[8023-10GEPON] FEC, and the need to scramble parity bytes.

[Frank Effenberger <feffenberger@xxxxxxxxxx>]

Dear All, 

At the meeting today, there was a small issue on the need to scramble the
parity bytes that a FEC algorithm produces.  As a reference point, I looked
at G.975, which defines the use of RS(239,255) in SDH systems.  It seems
that this standard suggests that scrambling the FEC output is not necessary.
(section 6.4.3, first paragraph). 

There are two complications that I'd like to point out in fairness. 
1. G.975 operates by interleaving n encoders (e.g., n=16), so the parity is
not transmitted in quite the way that we would do.  However, the parity is
transmitted all in a big bunch at the end of the interleaved block, so if
the parity had some kind of pathological pattern, it would be revealed in
these systems.
2. Section 6.4.3 mentions that in some systems, the main SDH scrambler may
be disabled, and then in that case a FEC layer scrambler is defined.  But,
that is really a matter of implementation choice, and I don't believe it
contradicts the main idea that parity that is calculated on scrambled data
is effectively also scrambled.  

So, I would say that we do not need to worry about scrambling the parity. 

Sincerely,
Frank E.