Signal Encoding Techniques - Data Communications - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Chapter 5

Signal Encoding Techniques

Encoding Techniques

• Digital data, digital signal
• Analog data, digital signal
• Digital data, analog signal
• Analog data, analog signal

Terms (1)

• Unipolar —All signal elements have same sign
• Polar —One logic state represented by positive voltage the other by negative voltage
• Data rate —Rate of data transmission in bits per second
• Duration or length of a bit —Time taken for transmitter to emit the bit

Terms (2)

• Modulation rate —Rate at which the signal level changes —Measured in baud = signal elements per second
• Mark and Space —Binary 1 and Binary 0 respectively

Comparison of Encoding

Schemes (1)

• Signal Spectrum —Lack of high frequencies reduces required bandwidth —Lack of dc component allows ac coupling via transformer, providing isolation —Concentrate power in the middle of the bandwidth
• Clocking —Synchronizing transmitter and receiver —External clock —Sync mechanism based on signal

Comparison of Encoding

Schemes (2)

• Error detection —Can be built in to signal encoding
• Signal interference and noise immunity —Some codes are better than others
• Cost and complexity —Higher signal rate (& thus data rate) lead to higher costs —Some codes require signal rate greater than data rate

Nonreturn to Zero-Level (NRZ-L)

• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval —no transition I.e. no return to zero voltage
• e.g. Absence of voltage for zero, constant positive voltage for one
• More often, negative voltage for one value and positive for the other
• This is NRZ-L

Nonreturn to Zero Inverted

• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of signal transition at beginning of bit time
• Transition (low to high or high to low) denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding

Differential Encoding

• Data represented by changes rather than levels
• More reliable detection of transition rather than level
• In complex transmission layouts it is easy to lose sense of polarity

NRZ pros and cons

• Pros —Easy to engineer —Make good use of bandwidth
• Cons —dc component —Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission

Pseudoternary

• One represented by absence of line signal
• Zero represented by alternating positive and negative
• No advantage or disadvantage over bipolar-AMI

Bipolar-AMI and Pseudoternary

Biphase

• Manchester — Transition in middle of each bit period — Transition serves as clock and data — Low to high represents one — High to low represents zero — Used by IEEE 802.
• Differential Manchester — Midbit transition is clocking only — Transition at start of a bit period represents zero — No transition at start of a bit period represents one — Note: this is a differential encoding scheme — Used by IEEE 802.

Manchester Encoding