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RE: Chapter 46: preamble length


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.


		-----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


		At 08:39 PM 03/27/2001 -0800, Booth, Bradley wrote:
		>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
		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

		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


		> -----Original Message-----
		>From: 	Sanjeev Mahalawat [mailto:sanjeev@xxxxxxxxx] 
		>Sent:	Tuesday, March 27, 2001 8:08 PM
		>To:	Grow, Bob; 'Danielle Lemay';
		>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