1. Data Encoding
Data Encoding refers the various techniques of impressing data (0,1) or information on an electrical, electromagnetic or optical signal that would propagate through the physical medium making up the communication link between the two devices.

2. Plan for the Lecture Why Encoding, Encoding Issues
Digital Data and Digital Signals
Analog Data and Digital Signals
Digital Data and Analog Signals
Analog Data and Analog Signals

3. Data and Signals Two types of data Analog and Digital
Two types of Signals (transmission techniques) Analog and Digital

4. Advantages of Digital Transmission
The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over one line
Higher bandwidth is possible with data compression

5. Advantages of Analog Transmission
Most mediums support analog transmission - used for wireless communication
The telephone infrastructure provides a relatively cheap “individual point-to-point” transmission

6. Interpreting Signals Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting of signals
Signal to noise ratio
Data rate
Bandwidth

7. Comparison of Encoding Schemes (1)
Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling via transformer, providing isolation
Concentrate power in the middle of the bandwidth
Clocking
Synchronizing transmitter and receiver
External clock
Sync mechanism based on signal

8. Comparison of Encoding Schemes (2)
Error detection
Can be built in to signal encoding
Signal interference and noise immunity
Some codes are better than others
Cost and complexity
Higher signal rate (& thus data rate) lead to higher costs
Some codes require signal rate greater than data rate

10. Analog Signals Carrying Analog and Digital Data

11. Digital Signals Carrying Analog and Digital Data

13. Digital Data - Digital Signal
It is logical to represent digital data with a digital signal
Digital signal
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements
Signal changes value as the data changes value from 0 to 1 and 1 to 0
Several line encoding schemes are possible. Each has pros and cons

14. Encoding Schemes Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester

15. Digital Data - Digital Signal

16. NRZ pros and cons Pros Cons Used for magnetic recording
Easy to engineer
Make good use of bandwidth
Cons
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission

17. Problems With NRZ
Difficult to determine where one bit ends and the next begins
In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses
Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

18. Biphase Manchester Differential Manchester
Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
Differential Manchester
Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents one
Note: this is a differential encoding scheme
Used by IEEE 802.5

19. Digital Data - Digital Signal

20. Digital Data, Analog Signal
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)

21. Modulation Techniques

22. Amplitude Shift Keying
Values represented by different amplitudes of carrier
Usually, one amplitude is zero
i.e. presence and absence of carrier is used
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber