What is Far-End Crosstalk and Near-End Crosstalk

If you are at all familiar with copper cable testing, you have likely heard of crosstalk – the phenomenon where a signal transmitted on one pair or channel has an undesirable effect on another pair or channel.

Crosstalk can interfere with a specific pair of wires or the entire cable, causing bit errors or data failure. For example, have you ever heard someone else talking on the phone? This is caused by interference between adjacent phone lines.

But do you understand the difference between the NEXT and FEXT parameters when testing balanced copper network cabling systems? We thought it would be worth taking a closer look.

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Near-end crosstalk, abbreviated as NEXT, is a performance parameter for a single link/channel that measures the signal coupled from one pair of wires to another pair of wires. The pair causing interference is called the "disturbant pair", while the pair affected by the crosstalk is called the "disturbed pair".

NEXT is expressed in decibels (dB) and varies with the transmission frequency, with higher frequencies producing more interference. The larger the dB value, the less crosstalk is experienced by the interfered link/channel. For example, a Category 5e cable that can transmit up to 100 MHz may have a NEXT value of 45.8 dB at 20 MHz and a NEXT value of 35.3 dB at 100 MHz, indicating better NEXT performance at lower frequencies.

The measurement result is called "near-end" crosstalk because the crosstalk is measured at the source end of the link/channel.

Twisted pairs help to cancel out NEXT—different twist ratios on each pair prevent pairs from picking up signals from adjacent pairs. So it’s important to keep the pairs twisted as close to the terminals as possible. The twist ratio for each category is also optimized for crosstalk performance and improved isolation. So for a Category 6 cable that supports 250 MHz, the NEXT value at 100 MHz is 44.3 dB, while Category 5e has a NEXT value of 35.3 dB at the same frequency.

Power Sum Near-End Crosstalk, abbreviated as PSNEXT, is a simple sum of the NEXT measurements of all adjacent pairs. NEXT represents the crosstalk caused by three of the four pairs in the cable on the other pairs. PSNEXT is the sum of the crosstalk values ​​of all three adjacent pairs, indicating how severe the impact is on one pair when all four pairs in the network are transmitting signals (such as 1000BASE-T).

Last but not least, the NEXT test includes PSACRN – the Near-End Power and Attenuation Crosstalk Ratio (formerly PSACR, but renamed to distinguish it from PSACRF, described below). Calculated using PSNEXT and the insertion loss value (see the “Cable Testing 101 Q&A” series on insertion loss), it tells you the difference between the attenuation of each pair and the combined crosstalk received from the other three pairs. The goal is to ensure that the received signal is strong enough compared to the noise in the cable. The higher the PSACRN value, the better the performance.

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Far-end crosstalk, abbreviated as FEXT, is also measured within a channel. Far-end crosstalk has many similarities to NEXT, but is measured at the far end of the channel. However, FEXT alone does not account for the attenuation of the signal over a certain distance.

To provide a more informative result, attenuation (insertion loss) is removed from the FEXT result and is called equivalent far-end crosstalk. In recent years, TIA renamed this parameter as far-end attenuation-to-crosstalk ratio, or ACRF for short.

As with NEXT, the ACRF measurements for each of the three disturbing pairs are summed to produce the Power Sum ACRF (PSACRF). The PSACRF parameter used to be referred to as Power Sum ELFEXT (PSELFEXT) before TIA changed the parameter name.