encoding data onto digital signals: sending digital data with digital signals
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Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals. Based on Chapter 5 of William Stallings, Data and Computer Communication, 8 th Ed. Kevin Bolding Electrical Engineering Seattle Pacific University. Ideal. Source: Stallings, Fig. 3.7. Actual. Digital Signals. - PowerPoint PPT PresentationTRANSCRIPT
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
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
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• 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
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
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
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)
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
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
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