Chapter 2 The Physical Layer Encoding Schemes Physical

Chapter 2 The Physical Layer, Encoding Schemes: -Physical Transmission of Bits- Chapter 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) CSC 3352 Computer Communications Chapter 2 2

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: n n n 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 n Computer Communications CSC 3352 3

Encoding Techniques Encoding techniques depend on the type of data to transmit and the medium being used: n Digital data on digital signal (our focus in this brief description. . . ) n Analog data on digital signal n Digital data on analog signal n Analog data on analog signal CSC 3352 Computer Communications Chapter 2 4

Digital Data, Digital Signal Digital signal: n Discrete, discontinuous voltage pulses n Each pulse is a signal element n Binary data encoded into signal elements CSC 3352 Computer Communications Chapter 2 5

A Simple Encoding Scheme 0 is Vo (some voltage) 1 is V 1 Example : let us encode 10011101 Clock Vo V 1 CSC 3352 Computer Communications Chapter 2 6

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? CSC 3352 Computer Communications Chapter 2 7

Bipolar Encoding (RZ Signal) +0. 85 V Hight. V + No. V –» 1 bit 1 is 0 0 Low. V + No. V –» 0 bit 0 is -0. 85 V Example : let us encode 01100010 Clock CSC 3352 Computer Communications Chapter 2 8

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: CSC 3352 Computer Communications Chapter 2 9

Alternative 1: Non-Return to Zero Two different voltages for 0 and 1 bits Voltage constant during bit interval n 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. . . CSC 3352 Computer Communications Chapter 2 10

Alternative 1: NRZ-Manchester Encoding 1 is 0 is +0. 85 V 0 Hight. V + Low. V –» 1 bit -0. 85 V 0 Low. V + High. V –» 0 bit Example : let us encode 01100010 Clock CSC 3352 Computer Communications Chapter 2 11

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 CSC 3352 Computer Communications Chapter 2 12

Differential Manchester Encoding 0 is 1 is 0 (Presence of transition) 0 (Abscence of transition) A transition in the middle of the bit is required anyway… Example : let us encode 10011101 Clock CSC 3352 Computer Communications Chapter 2 13