Digital Signaling: Encoding Schemes and Characteristics, Lecture notes of Engineering

Download Digital Signaling: Encoding Schemes and Characteristics and more Lecture notes Engineering in PDF only on Docsity! ECE230X Lecture 9 D. Richard Brown III Worcester Polytechnic Institute Electrical and Computer Engineering Department Adapted from Prentice Hall instructor resources Data and Computer Communications Eighth Edition By William Stallings Section 5.1 – “Digital Data, Digital Signals” Basics of Signal Encoding • Important function of the physical layer: Convert data (e.g. bits) to signals (e.g. voltages). • The signal must be designed to efficiently propagate through the medium. • The signal must also be designed so that the receiver can correctly interpret it. Signals->dataData->signals medium Data generated by higher layers Data received by higher layers Characteristics of Digital Signal Encoding Schemes • Signal spectrum  Less high frequency content means we can use cheaper cables or go longer distances without repeaters. • Clocking  The receiver needs to know where the start and end of each bit occurs.  Some signaling techniques make it easy on the receiver to determine the timing of the bits. • Error detection  Features built into the signaling scheme to detect errors. • Noise immunity • Cost and complexity Some Common Encoding Schemes NRZ-L NRZI Bipolar-AMI (most recent preceding 1 bit has Differential Manchester (O; 1) 0:0; 1; 1, 0;0, 0:41; py k ao¢ Nonreturn to Zero-Level (NRZ-L) • Two different voltages:  Logical 0 -> V1  Logical 1 -> V2 • Signal voltage held constant during bit interval  Unipolar: either V1 or V2 is equal to zero. The other voltage is usually positive, e.g. +5V.  Bipolar: V1 = -V2 Multilevel Binary: Bipolar-AMI • Three voltage levels: +V, 0, -V  Logical 0 -> output zero voltage  Logical 1 -> pulse at voltage +V or -V  Pulse transmitted with opposite polarity of last pulse  Signal voltage held constant during bit interval • Properties:  No loss of sync if a long string of ones  Long runs of zeros still a problem  No DC (zero-frequency) component  Better spectral properties than NRZ-L & NRZI  Some built-in error detection  e.g. two consecutive positive pulses: illegal! Multilevel Binary: Pseudoternary • Same idea as Bipolar-AMI • Three voltage levels: +V, 0, -V  Logical 1 -> output zero voltage  Logical 0 -> pulse at voltage +V or -V  Pulse transmitted with opposite polarity of last pulse  Signal voltage held constant during bit interval • Same properties of Bipolar-AMI  No advantage or disadvantages  Each used in different applications Multilevel Binary Issues • Loss of synchronization with long runs of 0’s or 1’s  Workaround: insertion of bits or scrambling • Not as efficient as NRZ  Each signal element only represents one bit  receiver distinguishes between three levels: +V, -V, 0  A 3 level system could represent log23 = 1.58 bits in each bit period  Requires approx. 3dB more signal power than NRZ for same of bit error rate (BER) Biphase Pros and Cons • Pros  Self clocking: every bit period guaranteed to have a mid-bit transition  No DC (zero-frequency) component  Some built-in error detection capabilities • Cons  Poor spectral containment (requires more bandwidth)  At least one transition per bit period (and possibly two)  Maximum modulation rate is twice NRZ Modulation Rate ————————————————————— 5 bits = 5 psec < > 1 1 1 1 1 NRZI <+—_> I bit = 1 signal element = I psec Manchester +> I bit = 1 signal element = I psec 0.5 usec Scrambling: A workaround for problems with multilevel modulation • Use scrambling to replace sequences that result in long periods of constant voltage • The replacement sequences must  produce enough transitions to maintain sync  be recognized by receiver & replaced with the original (intended) sequence  be same length as the original sequence • Design goals  have no dc (zero-frequency) component  have no long duration of constant voltage  have no reduction in data rate  provide some error detection capability B8ZS and HDB3 —_————— :1{150 1050 0:0 {0:0 {0:1 '1:0 {0:0 0:0 i1:0: Bipolar-AMI ooovBoVE B8ZS 00:0 :V:Bi0:0iV; | {Bi0:0 iv: HDB3 (odd number of 1s since last substitution) | B = Valid bipolar signal V = Bipolar violation Spectrum Comparison a Ci 1.4 -—- = B8ZS, HDB3 AMI = alternate mark inversion 3 arnt B8ZS_ = bipolar with 8 zeros substitution & 125 f y: HDB3 = high-density bipolar—3 zeros q NRZ-I i % NRZ-L = nonreturn to zero level = NDOT! at ‘ NRZI_ = nonreturn to zero inverted S OER NRO cents ® = = € = 12 SA¥ f = frequency = ro ™ R = data rate a “A ‘ & 0.8 | ; : 3 \ = ps <\ AML, pseudoternary > n2 i — ! ut z 0.6 ! ; A s 7 ot = i . c a? 41 g 04 rf af Manchester, = +} uA differential Manchester i sh 02h ff Nh “f A a ay sy BimiGe® eens iE | ] ] (Hoc pAeaie = peal 0.2 04 0.6 0.8 1.0 1.2 14 1.6 1.8 Figure Normalized frequency (f/R) 5.3 Spectral Density of Various Signal Encoding Schemes 2.0