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IEEE P802.3ap Task Force Members,
I apologize for not being able to join that channel model ad hoc call this morning. It is my understanding that we continue to have disagreement on the placement of test points for the purpose of transmitter, channel, and receiver definition.
I cannot over-emphasize the need to find the common ground on this issue, put a stake in the ground, and move on to other pressing topics.
After observing the discussion on the reflector, I tried the summarize the salient points of each principal's argument and attempted to find the common ground between them. This summary is in the reflector message (http://ieee802.org/3/bladesg/email/msg00166.html) dated the 26th of August. I received no feeback on this summary but clearly there are still issues with this approach.
I have copied the entirety of the summary message for easy reference. I strongly encourage those with reservations regarding this proposed approach to state those reseverations clearly and offer a proposal for resolution that can be evaluated by the Task Force.
Finally, I hope it is clear the proposal below is not the position of the Task Force chair nor is it necessarily my personal preference. It is simply my perception of the common ground which I have summarized as an aid to generating concensus. Please, take the time to review it carefully, provide constructive feedback, and help me find the common ground so that we may move on.
Thank you,
Adam
Title: Test Point Discussion Summary
IEEE P802.3ap Task Force Members,
I am just now catching up on the test point discussion. My eyes may be deceiving me, but after carefully reading the thread, it appears that we may be close to consensus on this. Let my try to summarize what I saw as the salient points of this discussion and see if I can flush out any remaining points of disagreement.
1. The convention used for the DEFINITION, SIMULATION, and SPECIFICATION of the transmitter, channel, and receiver, is that the AC-coupling capacitor is between TP4 and the receiver.
1a. Transmitter specifications are defined looking into the transmitter from TP1.
1b. Receiver specifications are defined looking into the receiver from TP4.
1c. Channel model ad hoc defines the signal path from TP1 to TP4.
Discussion:
I believe this is consistent with Mike Altmann's observations about how devices are actually tested by third parties and during interoperability plug-fests. The observations points in such environments are the test points provided on the evaluation board, and that typically includes some type of connector (SMA), some distance of PCB trace, the landing pad, and on the receiver side, typically an AC-coupling capacitor and associated mounting structure. These are connected to "test" backplanes, examples of which we have seen presented by Joel Goergen, John D'Ambrosia, David McCallum, and Steve Anderson. Under this convention, we are still able to use such environments to demonstrate feasibility and interoperability.
2. We define the signal path from the transmitter to TP1, and from the receiver to TP4 to be as "transparent" as possible.
2a. Of course we realize that these paths will not be completely transparent. To borrow a phrase from Rich Mellitz, we need to define the signal paths "whose error terms are acceptable to all." It is my recommendation that these signal paths are defined as part of the work of the signaling ad hoc and that we use Rich's "super-cap" model as the basis for this work.
2b. Transmitter and receiver electrical specifications as defined at TP1 and TP4 take the reference structures defined per (2a) into account to avoid penalizing the components for any lack of "transparency".
2c. All performance simulations include these reference structures.
Discussion:
I believe that this is consistent with the recommendations made by Rich Mellitz, John D'Ambrosia, Joel Goergen, and Charles Moore (option #2).
3. Channel specifications span TP1 to TP4. The specifications include a footnote or an appendix describing the environment defined in (2) above for which the specifications were verified (equivalent circuit, return loss/insertion loss curves for Tx-to-TP1 and TP4-to-Rx, etc.).
Discussion:
As Mike Altmann points out, TP1 and TP4 are the demarcation points for the standard, and we need to ensure that they "match up" in order to guarantee interoperability. Per the statements above, ownership of the AC-coupling cap and associated mounting structure to the receiver "block". However, as Rich and others have observed, per the system designers choice, the channel may not include the AC-coupling cap or utilize different placement or a lower cost/performance alternative to what we define above. The tricky part is making it clear that if you "cheapen up" on the AC-coupling structure, you need more margin from TP1 to TP4 to make up for that. Adding informative statements related to the specification assumptions for Tx-to-TP1 and TP4-to-Rx are one way to address this as the allow to interested system designer to understand the trade-off. I say "informative" because making such statements normative implies that we are mandating a particular AC-coupling structure, which is not what we want to do.
I think this sums it up. If I misquoted any of you, I apologize and please correct me as appropriate. If any of you have a fundamental disagreement with any the above points, please air it out now so that we can address it. This Task Force has a great deal of work to do and schedule to keep. The time has come to put a stake in the ground on this and put it behind us.