[802.3ae] RE: [802.3ae_Serial] 802.3ae PRBSs are upside down
hehe --- we definitely have a bit of a mess here.
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.
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.
By specifying the PRBS31 with it's output from the input stage and referencing
O.150, you are implying that the PRBS23 should be generated the same way.
This will result in a different WIS test pattern payload that if the output
is the MSB.
The polarity of the PRBS23 is not clear. O.150 says it should be inverted,
and 802.3ae says that the "PRBS output" should be inverted before it is placed
in the second frame. Is the PRBS output the already inverted O.150 output?
The matching of the polarity of the CID bits and the payload
would be changed if the PRBS23 lsfr output were inverted or not inverted.
The x**7 + x**6 + 1 scrambler is the worldwide specified SONET payload scrambler.
It is specified in ANSI T1.105-1995, and ITU-T G.707.
It also is MSB out and has no inversions. In this case (and the WIS PRBS23) the
output bit placement (MSB, etc) is important since the lsfr is reset to all 1's for
each frame.
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.
2. The output bit position is important in the PRBS23 scrambler since it is
reset to all 1's after every frame. It's polarity is important relative to the CID.
The PRBS23 "prbs output" should be the MSB inverted.
That is, the first frame of the WIS test pattern will contain the shift
register MSB inverted. This is important if you want the CID to match the
prbs pattern, and meet the letter of O.150.
Bruce
> -----Original Message-----
> From: piers_dawe@xxxxxxxxxxx [mailto:piers_dawe@xxxxxxxxxxx]
> Sent: Monday, March 04, 2002 11:23 AM
> To: stds-802-3-hssg@xxxxxxxxxxxxxxxxxx
> Cc: stds-802-3-hssg-serialpmd@xxxxxxxx
> Subject: [802.3ae_Serial] 802.3ae PRBSs are upside down
>
>
>
> Calling 10GE chip designers:
>
> Pseudo random bit sequences of length 2^31-1, 2^23-1 are each uniquely
> defined in ITU-T's O.150 and implemented in test equipment,
> as is a PRBS7.
> Meanwhile 802.3ae D4.1 has defined:
>
> In clause 49, a PRBS31 which differs from O.150 in that for every 1 in
> O.150, D4.1 has a 0. We think the scrambler polynomial is the same.
> In clause 50, a PRBS31 identical to the one in clause 49 but
> independently
> defined.
>
> In clause 50, a mixed frequency test pattern based on O.172
> and G.957 using
> a PRBS23.
>
> In clause 48, a PRBS7, X^7 + X^3 + 1 or X^7 + X^6 + 1, which
> drives the
> A,K,R randomizer. And has no other purpose in the standard?
>
> The 58 bit scrambler in clause 49 is neither used as a PRBS
> alone, nor does
> it compete with any 58 bit scrambler or PRBS in O.150. So we
> are free to
> stay with our own choices for this.
>
> Are there any other PRBSs to be considered?
>
> Almost no-one knows exactly what the actual bits in these
> patterns are,
> no-one wants to. But we all expect that pressing e.g. the
> PRBS31 button on
> a test equipment will get us to a working test. It would be
> particularly
> inconvenient for manufacturers and networks which use both SONET and
> 10GEthernet to let this discrepancy continue indefinitely.
> Therefore, we
> should make the O.150 PRBS31 the preferred variant in our
> standard. We
> should allow its inverse, the one currently in D4.1, so as
> not to disqualify
> implementers who have implemented it in good faith. We may
> wish to define
> optional inversion bits in clause 45. This way, over time, we can all
> migrate to the consistent set of PRBSs which are in the test
> equipment, and
> not have to train users, customers and testers in this detail.
>
> To avoid snarl-ups like this we should not define things that
> are already
> defined (just as, "we don't do fiber standards". We should refer
> normatively to O.150 where appropriate, yet repeat the information,
> informatively, in our standard to assist the reader.
>
> Some test equipment can invert its PRBSs. We don't know if
> all makes can -
> input welcome.
>
> For those who want to know more, O.150 is available for about
> US$12 from
> http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&;
parent=T-REC-O
.150 . Each PRBS has one run of identical bits as long as the generator
polynomial, of one type only. For example, PRBS31 has one run of 31 zeroes
per pattern, in the O.150 definition, or one run of 31 ones per pattern, in
the D4.1 definition.
Does anyone know where the PRBS7 in the test equipment is defined? I
believe some, maybe all, test equipment uses !(X^7 + X^6 + 1) (i.e. contains
a run of 7 zeroes) which is O.150 style, by default. Are all makes of test
equipment the same on this? Can all invert the pattern (generating and
checking)?
Piers