Di Digital Transm smissi ssion on 01204325 Data Communications and Computer Networks Chaipo Chaiporn J n Jaik aikae aeo Department of De f Computer Engineering Kasetsart Unive versity Based on lecture materials from Data Communications and Networking, 5 th ed., Behrouz A. Forouzan, McGraw Hill, 2012. Revised 2019-08-12
Out Outline line • Line coding • Encoding considerations • DC components in signals • Synchronization • Various line coding methods • Analog to digital conversion 2
Line Cod Li Coding • Process of converting binary data to digital signal 3
Si Signa gnal v vs. D . Data E Eleme ments 1 data element = 1 symbol 4
En Encod oding Con Consi siderati tion ons • Signal spectrum ◦ Lack of DC components ◦ Lack of high frequency components • Clocking/synchronization • Error detection • Noise immunity • Cost and complexity 5
DC Com DC Compon onents ts • DC components in signals are not desirable ◦ Cannot pass thru certain devices ◦ Leave extra (useless) energy on the line ◦ Voltage built up due to stray capacitance in long cables v Signal with DC component t v Signal without DC component t 6
Synchronization Sy • To correctly decode a signal, receiver and sender must agree on bit interval 0 1 0 0 1 1 0 1 Sender sends: v 01001101 t 0 1 0 0 0 1 1 0 1 1 Receiver sees: v 0100011011 t 7
Pr Providing Synchronizat ation • Separate clock wire data Sender Receiver clock • Self-synchronization 0 1 0 0 1 1 0 1 v t 8
Li Line Cod Coding Me Meth thod ods • Unipolar ◦ Uses only one voltage level (one side of time axis) • Polar ◦ Uses two voltage levels (negative and positive) ◦ E.g., NRZ, RZ, Manchester, Differential Manchester • Bipolar ◦ Uses three voltage levels (+, 0, and –) for data bits • Multilevel 9
Uni Unipo polar • Simplest form of line coding • Only one polarity of voltage is used • E.g., polarity assigned to 1 (TTL) 0 1 0 0 1 1 0 0 5V t 10
Po Polar Encoding • Two voltage levels (+,-) represent data bits • Most popular four ◦ Nonreturn-to-Zero (NRZ) ◦ Return-to-Zero (RZ) ◦ Manchester ◦ Differential Manchester 11
NR NRZ E Enc ncoding ding • Nonreturn to Zero ◦ NRZ-L (NRZ-Level): Signal level depends on bit value 0 1 0 0 1 1 1 0 t ◦ NRZ-I (NRZ-Invert): Signal is inverted if 1 is encountered ? 1 0 0 1 1 1 0 t 12
RZ RZ Encoding • Return to Zero ◦ Uses three voltage levels: +, - and 0, but only + and - represent data bits ◦ Half way thru each bit, signal returns to zero 0 1 0 0 1 1 0 0 t 13
Ma Manchester r En Encod oding • Uses an inversion at the middle of each bit ◦ For bit representation ◦ For synchronization = 0 0 1 0 0 1 1 0 1 t = 1 14
Di Differential Manchester Encod oding • The inversion on the middle of each bit is only for synchronization • Transition at the beginning of each bit tells the value 0 1 0 0 1 1 0 1 t 15
Bi Bipol olar r En Encod oding • Bipolar encoding uses three voltage levels: +, - and 0 • Each of all three levels represents a bit • E.g., Bipolar AMI (Alternate Mark Inversion) ◦ 0V always represents binary 0 ◦ Binary 1s are represented by alternating + and - 0 1 0 0 1 1 0 1 t 16
B n ZS ZS Schemes • B n ZS – Bipolar n -zero substitution ◦ Based on Bipolar AMI ◦ n consecutive zeros are substituted with some +/- levels ◦ provides synchronization during long sequence of 0s ◦ E.g., B8ZS 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Bipolar AMI t 0 0 0 V B 0 V B B8ZS t V – Bipolar violation B – Valid bipolar signal 17
m B n L Sche Scheme mes • m data elements are substituted with n signal elements • 2B1Q (two binary, 1 quaternary) Bit sequence Voltage level 00 11 01 10 01 10 11 00 00 -3 +3 +1 01 -1 t 10 +3 -1 11 +1 -3 • 8B6T (eight binary, six ternary) 18
Bl Bloc ock Cod Coding • Improves the performance of line coding • Provides ◦ Synchronization ◦ Error detection Line Division Substitution Coding t …01011010001… : : 0010 10110 1101 01011 0001 01010 : : 19
4B/ 4B/5B 5B Encod oding Table Data Code Data Code Data Code 0000 11110 1000 10010 Q (Quiet) 00000 0001 01001 1001 10011 I (Idle) 11111 0010 10100 1010 10110 H (Halt) 00100 0011 10101 1011 10111 J (start delimiter) 11000 0100 01010 1100 11010 K (start delimiter) 10001 0101 01011 1101 11011 T (end delimiter) 01101 0110 01110 1110 11100 S (Set) 11001 0111 01111 1111 11101 R (Reset) 00111 20
An Analog to Digital Conversion • Pulse Amplitude Modulation (PAM) ◦ Converts an analog signal into a series of pulses by sampling PAM PAM signal Analog signal (Sampled analog data) 21
Pu Pulse Code Modulat ation (PCM) • Converts an analog signal into a digital signal ◦ PAM ◦ Quantization ◦ Binary encoding ◦ Line coding 22
PC PCM: Q : Quan uantiz izatio ion • Converts continuous values of data to a finite number of discrete values 6 4 Output 2 0 1 2 3 4 5 6 7 Input 23
PC PCM: Q : Quan uantiz izatio ion Quantization 24
Qua Quantization E n Error • Assume sine-wave input and uniform quantization ◦ n b is the number of bits per sample • Known as the 6 dB/bit approximation See also: http://en.wikipedia.org/wiki/Quantization_error#Quantization_noise_model 25
Ex Example: Quanti tizati tion on Err Error or • A telephone subscriber line must have an SNR dB above 40. What is the minimum number of bits per sample? Solution We can calculate the number of bits as Telephone companies usually assign 7 or 8 bits per sample. 26
PC PCM: B : Binar inary E y Enc ncoding ding • Maps discrete values to binary digits 27
PC PCM: T : The Who he Whole P le Proces cess 28
Mi Minimum Sampling Rate • Nyquist Theorem: Sa Samp mpling ra rate mu must b t be g gre reater th r than tw twice th the highe highest fr frequency Ex. Find the maximum sampling interval for recording human voice (freq. range 300Hz – 3000Hz) t sampling interval 29
Ny Nyquis quist’s Sampling The Sampling Theorem Sampling demonstration See also: Wagon-wheel effect 30
Ex Example: Sa Sampling and Bit Rate • Calculate the minimum bit rate for recording human voice, if each sample requires 60 levels of precision 31
Summa Summary • Line coding and block coding • Digital signal consideration ◦ Bit rate ◦ Symbol rate ◦ DC component ◦ Synchronization • Analog-to-digital conversion ◦ Pulse-Code Modulation ◦ Minimum sampling frequency 32
Recommend
More recommend
