Re: Chapter 46: preamble length
Brad,
In my opinion, after reading the drafts, I would say that an
implementation which chose to change the length of the preamble,
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?
Ben
"Booth, Bradley" wrote:
>
> Pat,
>
> I agree that within the context of what we have document in 802.3ae, that we
> are allowing for adjustments of clock difference only by deleting idles and
> not preamble. I can think of a case with our new minimum IPG over XAUI that
> depending on the structure of the implementation for compensating for clock
> tolerances, that an implementer could be forced to shrink the preamble due
> to the inability to shrink the IPG. I personally thought the WAN PHY might
> see this issue too, but that was only a personal opinion. Others may have
> differing opinions on the need for shrinkage of preamble, and I fully
> understand and accept that because it is based on how you wish to implement
> 802.3ae.
>
> I admit that the discussion has moved away from the real question, and I've
> done a very poor job of keeping the focus on the MAC's need to receive 8
> bytes of preamble. The case is that IPG and preamble are permitted to
> shrink, and if you believe that there is any possibility that could happen,
> then you need to design for it. Preamble's use is not the same now as it
> was when it was originally specified; although, I'm sure I don't have to
> tell you that, as you have more experience with Ethernet than I do. The
> fact is that in GbE I saw some implementers design their MACs to only
> receive Ethernet packets with 8 bytes of preamble, when the standard
> actually permitted 7 bytes of preamble. My point is that even if the
> transmitter specifies sending 8 bytes, the receiver just discards them.
>
> Cheers,
> Brad
>
> -----Original Message-----
> From: pat_thaler@xxxxxxxxxxx [mailto:pat_thaler@xxxxxxxxxxx]
> Sent: Wednesday, March 28, 2001 6:05 PM
> To: bbrown@xxxxxxxx; stds-802-3-hssg@xxxxxxxx
> Subject: RE: Chapter 46: preamble length
>
> Ben,
>
> Brad is mistaken. Nothing can change the preamble length. Adjustment for
> clock difference is done only by deleting idles or, when continuous ordered
> sets are being received, ordered sets.
>
> Pat
>
> -----Original Message-----
> From: Ben Brown [mailto:bbrown@xxxxxxxx]
>
> Brad,
>
> Just to correct something you said: "In 10GbE, truncation of the
> preamble can occur due to the asynchronous timing associated with
> the WAN PHY." If bits are lost in the WAN, the 66-bit blocks are
> going to be corrupted and the PCS will lose sync. There is nothing
> that currently affects the length of the preamble. If this is not
> the case, please educate me.
>
> Ben
>
> "Booth, Bradley" wrote:
> >
> > Sanjeev,
> >
> > 10GbE does not support CSMA/CD. In half duplex implementations, some
> > preamble bits could be lost due to sampling, asynchronous timing and clock
> > recovery effects. From my understanding, the preamble was also to permit
> > the Manchester encoder/decoder to lock to the incoming stream.
> >
> > In a full duplex system without manchester encoding (like GbE and 10GbE),
> > the need for all the preamble bits is greatly decreased. Some GbE MACs
> were
> > designed to be able to receive SFD immediately following the reception of
> > one byte of preamble. In 10GbE, truncation of the preamble can occur due
> to
> > the asynchronous timing associated with the WAN PHY.
> >
> > Cheers,
> > Brad
> >
> > -----Original Message-----
> > From: Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx]
> > Sent: Wednesday, March 28, 2001 12:09 PM
> > To: Booth, Bradley; stds-802-3-hssg@xxxxxxxx
> > Subject: RE: Chapter 46: preamble length
> >
> > Booth,
> >
> > At 08:39 PM 03/27/2001 -0800, Booth, Bradley wrote:
> > >
> > >Sanjeev,
> > >
> > >You may wish to re-read 802.3, as Ethernet has always had
> > the ability to
> > >shorten the preamble. This was very true in the CSMA/CD
> > half duplex days of
> > >Ethernet, and it has remained a part of full duplex
> > Ethernet. The transmit
> > >side of the MAC generates the full preamble, but the
> > receive side never
> > >requires reception of the full preamble.
> >
> > Does 10GE supports CSMA/CD? Has everything been same in
> > 802.3's
> > every version of Ethernet? What is the reason in 10GE for
> > preamble to be shortnened?
> > Are you suggesting that there is no reason but just
> because
> > it was so in the older
> > versions, therefore, 802.3ae HAS to allow preamble to be
> > truncated?
> >
> > And though many things have been added and/or removed from
> > version to version BUT preamble has to be allowed to be truncated without
> > any reason, right? If this is the case then I do not have any further
> > question.
> >
> > Thanks,
> > Sanjeev
> >
> > >
> > >Cheers,
> > >Brad
> > >
> > > -----Original Message-----
> > >From: Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx]
> > >Sent: Tuesday, March 27, 2001 8:08 PM
> > >To: Grow, Bob; 'Danielle Lemay';
> > stds-802-3-hssg@xxxxxxxx
> > >Subject: RE: Chapter 46: preamble length
> > >
> > >
> > >Hi Bob,
> > >
> > >At 02:21 PM 03/27/2001 -0800, Grow, Bob wrote:
> > >>
> > >>On transmit, a conforming implementation will send seven
> > preamble plus the
> > >>SFD.
> > >>
> > >>On receive, there is no current function that will
> change
> > that length, but
> > >>the concensus of the committee was to keep the option
> > open. (In 802.3z we
> > >>did change preamble length for idle alignment.) The
> D3.0
> > text should make
> > >>it clear that an implementation should be tolerant to
> > changes in preamble
> > >>length, though it can still rely on lane alignment
> (Start
> > in lane 0, SFD in
> > >>lane 3). Text was added to warn that the Start and SFD
> > could appear in the
> > >>same column.
> > >
> > >What is the reasoning behind letting a layer lower than
> > >MAC to touch the preamble?
> > >
> > >Since preamble is coded as data it belongs to MAC
> > >and no lower layer should be allowed to change
> > >and/or remove the length of preamble.
> > >
> > >Thanks,
> > >Sanjeev
> > >
> > >
> > >>
> > >>--Bob Grow
> > >>
> > >>-----Original Message-----
> > >>From: Danielle Lemay [mailto:dlemay@xxxxxxxxxxxxxxxxx]
> > >>Sent: Monday, March 12, 2001 10:38 AM
> > >>To: stds-802-3-hssg@xxxxxxxx
> > >>Subject: Chapter 46: preamble length
> > >>
> > >>
> > >>
> > >>
> > >>
> > >>Is it possible for the preamble+SFD to be less than 8
> > bytes ?
> > >>
> > >>thanks,
> > >>Danielle
> > >>
> > >>
> > >>
> > >>*******************************************
> > >>Danielle Lemay
> > >>Design Engineer, Nishan Systems
> > >>dlemay@xxxxxxxxxxxxxxxxx
> > >>408-519-3985
> > >>
> > >
>
> --
> -----------------------------------------
> Benjamin Brown
> AMCC
> 2 Commerce Park West
> Suite 104
> Bedford NH 03110
> 603-641-9837 - Work
> 603-491-0296 - Cell
> 603-626-7455 - Fax
> 603-798-4115 - Home Office
> bbrown@xxxxxxxx
> -----------------------------------------
--
-----------------------------------------
Benjamin Brown
AMCC
2 Commerce Park West
Suite 104
Bedford NH 03110
603-641-9837 - Work
603-491-0296 - Cell
603-626-7455 - Fax
603-798-4115 - Home Office
bbrown@xxxxxxxx
-----------------------------------------