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Re: Clause 46: Start of frame reception


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.


> thanks,
> Danielle
> *******************************************
> Danielle Lemay
> Design Engineer, Nishan Systems
> dlemay@xxxxxxxxxxxxxxxxx
> 408-519-3985

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