1. Dr. Clincy Professor of CS
Chapter 4 Handout #4
Dr. Clincy
Professor of CS
Dr. Clincy
Lecture

2. Data Vs Signal
Fully explain the difference between signal and data before getting into the details
Today’s Lecture (Digital Transmission)
DATA SIGNAL
D D (still covering) A
Next Lecture (Analog Transmission)
A D
Dr. Clincy
Lecture

3. DIGITAL-TO-DIGITAL CONVERSION
Can represent digital data by using digital signals. The conversion involves three techniques:
line coding – converting bit sequences to signals (Covered already)
block coding – adding redundancy for error detection
scrambling – deals with the long zero-level pulse issue
Line coding is always needed;
Block coding and scrambling may or may not be needed.
Dr. Clincy
Lecture

4. Block coding concept
Block coding provides redundancy for synchronization and error detection
Block coding changes a block of m bits into a block of n bits (where n>m)
Block coding is also called mB/nB encoding
Dr. Clincy
Lecture

5. Using block coding 4B/5B with NRZ-I line coding scheme
Fixes that PROBLEM of long stream of 0s
Use 4B/5B to change the long stream of 0s prior to using NRZ-I
For example, for 4B/5B encoding, 4-bit groups or replaced with 5-bit groups and those 5-bit groups are re-combined – NOTE: the 5-bit code could be completely different from the original 4-bit code
Dr. Clincy
Lecture

6. 4B/5B mapping codes
Because the 5-bit code has 25 = 32 codes, the extra codes can be used for control sequences and error detection
For example, for 4B/5B encoding, 4-bit groups or replaced with 5-bit groups and those 5-bit groups are re-combined – NOTE: the 5-bit code could be completely different from the original 4-bit code
Dr. Clincy
Lecture

7. 8B/10B block encoding Eight binary, ten binary encoding scheme
If there are more consecutive 0s over 1s (or vice versa), controller detects and complements either the 0s or 1s – uses 768 redundant bit groups for this
Eight binary, ten binary encoding scheme
8-bit codes replaced with 10-bit codes
Provide greater error detection
5 most significant bits are fed to 5B/6B encoder
3 least significant bits are fed to 3B/4B encoder
Done to simplify mapping table
Dr. Clincy
Lecture

8. Scrambling
The biphase encoding schemes are suited for long-distance communication due to bandwidth requirement.
However, bipolar AMI encoding is good because of the narrow bandwidth requirement – however, long streams of 0s could throw off the synchronization
In dealing with synchronization issue, we could substitute long zero-level pulses with a combination of other levels to provide synchronization
This is called scrambling
Dr. Clincy
Lecture

9. AMI used with scrambling
Unlike block coding, scrambling is done at the SAME time encoding is done
System inserts the require pulses based on “scrambling rules”
Two techniques: (1) bipolar with 8-zero substitution (B8ZS), (2) high-density bipolar 3-zero (HDB3)
Dr. Clincy
Lecture

10. Two cases of B8ZS scrambling technique
Takes 8 consecutive zeros and replace with 000VB0VB where V denotes violation (a non-zero voltage not in accordance with the AMI rule) and B denotes bipolar (a non-zero voltage in accordance with AMI rule)
Recall the Bipolar AMI scheme on page 110 [Alternate Mark Inversion (AMI) scheme – neutral zero voltage is 0 and alternating positive and negative voltage represents 1]
Dr. Clincy
Lecture

11. Different situations in HDB3 scrambling technique
For HDB3, 4 consecutive zeros are replaced with 000V or B00V
With the two choices, an even number of non-zero pulses can be maintained
Rule 1: if the # of non-zero pulses is odd after the last substitution, use pattern 000V – which will make the total number even
Because # of non-zero pulses here is even, used B00V.
Now we have only 1 non-zero pulse (odd), so use 000V
Since there are no non-zero pulses after the 2nd substitution, the 3rd substitution is B00V because this is an even case
Rule 2: if the # of non-zero pulses is even after the last substitution, use pattern B00V – which will make the total number even
Dr. Clincy
Lecture

12. Dr. Clincy Professor of CS
Chapter 4 Handout #4
Dr. Clincy
Professor of CS
Dr. Clincy
Lecture

13. ANALOG-TO-DIGITAL CONVERSION
We have seen in Chapter 3 that a digital signal is superior to an analog signal. The tendency today is to change an analog data to a digital signal. In this section we describe two techniques, pulse code modulation and delta modulation.
Dr. Clincy
Lecture

14. Components of PCM encoder
PCM – Pulse Code Modulation
1st: analog signal is sampled
2nd: sampled signal is quantized
3rd: quantized values are encoded as bit streams (or codes)
Analog signal is sampled every Ts seconds
Sample rate is fs = 1/Ts
Dr. Clincy
Lecture

15. Three different sampling methods for PCM
High-speed switch used – able to retain the shape of the signal
Ideal but complex
Sample-and-hold method that creates flat-top samples by using a circuit
Sampling process also called pulse amplitude modulation (PAM)
Dr. Clincy
Lecture

16. Recovery of a sampled sine wave for different sampling rates
Nyquist theorem states that the sampling rate must be at least 2 times the highest frequency of the signal
Catches the essence of the signal
Doesn’t improve the case
Doesn’t capture the essence of the signal
Dr. Clincy
Lecture

17. Quantization and encoding of a sampled signal
actual-amplitude/D
actual amplitude
Quantization steps:
Determine Vmin and Vmax
Divide range into L zones, each of height D
D = [Vmin - Vmax]/L
3. Assign quantized values of 0 to L-1 to midpoint of each zone
4. Map the sample value to a quantized value
Norm. Actual
Error between actual and nornalized
Quant. value for code
Code that represents the voltage level
Assume sample amplitudes between -20V and +20V
Let L = 8 (levels) – therefore, D = [ ]/8 = 5
Quantization error can contribute to Shannon’s SNR: SNRdB = 6.02nb where nb is bits per sample
Bit rate = sampling rate x # of bits per sample = fs x nb
Dr. Clincy
Lecture

18. Components of a PCM decoder
PCM decoder recovers the original signal
Smooths out the staircase signal
What is the minimum bandwidth of the filter the digitized signal will need ?
Bmin = c x nb x 2 x Banalog x 1/r (nb = # bits per sample)
If 1/r=1 and c=1/2, Bmin=nb x Banalog
If the data rate and number of signal levels are fixed, minimum bandwidth is
Bmin = N / [2 x log2 L]
Dr. Clincy
Lecture

19. The process of delta modulation
PCM is more complex than Delta modulation
PCM finds the amplitude of the signal; delta modulation simply finds the change in the signal from the previous sample
Delta modulation doesn’t use codes – bits are sent one after another
Positive changes are encoded as 1; negative changes are encoded as 0
Dr. Clincy
Lecture

20. TRANSMISSION MODES
The transmission of binary data across a link can be accomplished in either parallel or serial mode.
In parallel mode, multiple bits are sent with each clock tick.
In serial mode, 1 bit is sent with each clock tick.
While there is only one way to send parallel data, there are three subclasses of serial transmission: asynch, syn and iso approaches (asynchronous, synchronous, and isochronous.)
Dr. Clincy
Lecture

21. Parallel transmission
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Lecture

22. Serial transmission
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Lecture

23. Asynchronous transmission
In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.
Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.
Dr. Clincy
Lecture

24. Synchronous transmission
In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.
Dr. Clincy
Lecture

25. Isochronous Transmission
For realtime audio and video, uneven delays between frames is not acceptable – so synchronous transmission doesn’t work well
The entire stream of bits must be synchronized – this is isochronous transmission
Isochronous transmission guarantees data at a fixed rate
Dr. Clincy
Lecture