RE: Chapter 46: preamble length
Bob,
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?
Thanks,
Sanjeev
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.
>
>--Bob Grow
>
>-----Original Message-----
>From: Devendra Tripathi [mailto:tripathi@vidyaweb.com]
>Sent: Wednesday, March 28, 2001 10:36 AM
>To: Sanjeev Mahalawat; Grow, Bob; 'Danielle Lemay';
>stds-802-3-hssg@ieee.org
>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.
>
>Regards,
>
>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@ieee.org
>> [mailto:owner-stds-802-3-hssg@ieee.org]On Behalf Of Sanjeev Mahalawat
>> Sent: Tuesday, March 27, 2001 6:08 PM
>> To: Grow, Bob; 'Danielle Lemay'; stds-802-3-hssg@ieee.org
>> 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@nishansystems.com]
>> >Sent: Monday, March 12, 2001 10:38 AM
>> >To: stds-802-3-hssg@ieee.org
>> >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@NishanSystems.com
>> >408-519-3985
>> >
>>
>