What is the cable NEXT, FEXT, PS-,  ACR?

Crosstalk is signal interference between pairs that may be caused by a pair picking up unwanted signals from either adjacent pairs of conductors or nearby cables.

For example, this interference can result from the magnetic field that surrounds any current-carrying conductor. The crosstalk interference can be intelligible or unintelligible depending on the coupling modes.

Two types of crosstalk coupling losses are of interest to the telephone circuit designer, Near End Crosstalk(NEXT) and Far End Crosstalk (FEXT).

Near End Crosstalk (NEXT) is measured by applying and measuring the signal transfer level at the same end.

Far End Crosstalk (FEXT) is measured by applying the disturbing signal on one pair at the near end and measuring the signal transfer level on the disturbed pair at the far end.

Crosstalk may be reduced by twisting the pair. Where there is more than one pair in a cable, crosstalk between them is reduced by twisting them at different rates. For higher frequency use, the twist rate must be increased. The difference in twist rates also becomes more important. Screening and shielding are also used to reduce crosstalk.

Power-sum NEXT (PSNEXT)(or Power-sum FEXT (PSFEXT)) is actually a calculation, not a measurement. PSNEXT is derived from an algebraic summation of the individual NEXT effects on each pair by the other pairs.

The signal to noise relationship may also be expressed as the attenuation to crosstalk ratio (ACR).

ACR is the difference between the attenuated signal and the unwanted coupled signal and can be calculated as follows:

ACR = Minimum NEXT – Maximum Attenuation

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NEXT: Near-End Crosstalk

Definition: Near-End Crosstalk (NEXT) refers to the interference caused when a signal transmitted on one wire pair induces an unwanted signal on an adjacent wire pair at the same end of the cable where the transmission originates. 
How It Happens: Signals traveling through a cable generate an electromagnetic field. If the cable pairs are not properly shielded, this field induces crosstalk in neighboring pairs. 
Measurement: NEXT is measured in decibels (dB) and represents the ratio of the transmitted signal's power to the crosstalk power. Higher NEXT values indicate better cable performance. 
Formula: NEXT = 10 * log10(Psignal / Pcrosstalk) 
Importance: NEXT is critical for ensuring clear signals at the transmitting end, especially in high-speed networks.
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FEXT: Far-End Crosstalk

Definition: Far-End Crosstalk (FEXT) refers to the interference caused by a signal on one wire pair that induces an unwanted signal on another pair, measured at the far end of the cable. 
How It Happens: Similar to NEXT, but the measurement is taken at the receiving end, where the transmitted signal has already weakened due to attenuation. 
Measurement: FEXT is also expressed in decibels (dB). Higher FEXT values indicate lower interference at the far end. 
Formula: FEXT = 10 * log10(Psignal / Pinterference) 
Importance: FEXT impacts the signal-to-noise ratio (SNR) at the receiver, which affects data transmission reliability.
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PS-NEXT: Power Sum Near-End Crosstalk

Definition: Power Sum NEXT (PS-NEXT) is the cumulative interference caused by all adjacent pairs in a cable at the near end. 
How It Happens: Each wire pair in a multi-pair cable generates crosstalk. PS-NEXT accounts for the combined effect of all pairs on a single pair. 
Measurement: PS-NEXT is calculated as the power sum of the NEXT values of all adjacent pairs and is also measured in decibels (dB). 
Formula: PS-NEXT = 10 * log10(Σ(Psignal / Pcrosstalk)) 
Importance: It is particularly significant in environments with high cable density, such as data centers.
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ACR: Attenuation to Crosstalk Ratio

Definition: Attenuation to Crosstalk Ratio (ACR) is the difference between the signal attenuation (loss) and the level of interference (NEXT). 
How It Happens: As the signal travels through the cable, it weakens due to attenuation while being affected by crosstalk. ACR ensures the signal strength remains above the noise level. 
Measurement: ACR is measured in decibels (dB). Positive ACR values mean the signal is stronger than the interference. 
Formula: ACR = NEXT - Attenuation 
Importance: Higher ACR values improve signal quality and ensure error-free data transmission.
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Summary of Metrics

• NEXT: Near-end interference between cable pairs. 
  
• FEXT: Far-end interference between cable pairs. 
  
• PS-NEXT: Combined near-end interference from all other pairs. 
  
• ACR: Difference between signal strength and interference. 
  

Key Point: Higher dB values for these metrics mean less interference, better performance, and reliable data transmission.
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