14-06-2014, 03:02 PM
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Data Communications &
Computer Networks
Data Communications.pdf (Size: 304 KB / Downloads: 726)
Encoding Techniques
There are a number of transmission options
available today, depending on the encoding
technique
• There are four possible combinations of
encoding techniques
—Digital data, digital signal
—Digital data, analog signal
—Analog data, digital signal
—Analog data, analog signal
• We shall examine only the first two techniques
Unipolar
—All signal elements have same sign, i.e. all positive or
all negative
• 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
—eg. For a data rate R, the bit duration is 1/R
Interpreting Signals
• Receiver needs to know
—Timing of bits - when they start and end
—Signal levels
• What factors determine how successful the
receiver will be interpreting the incoming signal?
—Signal to noise ratio
—Data rate
—Bandwidth
—Encoding Scheme
Encoding Schemes
considerations
Signal Spectrum
—Lack of high frequencies reduces required bandwidth
—Lack of dc component also desirable since it allows ac
coupling via transformer, providing electrical isolation
—Concentrate tx power in the middle of tx bandwidth
• Clocking
—Synchronizing transmitter and receiver
—External clock
—Sync mechanism based on tx signal with suitable
encoding
Differential Encoding
• Data represented by changes rather than levels
• Benefits
—More reliable detection of transition in the presence
of noise rather than to compare a value to a
threshold level
—In complex transmission layouts it is easy to loose
sense of polarity of the signal
Disadvantages of Multilevel
Binary
• Not as efficient as NRZ
—Each signal element only represents one bit
—The line signal may take on one of 3 levels
—Each signal element, which could represent log23 = 1.58 bits
bears only one bit of information
• Receiver must distinguish between three levels
(+A, 0, -A) instead of two in NRZ
• Requires approximately 3dB more signal power for same
probability of bit error
—bit error for NRZ at a given SNR is much less than that for
multilevel binary