[802.3_EPOC] Early comments on dai_01_1112.pdf
Hi Eugene and Marek,
Concerning your joint presentation, we would like to point out a few things in advanced. We would appreciate if you could clearly explain or justify your assumptions during the presentation. We believe that your conclusions are based on technical incorrect assumptions:
- Multiple modulation profiles (MMP) do not require additional clock signals. The PHY layer clock rate is related to the sampling clock of the analog-digital converter and the digital-analog converter. This clock does not change with the order of the modulation scheme. MMPs would be implemented by (de)multiplexing and buffering, which is also required for a single modulation profile (SMP).
- Your proposal of stronger FEC for symbols with higher modulation order seems to be ill-posed. If bit loading is applied, you would encode all bits with the same rate. Actually, you use bit loading in order to use the same FEC for all bits and still be efficient.
- SNR measurements for a single carrier transmission scheme (6/8 MHz channels) actually provide a good indication of what gains could be seen for MMP. Such measurements average the SNR over 6/8 MHz. With a finer bandwidth resolution for SNR measurements, the benefits of MMP will increase. Meanwhile, there have been several presentations pointing out that there are gains.
- The different in-home configurations that you describe actually motivate the use of MMP because SNR will vary much more for different users in the same plant. Hence, this is the best example where you need MMP to make EPoC work. It will be difficult to mandate the use of only 1x2 splitters in-home. There will be always users disobeying. Also, replacing all AM fiber would be very costly and not desired by many MSOs.
- Your proposal in slide 16 is not a solution of the problem you posed. You propose to do MMP for different frequencies, but the problem you described motivates MMP for different users.
- End users do benefit from MMP in one way or the other. RF bandwidth is limited and the subscribers (end users) will have to share a certain amount of RF bandwidth. With MMP, the aggregated data rate of all end users is higher. Hence, each user gets a larger share of data rate (unless the system is not fully loaded and all the end users are limited by the maximal rate of their subscription). Alternatively, an MSO could increase the number of users in a plant.
- The choice of MMP is automated based on SNR and the allowed profiles. The PHY will automatically make sure that it operated reliably at the highest rate supported. We believe it is desired by (most/all?) MSOs to avoid manual monitoring and configuration, which would add to the operational costs.
- Multicast and broadcast could be done with a profile common to the worst user. There is no need to duplicate data or rescaling of profiles in addition to what needs to be done for MMPs anyways.
- It seems that only your universal modulation profile refers to what is commonly referred to as single modulation profile for a plant. A CLT broadcasts into multiple segments of the cable plant, i.e. you have the same DS signal on all segments. Your single modulation profile requires all the features your MMP scheme needs.
Thanks,
Christian
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