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RE: Clarification on added B2/B3 bytes in WIS


To understand the method of computing the bit interleaved
parity for the B2 fields, I think it might be easier to simply
use the definition of "BIP-N" in the Definitions subsection
of P802.3ae, subclause 1.4:

" Bit interleaved parity N (BIP-N): A method of error
monitoring using even (or odd) parity, such that an N-bit
codeword is generated over a specified portion of an input
data stream in such a manner that the i-th bit of the codeword
provides even (or odd) parity over the i-th bit of all N-bit
sequences in the covered portion of the data stream."

The 192 B2 bytes, taken together, form the "N-bit codeword".
This field has a total width of (192 x 8) bits, or 1536 bits. In
order to compute the B2 field from the WIS frame, therefore,
we do the following:
- take the entire WIS frame and divide it into 1536-bit chunks,
starting at the top left (the first A1 byte) and working left
to right, top to bottom
- throw away the 1536-bit chunks corresponding to the first
576 columns of the first 3 rows (13824 bits altogether)
- XOR all the remaining 1536-bit chunks together, resulting
in a single 1536-bit (192 byte) vector

The result will provide the even parity vector that must be
placed in the B2 bytes. Formally, the kth bit of the ith byte
of the B2 field in the Line Overhead represents the even
parity computed over the kth bit of the ith byte of all 192-byte
aligned, contiguous and non-overlapping subsequences in the
preceding WIS frame, excluding the Section Overhead. This is
actually quite straightforward to implement from a logic
standpoint (1536 register bits and some XOR gates, essentially).
No awkward byte addressing issues are involved.

I believe your confusion might arise from interpreting the term
"concatenated" as meaning "placed back-to-back". In this
context, however, it indicates that all of the STS-1s in the
STS-192 frame are chained (or glued) together - hence the use
of the term "concatenated" - to create a new, single, large frame
carrying a single stream of payload data, as opposed to the
"multiplexed" scenario where the individual STS-1s are separate
entities carrying separate data streams. ANSI T1.105,
referenced by Clause 50 (see 50.1.1), explains the differences
in detail and gives various examples.

Effectively, the last paragraph in your e-mail is correct: the B2
calculation is made in an interleaved fashion over the whole
WIS frame (not interleaved over the Line Overhead and in
contiguous blocks over the payload, as you initially assumed).
Note however, that there is an error in your column numbers:
byte #2 of the B2 field is calculated from columns 2, 194 and
386 of rows 4-9 of Figure 50-8, and columns 2, 194, 386, ... 16514
of all rows of Figure 50-4.

I hope this clarifies things. Apologies for the length of this e-mail.

Best regards,

- Tom Alexander

-----Original Message-----
From: Frank Moore [mailto:fmoore@xxxxxxxxxxxx]
Sent: Tuesday, February 13, 2001 11:47 AM
To: stds-802-3-hssg@xxxxxxxx
Subject: Re: Clarification on added B2/B3 bytes in WIS

Apologies in advance for still not understanding how to
calculate B2 for the STS192c in the WIS (I have no background
in telecommunications and am having to learn fast for this

The way I understand it, B2 for an STS1 is calculated over the
line overhead (columns 1-3, rows 4-9) and the STS1 payload
(columns 4-90, rows 1-9).

My confusion arises over what this means for a concatenated
STS192. Concatenated seems to imply that the STS1 payloads
are placed back to back in the Synchronous Payload Envelope
of Figure 50-4 in section 50.3.1 If this is so, then I guess
STS1 number 1 incorporates columns 1-87, rows 1-9, and STS1
number 2 incorporates columns 88-174 and so forth.

Does this mean then that the B2 of column 2 row 5
(STS1 number 2) is calculated from columns 2, 194 and 386,
rows 4-9 of Figure 50-7, section 50.3.2 and columns 88-174,
rows 1-9 of Figure 50-4?

On the other hand this seems really awkward from a logic
standpoint. Since the section and line overhead for the
concatenated STS192 are still interleaved, is the B2
calculation on the payload also still interleaved?
This would mean that B2 of column 2 row 5 is calculated
from columns 2, 194 and 386 rows 4-9 of Figure 50-7 and
columns 2, 89, 176, etc rows 1-9 of Figure 50-4.

Thanks for the help,

Frank Moore
Mitsubishi Electronics
Durham, NC