Fluke DSP-2000 - Manual (Page 121)

Basic Cable Testing
Crosstalk and Near-End Crosstalk (NEXT)
7
7-9
Crosstalk and Near-End Crosstalk (NEXT)
Crosstalk is undesirable signal transmission from one cable pair to another nearby
pair. Like electrical noise from outside sources, crosstalk can cause
communication problems in networks. Of all the characteristics of LAN cable
operation, crosstalk has the greatest effect on network performance.
The test tool measures crosstalk by applying a test signal to one cable pair and
measuring the amplitude of the crosstalk signals received by the other cable pairs.
The crosstalk value is computed as the difference in amplitude between the test
signal and the crosstalk signal when measured from the same end of the cable.
This difference is called near-end crosstalk (NEXT) and is expressed in decibels.
Higher NEXT values correspond to less crosstalk and better cable performance.
All signals transmitted through a cable are affected by attenuation. Because of
attenuation, crosstalk occurring at the far end of a cable contributes less to NEXT
than crosstalk occurring at the near end of a cable. To verify proper cable
performance, you should measure NEXT from both ends of the cable.
Locating NEXT Problems
If the test tool reports a NEXT failure on a cable pair, you can use the TDX
analyzer to locate the source of the crosstalk problem.
Like the TDR results, the TDX analyzer results are presented in both a list and a
plot format. The list format shows the cable pairs tested, the peak magnitude of
crosstalk detected on the pairs, and the distance to the peak magnitude.
The TDX analyzer plot shows the location and magnitude of all crosstalk sources
detected on the cable. An example of a TDX analyzer plot from a test on a good
twisted pair cable is shown in Figure 7-6.