Alien crosstalk dialogue
With the 10-GBASE-T standard on the verge of being approved, understanding the various nuances of AXT testing will become critical.
May 1, 2006
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As the 10GBASE-T standard nears completion, it is important to understand the implications of alien crosstalk testing for end users in support of 10 Gb/s Ethernet.
Although there is no guarantee that existing cabling will meet the new alien crosstalk performance criteria, there is an expectation that the requirements can be met for Category 6 cabling for distances up to 55 meters under “reasonable worst case” conditions.
What is reasonable is assumed to mean cables that are loosely bundled (tie-wrapped every five feet) or installed in a conduit with a maximum 40% fill. The problem is that there has been limited field test data to fully validate these assumptions.
A few vendors have contributed alien crosstalk measurement data to the TIA TR 42.7 subcommittee that is developing TSB-155 “Guidelines for the Assessment and Mitigation of Installed Category 6 Cabling to Support 10GBASE-T.”
These measurements indicate that the alien crosstalk performance criteria are not always met under some conditions in the field.
The culprit tends to be the alien Far End Crosstalk, which is very much affected by how tightly the cables are bundled together. The most recent information indicates that there is a grey zone between 37 metres and 55 metres where Category 6 cabling may or may not meet the alien crosstalk guidelines in TSB-155.
An example of such a condition would be bundled cables that are installed in a vertical run supported by Kellum’s grips (a wire mesh that grips the cables) on every floor or every two or three floors.
IEEE is on schedule to publish the 10GBASE-T standard sometime this summer. There is also a chance that TIA will approve the TSB-155 bulletin at our next meeting in June or by October at the latest.
Both these documents recommend a certain amount of alien crosstalk testing to assess the performance of existing installations in the field. It is also a necessary step to validate the effectiveness of mitigation techniques.
Field instruments for measuring alien crosstalk are not yet commercially available, but are expected to be available soon.
And while there were some product announcements at the BICSI show in Orlando, it may not be necessary to purchase a brand new test tool.
Field test strategy
Testing could be implemented using a specific adapter and upgraded software. We have had some good experience using a prototype evaluation unit from a field test vendor.
Alien crosstalk measurements in the field can be time consuming. An exhaustive analysis for all possible alien crosstalk combinations is often not practical.
The overall field test strategy consists of identifying worst case bundling conditions (from the cabling topology and through visual inspection) and then testing a sufficient number of ports to assure performance using statistical techniques.
Both alien NEXT and alien FEXT should be tested using channels adjacent to the disturbed channel where a large number of cables join (i.e., a patch panel).
The alien crosstalk noise levels are dependent on the number and proximity of adjacent cables and connectors.
The alien crosstalk is reduced as the separation between adjacent cables and connectors increases.
If the channel does not meet either the PSANEXT or PSAELFEXT limits independently, then the combined alien crosstalk noise is computed and compared to the alien crosstalk noise budget for 10GBASE-T in the frequency range from 10 MHz to 400 MHz.
The Alien Crosstalk Margin Computation (ACMC) needs to be greater than zero for the worst pair and for the average of all four pairs in the disturbed channel.
A number of test strategies to mitigate alien crosstalk in the field include selective deployment of 10GBASE-T utilizing non-adjacent patch panel positions, separating the equipment cords and the patch cords and un-bundling the horizontal cabling, utilizing equipment cords sufficiently specified to mitigate the alien crosstalk coupling such as Augmented Category 6, reconfiguring the cross-connect as an interconnect, and replacing connectors with Augmented Category 6.
I am looking forward to seeing what develops in the industry as 10GBASE-T equipment becomes available in the market.
Something that appears to be alien today may be quite commonplace or familiar in a year from now.
Or as they say in Europe, something that is exogenous (alien) today may be indigenous to our industry in a few years.
Paul Kish is Director, Systems & Standards at Belden CDT. He is also vice chair of the TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.