A white paper recently published by Superior Essex explains why, for 10GBase-T, alien crosstalk is a twisted-pair cabling system’s most-critical performance characteristic. The document, titled “Alien Crosstalk: The Limiting Noise Factor in Category 6A Channel Performance,” describes the importance of alien crosstalk, particularly in light of 10GBase-T transceivers’ ability to cancel internally generated noise.
Midway through the 11-page paper, Superior Essex explains, “Today’s 10GBase-T active components (PHY) employ cancellation technologies (>55 dB RL [return loss]; >40 dB NEXT [near-end crosstalk]; >20 dB FEXT [far-end crosstalk]) that are sufficient to effectively push internal RL/NEXT/FEXT well below the allowed AXT [alien crosstalk] noise level … As a result of this internal noise cancellation, the noise contribution from PSNEXT [power-sum near-end crosstalk] is more than 10 dB below the noise contribution in the channel from AXT. At the 40 dB NEXT cancellation level (typical of today’s PHY technology), internal NEXT becomes an insignificant contributor to total noise in the channel.” With that being the case, the paper says, cabling-system performance margin beyond Category 6A requirements for internal NEXT provides little if any benefit to 10GBase-T transmission. Rather, the cabling system’s alien crosstalk performance is key.
Additionally, the paper puts forth some in-the-field cabling-system deployment scenarios that further bolster the case for alien-crosstalk’s performance. “In the TIA-568-C.2 requirement for AXT, Cat 6A designs are tested in a 6-around-1 design using identical cables,” the paper says. “In actual installations, different Category cables, such as Cat 5e or Cat 6, are often installed adjacent to these Cat 6A cables. The impact of these adjacent cables can be significant if the Cat 6A cable is not sufficiently immune to AXT. All of these factors can introduce additional noise that the typical Cat 6A cabling system is not necessarily designed to protect against.” The paper includes results of tests Superior Essex conducted of a Category 6A-compliant cable surrounded by six Category 6 cables; the Category 6A cable failed the alien-crosstalk parameter by 7 dB. “This cable is not designed to protect against AXT from Category 6 or Category 5e cables,” the paper explained.
“Typical Cat 6A UTP cables are designed specifically to reject AXT from adjacent cables of the same design, i.e., other identical Cat 6A cables,” Superior Essex further explained. “When disturber cables are Cat 6 cables or from other manufacturers, AXT performance may suffer.”
Cables do exist, the paper points out, that are designed to suppress all forms of alien crosstalk. The paper reaches a firm conclusion: “The most effective way to add margin to the channel performance is to choose a cabling system that provides guaranteed AXT margin above the TIA-568-C.2 minimum requirements. The corollary to this finding is that Cat 6A cabling systems that provide guaranteed margin for NEXT, PSNEXT, FEXT, PSFEXT and RL are not providing the system with any additional true benefit in terms of lowering the total noise level in a Cat 6A channel.”