RE: Chapter 46: preamble length
Since we seem to be taking a poll - I agree with Brad. Any device causing
preamble shrinkage in 10 Gig Ethernet would be non-compliant. All the data
manipulating physical layers (meaning those that handle encoding and
decoding) have state machines specifying how the encoding takes place and
those state machines show that when data is received, the data is encoded
and sent. We have text that explicitly allows deleting idles and, in some
circumstances, ordered sets. We don't have any text allowing deletion of any
kind of data and the layers below the RS have no knowledge of preamble.
WIS, the PMAs, and the PMDs do not do deletion.
Furthermore, we have made sure that there are plenty of idles available for
deletion. On the other hand, preamble is only 8 bytes and deletions can only
be done in 4 byte chunks. If devices were relying on the ability to delete
preamble, there is only one chance per frame so if a device higher on the
stack deleted one, there are no chances per frame.
If you believe that the current draft text allows deletion of preamble, then
it also allows deletion of an arbitrary 4 bytes of data because there is no
text in any of the PCS clauses that treats preamble differently than any
other data byte.
Also, note that if a device deleted preamble, it would be a different kind
of deletion. Normally deletion happens to a column of idle or an ordered
set. Neither of the preamble containing columns is deletable because one
contains the /S/ and the other contains the SFD. An implementation deleting
preamble would have to delete some bytes from one column and some from the
next column combining the ends of the columns to make a new column.
From: Booth, Bradley [mailto:bradley.booth@xxxxxxxxx]
Sent: Thursday, March 29, 2001 8:02 AM
Subject: RE: Chapter 46: preamble length
Yes, I would disagree with that. I agree with what Pat said about MAC
transmitter being required to send the full preamble, but the MAC receiver
being tolerant to preamble shrinkage. I remember that Gigabit Ethernet MAC
receivers that were designed for only 8 bytes of preamble, even though the
standard permitted 7, had some interoperability problems with systems that
occasionally generated 7 bytes of preamble. Thankfully, the UNH IOL caught
these issues in a non-biased manner.
As for where in the draft, I don't think there is any text that directly
tells you how to do preamble shrinkage. There is text that describes the
occurrence of preamble shrinkage (hence this thread). Another question
would be: is there any text in the draft that strictly forbids preamble
From: Ben Brown [mailto:bbrown@xxxxxxxx]
Sent: Wednesday, March 28, 2001 10:44 PM
Subject: Re: Chapter 46: preamble length
In my opinion, after reading the drafts, I would say that an
implementation which chose to change the length of the
for any reason, would be non compliant. I think you would
disagree with this statement. I wonder how many others would
disagree with it. Also, where in the draft does it allow an
implementation to change the length of the preamble?