Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals

transcript

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 Signals• Digital signals are sequences of discrete voltage pulses

• Discrete - Takes on one of a finite number of voltages levels

• 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 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

• 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

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

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)

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

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

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 Encoding SummaryEfficiency No DC Self-clocking Error detect

Bipolar AMI

Manch.

Maximum

Maximum(differential)

Needs 3-4dBextra SNR

Needs 2xbandwidth

Major DC

DC only at 0V

No sustained DC

Not for stringsof zeros

Yes, but nottrivial

Yes. >=1 edgeper bit

Needs extrabitsNeeds extrabits

Bit-flipinvalid code

Bit-flipmiss transition

Encoding Data onto Digital Signals: Sending Digital Data with Digital Signals

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