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The NEXT value for a given cable type is generally expressed in decibels per feet or decibels per 1000 feet. NEXT value varies with the frequency of transmission. The higher the NEXT value, the greater the cable’s ability to reject crosstalk at its local connection.

I'm trying to understand how one would calculate NEXT (Near-end Crosstalk). I assume the effect would be greatest at the source of the noise?

So, given some arbitrary figures:

  • signal power at source 1 watt
  • NEXT crosstalk 10dB/1000 feet.

I have been told that it's similar to a SNR ratio.

Does this mean that at 1000 feet, we would expect one tenth of the signal to leak to the adjacent pair?

How would one calculate the crosstalk closer to the source?

I suspect I'm missing a couple of pieces of the puzzle, any help would be greatly appreciated.

T.I.A.

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When you send a signal on a wire pair, a copy of that signal is induced on the adjacent pairs in two directions. The induced signal back toward the sending point is call NEXT (Near-end Crosstalk), and the induced signal heading away from the sending point is called FEXT (Far-end Crosstalk). This signal induction happens along the length of the cable (a long wire is an antenna). When you send a signal from one end of a cable, you measure NEXT at that end of the cable, and you measure FEXT at the other end of the cable. It's not simply the signal induced at the near end which is measured, but from the whole cable length, since the far end of the cable will also be inducing a signal back toward the sending end, as will happen along the whole cable length. That's why NEXT is measured per a particular length.

You measure NEXT and FEXT to determine the resistance of the cabling to a signal induced from within the cable sheath. These values are measured on all pairs, and the worst values are used to rate the cable. Since NEXT is a measure of difference in signal strength between a disturbing pair and a disturbed pair, a larger number (less crosstalk) is more desirable than a smaller number (more crosstalk). NEXT varies by frequency, and you must measure it at different frequencies.

It's not like SNR, which would compare a received signal (on the far end) to noise on the line.

Given your 10 dB number for NEXT, that would mean the signal returning toward the sender on a different pair is 1/10 as strong as the signal sent out. This would be a very bad NEXT number. For example, on Category-5E horizontal cabling, you need a minimum NEXT value of 50.3 dB at 10 MHz per 100 meters, and 35.3 dB at 100 MHz per 100 meters.

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