Re: Clause 46: Start of frame reception
Danielle,
Frame 1 would end as soon as it saw a control character
other than /E/, even though it hadn't seen an SFD in lane
3. Therefore, in the cases you've described, neither
frame would be received correctly.
Ben
Danielle Lemay wrote:
>
> Hi Ben,
>
> It doesn't seem like the rules for reception are strict enough to keep a bad
> frame from being interpreted as good. Let's say the implementation requires
> both rules (/S/ in lane 0, /SFD/ in lane 3) and take this case for example:
>
> Lane 0 1 2 3
> /S/ P P P
> /SFD/ D D D <--Frame 1 is not received, bad /SFD/
> D D D D
> ...
> D T I I
> I I I I
> I /S/ P /SFD/ <--Frame 2 received, good /S/ from
> D D D D *previous* frame plus good /SFD/
> D D D D this frame
> ...
> D D T I
>
> Is it correct to receive "Frame 2" as a good frame, even though its /S/
> character did not occur in Lane 0 ?
>
> thanks,
> Danielle
>
> *******************************************
> Danielle Lemay
> Design Engineer, Nishan Systems
> dlemay@xxxxxxxxxxxxxxxxx
> 408-519-3985
>
> > -----Original Message-----
> > From: Ben Brown [mailto:bbrown@xxxxxxxx]
> > Sent: Friday, May 11, 2001 11:00 AM
> > To: Danielle Lemay
> > Cc: stds-802-3-hssg@xxxxxxxx
> > Subject: Re: Clause 46: Start of frame reception
> >
> >
> >
> >
> > Danielle,
> >
> > Danielle Lemay wrote:
> > >
> > > 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
> > > ...
> >
> > It depends upon your implementation. If your implementation
> > is designed to really know nothing about lanes, then your RS
> > only checks that /S/ comes in lane 0 then sends everything else
> > to the MAC. The MAC will parse the data as it arrives, find the
> > first SFD and start the packet on the first D.
> >
> > If your implementation cares about lane alignment, then
> > your RS is allowed to ignore the data on lanes 1 & 2 and
> > discards all bytes up to the second SFD.
> >
> > The case you've described above is arguably a no win case.
> > The preamble should never have been shortened by a single
> > byte so it should never occur in the first place. An SFD
> > that falls where your first SFD does is likely a bit error
> > somewhere converting the 0x55 to a 0xD5. Also, if the
> > second byte in lane 3 is not an SFD, it is probably also
> > due to a bit error. The second SFD in your example is
> > probably part of the SA but now you're thinking it is
> > the SFD so you'll likely get a CRC error on the packet.
> >
> > The bottom line is that this scenario is, in all likelihood,
> > a bad packet that will not be correctly received. However,
> > the first 2 paragraphs above describe how you're supposed
> > to handle it.
> >
> > Regards,
> > Ben
> >
> > >
> > > 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
> > > >
> > > >
> > > > --
> > > > -----------------------------------------
> > > > Benjamin Brown
> > > > AMCC
> > > > 2 Commerce Park West
> > > > Suite 104
> > > > Bedford NH 03110
> > > > 603-641-9837 - Work
> > > > 603-491-0296 - Cell
> > > > 603-626-7455 - Fax
> > > > 603-798-4115 - Home Office
> > > > bbrown@xxxxxxxx
> > > > -----------------------------------------
> > > >
> >
> >
> > --
> > -----------------------------------------
> > Benjamin Brown
> > AMCC
> > 2 Commerce Park West
> > Suite 104
> > Bedford NH 03110
> > 603-641-9837 - Work
> > 603-491-0296 - Cell
> > 603-626-7455 - Fax
> > 603-798-4115 - Home Office
> > bbrown@xxxxxxxx
> > -----------------------------------------
> >
--
-----------------------------------------
Benjamin Brown
AMCC
2 Commerce Park West
Suite 104
Bedford NH 03110
603-641-9837 - Work
603-491-0296 - Cell
603-626-7455 - Fax
603-798-4115 - Home Office
bbrown@xxxxxxxx
-----------------------------------------