The Physical Layer , Encoding Schemes:

1. Chapter 2 The Physical Layer, Encoding Schemes: -Physical Transmission of Bits-

2. What To Do Against Attenuation, Distortion and Noise? • A media adapted to the distance/bit rate (we see it earlier…) • Encoding schemes (we well see it now) Chapter 2

3. Encoding Techniques • Encoding schemes deal with how to transport bits over the physical media… • We must deal and manage many issues to well represent and interpret the bits: • Timing of bits (start and end, duration, Signal levels) • Clocking ( Synchronizing transmitter and receive, External clock, Sync mechanism based on signal) • Error detection, Signal interference and Noise immunity • Cost and complexity Chapter 2

4. Encoding Techniques • Encoding techniques depend on the type of data to transmit and the medium being used: • Digital data on digital signal (our focus in this brief description...) • Analog data on digital signal • Digital data on analog signal • Analog data on analog signal Chapter 2

5. Digital Data, Digital Signal • Digital signal: • Discrete, discontinuous voltage pulses • Each pulse is a signal element • Binary data encoded into signal elements Chapter 2

6. A Simple Encoding Scheme • 0 is Vo (some voltage) • 1 is V1 • Example : let us encode 10011101 Clock Vo V1 Chapter 2

7. Problem? • Let us now try to encode two bytes that come one 2 seconds after the other: 10011101 and 00001101 • What is the problem with this encoding scheme? • What is the solution? Chapter 2

8. Bipolar Encoding (RZ Signal) +0.85V • 1 is • 0 is • Example : let us encode 01100010 HightV + NoV –» 1bit 0 0 LowV + NoV –» 0bit -0.85V Clock Chapter 2

9. Problem? • The problem with this encoding scheme is that we have to return to 0 each time the transmission is done which generates some complexity and cost in implementing and managing this technique • We should look for a better technique… We have 2 alternatives: Chapter 2

10. Alternative 1: Non-Return to Zero • Two different voltages for 0 and 1 bits • Voltage constant during bit interval • no transition i.e. no return to zero voltage • e.g. Absence of voltage for zero, constant positive voltage for one • More often, negative voltage for one value and positive for the other • This is NRZ also known as Manchester Encoding... Chapter 2

11. Alternative 1: NRZ-Manchester Encoding +0.85V • 1 is • 0 is • Example : let us encode 01100010 HightV + LowV –» 1bit 0 -0.85V 0 LowV + HighV –» 0bit Clock Chapter 2

12. Alternative 2: Differential Encoding • Data represented by changes trends rather than levels of voltage • More reliable detection of transition rather than level • In complex transmission layouts it is easy to lose sense of polarity Chapter 2

13. Differential Manchester Encoding (Presence of transition) 0 • 0 is • 1 is • A transition in the middle of the bit is required anyway… Example : let us encode 10011101 0 (Abscence of transition) Clock Chapter 2