Attenuation-to-crosstalk ratio (ACR), also called headroom, is the difference, expressed as a figure in decibels (dB), between the signal attenuation produced by a wire or cable transmission medium and the near-end crosstalk (NEXT).
In order for a signal to be received with an acceptable bit error rate, the attenuation and the crosstalk must both be minimized. In practice, the attenuation depends on the length and gauge of the wire or cable transmission medium, and is a fixed quantity. However, crosstalk can be reduced by ensuring that twisted-pair wiring is tightly twisted and is not crushed, and by ensuring that connectors between wire and cable media are properly rated and installed. Crosstalk can sometimes be reduced by replacing unshielded twisted pair (UTP) wiring with shielded twisted pair (STP) wiring.
The ACR is a quantitative indicator of how much stronger the attenuated signal is than the crosstalk at the destination (receiving) end of a communications circuit. The ACR figure must be at least several decibels for proper performance. If the ACR is not large enough, errors will be frequent. In many cases, even a small improvement in ACR can cause a dramatic reduction in the bit error rate.