Topics discussed in this section: 4-1 DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed. Topics discussed in this section: Line Coding Line Coding Schemes Block Coding Scrambling 4.#

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: running average of the received signal power 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

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

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 4.14 Block coding concept

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

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 CONVERSION The tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation.

Figure 4.21 Components of PCM encoder

Figure 4.26 Quantization and encoding of a sampled signal #(bits)/sample: nb=log2L data rate = sampling rate x nb

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

5-1 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 5.2 Types of digital-to-analog conversion

Figure 5.12 Concept of a constellation diagram

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) If we use m amplitudes, n phases, L = m x n and r = log2L

5-2 ANALOG-TO-ANALOG CONVERSION 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

6-1 MULTIPLEXING Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic.

Figure 6.2 Categories of multiplexing

FDM is an analog multiplexing technique that combines analog signals. Figure 6.4 FDM process FDM is an analog multiplexing technique that combines analog signals.

Figure 6.5 FDM demultiplexing example

Example 6.2 Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 × 100 + 4 × 10 = 540 kHz,

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.

Rules for synchronous TDM input unit size = output unit size input bit duration = 1/input bit rate input unit duration = input bit duration x input unit size output unit duration = (1/n) x input unit duration frame duration = input unit duration frame rate = 1/frame duration frame size = n x input unit size ( + 1) (depending on synchronizing bit or not) output data rate = frame rate x frame size

Figure 6.17 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 6.22 Framing bits

Bss >> B Figure 6.27 Spread spectrum 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)