Re: [802.3_EPOC] Why multiple simultaneous MCS
Eugene, Matt, and Marek -- There are lots of ways to do this way above MCS.
I return again to wireless where there are mechanisms to broadcasts and
there are common radio link control channels and then common physical link
(ds) channels.
-Victor
From: Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxx]
Sent: Thursday, October 11, 2012 11:51 AM
To: 'Victor Blake'; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Victor,
Think about how DPoE resolves multicast in phase 2. I'd think we would like
to have similar mechanism in EPoC as well, but it relies on LLID transport,
which implies precisely what Euguene was saying - once you start using MCS,
you lose the benefit of broadcast delivery mode and have to single-cast or
at most create multiple multicast groups
Marek
From: Victor Blake [mailto:victorblake@xxxxxxxxxxxxxxx]
Sent: Thursday, October 11, 2012 11:26
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Hi Eugene,
I don't think we've met, but I've been following your comments with
interest.
I don't see the relationship to "multicast video." Unlike HFC today, the
goal is not to FDM video and data, it's one big fat data pipe which is
bandwidth managed by IP and Ethernet. Not that it's at all related to EPoC
(because I don't think it is), but one strict fix for that is simply an LLID
for video. That would all happen above the PHY without respect to other than
the obvious fact that one cannot administrative configure an LLID to support
more bandwidth than the PHY supports and that obviously one must do
calculations (either in engineering or admistratively on the box - again
without reference to the protocol) as to how to share the shared bandwidth
(aka configuration management).
Victor R. Blake
Independent Consultant
victorblake@xxxxxxxxxxxxxxx
http://www.victorblake.com <http://www.victorblake.com/>
http://twitter.com/victorblake
540-338-1076
From: Dai, Eugene (CCI-Atlanta) [mailto:Eugene.Dai@xxxxxxx]
Sent: Thursday, October 11, 2012 10:38 AM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Duane: I suggest that we consider some realistic examples, instead of this
kind of wild argument. All DOCSIS CM support 256QAM today, some may support
1024 QAM in the future. Assuming ~100MHz DS bandwidth allocation for EPOC,
with 1024 QAM we get 1 Gbps raw bandwidth, with 256 QAM the raw bandwidth
will be 800 Mbps. Where is 250Mbps in your example come from?
We can make MAC to aware PHY or more preciously PHY rate, but what price we
have to pay? The impacts are not limited at PHY layer; it will propagate to
system level, for example how to deal with multicast video? Unnecessarily
duplicate multicast video streams to different PHY profiles could consume
more bandwidth than possible gain. Backward compatibility is another
concern.
The ultimate goal is to improve outside plant condition to accommodate a
reasonably higher modulation orders.
Eugene Dai PhD
Principle Transport Architect
COX Communications
Tel: 404-269-8014
From: Duane Remein [mailto:Duane.Remein@xxxxxxxxxx]
Sent: Wednesday, October 10, 2012 9:51 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Marek,
A simple example.
An EPoC network with 10 CNUs, 9 can operate at 1G, 1 can only attain 250M
With 2 MCS profiles you can attain an average instantaneous network data
rate of 950 Mbps or 95 Mbps per user.
With LCD you can only attain an average instantaneous network data rate of
250 Mps or 25 Mbps per user.
Best Regards,
Duane
FutureWei Technologies Inc.
duane.remein@xxxxxxxxxx
Director, Access R&D
919 418 4741
Raleigh, NC
From: Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxx]
Sent: Wednesday, October 10, 2012 6:20 PM
To: Duane Remein; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_EPOC] Why multiple simultaneous MCS
Duane,
Could you please kindly point to me to any study which shows that
"significantly lower" bandwidth ? I failed to find any such contribution
submitted to EPoC TF / SG. Perhaps I missed something along the way.
Marek
From: Duane Remein [mailto:Duane.Remein@xxxxxxxxxx]
Sent: Wednesday, October 10, 2012 16:23
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_EPOC] Why multiple simultaneous MCS
Matt,
I suspect another argument for multiple profiles is that you can offer a
higher level of service. Like it or not, networks are assessed based on the
average sustained data rate that can be delivered to a customer. With a
lowest common denominator approach this would be significantly lower.
Best Regards,
Duane
FutureWei Technologies Inc.
duane.remein@xxxxxxxxxx
Director, Access R&D
919 418 4741
Raleigh, NC
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Tuesday, October 09, 2012 11:25 PM
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: [802.3_EPOC] Why multiple simultaneous MCS
Based on some of the discussions, I think it might be worthwhile to spend a
moment going into the chain of reasoning that led myself (and others) to the
conclusion that an advanced next generation HSD system for HFC (such as
EPoC) should include the ability to support multiple simultaneous Modulation
and Coding Schemes (MCS).
If I were to sum it up in 2 words (okay, technically 4), it would be this:
SNR Headroom.
In a system that requires all end stations to operate with the same MCS (and
to receive all downstream transmissions), the great danger is that for
whatever reason the SNR to a given end station will change sufficiently that
it will drop offline. That simply cannot be allowed to happen. As a
result, operators are required to maintain a significant amount of "SNR
Headroom" when they setup and deploy an end station. Common practice is to
allow on order of 8 db of SNR headroom when deploying a modem (some a little
more, but I'm not aware of any that do less).
What this means is the following. Let's say that you needed 25 dB of SNR to
operate 1024 QAM with a 3/4 LDPC FEC code (which I believe is the case with
DVB-C2, and serves as a useful reference point here). An MSO would not
actually use that MCS unless they could get their end stations at 33 dB of
SNR (25+8) when deploying them. If they couldn't get to that level when
deploying the end station, they wouldn't be able to use that MCS.
I pulled out that particular number because, based on the SNR distributions
that were shared previously, even if you assumed you could "bring up" the
bottom 2.1% of modems, that's the best you could expect to do. And even if
you could further improve SNR, you're still always going to be operating in
effect 8 dB below what you could theoretically do. For example, just to get
to 4096 QAM with a 5/6 LDPC FEC, you'd need to get every single end station
to operate at about 40dB or higher of SNR.
Now, if you could get away from requiring that 8 dB of headroom, it would be
huge, considering that 6 dB is an order of modulation with the same FEC, an
increase of 2 bits per symbol.
One way to achieve that end is to support multiple MCSs, but only one at a
time, which is set based on the "lowest common denominator". In this
approach, if an end station were to have a sudden drop in SNR, then the
system would need to drop to a lower "lowest common denominator", until such
time as the problem could be addressed. By having that fall back, you could
in theory operate closer to the theoretical maximum and reduce the amount of
headroom required.
However, there are some issues here. First, one bad end station coming onto
the network - or a single problem in a single household - would affect the
entire network, degrading the service of every single end station. Second,
particularly in the scenario where an end station encounters a problem after
it's already online, there's no guarantee that it would even still be able
to communicate that it was having a problem and that it needed the MCS to be
changed. That risk is then going to force you to leave more headroom,
mitigating any potential gains.
The only approach that we've come up with so far that allows you to
significantly reduce that headroom and operate close to theoretical levels -
while not adversely affecting the entire system when there's a problem - is
to create a system in which a single end station is able to fall back to a
more robust MCS if it encounters a problem. In that way, only the single
device having an issue is affected, and unless the problem is truly
catastrophic you can reliably keep the mode online (meaning that you have an
opportunity to fix the problem while the customer remains online and
operational).
We could argue back and forth for quite a while about SNR spreads on the
network, how much variation there will be between end stations in a given
service group, etc. And the ability to operate each device to its maximum
potential is very appealing to be sure. But what really makes this
decision, in my opinion, is the SNR headroom issue, and in particular the
ability to significantly reduce it while actually improving robustness and
the ability to keep customers on line.
Thanks.
Matt
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