Six Around One
Pete noted, “Another thing that happens when you have multiple signals in a cable—and we have four signal pairs here—is crosstalk. This is when signals from one wire couple onto another wire. There are various forms of crosstalk. Within a cable, there are different levels of crosstalk, or XT, energy at the near end of the cable than the far end. We call that NEXT and FEXT. You can also get crosstalk from one cable to another—that’s the infamous alien crosstalk.
Alien crosstalk is noise introduced from an adjacent channel. One way we introduce that is to intentionally create a worst-case scenario. See the cables we’ve bundled and tightly wrapped around this rack? There’s a red cable in the bundle called the victim, and there are six aggressors, or disturbers, around it. We send traffic over the six pairs around the one. Colloquially, we call it a ‘six around one’ test. We try to couple as much alien energy into that cable in the center as possible and see how well we do. This isn’t designed in the IEEE standard, but it’s more of a real-world type test.”