In our previous article, we discussed types of crosstalk, and especially Near-End Crosstalk (NEXT) and Far-End Crosstalk (FEXT).
If you still want to read about crosstalk 101, and NEXT & FEXT, click here to read our first article in this series!
Today, let’s delve even deeper!
Attenuation-to-Crosstalk Ratio (ACR-F and ACR-N)
This ratio is an indication of how much bigger a received signal is when it is compared to the FEXT or NEXT crosstalk on the same pair. ACR is also known as the signal-to-noise ratio (SNR) as the value represents the ratio between the strength of the received data signal and the strength of the noise due to crosstalk from end signals.
Technically, SNR incorporates not only noise generated by the data transmission, but also outside interference. True SNR and ACR are functionally identical for all practical purposes, except in environments with high levels of EMI.
Despite the name however, it is not really a ratio, but rather the mathematical difference obtained when subtracting the crosstalk value from the attenuation value at a given frequency. This value is calculated and ACR can’t be measured directly. ACR-N is calculated by subtracting the attenuation of the disturber pair from the near-end crosstalk (NEXT) on the disturbed pair.
ACR-F (formerly known as ELFEXT) is calculated by subtracting the attenuation of the disturber pair from the far-end crosstalk (FEXT) on the disturbed pair. Note that although ACR-N, and its power-sum equivalent, PSACR-N, is not a required parameter in TIA/EIN-568-C, it is required in ISO/IEC 11801 Ed. 2.2.
KEY TERM – headroom
As ACR represents the minimum gap between crosstalk and attenuation, headroom represents the difference between the actual ACR and the minimum ACR performance values.
Bigger headroom is wanted as it provides an additional performance margin that can compensate for the sins of sloppy termination practices and cheap connectors. It also results in the maximum bandwidth of the cable increasing slightly.
The differential between the attenuation (loss of signal) and the crosstalk (noise) is important as it guarantees that the signal that is sent down a wire is stronger at the receiving end than any interference that might be imposed by crosstalk or any other noise.
The relationship between NEXT and attenuation is shown in ﬁgure 1 below and illustrates ACR for Category 5. (Category 5e, Category 6, and Category 6A will produce similar graphs.)
Figure 1: Attenuation-to-crosstalk ratio (ACR) for CAT5e
Notice that as the frequency increases, the NEXT values decrease while the attenuation values increase. The difference between the NEXT and attenuation lines on the graph is the ACR-N. Note that a theoretical maximum bandwidth exists for all cables that is bigger than the maximum specified in the standards. This is appropriate conservative engineering.
The measured value of FEXT is used to determine equal-level far-end crosstalk (ACR-F). Less FEXT will be seen on a longer cable than a shorter one because the signal at the receiving side will have more distance over which it can attenuate.