RE: Clause 46: Start of frame reception
Ben,
If a MAC receives this frame and only /S/ is required for reception,
where is the frame supposed to start ?
Lane 0 1 2 3
/S/ P P P
P P /SFD/ D
D D D D
D D D /SFD/
D D D D
...
thanks,
Danielle
> -----Original Message-----
> From: Ben Brown [mailto:bbrown@xxxxxxxx]
> Sent: Friday, May 11, 2001 6:58 AM
> To: Danielle Lemay
> Cc: stds-802-3-hssg@xxxxxxxx
> Subject: Re: Clause 46: Start of frame reception
>
>
>
>
> Danielle,
>
> I think you have it backwards:
>
> Danielle Lemay wrote:
> >
> > Hi,
> >
> > What are the rules for receiving the start of a frame?
> >
> > On transmission, the following rules apply:
> >
> > a) /S/ must appear in lane 0
> > b) /SFD/ must appear in lane 3
> > c) Only Preamble may appear between /S/ and /SFD/
> > d) Preamble may be of any length
>
> All but rule c are applicable to the RECEIVE side of the RS
> (Clause 46). The RS doesn't check the content of the preamble
> other than to verify that there is an /S/ in lane 0. It is
> allowed to discard packets that don't have the SFD on lane 3
> but even this is not necessary. The MAC is defined as 1 bit
> wide so it knows and cares nothing about lanes.
>
> On transmission, a MAC will transmit exactly 7 bytes of preamble
> and 1 byte of SFD. The RS converts the first byte of preamble
> to /S/ and aligns the /S/ to lane 0 by modifying the IPG
> according to the rules of the Deficit Idle Counter (DIC). The
> RS never modifies the length nor the content of the preamble
> other than the first byte. For that matter, there is nothing
> currently in the standard that allows for the preamble length
> or content to be modified by any sublayer. However, the receive
> RS must be able to receive a preamble of a modified length with
> modified content should it receive one. (Go figure :)
>
> >
> > Clause 46 suggests that only (a) is necessary to receive a frame.
> > Is that the intention ?
>
> Yes, rule a is the only necessary rule. Rule b is allowed but
> not necessary.
>
> Regards,
> Ben
>
> >
> > thanks,
> > Danielle
> >
> > *******************************************
> > Danielle Lemay
> > Design Engineer, Nishan Systems
> > dlemay@xxxxxxxxxxxxxxxxx
> > 408-519-3985
>
>
> --
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> Benjamin Brown
> AMCC
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