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All About PSAXTIR

With the industry at the threshold of exciting new developments, cabling performance will need to be handled much differently.


November 1, 2004  


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There is a new measure of cabling performance that will become increasingly im-portant for the next generation of cabling systems, commonly referred to as augmented Category 6 cabling.

Called PSAXTIR (Power Sum Alien Crosstalk-to-Insertion Loss Ratio), it supplants PSACR (Power Sum Attenuation-to-Crosstalk Ratio) as the determining factor that limits the reach for 10 Gigabit Ethernet (10GBASE-T) systems over balanced twisted pair cabling.

PSAXTIR is the difference between the signal level and the noise level at the receiver after cancellation of NEXT (Near End Crosstalk) and FEXT (Far End Crosstalk) within a channel.

10 Gigabit Ethernet will employ 12 NEXT cancellers and 12 FEXT cancellers in the receiver circuitry to remove substantially all of the internally generated crosstalk within the cabling. What is left over is the alien crosstalk noise from surrounding cables and the ambient noise from the environment.

The traditional measure of the available bandwidth has been PSACR, which is the difference between the signal level and the power sum NEXT noise level at the receiver. The available bandwidth is usually expressed as the frequency range where the PSACR is positive.

The chart shows both the PSACR and the PSAXTIR for 100 meters of Category 6 and augmented Category 6 channels.

As you can see, the available bandwidth is increased from 200 MHz to 500 MHz for augmented Category 6 if we eliminate the PSNEXT noise entirely. The power sum alien NEXT for Category 6 cabling is 47 dB at 100 MHz compared to 60 dB for augmented Category 6.

The maximum information capacity for a channel is determined using Shannon’s theorem. Using the curves in the chart, the information capacity can be determined from the area between the PSANEXT and the IL (Insertion Loss) curves.

In the case of augmented Category 6 cabling the maximum information capacity is ~ 18 Gb/s for a 100 meter channel.

How do we measure the alien crosstalk? The standards bodies have yet to determine a “standard” methodology for testing alien crosstalk in a channel or at the component level. Currently, there is no commercially available method to field-test for alien crosstalk.

In order to measure alien crosstalk on a channel, adjacent channels must be carrying a signal.

This “signal” on adjacent channels must be generated by either the active equipment (NIC’s, servers, etc..) or by the tester itself. The amount of adjacent channels can be quite significant, e.g. between all ports on a 24-port switch.

To reduce the amount of testing, it is likely that the alien noise will be measured with all adjacent channels energized. It is also expected that 10GBASE-T equipment will have some built-in diagnostic testing capability.

We are at the threshold of some exciting new developments in cabling and we have to look at cabling performance in a different way than we did in the past.

The new metric is PSAXTIR with an available bandwidth that extends up to 500 MHz and beyond.

Paul Kish is Director, IBDN Systems & Standards at NORDX/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.