRZ scheme

Figure 4.6 Polar NRZ-L and NRZ-I schemes

Figure 4.7 Polar RZ scheme

Figure 4.8 Polar biphase: Manchester and differential Manchester 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

Figure 4.9 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 4.10 Multilevel: 2B1Q scheme

Figure 4.13 Multitransition: MLT-3 scheme

Table 4.1 Summary of line coding schemes Polar

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

Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme

Figure 4.14 Block coding concept

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

Figure 4.19 Two cases of B8ZS scrambling technique B8ZS substitutes eight consecutive zeros with 000VB0VB

Figure 4.20 Different situations in HDB3 scrambling technique HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.

2. ANALOG-TO-DIGITAL CONVERSION • The tendency today is to change an analog signal to • digital data. • pulse code modulation • delta 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

Contribution of the quantization error to SNRdb SNRdb= 6.02nb + 1.76 dB nb: bits per sample (related to the number of level L) 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 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

3. TRANSMISSION MODES 1. The transmission of binary data across a link can be accomplished in either parallel or serial mode. 2. In parallel mode, multiple bits are sent with each clock tick. 3. In serial mode, 1 bit is sent with each clock tick. 4. there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

Figure 4.31 Data transmission and modes

4. DIGITAL-TO-ANALOG CONVERSION Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data.

Figure 5.1 Digital-to-analog conversion

Figure 5.2 Types of digital-to-analog conversion

Data element vs. signal element • What is a signal element here? • 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 Binary amplitude shift keying B = (1+d) x S = (1+d) x N x 1/r

Figure 5.4 Implementation of binary ASK

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 buy a single signal change • Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes)

Figure 5.14 Constellation diagrams for some QAMs

5. ANALOG AND DIGITAL Analog-to-analog conversion is the representation of analog information by an analog signal. Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us. Example: radio stations

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 5.17 AM band allocation

Figure 5.18 Frequency modulation

Figure 5.19 FM band allocation The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B. β has a common value of 4

Digital Transmission & Analog Transmission