ELTCTL is one of two balance measurements found in ANSI/TIA-568-C.2, ANSI/TIA-1005 and ISO/IEC 11801 Edition 2.2 2011-06. The other is TCL.
The measurement is made by injecting a differential mode signal (DM) into a twisted pair, then measuring the common mode signal (CM) at the far end of the link on that same twisted pair. Technically speaking, that is TCTL. Since the amount of CM signal at the far end of the link is length dependent, the standards apply an equalization to take into account the Insertion Loss of the link. So what is actually reported is ELTCTL, a more meaningful measurement than TCTL.
The smaller the CM signal measured at the far end, the better the ELTCTL measurement (balance).
If it looks similar to an Insertion Loss measurement, you're right. Except an Insertion Loss measurement is made by injecting a DM signal into the pair and measuring the DM signal at the far end. Here we are measuring the CM signal at the far end, which is TCTL, then applying equalization based on the Insertion Loss of that pair to give ELTCTL.
Field test requirements for ANSI/TIA-568.2-D are deferred to ANSI/TIA-1152. ELTCTL is especially important for UTP cables in industrial premises as described by TIA-1005A and ISO 11801:3 for E1, E2 and E3 zones. E3 zones are generally near EMI emitting devices (VFDs, for example) and related wiring, and E1 is a data or control room environment. E2 is factory environment between zones E3 and E1. Field testers are normally capable of DM measurements only - the DSX CableAnalyzer is capable of both DM and CM measurements, hence its ability to measure ELTCTL and TCL.
If you wish to add ELTCTL and TCL to your standard Category 5e, 6, 6A or Class D, E or EA test, select your ISO or TIA folder in DSX and look for a 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. With a fantastic margin of 45.1 dB, you can feel confident that this link will perform well, even in a noisy environment. Don't forget to view the TCL measurement as well.
Why is balance important?
Ethernet transmits a balanced signal. The idea is that any noise injected into the cable is cancelled out, assuming the link has good balance. It also indicates how much signal the link emits.
If the link has poor balance, the noise injected into the cable becomes part of the signal. The imbalance in the link causes the injected voltage on the pairs to be unequal.
This has the potential of creating errors on the network, resulting in re-transmission of signals and slower network performance. This can be particularly problematic in Industrial Ethernet applications where latency is critical. In a data center where it is also particularly noisy and where transaction times are measured in mS, re-transmitting a signal could result in a delayed transaction.