[802.3ae_Serial] Consistent treatment of sensitivities and margins
I had an action with Tom Lindsay to document the proposed consistent
treatment of sensitivities and margins which has gained consensus on the
serial PMD conference calls. Here it is:
First, a description of where we are
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Apart from the errors, the position in draft 3.2 and link model 2.4.1 is:
Budget = Tx power - Informative Rx sensitivity
Budget = Impairments + Losses + Margin
Where informative Rx sensitivity is also known as "nominal" or
"unstressed" sensitivity, and here "Impairments" are penalties apart from
(broadband) optical attenuation or loss.
Also draft 3.2 and link model 2.4.1 have (slightly simplified):
Stressed Rx sensitivity = Tx power - Losses
The principle is that the impairments are recreated by creating the
stressful test eye. However, we noticed that margin was treated differently
in the stressed and nominal sensitivities. Also, impairments that are not
recreated by the test eye are effectively being ignored in the stressed Rx
sensitivity calculation.
Second, a description of where we think we should be
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Stressed Rx sensitivity = Informative Rx sensitivity + impairments included
by the shape of the stressful test eye
and also
Stressed Rx sensitivity = Tx power - Losses - impairments not included by
the shape of the stressful test eye - Margin
Subtracting the first equation from the second we get
Informative Rx sensitivity = Tx power - Losses - all impairments - Margin
as before.
To simplify this description, questions of measurement at TP3 versus TP4,
mean power versus OMA, and triple trade offs, are not mentioned. They are
orthogonal questions to this one, which is what do we want stressed Rx
sensitivity to mean?
Now we had to split the impairments into two categories.
Impairments which are included by the shape of the stressful test eye:
Inter symbol interference penalty P_ISI
Deterministic jitter penalty P_DJ
The part of Rx baseline wander which is exacerbated by the shape of
the stressful test eye
This class are all pattern dependent penalties. The first two items combine
to the "vertical eye closure penalty" in D3.2 52.9.13 Conformance test
signal at TP3 for receiver testing.
Impairments which are not included by the shape of the stressful test eye:
Modal noise
Reflection noise
Mode partition noise
Relative intensity noise
Anticipated Tx baseline wander
The second class are noise-like, mainly non-pattern dependent penalties.
The proposed link model 10GEPBud3_1_14.xls implements nearly all of this.
The fine detail of the baseline wander (in Pcross) is simplified and we have
found an error in the Rx stressed OMA column: it should not contain Pmn.
This error does not affect anything but the stressed sensitivities in cases
with multimode fiber.
In the draft standard there is no line item called "Margin" any more. In
general we have spent most of it on penalties. But it was worth going
through this exercise to account properly for all the noise-like terms in
the stressed sensitivity test which is now our only normative receiver
sensitivity criterion.
Piers