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(you might want to note that there is a message length limit on the reflector – hence much of what went before is truncated.)
It is worth re-iterating that the limit lines provided in TIA & ISO specs are defined, not as models, but in fact, as limiting specifications, against which real cables are evaluated at each frequency, over a specified bandwidth. This means a number of things, some of which are pointed out:
1) That real cables will generally touch the limit line at a small number of frequencies (often a single point) over the specified bandwidth for the measurements. (Xiaopeng – this is the PRIMARY reason that capacity with real models is significantly greater than your limit-line models. You may continue to believe otherwise, but the numbers are clear, and there is a valid physical reason for it – namely the noise is lower everywhere that the real transfer function doesn’t touch the limit line)
2) That, as a result, in general, real cables will be better than the measurement, since manufacturers design with margin, and qualification specifications are designed so that installers have margin as well.
3) That extension of these lines beyond the specified frequency range has limited value, and representative measurements would be preferred (real cables can be better or can be worse than an extrapolation – we’ve seen both, depending on the characteristic)
4) That any scaling to a specification probably out to respect the frequency range over which the spec is defined.
It has been our experience that, in fact, the limit lines are most often most closely approached in the low frequency bands (under 10 MHz) – this actually detracts from any attempts at extrapolation to high frequencies. For existing cabling, no doubt we will ask for some form of qualification measurement. I do not expect that it will require strict adherence to an extrapolated limit line, and would oppose such a qualification as unnecessarily restrictive. Meanwhile, a simple total power measurement doesn’t quite provide the right level of input either. I suspect that some coarser, relaxed frequency averaging, perhaps as a ratio of insertion loss to impairment (e.g., NEXT) level will be the solution. This is work for the Task Force, and for communications with the cabling standards bodies.
For informational purposes, the material presented for capacity in the November tutorial were scaled by adjusting the levels of the insertion loss, NEXT, and ELFEXT specifications were scaled to touch the limit lines defined for the category of cable (Cat5E) within the 1 MHz to 100 MHz region of the frequency band. The exception to this scaling was Alien NEXT, as discussed at the time of the tutorial, the variance between observed results, and the rather loose limit line recently put forward is not a small amount for margin, but a rather large amount to account for the uncertainty. Estimates on achievable cancellation can only be put forward in conjunction with an observed model, hence it is also unreasonable to apply the 8 dB number we used to a situation where the Alien NEXT is higher. I would conjecture that it is likely that effects resulting in higher alien NEXT would increase the pair-to-pair correlation, thereby increasing the potential for cancellation.
We feel strongly that simply scaling the within-sheath NEXT response is inadequate for alien NEXT measurements, because we have observed both a different frequency shape in the high frequency bands), and because, if you take the NEXT response at -3 dB relative to the within-sheath response, and assume that that case, in fact, is valid, you will rapidly come to such odd, and contradictory conclusions that NEXT cancellers are unnecessary for 1000BASE-T, or that 1000BASE-T will not work due to Alien NEXT either, both of which are known falsehoods. I know that folks in the cabling community are working hard on alien NEXT modeling, measurement methodology & new specifications. These will undoubtedly come out of the work from TIA and other groups in the near future. Meanwhile the work in IEEE can move forward to provide them with meaningful targets to shoot for.
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In this regard, as the frequency band of interest expands considerably,
wouldn't it be beneficial to introduce an integral limit for the NEXT,
FEXT and ANEXT power (say max curve RMS in a given freq. band [500 MHz]).
This would be complimentary to the existing practice of the limit envelop
The integral metric would be easier to make both strict and at the same time
much less conservative than the envelop. The RMS spec would not be allowed to
exceed, whereas failing to meet the envelop at the specific frequency point(s)
(say beyond the conventional band) could still be well tolerated in the system,
and therefore could be made discretionary, subject to application.
(Integral impairment metric for NEXT, FEXT and ANEXT would also be consistent
with other UTP systems, such as EFM MC and SC, etc)
William Jones wrote:
Thanks for making this crystal clear with your example. I believe this brings to the forefront the distinction between limit lines and models. Specifically, limit lines must be extremely pessimistic since the cabling folks must necessarily insure, with extremely high probability, the deployed cable will not exceed these limits at any point in frequency. Further these line must be smooth due to the unpredictable structure of the xtalk. The effect is even more pronounced in the case of alien xtalk. See, for example, the tutorial where the peak alien xtalk measurements are almost 16 dB below the limit line.