[8023-10GEPON] Idle insertion in Rx PCS
All,
1. In the course of the discussion of comment 1458 in Munich, the
issue of maintaining acceptable latency variation in the Rx PCS came up
once again.. It was noted that the current state diagrams are broken
and that a jitter buffer is required to guarantee uniform latency.
Attached are revised figures 92-22, 92-26 and 92-27 (numbering from
draft d1_8022) which include the jitter buffer.
2. This fix is independent of the method used for FEC_overhead()
equations. However, any analysis done on the FEC overhead equations
should take this into account.
3. The basic idea is that the II_FIFO[] is the length of 8 parity
regions (ie. the maximum number that can occur in a packet). All
received codes pass through the FIFO. The location of the parity
regions is noted and the parity regions are counted up. After the frame
terminates, the PCS inserts number of IDLEs corresponding to the amount
of deleted parity. Put even more simply: the receiving PCS inserts the
IDLEs following the frame, just as the transmitting MPCP ensures that
the extra IDLEs follow the frame.
4. The figures in the current draft lack this buffering, and
consequently there is significant variability in the frame latency
depending on the length of the frame ie. the existing Figure 92-26 bumps
the FrameReadyCount right after /T/ is received from the 66b decoder.
In the case where a 64byte frame terminates in the last octet of a FEC
codeword, the frame will be delayed one parity region (ie. 264 PMA bit
times). Whereas a 1522 byte frame which terminates in the first octet
of a FEC codeword will be delayed 8 full codewords (ie. 16320 PMA bit
times). This delay variation is much greater than the 1TQ that is
permitted.
Specifically, if we look at lines 17-18 of current figure 92-27, we see
that the amount of time that a frame is retained in the II_Fifo depends
on when FrameReadyCount is bumped, for which we look to current figure
92-26 to see that it depends directly on the length of the frame.
5. Consequently, this change is necessary for technical completeness of
the draft.
Feedback is appreciated.
Thanks,
- Jeff Mandin
PMC-Sierra
Rx_Pcs_Idles.pdf