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Re: Chapter 46: preamble length


"Booth, Bradley" wrote:
> Ben,
> Yes, I would disagree with that.  I agree with what Pat said about MAC
> transmitter being required to send the full preamble, but the MAC receiver
> being tolerant to preamble shrinkage.  I remember that Gigabit Ethernet MAC
> receivers that were designed for only 8 bytes of preamble, even though the
> standard permitted 7, had some interoperability problems with systems that
> occasionally generated 7 bytes of preamble.  Thankfully, the UNH IOL caught
> these issues in a non-biased manner.

Clause 36 explicitly states that the preamble can be shortened,
if not in words than in the description of the TX state machine.
People who made this mistake in 1G were clearly in the wrong.
However, I contend that if those same designs were made to accept
either 7 or 8 bytes (preamble + SFD) then they would have worked
fine. No implementation should ever remove more than 1 byte of
preamble so a MAC designed to expect no fewer than 7 bytes should
be fine.

I think this same idea exists in 10G. No implementation should
ever remove any bytes of preamble so a MAC designed to expect no
fewer than 8 (or even exactly 8) should be fine. If it stops working
because someone else's PHY removes bytes of preamble, than it should
be easy to show that PHY is non compliant.

> As for where in the draft, I don't think there is any text that directly
> tells you how to do preamble shrinkage. There is text that describes the
> occurrence of preamble shrinkage (hence this thread).  Another question
> would be: is there any text in the draft that strictly forbids preamble
> shrinkage?

There is also no text in the draft that says a PHY can't perform
clock tolerance by discarding data from the packet. However, a PHY
that did so, even if it corrected the CRC somehow, would immediately
be considered non compliant. The way we're handling preamble in
these PHYs is identical to the way we're handling data. It should
never be touched. Therefore, a MAC designed to only expect 8
bytes of preamble should never be deemed non compliant.


> Cheers,
> Brad
>                 -----Original Message-----
>                 From:   Ben Brown [mailto:bbrown@xxxxxxxx]
>                 Sent:   Wednesday, March 28, 2001 10:44 PM
>                 To:     stds-802-3-hssg@xxxxxxxx
>                 Subject:        Re: Chapter 46: preamble length
>                 Brad,
>                 In my opinion, after reading the drafts, I would say that an
>                 implementation which chose to change the length of the
> preamble,
>                 for any reason, would be non compliant. I think you would
>                 disagree with this statement. I wonder how many others would
>                 disagree with it. Also, where in the draft does it allow an
>                 implementation to change the length of the preamble?
>                 Ben

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