Attenuation-to-Crosstalk Ratio (ACR)

Attenuation-to-crosstalk ratio (ACR) is an indication of how much larger the received signal is when compared to the NEXT (crosstalk or noise) on the same pair. ACR is also sometimes referred to as the signal-to-noise ratio (SNR). It is a calculated value; you can't directly measure ACR. Also, despite the name, it's not really a ratio. It is the mathematical difference you get when you subtract the crosstalk value from the attenuation value at a given frequency. Note that ACR, and its power-sum equivalent, PSACR, is not a required parameter in TIA/EIN-568-B. Technically, SNR also incorporates not only noise generated by the data transmission but also outside interference. For practical purposes, the ACR and true SNR are functionally identical, except in environments with high levels of EMI.

KEY TERM: headroom

Because ACR represents the minimum gap between attenuation and crosstalk, the headroom represents the difference between the minimum ACR and the actual ACR performance values. Greater headroom is desirable because it provides additional performance margin that can compensate for the sins of cheap connectors or sloppy termination practices. It also results in a slight increase in the maximum bandwidth of the cable.

The differential between the crosstalk (noise) and the attenuation (loss of signal) is important because it assures that the signal being sent down a wire is stronger at the receiving end than any interference that may be imposed by crosstalk or other noise.

Figure 1 shows the relationship between attenuation and NEXT and graphically illustrates ACR for Category 5. (Category 5e, Category 6 and 6A would produce similar graphs.) Notice that as the frequency increases, the NEXT values get lower while the attenuation values get higher. The difference between the attenuation and NEXT lines is the ACR. Note that for all cables, a theoretical maximum bandwidth exists greater than the specified maximum in the standards. This is appropriate conservative engineering.

Figure 1: Attenuation-to-crosstalk ratio for a Category 5e channel link

 

Solving problems relating to ACR usually means troubleshooting NEXT because, short of replacing the cable, the only way to reduce attenuation is to use shorter cables. PSACR is similar to ACR but is the difference between insertion loss and NEXT.