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RE: Chapter 46: preamble length




Hi Bob,

I agree with the fact that MACs have not assumed 7 byte preamble size so
far. Also
that initially the preamble pattern was meant to give enough transition for
PHY
to lock in. Also that half duplex repeaters were allowed to eat into
preamble
bits. But that was till 1G.

Now with 10 G, there is neither half duplex nor repeater so why not make
things simple
now. In terms of standard, strictly speaking preample is a payload to
PCS(the table 46-3,
46-2 and PLS_Data.Indicate do not have any indication on what is data and
what is preamble).

Also I read 46.3.2 again and I do not notice exception pointed out (may be
it is some
where else).

All this may sound too much ado about nothing but I am scared if somehow
this allowance
of preamble shrinkage (when the reasons for allowing them are not clear),
will
create further "options" in PHY layer which then a chip vendor has to
support.

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: Grow, Bob [mailto:bob.grow@intel.com]
> Sent: Wednesday, March 28, 2001 10:59 AM
> To: 'Devendra Tripathi'; Sanjeev Mahalawat; Grow, Bob; 'Danielle Lemay';
> stds-802-3-hssg@ieee.org
> Subject: 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@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
> > >
> >
>
>
>