Your points about and why everyone believe LRM modules require DDJ compliance but somehow limiting link
don't care puzzles me too! Attach 802.3ae and 802.3aq link simulation shows both DDJ and DDPWS impact the
TDP/TWDP, my observation are DDPWS has more linear relationship to the penalty but in the case of TDP
it is pretty resilient to DDJ to some point then you start seeing hockey stick penalty. May the reason DDJ is
discarded for limiting link is it's insensitivity at low DDJ values?
If the output eye has significant closure and jitter it will impact both DDJ and TWDP. I have also added Norm
as he was involved heavily with LRM and might be able to shed some more light on this subject.
(Just remember TDP results are based on 300 m per definition of 802.3ae with 55 ps FIR filter).
Thanks,
Ali
Brian Misek wrote:
Ali,
You bring up the comparison of SFF-8431 which is for a link that must support a linear (LRM) interface and host Rx. That type of link, I have been told, is more sensitive to this phenomena. Is this a true understanding or am I miss informed. If we are going to add additional requirements at this date to this interface I would like to be VERY sure they apply to the LIMITING nature of the nPPI interface and J2 and J9 and the eye mask do not sufficiently bound the problem. I would appriciate the insight from module suppliers as to the way this signal would cause them to fail TP2 tests.
Cheers,
Brian Misek
On Tue, Sep 1, 2009 at 3:09 PM, Ali Ghiasi <aghiasi@xxxxxxxxxxxx> wrote:
Hi John and I had side discussion if the current TP1a specifications of J2 and DDPWS are sufficient for TP2 compliance regardless of input de-emphasis or not. One condition that I am concerned is the case of a host with low RJ and with significant over emphasis 3-5 dB, the DDJ in these case could be in the range of 0.15 to 0.17 UI. An example TP1a eye diagram with about 5 dB is shown below. I have seen with SFP+ some module much more sensitive to over emphasis than others, in SFF-8431 max DDJ does not allow this type of setting and is controlled. If all of the module suppliers building 40GBase-SR4/SR10 have no issue with over-emphasis at TP1 by as much as 5 dB then the current specification is sufficient. On the other if other have the same concern we could pile on the unsatisfied comments 216 and 218 against on D2.1. Thanks, Ali
Ghiasi, Ali BroadcomCl 86A SC 86A.4.1 P 408 L 30 # 216Comment Type TRIf the transmitter has very low RJ~0 then DDJ will approch J2 or 0.18 UIdue to overemphasis. Over emphasis can result in sever eye degradation depending onthe laserdriver gain, etc.SuggestedRemedyTo protect aginst these over-emphasis scinearios DDJ must be added withpropose valueof 0.12 UI.REJECT.The combination of other specifications, including the eye mask mayprotect against this.There was no consensus to add this parameter.Further study of this issue is invited.Comment Status RResponse Status UGhiasi, Ali BroadcomResponseCl 86A SC 86A.4.1 P 410 L 19 # 218Comment Type TRIf the transmitter has very low RJ~0 then DDJ will approch J2 or 0.18 UIdue to overemphasis. Over emphasis can result in sever eye degradation depending onthe laserdriver gain, etc.SuggestedRemedyTo protect aginst these over-emphasis scinearios DDJ must be added withpropose valueof 0.12 UI.REJECT. [Reclassified from 86A.4.2]See response to comment 216.Response
