Re: [EFM] EFM OAM on Preamble
Thank you for the question.
Indeed, during my presentation, there was a discussion on Preamble
In the current PCS/RS/GMII clauses of existing 802.3 spec clearly describes
PHY may shrink preamble to only SFD and RS has to accept a frame with any
For these comments, I made it clear that
- OAM on Preamble is applied to Full duplex mode only. Not for half-duplex
/ CSMA-CD mode.
Preamble shrinkage to "any" size happens only for CSMA-CD mode.
And EFM environments and SP provider circumstances, any half duplex mode
does not make sense
due to distance limitation. So this assumption is valid, I hope.
Only thing to modify the spec is that we state something like in RS/GMII
clause ( where OAM handler locates )
"Preamble not be shrinked by PHY in full duplex mode, when OAM is used"
-EFM Cupper P2P, EPON will have PCS change as well as PMD, so we can make
sure preamble transparency in the new spec.
-For P2P GE, there is the following issue, as you anticipate.
( this was not discussed during presentation but
discussed off-line during the meeting)
Since Idle detection is done by every 16bits, Aligned / Unalined frame
may lose the first byte of Preamble, where
the second byte be replaced with /S/.
We have 2 options for this.
a) Not to use this byte. This means we have only 6byte preamble (
including SFD byte.) to carry OAM.
Since we have 2 reserved byte, still OK with it. This actually was
discussed off-line last week.
b) PCS recovers /S/+7byte preamble ( including SFD byte ), when Unaligned
This is simply done by having additional 3-4 byte FIFO in PCS.
We will discuss about these options in detail in Jan meeting.
Note this occurs only in GE. Not in 100baseX, 10GE.
-For 802.3ae 10GE, it is clearly stated in the spec, no preamble shrinkage.
If preamble is shrinked, MAC can discard whole frames.
Hope this helps.
At 09:48 AM 11/20/2001 -0500, Ben Brown wrote:
>I did not attend your presentation entitled "EFM OAM on Preamble"
>but I've seen a copy of your slides. I wonder if this question
>was asked during the presentation and would like to hear your
>How is this protocol handled when preamble bytes are lost?
>Specifically in the case of 1GE when the first byte of
>preamble is discarded around 50% of the time.
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