Re: [802.3_EPOC] Questions on varanese_01_0912.pdf
Again, excellent presentation Andrea. It does capture the essence of
important channel model fundamentals that we need to quantify to select a
proposal and optimize a system design.
Below in line is the written feedback version of my comment with respect to
the direct relevance of the results shown to our application, as they
relate to some of Marek's Q's. Perhaps a better way to put it is that it
is relative to what I view as the identified use case with the highest
probability or highest priority in practice for NA cable applications.
Rob H
On Fri, Sep 28, 2012 at 5:47 AM, Marek Hajduczenia <marek.hajduczenia@xxxxxx
> wrote:
> Thank you Andrea, ****
>
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> Please see inline ****
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> Marek****
>
> ** **
>
> *From:* Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxx]
> *Sent:* Friday, September 28, 2012 10:29
> *To:* STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
> *Subject:* [802.3_EPOC] Questions on varanese_01_0912.pdf****
>
> ** **
>
> Dear colleagues, ****
>
> ** **
>
> Here are some questions on the varanese_01_0912.pdf presentation which did
> not get sufficient time for discussion. I’d appreciate if they were
> answered via reflector so that everybody benefits from these clarifications:
> ****
>
> ** **
>
> **- **How many taps have been examined in total in this study? I
> do not care much about names, types but rather to see what the sample size
> we are looking at and whether it is representative of a large network as a
> whole rather than a single CMT port or not. ****
>
> *[AG] I think it is mentioned in the slides, we have 140 subscriber (CNU)
> ports for the model, we considered all of them in the analysis. As you can
> see from the curves, since this is a model and small variability have not
> been included, SNR curves overlap when users are attached to same last
> splitter – in reality, the CDF is more continuous like shown in the
> measured valued, rather than step-wise. Does that answer your question?*
>
> ** **
>
> [mh0928] This begs a question then. Is this a number of CNUs that you’d
> consider typical? What I am trying to understand whether the scenario that
> was presented is the worst-case, best case or average (what can be expected
> in majority of deployments)? Would it be possible for an operator to use
> more tailored service groups to optimize them for SNR performance and to
> avoid complicating the design of devices, allowing for more optimized
> performance, rather than complicating the design of active devices?
>
*[RH]* There are several important architecture and system deltas between
the analysis, which accurately portrays residential SNR dynamics today, and
then extrapolates to the band >1 GHz for the tap models shown (which can
vary by manufacturer). The differences tend to limit the range of SNR
across CPE and translate to less extra capacity from bit-loading on a
per-user basis.
Key differences influencing results are:
1) Digital optics for EPoC into the plant vs the linear optics that play
the dominant role in link SNR. The optical link in typical HFC design
tends to set the SNR and would be reflected in these results.
2) RF actives vs No RF Actives. Amplifiers are extremely linear, but
nonetheless a cascade will tend to contribute some SNR degradation in the
form of CCN (digital distortion build up). The result is that near the
node versus end of line there will be a plant-contributed SNR delta. That
would be reflected in these results.
3) For a digitally optically fed OCU in the plant, or inside a node module
in the plant, its RF chain has only its own load to support. There is no
broadband load to share with legacy, and thus no sharing a fixed dynamic
range of a laser, D/A or RF stage.
4) CPE in residential may be deep in a home and the NF begins to contribute
or even dominate the reported SNR. My understanding is the devices used in
this study extract these low RF level cases for the most part by choice of
device (which is good).
5) As a frame of reference a residential HFC N+0 design would be in the
range of 50 or less homes passed using the reach of typical levels,
bandwidths, and tap strings. High bands would drop the reach. Other dB
offsets for different levels, fewer taps, lower CPE NF, and presumably the
opportunity to use a higher quality drop to a new user can be calculated.
As mentioned, the best effort "excess" band >1 GHz clearly works against
SNR uniformity. In that band, low SNRs due to roll-off are also
accompanied by high group delay, and poor return loss and reflections,
which will mean a different set of optimal OFDM parameters. I would add
that most cable systems are still 750 MHz or 870 MHz, and the limitation is
the RF actives. In these systems, passive coax bandwidth between 750/870
MHz and 1 GHz may be available that is in spec of the Taps and not
degraded. There are also taps and approaches to upgrade taps to get them
to 1.5-1.7 GHz.
Good starting point for an important discussion that the channel model work
aims to clarify.
****
>
> ** **
>
> **- **What is the most probably SNR distribution for a much
> smaller population of CNUs connects to a single CLT port? I assume that you
> will see some difference in SNR but it is not very likely to be as high as
> it was presented at the meeting for the whole measured population of taps
> and ports. ****
>
> *[AG] That may be the case, depends on how the few users are distributed
> in the plant. However, the question is whether this would be representative
> of a realistic plant – measurements are over population of ~240 modems, so
> it seemed to us that 140 (already smaller) was in the correct ballpark. Do
> you see any use case for much smaller plants, we could include in the
> analysis? *
>
> ** **
>
> [mh0928] This is something that operators should speak to. However, when I
> look at the OLT driven deployment model with several CLTs deployed in
> field, I’d not expect each CLT to be connected to 150+ CNUs. That would
> easily reach thousands of CNUs visible to a single OLT, which brings the
> available bandwidth down drastically, while burning a lot of bandwidth on
> scheduling overhead. I’d like to understand the trade-off here, that is
> all. ****
>
> ** **
>
> Please consider presenting more focused study for the next meeting,
> focusing on a number of drop sections to show what is expected to be seen
> on a single CLT port. While I am not against adaptive loading on per CLT
> port, I do not believe that this contribution has sufficient footing to
> justify adaptive loading on per CNU basis. ****
>
> *[AG] What we shown is one CLT port and 140 CNU ports attached for the
> modeled plant – for measured values, each plant is one node and ~240 CM all
> attached to the same coax distribution tree (Comcast may provide more
> clarifications in case) – we can make it more clear and improve in the next
> steps.*
>
> ** **
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> ** **
>
> Regards****
>
> *Marek Hajduczenia, PhD*
> * *
> ZTE Portugal
> *Technology Strategy Department
> *Edifício Amoreiras Plaza,
> Rua Carlos Alberto da Mota Pinto, nr. 9 - 6 A,
> 1070-374 Lisbon, Portugal
> * *
> *Office: +351 213 700 090
> Fax: + 351 213 813 349
> Mobile: +351 961 121 851 (Portugal)*****
>
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