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Fig. 7.19 Slew rate limiting to reduce induction

much less at the end of a long cable. This effect can be reduced
by using thicker cable that has less electrical resistance.

• Radiation The cable gets more and more like an aerial as its
length increases, and picks up significantly more radiated energy.
This effect can be reduced by using higher quality screening on
the cable.

• Induction/Capacitance As the length increases, the cable no longer
passes a true signal. In particular, the transition slopes are
modified and may cease to conform to the recommendation even
though they were generated correctly.

It is rather like a radio receiver being moved further and further from the
transmitter: eventually the receiver becomes unable to distinguish the
real signal from all the false signals.

The reason that V.28 is not explicit about length is that it is not possible
to be precise. There are so many factors affecting transmission, most of
which are invisible and difficult to measure. It is only possible to be
pessimistic and quote short lengths, in the full knowledge that longer
lengths are usually possible depending on the actual installation.

There are test instruments that try to determine the quality of the line,
but the only truly practical way of determining whether it is too long is
to run it with the DTE and DCE that will use it. The error rate should
then be carefully monitored by whatever means are available. This might
be done by looking at the number of Reject frames on an X.25 line, or by
gathering statistics from the application.

If there is a problem and the error rate is not acceptable, then line
drivers or modems will have to be used.

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