Definition of Near-end Crosstalk (NEXT) on Network Encyclopedia.
What is NEXT (near-end crosstalk)?
NEXT is a measurement of the ability of cabling to reject crosstalk. Crosstalk is an undesirable condition in which the signals traveling through adjacent pairs of wire in twisted-pair cabling interfere with each other.
Near-end crosstalk (NEXT) measures the ability of a cable to reject crosstalk between pairs of wire at the near end of the circuit.
The pair causing the interference is called the “disturbing pair,” while the pair experiencing the interference is the “disturbed pair.” Channel NEXT is the NEXT value measured between one wire pair and another in the same cable; it is measured at both ends of the wire.
The NEXT types are as follows:
- Pair-to-Pair NEXT: NEXT between adjacent pairs of wire in a twisted-pair cable. A typical four-pair (eight-wire) unshielded twisted-pair (UTP) cable has six possible values for pair-to-pair NEXT, which are then averaged. This simple measurement is not adequate, however, because every pair of wire generates crosstalk with every other pair in the cable.
- Power Sum NEXT (PS NEXT): A more rigorous way of rating a cable’s crosstalk that measures the total amount of crosstalk between one wire pair and all its neighboring pairs in the same cable. PS NEXT is particularly important for cabling used in high-speed networks such as Gigabit Ethernet and Asynchronous Transfer Mode (ATM) networks.
- Far-End Crosstalk (FEXT): A measurement of how the far end of one wire pair affects the near end of another pair.
Desired Data Rate and the Cable’s Minimum NEXT Value
|Frequency||Minimum NEXT Value|
|4 MHz||53 dB/1000 feet|
|10 MHz||47 dB/1000 feet|
|20 MHz||42 dB/1000 feet|
|1000 MHz||32 dB/1000 feet|
To minimize NEXT in installations of category 5 cabling, do not expose more than 2 inches of wire pairs at the termination point of the cable (the patch panel, wall plate, or RJ-45 connector). Also, do not untwist the wire pairs more than half an inch.