Geoff –d thanks for the clarification, and this is a fine point.
First, I think we agree that the MDI referred to in standards is an interface plane, a reference point, not the actual physical connector, although common usage may use the term “MDI” to refer to what would be more precisely the “MDI connector”.
Technically, the MDI (reference point) cuts right through the middle of a physical connection to the medium, at its mating surface, and is therefore hard to measure. Always a sticking point, because, as you point out, the cabling standards
don’t include even the cabling side of the connector (a plug in the case of RJ-45).
However, I disagree that the link segment includes the MDI connector for the following reasons:
First, the electrical characteristics of the MDI connector for most PHYs are defined separately. (they’re usually in a xxx.8 whereas the link segment is in xxx.7).
Second, the definition of the ‘link segment’ in clause 1.4 is “the medium connection between” two MDIs. To my understanding, the words “medium” and “between” exclude the connectors which define what the “medium” is “between”. when the
connector is a jack/plug arrangement, with the jack on the equipment, the jack side of the interface plane is part of the equipment (or “port”) which we specify, and the plug side is part of the link segment (whose composition we do not).
Third, it is not possible to measure the link segment characteristics, independent of the equipment, and therefore impossible to specify them, if you include the port side of any connector.
- again, this is a fine point. As far as the discussion we were having in the ad hoc today (which regarded the inliners vs. the cable segments themselves), I think we agree. they are all parts of the link segment.
-george
From: Geoff Thompson <thompson@xxxxxxxx>
Sent: Tuesday, December 08, 2020 10:27 AM
To: George Zimmerman <george@xxxxxxxxxxxxxxxxxxxx>
Cc: Geoff Thompson <thompson@xxxxxxxx>; STDS-802-3-B10GAUTO@xxxxxxxxxxxxxxxxx
Subject: Re: [802.3_B10GAUTO] Link segment and cabling terminology (was [802.3_B10GAUTO] P802.3cy ad hoc meeting follow-up)
George-
Actually, to be precisely correct,
The "Link Segment" is the MDI to MDI piece and includes the connector.
Just as the MDI on the DTE includes the connector.
MDI means exactly what it stands for, Media
Dependent INTERFACE as distinct from the "MDI connector."
Interface means "at the connecting surface." The distinction is very important for copper connectors which generally attach pins to the cable conductors. Not as important, but not of zero importance in fiber connectors where
the connector in usually used to position the cable fiber directly against the mating surface.
Our system separation is: Equipment - Terminated Medium - Equipment
Cabling standards are different, they put the edge of their standards and performance specs at the back side of the connector which is application specific to the DTE.
This difference is not huge but it is real. Again, it tends to be more significant with copper than it is with fiber.
Thus your statement about the definition of a "link segment" below is
incorrect.
Chris – I think the below is a great start, and yes, we need to be rigorous about saying “link segment”.
I haven’t heard Geoff (Thompson) on our calls in a while, but I’m hearing his voice in the back of my head, and in this case, his voice is one of wisdom. Words have power and meaning – we need to use them correctly. Good standards are
unambiguous to the reader, and do not rely on assumptions. I know you know this, and the below, it is for other’s benefit I expound.
To that extent, we talk about the components used, such as cable and connectors, in order to understand the technical feasibility of what we are requiring. However, I suggest that what we SPECIFY in the document is only at the link segment
level. I write the below with great trepidation, because someone will have a nit. The important thing is that the link segment is a defined thing and we don’t go within it – just as Chris said.
link segment: this would include inline connectors, and cable (or other conductor) segments, but not the MDI connectors. When the MDI is at the edge of the PCB, it does not include PCB
losses as well. It is a defined term in IEEE Std 802.3-2018 (see 1.4.309), so we can’t change it, it is point-to-point and includes everything between the two MDIs. 802.3 does not generally sub-divide the link segment into components in its requirements
(on occasion it may offer informative guidance on how components might relate to construction of a conforming link segment).
If we want to go further in specifying requirements and include more than just the link segment, “channel” (1.4.179) is often used for these purposes (but see below for a more rigorous approach - test points). “channel” has been a bit
of a hot-button in 802.3 specs, and, except for systems which use a notion of a channel as a frequency (like a radio or TV channel), is usually locally defined within clauses (“a defined path along which an electrical or optical signal passes”). As a result,
the precise meaning, depends on the clause and the context, and therefore, in my opinion, the word “channel”, while common, is better avoided in 802.3 standards, and needs extra precision. (because the reader THINKS they know it, but it has some specific,
precise meaning). However, for us, and often elsewhere, channel is useful to mean everything between the PMA/PMD (transmitter) and PMA/PMD (receiver), including PCB and MDI.
The rest of the words we use are not strictly defined in IEEE Std 802.3, they include “cabling” which is generally “cable and connectors” and roughly equivalent to link segment, except that it isn’t restricted to MDI to MDI connections
(“cabling” may be a subset of a link segment as a result), “cable” which is just the wiring of an individual segment (which may be a subset of a link segment), and “connectors”, “inliners”, or “connecting hardware” which are mostly self-explanatory for connecting
hardware within the link segment (not the MDI connector).
Components of the link segment are generally in the domain of other organizations. In this case it might be SAE, might be OPEN, might be IEC, … not really sure. This is where liaisons help, and if some of our automotive-industry-affiliated
participants can help clarify which groups write normative specifications that we might reference, that would be great.
Following the ad hoc meeting discussion; I suggest when we mean "link segment", we say "link segment".
If I understand correctly, methods for restricting micro-reflections in jonsson_3cy_01_12_08_20.pdf would yield restricting reflections in "link segments" characterized as residual
return loss (TBD). Towards baseline, residual return loss would be specified as a link transmission parameter (using 802.3ch terminology).
XXX.X Link segment characteristics
XXX.X.X Link transmission parameters
XXX.X.X.X Residual Return loss
Brief description on residual return loss...In order to limit the noise at the receiver due to residual return loss.....
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