101 Series: What are TCL and ELTCTL?

Maybe you’ve seen TCL and ELTCTL listed on your cable spec sheet, or perhaps you’ve noticed them on your DSX CableAnalyzer, and you’re simply not familiar with these parameters, what they indicate and if they are required. Let’s take a closer look.

What is Mode Conversion?

In our last 101 Series: Understanding Copper Cabling’s Balancing Act, we explained the difference between common mode (unbalanced) and differential mode (balanced).

Just to reiterate, Ethernet signals are applied in differential mode to conductors of a pair where opposite positive and negative voltages reference each other. In contrast, noise signals are injected into the conductors of a pair in common mode, which travels simultaneously and are referenced to ground.

When noise is injected into a cable, a portion of this common mode signal can be converted to differential mode and become a part of the Ethernet signal. This phenomenon, referred to as mode conversion, is detrimental to Ethernet transmission because it can cause the differential signal of Ethernet to no longer be balanced, leading to potential bit errors and retransmissions.

How is it Measured?

Mode conversion is measured via two mode conversion parameters – TCL and TCTL.

Transverse Conversion Loss (TCL)-, measures mode conversion within a pair at one end. This is done by injecting a differential mode signal onto the pair and measuring any common mode signal returned on the same pair. The smaller the common mode signal returned, the better the balance.

Transverse Conversion Transfer Loss (TCTL) measures mode conversion within a pair at the opposite end. Instead of measuring any common mode signal returned, TCTL is measured by injecting a differential mode signal onto the pair and measuring any common mode signal at the other end of the link. Again, the smaller the common mode signal, the better the balance.

Because the amount of common mode signal in TCTL testing is dependent on the length of the link due to insertion loss, equalization must be applied to take this into account. That’s where we get ELTCTL, or Equal Level TCTL.

What Do They Indicate?

Since noise is injected into a cable in common mode, TCL and ELTCTL are excellent indicators of noise immunity and whether a cable link will provide adequate performance in noisy environments – including alien crosstalk from neighboring cables in high speed 10GBASE-T applications or noise from other external sources such as those found in industrial environments.

Is TCL and ELTCTL Testing Required?

The short answer to this questions is yes and no. The latest industry standards require TCL and ELTCTL testing by manufacturers to establish conformance. That is why you are seeing these parameters listed on spec sheets.

The standards however do not require field testing of TCL and ELTCTL – most likely due to lack of knowledge surrounding these parameters and a lack of field testing capabilities at the time the standards were developed. But not field testing for TCL and ELTCTL means having to trust manufacturer claims.

The good news is that because of the DSX CableAnalyzer’s unique ability to measure both differential and common mode signals, it can test for both TCL and ELTCTL alongside standard field testing for all the other performance parameters you’re familiar with – NEXT, PSNEXT, insertion loss, return loss, etc.

Testing for these TCL and ELTCTL mode conversion parameters only adds about 6 seconds to the typical test time. Not only does this give you peace of mind in manufacturer claims, but this method of determining noise immunity is far less time consuming compared to complex field testing for alien crosstalk.