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RE: Chapter 46: preamble length

As Brad pointed out in his message, it would be a change to require that
preamble length be preserved.  Though generated by the MAC it's original
purpose was specifically for PHY layer requirements, not the MAC.  While
many DTE designs benefit from the extra memory accesses allowed by IPG and
preamble, no previous generation of Ethernet has assumed that seven preamble
bytes would be received at the MAC.  This was true even with the
continuously clocked media of previous Ethernet speeds, where preamble
shrinkage was studied in the design and budgeted in the topology rules.  In
some Ethernet port types, the PHY only changes IPG lengths, in others, it
changes both (even for full-duplex, continuously clocked, gigabit Ethernet).

The review and comments that have produced the D3.0 text were simply to
preserve a characteristic of Ethernet MACs.

--Bob Grow

-----Original Message-----
From: Devendra Tripathi [mailto:tripathi@xxxxxxxxxxxx]
Sent: Wednesday, March 28, 2001 10:36 AM
To: Sanjeev Mahalawat; Grow, Bob; 'Danielle Lemay';
Subject: RE: Chapter 46: preamble length

I think Sanjiv has very good point. It is regressive to go back and make
preamble open
for PHY layers. After we have entered in continuous transmission mode  (idle
or data),
I do not see any reasoning of allowing "option" in preamble field.


Devendra Tripathi
VidyaWeb, Inc
90 Great Oaks Blvd #206
San Jose, Ca 95119
Tel: (408)226-6800,
Direct: (408)363-2375
Fax: (408)226-6862

> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Sanjeev Mahalawat
> Sent: Tuesday, March 27, 2001 6:08 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
> >