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What is PSNEXT, ELFEXT, Impedance and ACR?

What is PSNEXT, ELFEXT, Impedance and ACR?. By: Thomas Olivotti. PSNEXT. PSNEXT stands for Power Sum Near End Crosstalk and is calculated by transmitting a signal down each wire pair in turn and summing all the NEXT values recorded on a particular wire pair.

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What is PSNEXT, ELFEXT, Impedance and ACR?

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  1. What is PSNEXT, ELFEXT, Impedance and ACR? By: Thomas Olivotti

  2. PSNEXT • PSNEXT stands for Power Sum Near End Crosstalk and is calculated by transmitting a signal down each wire pair in turn and summing all the NEXT values recorded on a particular wire pair. • Four PSNEXT values need to be calculated at each end of the link. Like NEXT, the higher the decibel value of PSNEXT the better, since this indicates less crosstalk.

  3. PSNEXT Continued On Gigabit 1000BASE-T LANs where all four wire pairs are used to transmit data, PSNEXT is an important measurement, although it is not required in the IEEE 802.3ae specification.

  4. ELFEXT • ELFEXT stands for Equal Level Far End Crosstalk and is calculated by subtracting the attenuation measured on the wire pair, the signal is sent on from the FEXT measured on an affected wire pair. Both these measurements are taken at the far end of the link. This removes the affect of cable length on the results.

  5. Impedance • Is a measure, in ohms, of the opposition to the flow of current in a cable. • Must be constant over the operating bandwidth of the cable. UTP is commonly 100ohms or 120 ohms. • This measurement is the key to the quality of the cable.

  6. Impedance Continued • Testing for average impedance can help identify: • Physical damage to the cable • Connector defects • Kinks • Improper setting of NVP

  7. ACR • 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).

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