Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: [802.3_4PPOE] Link section vs "channel" and link segment

Hi Geoff,

Many thanks.

Please see my response inline below.



From: Geoff Thompson [mailto:thompson@xxxxxxxx]
Sent: Thursday, May 11, 2017 1:38 AM
To: Yair Darshan <YDarshan@xxxxxxxxxxxxx>
Cc: Geoff Thompson <thompson@xxxxxxxx>; Geoff Thompson (thompsongeoff@xxxxxxxxxxxxxx) <thompsongeoff@xxxxxxxxxxxxxx>; Chad Jones (cmjones) <cmjones@xxxxxxxxx>; Fred Schindler <fred@xxxxxxxxxxxxxx>; Shariff, Masood (mshariff@xxxxxxxxxxxxx) <mshariff@xxxxxxxxxxxxx>; STDS-802-3-4PPOE@xxxxxxxxxxxxxxxxx
Subject: Re: Link section vs "channel" and link segment





I have copied your questions below

(and corrected some spelling and typographical items)

My answers are mixed in

(For those new to the conversation, these comments from Yair (Red) and my responses (Green) are all relative to my presentation from the March meeting,  )


Slide 2

1. I undestand that the term "channel" is ANSI/TIA term for the channel model with 4 connectors as defined in D2.4 page 211.

Why it is not in the list above?

(GOT) Because we write our standards for the international market with an eye towards eventual adoption by ISI/IEC.  That means we use international rather than national or regional standards for reference whenever possible.  In this particular case the definition in the ISO/IEC standard is technically equivalent to the TIA definition so there is no need to cite both.


Yair: OK.  We have ISO/IEC11801 in the list and is equivalent to TIA.

So why we can’t use the definition for channel per ISO/IEC instead of changing the whole draft from “channel” to “link segment”?

2. Does your plan is to change also the usage of "channel" and "permanent link" in clause 145.4.9 ?

(GOT) In a word, yes, but it is a little more complicated in my view.  First, the problem with figure 145-38 is that it is a cabling standard diagram and only (costly) correct in that context.

 Thus we face two choices (a) to acknowledge it as a stolen cabling drawing (and somewhat incorrect in our context)


-Why it is incorrect in our text? It is I believe a drawing from TIA/EIA or ISO/IEC which you said they are equivalent.

-To admit it is a stolen cabling drawing will not be so hard I guess. We just need to add reference to the source of the drawing correct?

or (b) to edit it into correctness.  

To do that we would have to do the following:  (a) redepict (at least) the end connectors from a single box with one "c" into two boxes (i.e. a mated pair) then (b) move the endpoint demarcation lines to align to the mating interface of the mated pair and (c) change the term "channel" to "link section".

"permanent link" should be changed to "link segment" (not "link section")  

Yair: I guess that this will be easy thing to do if we accept to change “channel” to “link segment” per your proposal. Figure 145-38 is extremely important to understand the matter of this clause.

3. Currently Midspans operated per the definitions of 145.4.9. How your 45 comments to change "channel" to "link segment" will affect it?

(GOT) My changes should fix the technical errors introduced by doing that.  We can still use the terms cross connect and interconnect without a problem (but I feel that they are unnecessary).

Yair: Can your repeat here what is the technical error?

Does channel per ISO/IEC can’t be ling segment? I see “channel” between Equipment and TE in figure 145-38 and to me the Equipment is Switch or PSE and the TE is the PD. Is this correct?

Does the differences is all about +/- one connector?

Slide 3

1. What is the link section model (how many connectors, cordage? cable?)
2. What is the link segment model (how many connectors, cordage? cable?)

(GOT) It doesn't matter.  The tradition in 802.3 (which is an equipment specification group, not a cabling group) has been to specify the media from MDI to MDI (or in the case of PoE from PI to PI) by overall performance of the link segment/section.  How a cabling standard chooses to divide that up inside their standard and between cables and cabling hardware is outside our scope.

Yair: I understand however, to us if it is 2 connectors +cable model or 0 connectors or 4 connectors +cable is important to know for the end to end unbalance model. Based on the 4-connector model, we have developed the requirements for unbalance as function of number of connectors vs length of cordage and cabling per the unbalance adhoc research.

3. Why you prefer to use terms (link segment/Link section) from ISO/IEC and not from ANSI/TIA

(GOT) See answer #1 for slide 2 above.

Yair: I understand.

Slide 4

1. I understand that this definition allows Switch without PoE and Midspan to be connected to the same PD correct?
2. I understand that this definition doesn't allow Endspan and Midspan to be connected to the same PD, correct?
3. Regarding (2), what if one of the PSEs (Endspan or Midspans is not active), does it meets the definition above? 
I am asking because we have equipment that work like this for backup purposes.

4. What is your opinion regarding to modify the definition above to

"Link section: The point-to-point medium connection between two and only two active Power Interfaces (PIs).

(GOT) I think your addition to my definition improves the definition.  It may need more work or a clarifying note to cover the case during the contention process while the PSE PI to be used when the system goes active is being chosen.

Yair: We need to close it before the meeting and bring something to the discussion that we are both OK with it since the whole subject has some holes in it and in my opinion it will be hard to accept all the comments marked with REF204 without addressing the other comments and issues related to it, i.e. in this s cycle if we are going to adopt your whole proposal, we must have at least the proposed addition that I have suggested (plus maybe other concerns that you will see through all this discussion), otherwise it can be interpreted that it is not allowed to connect Midspan and Switch to the same PD which is allowed for 15 year now. This is my main worry. In addition, the current definition introduces a problem to support two PSEs ALT A and ALT B to form 4-pair PSE operation.

 Slide 5

Yair: I like the current one since it is more flexible. It allows me to support my current equipment while the above doesn’t allow....which is my main problem unless you modify your fantasy definition to the one that I have suggested in the previous slide. 

(GOT) The current one is not correct.  That is because the "scope" of generic cabling does not include the "application specific connector" (i.e. the RJ-45 that is at each end of the "generic cabling"). 

Yair: Now I am a bit confused. If you look at Figure 145-38, the channel doesn’t include the equipment connector, so not clear to me why the current one is not correct?

And in slide 2 question 2 you have suggested to add box C to both ends to make the model correct and now you are saying that it shouldn’t include it.

If you can send me a drawing of Figure 145-45 and how it is sync with link segment definition as you see it, it will shorten the discussion (I guess).


1.4.248 link: The transmission path between any two interfaces of generic cabling. (From ISO/IEC 11801.) 

As I said above, I like your modification.  We will probably need to discuss this a little more in the meeting.

Yair: OK


Slide 9a

1. where from IEEE802.3AF/AT?

(GOT) Again, perhaps we should add the word "active" to the slide.  Otherwise, this is a trick question.  You know the answer much more intimately than I do. 

Yair: I looked at Figure 145-38 and Figure 149 and I see all options there so I don’t understand from where you took what you are define as the simple cases? Can you clarify?

 Slide 9b

Why it is more complex? This case is covered by IEEE802.3AF/AT and 802.3bt.. (Backoff time etc. It means that there is only one PSE active.

(GOT) I was under the impression that 802.3bt allows you to gang one active endspan PSE (Alt A) plus one active midspan PSE (Alt B) simultaneously to get additional power.

Yair: It may allow it, but I don’t see a problem with it. Why?

-We can explicitly disallow it by the additional text that I am proposing.

-If in In 802.3bt, you connect PSE on ALTERNATIVE A and PSE on ALTERNATIVE B when they are both active and connected to the same PD, this will never work because the unbalance will be huge and it will fail on overload etc.

The points are:

  • You cannot prevent from such a connection/configuration to happen and you can’t enforce it. Users may connect wrong configuration by mistake. All we need to make sure is to prevent damage to equipment and/or safety hazard. We already achieve this in the current spec. The detection mechanism and the protection system are specified per pairset and will prevent damage. There is no safety hazard.

 In that case, if we say that there is one and only link section it has (at least) a funny shape.

Yair. I agree. That is why I believe that we should use the term link segment and not link section. Link segment allows all configurations to exist under the condition that only one PSE is active.

When I mean one PSE, I mean to Endspan or Midspan. We also need to verify that there is no collision between the definition of connecting Midspan+Endspan to the one PD which are two PSEs connected to one PD but only one is active to the definition that PSE ALT A and PSE ALT B is supplying power over 4-pairs to a single PD which is OK (the purpose of 802.3bt). 


Slide 10

1. You can't prevent such a case from happening, it can be by mistake and most likely will not work over 4-pairs due to serious unbalance issues (It will fail PSE_Vdiff, Icon-2P_unb, Overload etc.)

(GOT) This is not a question.

2. If only one PSE is active, than there is no issues at all. Correct?

(GOT) Correct

3. I prefer using link segment instead of link section. What is your strongest argument against this proposal.

(GOT) Link segment and link section are exactly equivalent in the case of an endspan PSE.

Yair: OK

We invented the term "link section" during the development of 802.3af to be able to have a term for the power path when it was not the same length as the data path (i.e. for the midspan case).  

Yair: OK

We made the decision at the time that the term "link section" was defined to have it apply to either the endspan case or the midspan case.  We COULD have decided at that time to only make the new term apply to the midspan case but that is not the decision we made at that time.  That is not a decision that we can revisit at this point. 

Yair: Can you show me examples in the 802.3af/at spec how this decision that we took, was implemented by text or drawing or both?


I hope this answer your questions.  My guess is that to go any further on theses issues we need to discuss it during the meeting rather than by correspondence.  I look forward to seeing you in New Orleans.

Yair: I found few issue that could be a problem if we implement all your comments marked REF204 as is. As a result, I pulled out all the comments make them TFDT. Chad asked me to verify the whole subject with you and come to agreements prior the meeting in order to meet our time limits. So, I suggest to try to clear up the subject and come up with complete proposal prior the meeting and discuss it at the meeting.


Best regards,





On May 8, 2017, at 11:22 AMPDT, Yair Darshan <YDarshan@xxxxxxxxxxxxx> wrote:


Hi Geoff,


You have submitted many comments regarding replacing channel with link segment.

I have read your presentation and I have some questions and some concerns.

I would like to start a discussion with you about this much before the next meeting and I am attaching your presentation with questions/comment inline.

Ill appreciate if you can address them.


Many thanks



Darshan Yair

Chief R&D Engineer

Analog Mixed Signal Group

Microsemi Corporation


1 Hanagar St., P.O. Box 7220
Neve Ne'eman Industrial Zone
Hod Hasharon 45421, Israel
Tel:  +972-9-775-5100, EXT 210.

Cell: +972-54-4893019
Fax: +972-9-775-5111


E-mail: <mailto:ydarshan@xxxxxxxxxxxxx>.  



<Thompson_01_0317_with Yair comments.pdf>