If wires are not tightly twisted, the result is Near End Crosstalk (NEXT). Most of us have experienced a telephone call where we could hear another conversation faintly in the background. This is crosstalk. In fact, the name crosstalk derives from telephony applications where 'talk' came 'across'. In LANs, NEXT occurs when a strong signal on one pair of wires is picked up by an adjacent pair of wires. NEXT is the portion of the transmitted signal that is electromagnetically coupled back into the received signal.

Results Interpretation
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). Because NEXT varies significantly with frequency, it is important to measure it across a range of frequencies, typically 1 -100 MHz. If you look at the NEXT on a 50 meter segment of twisted pair cabling, it has a characteristic "roller coaster going uphill" shape. That is, it varies up and down significantly, while generally increasing in magnitude. This is because twisted pair coupling becomes less effective for higher frequencies.

If the NEXT curve crosses the pass/fail line at any point, then the link does not meet the stated requirement. Since NEXT characteristics are unique to each end of the link, six NEXT results should be obtained at each end.

Troubleshooting Recommendations
In many cases, excessive crosstalk is due to poorly twisted terminations at connection points. All connections should be twisted to within 13 mm of the point of termination according to ANSI/TIA/EIA 568-B. An additional note common to all standards is that the amount of untwist should be kept to a minimum. Experience has shown that 13mm does not guarantee a PASS when field testing.

The first thing to do in the event of a NEXT failure is to use the field tester to determine at which end the NEXT failure occurred. Once this is known, check the connections at that end and replace or re-terminate as appropriate. If this does not appear to be the problem, check for the presence of lower Category patch cords (such as voice grade cable in a Class D installation). Another possible cause of NEXT failures are split pairs. These can be identified automatically with the wiremap function of your field tester. Female couplers are another high source of crosstalk and should not be used in a data installation. If a cable is not long enough, replace it with a cable of the required length rather than adding another cable.

Sometimes a NEXT failure is caused by an inappropriate test being selected. For example, you cannot expect a Category 5 installation to meet Category 5e performance requirements.

The best method for troubleshooting NEXT is to use a tester with Time Domain capabilities. This gives the tester the ability to show the fault by distance, pinpointing the problem. This diagnostic function clearly identifies the cause of the NEXT failure, whether it's the patch cord, connection, or horizontal cable.

In the event you have eliminated all of the above NEXT sources and are still experiencing NEXT failures, contact the system designer for further assistance.