What is ALSNR and why should you care?

Nov. 1, 2016
In September the IEEE ratified the 802.3bz specification, widely known as "2.5 and 5GBASE-T." 

In September the IEEE ratified the 802.3bz specification, widely known as "2.5 and 5GBASE-T." BZ's primary value proposition is that the installed base of Category 5e and Category 6 will support 2.5- and 5-Gbit/sec operation. To that end, the bz standard references a TIA document, TSB-5021, titled Guidelines for the Use of Installed Cabling to Support 2.5GBASE-T and 5GBASE-T. As of late October TSB-5021 was in the standards-creation step known as default ballot. While it would be an exaggeration to say the default ballot process is a formality, it would be appropriate to say that all involved have a high level of confidence that when the default balloting closes on December 2, TSB-5021 will be ready for publication.

In the meantime the NBASE-T Alliance, the prime mover of the 802.3bz specification, produced a technical paper titled NBASE-T Performance and Cabling Guidelines. It provides guidelines on how to evaluate the readiness of existing Category 5e, 6 and 6A copper cabling infrastructure for 2.5 and 5G. Specifically, the paper states in part, "Certification of category cabling requires measurements of 'internal' parameters such as insertion loss, return loss, and crosstalk. These standards use the Category 5e internal cabling specifications for 2.5GBASE-T, specified to 100 MHz. Cabling requirements for 5GBASE-T extrapolate these requirements to 250 MHz, which is within the frequency range specified for Category 6. This means that Category 6 cabling supports the internal parameter requirements of both 2.5GBASE-T and 5GBASE-T. It does not mean that 5GBASE-T cannot operate over Category 5e, only that additional testing may be needed for assurance. Assessment of Category 5e channels for the internal cabling requirements of 5GBASE-T may require retesting, or if originally measured to 250 MHz or above, those results may be reevaluated."

The paper introduces and describes alien limited signal to noise ratio (ALSNR), which is "a calculation that combines insertion loss, alien NEXT and alien FEXT to estimate the response of the PHY. This determines if the channel has adequate SNR for supporting the new data rates under worst-case conditions."

The paper also discusses factors such as installation quality and cable bundling that can affect performance, and mitigation steps that users can take to reduce the risk of existing cabling links failing to support 2.5 and 5G.

Patrick McLaughlin
Chief Editor
[email protected]

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