1. 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.#

2. 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

3. 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

4. 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

5. Table 4.1 Summary of line coding schemes
Polar

6. 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

7. Figure 4.14 Block coding concept

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

9. 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

10. 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.

11. Figure 4.21 Components of PCM encoder

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

13. Contribution of the quantization error to SNRdb
SNRdb= 6.02nb 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

14. 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

15. 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.

16. Figure 5.2 Types of digital-to-analog conversion

17. Figure 5.12 Concept of a constellation diagram

18. 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

19. 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

20. Figure 5.15 Types of analog-to-analog modulation

21. 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.

22. Figure 6.2 Categories of multiplexing

23. 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.

24. Figure 6.5 FDM demultiplexing example

25. 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 × × 10 = 540 kHz,

26. 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.

27. 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

28. Figure 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.

29. Figure Framing bits

30. 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)