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I think the main problem is relaying the idea of de-rating the MAC without creating impression that we will do anything between 25G and 100G, i.e., we do not want (I am quite sure of that) 27.5G option or 31.5577G option or any odd-ball number that might be coming out of a broadly understood “de-rating”. This is what brought about the concern about open ended statement of data rate between 25G and 100G. This is also what prompted fixing the intermediate data rate of 50G – we know it is multiple of 25G baseline data rate and can be easily converted into whole number of data lanes.
The whole point right now is to specify the scope in such a way that (a) we clearly spell out what we set out to do, (b) we do not lock ourselves out right out of the bat from some of the technical choices that might be available down the road, and (c) we do not leave too much freedom to allow for “anything goes” proposals. I know these look quite contradictory … but this is what it is …
I know how 10GEPON works. I even got an award for it. (And what luxury - a whole piece of 8.5x11 paper, with color printing, even!)
Let’s not forget that everything in the standard is only a reference implementation. What you actually build is up to you, as long as it behaves as it should. This is particularly true for EPON.
So, for the concern on the “whole data path running at 100G” – come on… that’s not how it would be done.
Now, what’s wrong with saying that there is a 100G MAC, that then only uses a subset of its capability? In our view, that’s how you’d build any of these sub-rated things. I think there is a false argument here, that somehow people are going to build a dedicated 50G EPON. Do you think the industry is going to do such incremental advances? I don’t think so, particularly not for the silicon.
We expect that the 25G single channel technology will be developed, and that a way to combine them into higher speeds will be developed.
The 100G MAC is the large enough bucket that can accommodate 4 25’s, and four is a proven modularity.
And so, anybody who is interested in building any of these sub-rated systems would end up using 100Gb/s switch-ports.
I can say that some of this may depend on exactly how the channel combining is done. Some people may be thinking of “hard bonding” – that is, the designer decides how many channels will be tied together, and once they are combined, any ONU that wants to use those channels must listen to all of them. This is how 100G Ethernet works, and that is fine for point to point. They become an indivisible block, and that (maybe) motivates the thinking about a 50G MAC. I think this is a very poor design choice for PON. The whole point of PON is to allow bandwidth flexibility. It is much better to have a scheme of “soft bonding” – that is, the operator decides which ONUs work on which set of channels, and it can change over time. In addition, it is likely that there can be 1, 2, and 4 channel ONUs, all sharing the available channels in an efficient manner. If one “hard bonds” 2 channels together, then the single channel ONUs can’t listen to those channels – you’d have to have a single channel to take care of them. And then there is no space for the 4 channel bonded group. We would quickly paint ourselves into a corner.
So, back to the scope: If the scope is a maximum, then 100G is a fine maximum. If we do the right thing regarding how the 100G MAC gets reduced, all the desired use cases will be supported. And that is what really matters.
10G-EPON uses exactly the same MAC as is used in 10G point-to-point. This MAC runs at exactly 10Gb/s, no matter what the actual data throughput. The PHY adds an overhead due to FEC, so the effective throughput is lower. The data is throttled above MAC to make sure it does not overrun PHY capacity. But the MAC spits bits (idles if there is no data) out at exactly 10Gb/s. In other words, the data path in 10G-EPON runs at 10Gb/s.
Since the PAR scope is the upper bound and what the project is allowed to cover (as David clarified on the call), the existing scope limits us to only 25G and 100G MACs and nothing else.
If we don’t add 50G MAC, then we will have the MAC and the entire data path running at either 25Gb/s or 100Gb/s, no matter how many wavelengths are activated. This is what we try to avoid. We need to allow another generation between 25G and 100G.