February 2009


Eliminating alien crosstalk

Testing should be performed in the field when deploying 10GBASE-T.

by Dan Payerle

Alien crosstalk describes the general phenomenon where energy is coupled between cables in a common bundle or installation. Alien crosstalk was brought to light by the development of active network hardware that could provide 10-Gigabit Ethernet over twisted-pair cabling (10GBASE-T). Alien crosstalk becomes worse as the operational frequency increases.

Since the frequency range required to support 10GBASE-T is higher than that of 1000BASE-T, crosstalk, both internal and alien, has become a difficult obstacle to overcome when designing and installing LAN cabling to support the latest technologies.

Mitigation techniques
1. Proximity is the key contributor to alien crosstalk. When 10GBASE-T is being selectively deployed, avoid using adjacent ports in the patch panel. There may be no alternative, however, when deploying 10GBASE-T to workstations located near each other.

2. When deploying 10GBASE-T in adjacent ports of a patch panel, alien crosstalk testing should be performed in the field.

3. In the event the alien crosstalk test fails, take the following actions to reduce the level of alien crosstalk.

  • Separate equipment and patch cords and unbundled horizontal cables to increase the space between the cables.

  • If the cords cannot be separated, use either CAT 6A or CAT 6 screened twisted-pair (ScTP) cords.

  • Reconfigure any cross-connect as an interconnect.

  • Replace CAT 6 connecting hardware with CAT 6A.

  • Replace the CAT 6 horizontal cable with CAT 6A.

4. Retest the channel after performing any mitigation techniques to be sure that the techniques have brought the alien crosstalk margins back to acceptable levels.

Field testing
Alien crosstalk testing involves testing various combinations of links that are identified as victims and disturbers. Performing a test on a single victim cable involves at least six different testing configurations and as many as 12 depending on the manufacturer of the field test equipment.

Given that multiple tests are required for each victim link, requiring 100 percent testing of every link as a victim is not practical. When testing, if the first three victim/disturber combinations reveal a condition known as insignificant alien crosstalk, the test can be stopped, without finishing the 1 percent or minimum five victim links. Insignificant alien crosstalk is a condition where the measurement is below a certain level and may not be detectable by some field test instruments.

The selection of disturber links has to be done individually for every victim link. Select all links that run in the same cable bundle or are most consistently positioned relative to the victim cable. These bundles may be found in the patch panel, cross connect or conduit. Add any additional links that occupy adjacent positions in the patch panel or outlet.

When selecting links to test, in addition to the location of the links in the patch panel, the routing of the links also should be considered. The disturber links should be run in the same pathway as the victim link to have the most impact on alien crosstalk measurements.

The proper selection of links for alien crosstalk testing is critical and requires a certain degree of knowledge about the topology of the cabling plant. Without knowing where the various links are routed to within the building, the process of testing can be inaccurate, since the chosen disturber links may not be close enough to the victim to provide any significant data.

After deciding on the victim and disturber links to check, the field tester needs to be connected to the cabling according to the manufacturer’s directions. Some field testers require a personal computer to be attached to the field tester during the measurement process to gather the data and compute the alien crosstalk results. Additionally, the tester and computer may need to be moved to the opposite side of the link for the second half of the alien crosstalk testing process.

Because the number of links to test and the time to test each victim/disturber combination can be significant, choosing the right field tester can save time and trouble. A field tester that does not require a computer for data acquisition or double testing of each victim/disturber combination can cut 75 percent of the total alien crosstalk testing time, while also eliminating the need to bring a computer into the field.

Dan Payerle is senior product manager, DataComm test, IDEAL Industries, Sycamore, Ill.

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