RE: Chapter 46: preamble length
The way it is specified in older versions at receive
across MII/GMII is a start of frame with just SFD and also
a start of frame with 7 bytes of Preamble and 1 byte of SFD.
I do not see a preamble being /S/ + 2 bytes of Preamble (0x55)
+ 1 byte of SFD (0xD5) across MII/GMII. So, why do you think
/S/ + 2 bytes of preamble + 1 byte of SFD across XGMII
is NOT a CHANGE but keeping /S/ + 6 bytes of preamble
+ 1 byte of SFD is a CHANAGE and required?
At 10:59 AM 03/28/2001 -0800, Grow, Bob wrote:
>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.
>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
>for PHY layers. After we have entered in continuous transmission mode (idle
>I do not see any reasoning of allowing "option" in preamble field.
>90 Great Oaks Blvd #206
>San Jose, Ca 95119
>> -----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
>> >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