Properties Of Line Coding:
Digital data
can be transmitted by various transmission or line codes. Each has its
advantages and disadvantages. Among other desirable properties, a line code
should have the following properties:
1. Transmission
bandwidth: It should be as small as possible.
2. Power
efficiency: For a given bandwidth and a specified detection error
probability, the transmitted power should be as small as possible.
3. Error
detection and correction capability: It should be possible to detect, and
preferably correct, detection errors.
4. power
spectral density: It is desirable to have zero PSD at w = 0
(dc). because ac coupling and transformers are used at the repeaters.
Significant power in low-frequency components causes dc wander in the pulse
stream when ac coupling is used. The ac coupling is required because the dc
paths provided by the cable pairs between the repeater sites are used to
transmit the power required to operate the repeaters.
5. Adequate
timing content: It should be possible to extract timing or clock
information from the signal.
6. Transparency:
It should be possible to transmit a digital signal correctly regardless of
the pattern of 1's and 0's. We will see that a long string
of 0's could cause errors in timing extraction in on-off and bipolar
cases. If the data are so coded that for every possible sequence of data the
coded signal is received faithfully, the code is transparent. A
line code in which the bit pattern does not affect the accuracy of the timing
information is said to be a transparent line code.
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