CDNEXT (Common Mode to Differential Mode Near-End Xtalk) - DSX CableAnalyzer
CDNEXT is a troubleshooting tool only at this time for Channel Alien Xtalk testing issues. There are no test limits in ANSI/TIA-568-C.2, ANSI/TIA-1005 or ISO/IEC 11801:2010. However, you will find it discussed in TIA-TSB-1197.
Poor CDNEXT is primarily a connector choice issue. Although unlikley, it could be a workmanship issue too.
The measurement is made by injecting a differential mode signal (DM) into one twisted pair, then measuring the common mode signal (CM) on another pair. The smaller the CM signal returned, the better the CDNEXT.
If it looks similar to a NEXT measurement, you're right. Except a NEXT measurement is made by injecting a DM signal onto one pair and measuring the DM signal on another pair. Here we are measuring the CM signal on the other pair, which is CDNEXT.
Other balance measurements such as TCL, ELTCTL and CMRL are also available in the DSX-5000 CableAnalyzer.
If you wish to add CDNEXT to your standard Category 5e, 6, 6A or Class D, E or EA test, select your test limit with a suffix of (+ALL):
This will increase your AUTOTEST time.
The suffix of (+All) indicates a standard ANSI/TIA or ISO/IEC test with the addition of ELTCTL, TCL, Resistance Unbalance within a pair, Resistance Unbalance between pairs, CMRL and CDNEXT measurements.
In the example below from a DSX-5000 CableAnalyzer, a Category 6A Channel was tested.