Re: [802.3_EPOC] MMP issues with the MAC Layer Solution
Hi Ed,
Thanks for the summary.
Let's see if we can make a step forward in understanding better the concerns and how can they be addressed.
I inserted my feedback below, for further discussion.
Thanks,
Andrea
From: Ed (Edward) Boyd [mailto:ed.boyd@xxxxxxxxxxxx]
Sent: Tuesday, January 08, 2013 02:09
To: STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx<mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
Subject: [802.3_EPOC] MMP issues with the MAC Layer Solution
Jorge, Duane, and All,
I have some concerns about reshuffling the packets in the MAC based on destination. Here are a few of the issues that I brought up at the last meeting.
1) Not a PHY layer solution.
a. It won't work with existing standard compliant EPON OLT MACs. I believe that this is out of scope for our project and it severely impacts the economic feasibility for EPoC.
[AG] I tend to agree that OLT is out of scope of this project; in fact we have CLT and CNU, and we cannot assume the CLT is an OLT.
That said, I fail to see why it should not work - in the example we have shown there is no change to 802.3 MAC and I cannot see any particular change in the MPCP parts either: simply the rate adaptation (which will be needed for coax and still have to be defined in details) will use different computation parameter based on the active profile(s). The function will be the same and will need to be parameterized anyway as the coax rate can be quite variable from case to case even with single profile.
b. Higher layer solutions today assume point-to-point Ethernet with packet ordering happening at the bridging or routing layers. Having a packet reordering based on the destination of packets below the MAC with a variable delay goes against this direction.
[AG] In the solution we presented there is no reordering of packets, neither above nor below MAC. Simply the Multi-Point Transmission Controller apply a different algorithm (which I like to remind is as per today already proprietary and implementation dependent) when selecting which client to serve. This can be done by proper design of the scheduler which is listed in clause 77.2.1 ("The scheduling algorithm is implementation dependent, and is not specified for the case where multiple transmit requests happen at the same time."), and does not preclude any option and certainly allow an implementation to offer only single profile as well.
The downstream right now looks exactly like point to point Ethernet. We shouldn't decrease the performance and break this model. It isn't perform like EPON or Ethernet. This is not fiber performance.
[AG] In term of throughput, I would see MMP closer to fiber performance than SMP, as in average a larger data rate will be available on the coax since there is no need to run at the speed of the slowest user. In term of latency/jitter there may be some small increase at application level based on the particular implementation of the algorithm and of the system parameters, but this is well below the target of the current PHY layer assumptions.
2) Jitters the downstream packets
a. For the MMP, I proposed a PHY layer solution with packet time stamping. I had the packet time stamping so packets would be in the same order at the CNU with a fixed delay.
b. The MAC layer solution doesn't provide this function so there are a few issues.
[AG] In the MAC solution the order of packet is not changed: at each CNU the same order will be received as transmitted. Also there is no jitter as all operations of selection of the MAC Client for transmission are made before MAC, in the Multi-Point Transmit controller - when a client is selected, its transmission will be enabled as today and no jitter is introduced - when the packet leaves the MAC, it reached the CNU counterpart after a fixed delay. Therefore there is no need to timestamp to support MMP in this solution.
Right now, the assumption is that there is almost 0 jitter in the downstream for the MEF 23H jitter specification (3ms RTT). I used the entire 3ms in my analysis for the upstream polling jitter and discovery windows. Any jitter in the downstream will require a higher upstream polling rate or violate the specification.
c. My analysis showed that a small pipe of 24MHz will have multiple milliseconds of delay and in the MAC solution, it is jitter. It is impossible to meet MEF23H for small pipes and the larger pipes will require higher upstream polling to make up for it.
[AG] As commented last meeting, if the pipe is very small, a single profile can be configured and used (in addition the parameters of the MMP implementation can also be tuned) - I believe 24 MHz is rather the exception than then rule for a system which is supposed to be running at speeds of 1 Gb/s and above. Question for MSO: are we expecting a lot of deployments using only such a small bandwidth or are these corner cases? Appropriate configuration applies in any case, so I cannot see this to be a real issue.
d. Obviously performance monitoring protocols that work above the MAC will not work.
[AG] Could you explain why not? I expect a performance monitoring tool shall not assume how the system is implemented in the lower layers and shall actually work for various configurations and implementations.
3) GATE frame will break up the shuffled blocks of packets.
a. The scheduler in the OLT will send the GATE at the exact time that it should go out to decrease the RTT time to get the REPORT.
b. The scheduler is not aligned with the blocks of blocks out from the MAC so GATE frames will come out when another MMP block of packets is present.
[AG] According to the specification (see figure 77-4), the selection between GATE messages and data packets happens at the Control Multiplexer, which is in charge of prioritizing control frames (e.g. GATE) over data frames. In my understanding there is one instance of such control multiplexer for each MAC Client and whether a GATE or another frame is selected in there has nothing to do with the selection of the transmitting MAC Client itself by the multi-point transmit controller. Also the timing for the GATE messages is determined by the DBA agent, which is also proprietary and implementation dependent, I cannot see any issue in generating GATEs when they are needed, at most is a matter of optimizing proprietary algorithms to achieve better performance, something that should be done anyway.
c. Another short FEC termination will be needed for the GATE so the GATE frame can go out immediately. This will result in a lower efficiency based on the RATE of the upstream bursts. Based on the numbers in my presentation, it will significantly increase the data rate for GATE frames. Small bursts in the upstream are common with high data rate polling.
[AG] See above - all this is not needed, the control multiplexer still prioritize GATE messages over data and those are treated as any other frame of that profile.
d. If the GATE waits for its MMP block to come up, it will increase the RTT time for REPORT to GATE.
[AG] Maybe I am missing something here, but I do not see how this can be the case: to be able to send a REPORT, a GATE should first provide the CNU with corresponding grant - so the REPORT will be sent after getting the GATE and apart a possible delay of the very first message exchange when the CNU starts up, the RTT for REPORT to GATE (from XGMII at TX to XGMII at RX) remains the same and it is determined by the PHY processing and propagation time.
4) Small Pipes have long delays or poor efficiency.
a. As mentioned in my PHY solution, small pipes are inefficient or have long delays to support packet re-ordering.
b. If the pipe is 50% of the BW and the efficient is the same as a 1Gbps pipe, the delay for a block of packets would be doubled.
c. If the pipe is 50% of the BW and delay is constant, the overhead for the FEC would be doubled.
[AG] Again, the number of profiles and the choice to use one or more can be tuned accordingly, as well as the parameters of the MMP implementation can be properly selected. In our presentation we have shown examples of possible tuning and shown that the possible additional delay can be controlled.
Hope that helps as a starting point. I will try to be on the call at 6am. I can't imagine mixing this with the channel bonding proposal. The delay and jitter would be crazy and it would be very complex to implement. I think that the performance in the downstream shouldn't be compromised. Simple, cheap, fast, wide pipe is Ethernet.
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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