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RE: Chapter 46: preamble length


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.


 -----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
>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.


>--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 ?
>Danielle Lemay
>Design Engineer, Nishan Systems