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

International University Institute of Luxembourg (IUIL)Engineering

An overview of digital signaling, focusing on encoding schemes and their characteristics. Topics include the importance of signal encoding, the basics of digital signaling, and various encoding schemes such as Nonreturn to Zero-Level (NRZ-L), Nonreturn to Zero Inverted (NRZI), Multilevel Binary: Bipolar-AMI, and Multilevel Binary: Pseudoternary. The document also discusses issues with multilevel modulation and workarounds like scrambling.

Typology: Lecture notes

2021/2022

Uploaded on 08/05/2022

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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
Data and Computer Communications Eighth Edition
Eighth Edition
By William Stallings
By William Stallings
Section 5.1
Section 5.1 –
–
“
“Digital Data, Digital Signals
Digital Data, Digital Signals”
”
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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 Data and Computer Communications Eighth EditionEighth Edition By William StallingsBy William Stallings Section 5.1 Section 5.1 – – (^) ““Digital Data, Digital SignalsDigital 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. Data->signals (^) medium Signals->data Data generated by higher layers Data received by higher layers

Where is “digital” signaling used?

  • Often used for communication over

dedicated wired media

 Ethernet  RS-  Etc.

  • Not used for:  Wireless communication  Optical communication  Cable modems  Digital subscriber loops (DSL)

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

Nonreturn to Zero-Level

(NRZ-L)

  • Two different voltages:

 Logical 0 -> V

 Logical 1 -> V

  • 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 = -V

Nonreturn to Zero Inverted

  • Two voltages: V1 and V2 (can unipolar or

bipolar)

 Logical 1 -> transition from V1 to V2 or V2 to V  Logical 0 -> no transition

  • Signal voltage held constant during bit

interval

  • This is an example of “differential encoding”  data mapped to changes in signal level rather than actual levels  detection of a transition is often more reliable that detection of a level

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

BiPhase Encoding Method 1:

Manchester Encoding

  • Main idea: signal transition in middle of each bit period
  • Why do this? Transition serves as clock and data
  • Logical 1 -> transition from low to high
  • Logical 0 -> transition from high to low
  • Used by IEEE 802.3 (Ethernet LAN)

BiPhase Encoding Method 2:

Differential Manchester Encoding

  • Like regular Manchester: transition in each bit period
  • Differentially encoded  Logical 0 -> transition at start of bit period  Logical 1 -> no transition at start of bit period
  • used by IEEE 802.5 (token ring LAN)

Modulation Rate

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

High-Density Bipolar-3 Zeros

(HDB3)

  • Also based on bipolar-AMI
  • Scrambling specifics:  Data is buffered to detect strings of four consecutive zeros prior to transmission  Substitution based on polarity of preceding pulse (P) and number of ones transmitted since last substitution (N)  000- if P=- and N=odd number  000+ if P=+ and N=odd number  +00+ if P=- and N=even number  -00- if P=+ and N=even number  As before, these signals are illegal for bipolar-AMI. The receiver knows to interpret these patterns as four consecutive zeros.
  • B8ZS and HDB