ITT400 CHAPTER 3 (UITM)

1. ITT400
Introduction To Data Communication
and Networking
Chapter 3
Digital Transmission
Mazlan Osman, FSKM, UiTM (Terengganu) 2014

2. 4.2
4-1 DIGITAL-TO-DIGITAL CONVERSION
• In this section, we will discuss how we can
represent digital data by using digital signals.
• The representation involves three techniques:
• line coding
• block coding
• scrambling.

3. 4.3
LINE CODING
• Process of converting digital data (sequence of bits) to
digital signals
• At the sender, digital data are encoded into a digital signal;
at the receiver, the digital signal are decoded into digital
data.
Figure 4.1 Line coding and decoding

4. 4.4
DATA ELEMENT AND SIGNAL ELEMENT
• Data element : The smallest entity that can
represent a piece of information (bit).
• Signal element : The shortest unit (timewise) of
a digital signal to carry data element.
• Data elements are what we need to send; signal
elements are what we can send

5. 4.5
Figure 4.2 Signal element versus data element
DATA ELEMENT AND SIGNAL ELEMENT

6. 4.6
• Data rate : The number of data elements (bits) sent in 1s. Unit : bps
• Signal rate : The number of signal elements sent in 1s. Unit : baud
• Relation between data rate and signal rate (baud rate):
S = c x N x 1/r
• Example 4.1
A signal is carrying data in which one data element is encoded as
one signal element ( r = 1). If the bit rate is 100 kbps, what is the
average value of the baud rate if c is 0.5?
Solution
The baud rate is then
DATA RATE AND SIGNAL RATE

7. 4.7
DC Components
• Direct-current components
• The signal that have zero frequency and the
average amplitude is nonzero
Self-synchronization
• The method to correctly interpret the signals
received from the sender
DC COMPONENTS & SELF-SYNCHRONIZATION

8. 4.8
Figure 4.3 Effect of lack of synchronization
LACK OF SYNCHRONIZATION

9. 4.9
LINE CODING SCHEMES
Figure 4.4 Line coding schemes

10. 4.10
Uses only one voltage level
• Positive voltage defines bit 1 and the zero
voltage defines bit 0
Figure 4.5 Unipolar NRZ scheme
UNIPOLAR SCHEME

11. 4.11
NRZ (Non-Return-to-Zero)
• Have two versions of polar NRZ:
i. NRZ-L (NRZ-Level)
–Bit 1 is represented by negative voltage, bit 0
represented by positive voltage.
ii. NRZ-I (NRZ-Invert)
–Bit 1 is represented inversion of voltage, bit 0 is
represented by no change.
POLAR SCHEME

12. 4.12
Figure 4.6 Polar NRZ-L and NRZ-I schemes
POLAR SCHEME

13. 4.13
RZ (Return-to-Zero)
• Uses three values: positive, negative, and zero
• In RZ, bit 1 is represented by positive-to-zero
voltage, bit 0 is represented by negative-to-zero
voltage.
Figure 4.7 Polar RZ scheme
POLAR SCHEME

14. 4.14
Biphase : Manchester and Differential Manchester
• Best solution for synchronization problems
• Manchester (RZ + NRZ-L)
–Bit 1 is represented by negative-to-positive
transition; bit 0 is represented by positive-to-
negative transition.
• Differential Manchester (RZ + NRZ-I)
–Bit 1 is represented by no transition; bit 0 is
represented by transition.
POLAR SCHEME

15. 4.15
Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
POLAR SCHEME

16. 4.16
• Alternate Mark Inversion (AMI)
–A bit 1 is represented by positive and negative
voltage alternately; bit 0 is represented by zero
voltage.
–Advantages : DC component is zero and provide
synchronization for a long strings of 1s.
Figure 4.9 Bipolar schemes: AMI
BIPOLAR SCHEME

17. 4.17
Summary of Line Coding Scheme
Table 4.1 Summary of line coding schemes

18. 4.18
4-2 ANALOG-TO-DIGITAL CONVERSION
• Sometimes, we have to sent analog data
(signal) using digital signal.
• In this section, we will discuss how we can
represent analog signal by using digital signals.
• In this section we describe two techniques,
• Pulse Code Modulation
• Delta Modulation.

19. 4.19
PULSE CODE MODULATION
(PCM)
• Technique to change an analog signal to digital data (digitization)
• PCM encoder has three process:
1. The analog signal is sampled (sampling)
2. The sampled signal is quantized (quantizing)
3. The quantized values are encoded as streams of bit (encoding)
Figure 4.21 Components of PCM encoder

20. 4.20
SAMPLING
• Sampling is the process of measuring the non-
integral amplitude of analog signal at equal
intervals.
• The analog signal is sampled every Ts s, where Ts is
the sampling interval or period
• The inverse of the sampling interval is called the
sampling rate.
• There are three sampling methods : ideal, natural, and
flat-top
• The sampling process is sometimes referred to as
Pulse Amplitude Modulation (PAM).

21. 4.21
Figure 4.22 Three different sampling methods for PCM
SAMPLING

22. 4.22
Sampling Rate
• Based on Nyquist theorem ;
1. We can sample signal only if the signal is band-limited ->
signal with an infinite bandwidth cannot be sampled
2. Sampling rate must be at least 2 times the highest frequency
contained in the signal.
Example
What sampling rate needed for a signal with a bandwidth of
10,000 Hz (1000 to 11,000 Hz)?
• Solution
Sampling rate = 2 (11,000) = 22,000 samples/S
SAMPLING

23. 4.23
• The result of sampling is a series of pulses with
amplitude values between the maximum and minimum
amplitudes of the signal.
• Set of amplitudes can be infinite with non-integral
values and these values cannot be used in the encoding
process.
• Quantization is the method of assigning integral
values in a specific range to sampled instances.
• In quantization, we approximate the value of the
sample amplitude to the quantized values.
QUANTIZATION

24. 4.24
Figure 4.26 Quantization and encoding of a sampled signal
QUANTIZATION

25. 4.25
• After each sample is quantized and the number of bits per sample is
decided, each sample can be changed to an nb-bit code word
• A quantization code of 2 is encoded as 010; 5 is encoded as 101; etc
Bit rate = sampling rate x no. of bits per sample = fs x nb
• Example 4.14
We want to digitize the human voice. What is the bit rate, assuming
8 bits per sample?
Solution
The human voice normally contains frequencies from 0 to 4000 Hz.
So the sampling rate and bit rate are calculated as follows:
ENCODING

26. 4.26
4-3 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.

27. 4.27
Figure 4.31 Data transmission and modes
4-3 TRANSMISSION MODES

28. 4.28
PARALLEL TRANSMISSION
• Send data n bits at a time using n channels
• Conceptually: use n wires to send n bits at one
time
Figure 4.32 Parallel transmission

29. 4.29
SERIAL TRANSMISSION
• Send data one bit follows another using one
channel
• Serial occurs in one of three ways :
asynchronous, synchronous, and isochronous
Figure 4.33 Serial transmission

30. 4.30
• The timing of signal is unimportant
• A byte is sent with one start bit (0) at the beginning and
one or more stop bits (1) at the end of each byte.
ASYNCHRONOUS TRANSMISSION
Figure 4.34 Asynchronous transmission

31. 4.31
• The bit stream is combined into longer frames, which
may contains multiple bytes
• Bytes are sent one after another without start and stop
bits or gap
Figure 4.35 Synchronous transmission
SYNCHRONOUS TRANSMISSION

32. 4.32
EXERCISE
1. Assume a data stream is made of threes 0s followed by two 1s
followed by two 0s and another three 1s. Encode this stream using
the following encoding schemes: Unipolar, NRZ-L, NRZ-I, RZ,
Manchester, Differential Manchester and AMI.
2. Using Nyquist Theorem, calculate the sampling rate for the
following analog signal:
a) An analog signal with frequency from 2000 to 6000 Hz
b) A signal with horizontal line in the time-domain representation