Re: [STDS-802-3-EPOC] [802.3_EPOC] MMP implementation issues: Multiple FCUs and MMP, is it different?
Hi Andrea,
It is not guaranteed that the OLT can do the group shaping limit that you describe below. Because of requirements in other system standards, I think that all OLTs can shape to a single lower aggregate rate for all downstream traffic. While a single limit is fairly common, the ability to have 3 virtual pipes with aggregate limits as you describe below is not as common but it is possible on some of the OLTs. It is likely to be a shaping function in the switch or traffic management chip in the box. Using existing solutions, an operator could specify that it has group shaping for some number of pipes (3 in your example) and it seems reasonable that an OLT vendor could provide the functionality based on existing chips & systems. Requiring this function would reduce selection set of OLTs to the operators but solutions do exist.
On the other hand, the MMP shaping function required is not a standard function and it is far too complicated to implement behind the OLT MAC with off the shelf chips. I don't see how it could come up the proper FEC overhead for switching between modulation profiles and credit back differently per destination for the fluctuating rate. You would need to add this shaping function into the OLT MAC/EPoC PHY and then back-pressure the switch in some way. This is a major change.
Hope that helps,
Ed..
From: Garavaglia, Andrea [mailto:andreag@xxxxxxxxxxxxxxxx]
Sent: Wednesday, January 09, 2013 7:04 AM
To: Ed (Edward) Boyd; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
Subject: RE: [802.3_EPOC] MMP implementation issues: Multiple FCUs and MMP, is it different?
Hi Ed, all,
I still see one point that seems to me not fully clarified, talking about SMP (simplest case) and a deployment like the one shown in the figure.
Let me try to make an example, and see how this is supposed to work, for my understanding:
- EPON is 10 Gb/s and serves 3 FCUs and 2 ONUs
- Each ONU is compliant to 802.3av and can handle 10 Gb/s speed
- The FCU has an optical side also compatible with 802.3av, but cannot handle on the coax side 10 Gb/s - I think this is reasonable assumption
o Let's assume the following for FCUs, on coax (other example won't change the problem): FCU-1 runs at 2 Gb/s, FCU-2 runs at 1 Gb/s, FCU-3 runs at 1.5 Gb/s
o In each coax branch we use SMP as stated below, so CNUA, A', etc. can handle a link from FCU -> CNU of 2 Gb/s, CNU B, B', etc. handles 1 Gb/s and CNU C, C', etc. handle 1.5 Gb/s
- In my opinion the OLT shall not:
o Exceed 10 Gb/s delivery to any ONU - this is already guaranteed as being the upper limit of the EPON optical system
o Exceed 2 Gb/s aggregated traffic to FCU-1 - this is not guaranteed by EPON line rate alone
o Exceed 1 Gb/s aggregated traffic to FCU-2 - this is not guaranteed by EPON line rate alone
o Exceed 1.5 Gb/s aggregated traffic to FCU-3 - this is not guaranteed by EPON line rate alone
It seems to me that the OLT needs to shape traffic for each FCU according to the rules above, otherwise some of the FCU coax part will be overloaded - so it seems to me the OLT needs to shape traffic to each FCU and this is regardless the FCU particular architecture.
I assume the OLT shall and can do that - can be confirmed the OLT is able to do that?
Thanks,
Andrea
From: Ed (Edward) Boyd [mailto:ed.boyd@xxxxxxxxxxxx]<mailto:[mailto:ed.boyd@xxxxxxxxxxxx]>
Sent: Tuesday, January 08, 2013 19:59
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] MMP implementation issues: Multiple FCUs and MMP, is it different?
Matt,
Thanks for the reply. It doesn't change anything. You can say that A+B+C is more than 10Gbps but you won't be able to fully utilize the coax networks. The key point is that the shaping rate for sub-network (A, B, C) is a solid limit for each network. This is fairly easy to do with a switch.
The shaping equation is very different if MMP is allowed on the network. What is my bandwidth limit for network A if it has a mixture of modem types? I set the shaping rate based on 1024QAM or 4096QAM? I assume that you limit it to 4096QAM and decrement more credits if it is to a 1024QAM destination. Jorge's drawing suggest that we have a limit for 1024QAM modems across all networks A, B, and C. That doesn't prevent congestion on any one network. The shaping needs to be smarter than that. It would need to hierarchical (independent for A, B, and C) and it would need to take into account the MMP data rate for each CNU on the network. The more complicated decision is the FEC overhead from a short termination. How does the RS layer know when the PHY is going to insert a shortened last code word? I think that you would need to go back to the packet FEC that was used in 802.3ah.
The rate control is very simple for the RS layer in the current plan. We insert idles for a fixed FEC overhead for a fixed data rate pipe. It is programmed value based on the PHY Link up result. When you talk about MMP, I need to insert idles based on the spectrum configured, per-LLID/MMP modulation order, and insert idles based on the shortened FEC termination. The idles to insert for FEC is based on whether the packet is in the same FEC block on the same coax network. This is very complicated function and not available on any OLT. Jorge's drawing is not accurate for that reason. There is not OLT that can perform this function. Additionally, I think that it is very complicated or impossible for the RS to come up with this function for the FEC. It would need to be aware of the sub-networks and align its short FEC terminations with the PHY.
Thanks,
Ed...
From: Matthew Schmitt [mailto:m.schmitt@xxxxxxxxxxxxx]
Sent: Tuesday, January 08, 2013 9:13 AM
To: Ed (Edward) Boyd; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: Re: [802.3_EPOC] MMP implementation issues: Multiple FCUs and MMP, is it different?
Ed,
Thanks much for putting this out there, as it helps illustrate where we may be making different assumptions.
For me, I was not assuming that the sum of A+B+C is less than or equal to 10 Gbps; on the contrary, I've been assuming that the total available bandwidth for the sum of the FCUs would likely be greater than 10 Gbps.
If you change that assumption, does that change your conclusion about the difference between multiple FCUs and MMP?
Thanks!
Matt
From: "Ed Boyd (Edward)" <ed.boyd@xxxxxxxxxxxx<mailto:ed.boyd@xxxxxxxxxxxx>>
Reply-To: "Ed (Edward) Boyd" <ed.boyd@xxxxxxxxxxxx<mailto:ed.boyd@xxxxxxxxxxxx>>
Date: Monday, January 7, 2013 5:32 PM
To: "STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>" <STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>>
Subject: [802.3_EPOC] MMP implementation issues: Multiple FCUs and MMP, is it different?
Jorge, Duane, and all,
I started a new thread since I wasn't quite sure where to insert my reply. Let me start with the drawing with multiple FCUs question that you proposed.
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The assumption with multiple FCUs is that you are shaping 3 pipes (A, B, & C) inside the 10G EPON downstream. If you make that sum of those 3 less than the maximum rate, you can send them down the PON at the same with a single packet of jitter. The total bandwidth of the downstream is a constant since it is the sum of the 3 constants.
For Multiple sub-pipes: Total BW = A + B + C (Sum of 3 Constants)
In the case of MMP, you have a single pipe with a changing data rate based on the destination. The data rate from the OLT is not a constant or the sum of 3 pipes. It is going up and down based on the destination. A single shaping bucket is used but the debit for the reverse leaky bucket is different based on destination. The total bandwidth equation is an OR.
For MMP: Total BW = A | B | C. (The total bandwidth or maximum bandwidth is based on the traffic)
I hope that helps. I will send a separate email on the other issues related to the MAC layer solution.
Thanks,
Ed...
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Edward Boyd | Sr Technical Director
Broadcom Corporation | (O) 707-792-9008 | (M) 707-478-1146
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