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[EFM] An Important FEC Issue

Dear EFM'rs:

As you may know or not no there is a small group of us that have been
looking into FEC for EFM. The group includes myself, Lior Khermosh,
Frank Effenberger, Ajay Gummalla, Ali Abaye, Massoud Khansari, Pat
Thaler, Jonathan
Thatcher, Vipul Bhatt and a few others.  One of the main issues we have
run into is not the FEC itself but the effect of the lower line BER
(1E-4) on pre FEC decoder blocks, such as the CDR.  With the FEC that is
being proposed, the FEC decoder will correct a BER from 10E-4 to
10E-12.  In case you have forgotten, this is an optical coding gain of
about 3dB (more PON splits) and, equally or perhaps more important,
improves reach of a MPN limited link by 50% for links with MPN penalty
of about 2dB. Details on the improvement from FEC and its benefits are
covered in presentations already made before the group. Back to the
issue at hand.  Clearly the CDR operation will degrade (slower lock
time, etc) when it is asked to operate with a line BER of 10E-4, but by
how much?  So far we have not had much feedback on this issue from
within our little group.  This is not surprising since we are not PMA or
CDR experts.  For this reason we have decided to post this specific
issue to the larger EFM reflector in the hope of getting some feedback.
Does anyone have any thoughts on this issue?  Some of the performance
changes to consider are:

1. Changes in CDR lock time of the PMA under high BER conditions.
2. Changes in the ability to maintain CDR lock under high BER
3. Changes in the jitter characteristics under high BER conditions
(primarily TP4)
4. Changes in the synchronization lock time in the PCS under high BER
5. Changes in ability to maintain synchronization lock in high BER

One way to learn about this issue is to do some lab experiments with
existing transceivers.  But before we do that, perhaps some of the
answers already exist.


Larry Rennie