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an increase in attenuation.
You should keep this in mind in planning your design. The humidity effect is a
new consideration. Some cable manufacturers have stated that the humidity effect is
cumulative over time in PVC or plenum-rated PVC-compound jacketed cable and
may cause cable that originally tested good to fail after a period of exposure to high
humidity. The standard makes no allowance for the effects of humidity; the cable link
must pass the attenuation limits regardless of the humidity. This might be an argu-
ment against using the less expensive PVC insulation or plenum-rated PVC-com-
pound jacket and might also be a warning signal for some of the new copolymers that
contain other thermoplastics mixed with the more expensive TFE compounds.
Near-End Crosstalk Loss. The theory behind twisted-pair wiring is that the twist-
ing rotates the magnetic fields from the wire with the twist, and coupling to nearby
pairs or other objects is minimized. However, some coupling does exist. How much
coupling occurs between pairs of wires on the same cable is of particular interest. If
too much signal is coupled from, say, the transmit pair to the receive pair of a con-
nection, the receiver will not be able to distinguish the far-end signal from its own
transmissions. This coupling is illustrated in Fig. 15.4.
Coupling between pairs of a cable is called crosstalk . Because the crosstalk
between transmit and receive pairs is greatest when the transmitted signal is unaf-
fected by attenuation, the worst case occurs at the same end of the cable where a
test signal is transmitted. This is called the near end and yields the expression near-
end crosstalk, or NEXT.
NEXT is measured in dB below the test signal. Technically, NEXT would be a
negative dB number, but we turn it around by calling it NEXT loss. This means that
a large NEXT number in dB is considered low NEXT, as the amount of crosstalk is
much farther below the test signal. A high NEXT would be a smaller number in dB,
indicating much less isolation from pair to pair. Very logical, n'est-ce pas?
Signal
NEXT
FIGURE 15.4
The NEXT measurement. If too much signal is coupled from the transmit pair to the receive
pair of a connection, the receiver will not be able to distinguish the far-end signal from its own
transmission.
 
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