10 千兆铜缆以太网即将推出，很少人象Fluke Networks 的 Huge Draye那样知晓认证这些超速网络将面临的挑战。“外部串扰是在 10 千兆以太网较高频率时看到一种新现象，”Draye 说到……
10GigE to test cable limits
New installation and testing guidelines help combat alien crosstalk issues with copper networks.
by Hugo Draye
Today, most 10-Gigabit Ethernet (10GigE) use is in the data center, in campus backbones and over fiber, but widespread use of 10GigE over twisted-pair copper cabling (10GBASE‑T) is coming. The standards for 10GBASE‑T are still in the development stage, and expected to be finalized and published in mid-2006. Changes in the way twisted-pair cabling is certified in the field also are on the way.
Field certification of installed twisted-pair cabling for 10GBASE‑T includes all the test parameters that are currently specified in the TIA/EIA-568-B document for Category 6: insertion loss, return loss, pair-to-pair near-end crosstalk (NEXT), power sum NEXT, pair-to-pair equal level far-end crosstalk (ELFEXT), power sum ELFEXT, propagation delay, length and delay skew.
The 10GBASE‑T test limits are identical to the limits for CAT 6. The major difference is the frequency range and performance requirements for these tests are extended from 250 MHz to 500 MHz to support the higher data rates of the 10GigE technology. In addition, alien crosstalk test parameters must be included with the field-certification effort for 10GigE.
Crosstalk takes place between wire pairs in one cable. Alien crosstalk is the exact same phenomenon but the crosstalk coupling now occurs between wire pairs in different, adjacent cabling links. Alien crosstalk is the most significant disturbance or noise source for the 10GigE application when using UTP cabling.
Alien crosstalk occurs between every wire in a many-pair bundle, and the combined impact of all wire pairs in the bundle upon the wire pair under test (usually referred to as the victim wire pair) should be assessed. Unfortunately, in most cases, testing the alien crosstalk between all possible wire-pair combinations is not economically feasible or affordable.
Standards established by the IEEE call for the electronics (transmitters and receivers) to be specified in such a way that they properly operate with the specified cabling channel in a bundle of seven cables grouped in a circular configuration. A 55-meter CAT 6 channel is one of the cabling links that, by design, should meet the requirements for 10GBASE‑T.
Class E/CAT 6 UTP, up to 55 meters in length, will require alien crosstalk sample testing using the recommended test strategy rules. Class E/Class 6 ScTP, up to 100 meters, will meet alien crosstalk requirements with limited sample testing in the field for patch panel compliance. Class E/CAT 6 UTP, between 55 to 100 meters, must be included in the alien crosstalk field test. Augmented Class E/CAT 6 cable, up to 100 meters, is expected to require alien crosstalk sample testing using the recommended test strategy rules.
In other words, for the cabling infrastructure of a data center in which all links are 55 meters or less, existing CAT 6 UTP cabling may be satisfactory. The installation will need to be executed with meticulous attention to installation and termination quality. In addition, although the augmented CAT 6 or CAT 6A component specifications have not yet been finalized or published, specifying and using the newest patch panels and connecting hardware that is advertised as CAT 6A is advised in order to minimize alien crosstalk at the patch panels. Last, but not least, the cabling performance of such mission-critical installations should be certified in the field using a test strategy designed to accommodate alien crosstalk.
When using CAT 6 or CAT 6e cable, special attention should be applied to the design of the cabling system to minimize alien crosstalk. The recommendations to mitigate alien crosstalk disturbance focus on bundling and wire management. The test requirements for 10GBASE-T extend to 500 MHz, and all the established installation procedures to deliver quality cabling links should be meticulously observed.
Only cabling links in the same bundle are expected to contribute in a measurable way to power sum alien crosstalk (PSAXtalk). Therefore, PSAXtalk will be smaller when the number of cabling channels in a bundle is smaller. The test strategy will be more effective when the number of links per bundle is kept smaller. The optimum number of channels per bundle, especially of CAT 6 cabling channels, is generally considered 12 links; a bundle with more than 24 links is impractical to test.
Most of the alien NEXT occurs within the first 20 meters of the link, measured from the end from where testing begins. Modeling and verification of the models has shown that alien NEXT generated further away from the test end has virtually no impact on overall power sum NEXT unless the cables run in parallel the full length of the bundle. Therefore, the patch cords and patch panel arrangement of links, and the resulting wire management in the rack, can have a significant influence on the amount of alien crosstalk coupling between the links. Tight proximity increases alien crosstalk coupling.
Having knowledge of the cabling topology when testing alien crosstalk performance at a patch panel is important, as is knowing which of the cables run in the same bundles. A cable naming scheme can be designed and adopted that includes the identification of the bundle in which the cabling link is routed.
10GigE is going to be a demanding network technology for twisted-pair cabling. It will require a good CAT 6 cabling system that has implemented all of the mitigating measures to limit the impact of alien crosstalk disturbance; alternatively, it will require the newly proposed augmented CAT 6 cabling plant.
In either case, the workmanship of installation is going to play a significant role in assuring that the cabling installation will support this super-fast network technology flawlessly. Field certification of the in-channel requirements and of the alien crosstalk (between cables) requirements will be the only way to assure that the cabling system will support 10GigE.
Hugo Draye is marketing manager for Fluke Networks’ certification tools.