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Downstream micro-reflections

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OldDocsisGuy
Downstream micro-reflections

We have some inter-company arguing going on here about micro-reflections.
Some guys say a higher number is better, some say lower is better, and some say wtf are those, and who cares anyway.
Can someone point me to some relatively simple documentation about this subject that can be spread around to prove which is correct?
I've tried this site:
http://www.scte.org/TechnicalColumns/09-11-01%20what%20is%20a%20micro-re...
but met with mostly blank gazes.

Thanks!

ODG

Capm
Well

I cant remember where I read it, but I was reading about them a couple years ago, and 30 and below was desirable, anything below 80 is alright, above that and you've got problems. Ours are typically is 40 or below in our systems.

Msarmento
Good question!

Hi,

just to add a litte bit on the subject, first things that comes to my mind:

1) Considering that downstream is one-to-many, how to measure reflection on this scenario?
2) With downstream higher modulation rates, what is the distance where a reflection could cause interference? Does it really impacts the original transmission?

Anyway, Docsis specs say these are the operational parameters for Downstream reflections:

Micro-reflections bound for dominant echo
-10 dBc @ ≤ 0.5 μs
-15 dBc @ ≤ 1.0 μs
-20 dBc @ ≤ 1.5 μs
-30 dBc @ > 1.5 μs1

Looking for the other side, Upstream reflections are easy to measure. There is data available both at CM and CMTS. If you assume that the same impairment will cause reflections for both sides of the transmission, I could suggest that if the upstream path is clean of reflections, the downstream have a great chance to be clean as well.

Regards,

MS

ByteMe
Depends on the units

It really depends on the unit you're specifying the micro reflections in. If you express them in dBmV or dBm, meaning in their actual power/voltage: lower is better. Microreflections are however usually expressed in dBc, dB relative to the carrier, which is common to use with any kind of digital signal where you want to express how large/small the side effects are(e.g. if you want to specify the spurious free dynamic range, you'd put that in dBc). When you're talking in dBc, higher numbers are better, because that means a big distance between the carrier and the effects of the microreflection.

If you're referring to the MIB .1.3.6.1.2.1.10.127.1.1.4.1.6, that's expressed in dBc(source: http://support.ipmonitor.com/mibs/DOCS-IF-MIB/item.aspx?id=docsIfSigQMic...), so higher is better.

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