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Re: [802.3ae] RE: [802.3ae_Serial] 802.3ae PRBSs are upside down


Why is it that the 2 test patterns are not identical based simply
on where the output is taken? O.150 says take it from the MSB and
Clause 49 says take it from the input to the LSB. Isn't O.150's
output simply a 32-bit shifted version of Clause 49's output?
There are no modifications to the bit stream between the input
to S0 (Figure 49-9, D4.01, page 371) and the output of S31
(though this should be labeled S30) other than a time delay
based on the clock frequency of the shift register.

What am I missing?


Tim Warland wrote:
> "Nepple, Bruce" wrote:
> > hehe --- we definitely have a bit of a mess here.
> But it's easy to fix.
> > You interpret the "inverted signal" statements in O.150 to mean
> > that the output must be inverted.  Makes sense to me.
> >
> > In addition the PRBS31 specified in 802.3ae 4.01 is also not O.150
> > compatible since it's output sequence does not start at the proper place
> > in the sequence.  The output should be the inverted MSB, not the input.
> However, the starting point is trivial relative to having the correct data.
> Starting the PRBS at the wrong place impacts the time required to synchronize
> to the pattern not the quality of the results.
> > Section 4 states that the digital signal  is take from the output of the
> > shift register.
> > If you read Section 5.1 you will see that this sequence starts with the first
> > 1 of a sequence of 9 ones.  That is most easily done by resetting to all
> > 1's and taking the output from the MSB.
> FWIW the correct reference is section 5.8 which contains an error. It should
> recommend a thirty-one stage shift register not a twenty-nine stage shift
> register. Furthermore, section 5.8 does not suggest a starting value. The point
> remains that regardless of where you take the output, the pattern generated
> by a 2^31 PRBS generator should match the expected PRBS pattern which
> the majority of test equipment recognize. That pattern is 2^31 using the taps
> described in clause 49 and 50, with the output inverted.
> > <snip>
> >
> > In summary:
> >
> > 1.  The PRBS31 scrambler specified in 4.01 is drawn at odds with O.150 in that the
> > output is taken from the input, rather than from the MSB, and it is not inverted.
> But by adding a simple inverter, the two patterns become compatible (although as
> you say, perhaps not identical). Compatibility with existing test equipment is
> significant.
> --
> Tim Warland  P. Eng.
> Applications Engineer
> Quake Technologies   (613)270-8113 ext 2311
> Tough Times don't last, tough people do

Benjamin Brown
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