1. DIGITAL TRANSMISSION PART C

2. Review…
DELTA MODULATION (DM)
Conventional PCM- multiple bit codes required to represent the sample value.
DM-Uses a single-bit PCM code to achieve digital transmission of analog signals.
Only encodes and transmits one bit per sample time
Logic ‘0’ is transmitted if current sample is smaller than the previous sample
Logic ‘1’ is transmitted if current sample is larger than the previous sample
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL MODULATION

3. LINE CODES The line codes properties: Line code is an encoding scheme.
Many possible ways to encode 0s and 1s.
Converting standard logic level to a form more suitable to telephone line transmission.
Different types of encoding schemes or line codes can be used to improve signal’s noise immunity and to achieve higher data rates.
The line codes properties:
Transmission BW should be small as possible
Efficiency should be as high as possible
Error detection & correction capability
Transparency (Encoded signal is received faithfully)
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

4. Cont’d...
Six factors must be considered when selecting a line encoding format;
Transmission voltage & DC component
Duty cycle
Bandwidth consideration
Clock and framing bit recovery
Error detection
Ease of detection and decoding
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

5. Why Digital Signaling? Low cost digital circuits
The flexibility of the digital approach (because digital data from digital sources may be merged with digitized data derived from analog sources to provide general purpose communication system)
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

6. Digital Modulation Using Digital Signals to Transmit Digital Data
Bits must be changed to digital signal for transmission
Unipolar encoding
Positive or negative pulse used for zero or one
Polar encoding
Uses two voltage levels (+ and - ) for zero or one
Bipolar encoding
+, -, and zero voltage levels are used
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

7. Non-Return to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits.
Voltage constant during bit interval.
no transition, no return to zero voltage
More often, negative voltage for one value and positive for the other.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

8. Non-Return to Zero Inverted (NRZ-I)
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

9. Multilevel Binary (Bipolar-AMI)
zero represented by no line signal.
one represented by positive or negative pulse alternately.
No loss of sync if a long string of ones (zeros still a problem)
No net dc component
Lower bandwidth
Easy error detection
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

10. Pseudoternary One represented by absence of line signal
Zero represented by alternating positive and negative
No advantage or disadvantage over bipolar-AMI
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

11. Manchester
There is always a mid-bit transition {which is used as a clocking mechanism}.
The direction of the mid-bit transition represents the digital data.
1  low-to-high transition
0  high-to-low transition
Consequently, there may be a second transition at the beginning of the bit interval.
Used in baseband coaxial cable and CSMA/CD twisted pair.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

12. EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

13. Differential Manchester
mid-bit transition is ONLY for clocking.
1  absence of transition at the beginning of the bit interval
0  presence of transition at the beginning of the bit interval
Differential Manchester is both differential and bi-phase.
[Note – the coding is the opposite convention from NRZI.]
Used in (token ring) with twisted pair.
Modulation rate for Manchester and Differential Manchester is twice the data rate  inefficient encoding for long-distance applications.
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

14. EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

15. Example 5 Sketch the data wave form for a bit stream 11010 using NRZL
Bipolar AMI
Pseudoternary
EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION

16. Solution

17. END OF PART C EKT 231 : COMMUNICATION SYSTEM
CHAPTER 4 : DIGITAL TRANSMISSION