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SubjectRe: CRC errors between mvneta and macb
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Le 22/10/2018 à 18:34, Willy Tarreau a écrit :
> On Mon, Oct 22, 2018 at 05:15:21PM +0200, Richard Genoud wrote:
>> After analyzing the ethernet frame on the Davicom PHY's output (pin
>> TX+), I find out that the FCS errors occurs when the ethernet preamble
>> is longer than 56bits. (something like 58 or 60 bits)
>>
>> To say this in another way, instead of having 28 times 1-0 followed by
>> the SFD (10101011), I see 29 or 30 times 1-0 followed by the SFD.
>> (sometimes 29, sometimes 30)
>>
>>
>> Should a longer preamble be considered as an FCS error ? It seems a
>> little harsh since the point of the preamble is to synchronize the frame.
>
> That indeed seems a bit strange considering that you're not supposed to
> know what is before the preamble so it would very well contain random
> noise looking a lot like alteranted bits.
>
>> I don't know what the 802.3 standard says about that.
>
> Just found it :-)
>
> https://www.trincoll.edu/Academics/MajorsAndMinors/Engineering/Documents/IEEE%20Standard%20for%20Ethernet.pdf
>
> Page 132, #7.2.3.2 :
>
> The DTE is required to supply at least 56 bits of preamble in
> order to satisfy system requirements. System components consume
> preamble bits in order to perform their functions. The number
> of preamble bits sourced ensures an adequate number of bits are
> provided to each system component to correctly implement its
> function.
>
> So that totally makes sense since the purpose is to enable signal
> detection at the hardware leve, hence the problem definitely is on
> the receiver in your case.
>
> Willy
>
Great ! Thanks !
I'll check on the Marvell side

Richard

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