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Re: [HSSG] Higher speed trade offs


Sorry you won't be here this week.

At 01:06 PM 9/18/2006 , Stephen Bates wrote:
Hi All

Chris, thanks for the numbers on skew, they serve as a useful starting point. I also reviewed the presentations at this week's meetings. Unfortunately I have teaching commitments and cannot attend.

I did want to point out some results based on some initial analysis and some review of the presentation material.

It appears to me that any link aggregation technique will suffer from the BER of the underlying PHY technology. For example a 100G solution constructed from 10GBASE-T will exhibit a BER of 1e-12 unless we add additional complexity. 

or have somewhat different link budgets

This was the focus of a thread on BER targets a couple of weeks ago.

One interesting thing to note is that if we do aggregation above the MAC, adding any additional FEC may be difficult. As I understand it, typically the MAC will not output Ethernet frames with invalid CRCs?

I (personally) believe that aggregation above the MAC would be (or, at least, certainly be argued to be) out of scope for the group.

However, many existing MACs are configurable as to whether to discard or merely flag received packets with an invalid CRC.

Also at this point all soft-information has been lost so only hard-decoding would be possible.

In several presentations people suggest an aggregation scheme implemented above the PHY. In that case we could consider using an FEC. If you presume we can implement a high-rate code capable of correcting any single bit error in one of the data fragments then the BER changes from 1e-12 to

1 - (1-1e-12)^N - N*(1-1e-12)^(N-1)*1e-12

So for N=1024 (using the 1Kb skew length Chris suggested) we get a new BER of about 1e-19. Of course we have to assume bit errors are IID and this may not be the case if there is already an underlying FEC.

However the interesting initial conclusion is that if we do decide to do link aggregation it should be possible to utilize a high rate code that helps us achieve a reduced BER target.

It would be useful to develop some error models for some of the proposed underlying PHY schemes. Does anyone have any such models and be willing to share them with the group?




Dr. Stephen Bates PhD PEng SMIEEE

Department of Electrical and Computer Engineering Phone: +1 780 492 2691
The University of Alberta                         Fax:   +1 780 492 1811
Canada, T6G 2V4                            stephen.bates@xxxxxxxxxxxxxxx