re:A Minor NIT
So, to summarize...
The WIS treats the SONET/SDH data as bytes(octets). It extracts the
SPE from the SONET frame, which it sees as a block of 16704x9=150336
octets. The WIS processes and removes the POH, and removes the fixed
stuff. This leaves you with 16640x9=149760 octets, the payload (my
definition). This payload is sent to the PCS as a bit stream
(logically). This bit stream is a stream of concatenated 66-bit PCS
codewords. The PCS synchronizes to the 2-bit preamble to discover
the codeword delineations. Everything is good. If there are no
SONET pointer adjustments then the next SONET SPE is extracted in
exactly the same way as above, and the new payload is concatenated
with the previous payload to be sent to the PCS. The PCS
codewords will cross SPE boundaries but the PCS should not see any
indication that a new SONET frame has been parsed. It just sees a
constant (logically) bit stream.
Now, you get a negative pointer adjustment on the SONET pointer.
The pointer is decremented by one so there are 192 extra SPE bytes
stored in the 192 H3 positions. The WIS must include these as
part of the SPE (and they may be POH or fixed stuff). These 192
bytes should be inserted in the SPE following the last byte of
SPE preceding the first H1 byte, or, equivalently, preceding the
first byte of SPE following the last H3 byte. Note that the SPE
has not changed size, it has an extra 192 bytes stored in
the H3 overhead positions but ends 192 bytes earlier in the SONET
frame.
A positive pointer adjustment is just the reverse of above. The
pointer is incremented by one, so 192 byte times are unused (the
192 bytes following the last H3 byte) and 192 byte times are
added at the end of the SPE. Logically, the byte following the
last unused byte time follows the SPE byte preceding the first
H1 byte. The next SPE begins immediately following the last extra
byte time, 192 byte times later than in the previous SONET frame.
The other case is a New Data Flag (NDF) event. In this case, the
pointer can arbitrarily change. If the new pointer value is lower
than the current value, I suspect data will be lost since the
next SPE will overlap the current, which also means the PCS will
have to resync. If the new pointer value is higher than the
current value, there will be a gap between the current and next
SPEs. The WIS should not send any data during this gap. However,
as far as the PCS is concerned, the first bit proceeding the gap
(the 1st bit of the next SPE) follows the last bit preceeding the
gap (the last bit of the current SPE).
Does this make any sense?
Erik Trounce
Nortel Networks