Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

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

"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

Tim Warland  P. Eng.
Applications Engineer
Quake Technologies   (613)270-8113 ext 2311

Tough Times don't last, tough people do