Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals
description
Transcript of Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals
![Page 1: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/1.jpg)
Digital Data Encoding No. 1Seattle Pacific University
Encoding Data onto Digital Signals:Sending Digital Data with Digital Signals
Based on Chapter 5 of William Stallings, Data and Computer Communication, 8th Ed.
Kevin BoldingElectrical Engineering
Seattle Pacific University
![Page 2: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/2.jpg)
Digital Data Encoding No. 2Seattle Pacific University
Digital Signals• Digital signals are sequences of discrete voltage pulses
• Discrete - Takes on one of a finite number of voltages levels
Sou
rce:
Sta
lling
s, F
ig. 3
.7
Idea
lA
ctua
l
• Pulses - Ideally, square pulses with zero rise/fall time• Limited by finite bandwidth of medium • Approximated by a number of superimposed sine
waves A single digital signaltakes the entire bandwidth ofthe transmission medium
Digital signals cannot (normally) be combined in the same medium
![Page 3: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/3.jpg)
Digital Data Encoding No. 3Seattle Pacific University
Digital Signal Encoding• Goals:
• Efficient• bps maximized for given SNR
and bandwidth
• No DC component• Signal never stuck at a
constant voltage
• Self-clocking• Can recover clocking
information from the data stream
• Reality:• Tradeoff efficiency for other
goals
• Requires either:• Extra signal events• Extra levels in signal
• Requires regular transitions in signal
• Similar to No DC Component
• Error detection/recovery• Correct data in the
presence of errors
• Requires extra data• Subject of another
discussion
![Page 4: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/4.jpg)
Digital Data Encoding No. 4Seattle Pacific University
Non Return to Zero (NRZ)• NRZ
• Zero Voltage --> binary ‘1’• Nonzero Voltage --> binary ‘0’
• Multiple 0’s or 1’s in a row result in DC voltage
0 1 0 1 1 1 0 1 0 0 0 1 0 1
• NRZI (invert ones)• Transition at beginning of bit
time --> binary ‘1’• Differential coding
• Easier to detect transitions than levels
• Multiple 0’s in a row still result in DC voltage
0 1 0 1 1 1 0 1 0 0 0 1 0 1
NRZ:EfficientDC componentHard to recover clock
![Page 5: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/5.jpg)
Digital Data Encoding No. 5Seattle Pacific University
Multilevel Codes• Bipolar AMI (Alternate Mark
Inversion)• Three levels (-,0,+)• Zero Voltage --> binary ‘0’• +/- Voltage --> binary ‘1’• 1’s must alternate polarity
• Pseudoternary• Same thing, just switch 1’s
and 0’s
0 1 0 1 1 1 0 1 0 0 0 1 0 1
• Takes three levels, but only one bit per signal event• Not as efficient as NRZ (needs
3-4dB higher SNR)
Code violation
1’s alternate, but 0’s still can be flatlined
0 1 0 1 1 1 0 1 0 0 0 1 0 1
Multilevel codes:Needs 3-4dB higher SNRErrors produce code violationsOnly DC component is at zero
• Errors produce code violations (consecutive 1’s of same polarity)
0 1 0 1 1 1 0 1 0 0 0 1 0 1Bit Flip (1-> 0)
![Page 6: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/6.jpg)
Digital Data Encoding No. 6Seattle Pacific University
Modified Multilevel Codes• Bipolar AMI still has flatline for
multiple zeros• B-AMI has excess capacity (code
violations)• Substitute a special code when
too many zeros in a row
Substituted block
Multiple zeroes - Flatline
• B8ZS• If eight zeros in a row occur:
• If last ‘1’ was +, then• 000+-0-+
• If last ‘1’ was -, then• 000-+0+-
0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0
0 1 0 1 0 0 0 0 0 0 0 0 0 0 1 0
Code violations
Eight Zeros
B8ZS:Error detectionNo sustained DCDecent clock recoveryNeeds 3dB higher SNR
• Forces two code violations, so we know it’s not 00011011• One code violation might be a
regular error, but two is unlikely
![Page 7: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/7.jpg)
Digital Data Encoding No. 7Seattle Pacific University
Manchester Codes
• Manchester - Mid-bit change• Low-to-high --> ‘1’• High-to-low --> ‘0’• Consecutive 1’s or 0’s require a transition between bits
• Doubles the bandwidth requirementUp to two transitions per bit
Manchester codes:No DC componentEasy clock recoveryError detectionTwice the bandwidth needed
0 1 0 1 1 1 0 1 0 0 0 1
![Page 8: Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals](https://reader030.fdocuments.in/reader030/viewer/2022033022/5681632d550346895dd3a76d/html5/thumbnails/8.jpg)
Digital Data Encoding No. 8Seattle Pacific University
Digital Encoding SummaryEfficiency No DC Self-clocking Error detect
NRZ
NRZI
Bipolar AMI
B8ZS
Manch.
Maximum
Maximum(differential)
Needs 3-4dBextra SNR
Needs 3-4dBextra SNR
Needs 2xbandwidth
Major DC
Major DC
DC only at 0V
No sustained DC
No DC
No
No
Not for stringsof zeros
Yes, but nottrivial
Yes. >=1 edgeper bit
Needs extrabitsNeeds extrabits
Bit-flipinvalid code
Bit-flipinvalid code
Bit-flipmiss transition