
PM5317 SPECTRA-9953 Telecom Standard Product Data Sheet
Release
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2000741, Issue 5
114
Frequency offsets due, for example, to plesiochronous network boundaries, or the loss of a
primary reference timing source, and phase differences, due to normal network operation,
between the incoming data stream and the outgoing data stream are accommodated by pointer
adjustments in the outgoing data stream.
The SVCA also terminates the transport overhead. All the transport overhead bytes are set to
“00H” except A1,A2,H1,H2 and H3 bytes.
14.15.1
Elastic Store
The Elastic Store performs rate adaptation between the line side interface and the system side
interface. The entire incoming payload, including path overhead bytes, is written into a first-in-
first-out (FIFO) buffer at the incoming byte rate. Each FIFO word stores a payload data byte
and a one bit tag labeling the J1 byte. Incoming pointer justifications are accommodated by
writing into the FIFO during the negative stuff opportunity byte or by not writing during the
positive stuff opportunity byte. Data is read out of the FIFO in the Elastic Store block at the
outgoing byte rate by the Pointer Generator. Analogously, outgoing pointer justifications are
accommodated by reading from the FIFO during the negative stuff opportunity byte or by not
reading during the positive stuff opportunity byte.
After coming out of AIS, the SVCA may output one erroneous NDF indication. This is due to
several non-deterministic factors after coming out of AIS such as FIFO fill levels and FIFO J1
content. After the one possibly incorrect NDF indication, another NDF is issued and all
subsequent pointers and J1 indications are correct. Much of the same situation can occur when
reframing. Depending of the FIFO contents, the first J1 indication coming out of the SVCA
after reframing might take longer than one frame, but not longer than two frames. If there is
constant reframing, and the new pointer is continually less then the previous pointer, then the
SVCA can go multiple frames without a J1 indication. However, the H1/H2 values will always
be correct.
The FIFO read and write addresses are monitored. Pointer justification requests will be made to
the Pointer Generator based on the proximity of the addresses relative to programmable
thresholds. The Pointer Generator schedules a pointer increment event if the FIFO depth is
below the lower threshold and a pointer decrement event if the depth is above the upper
threshold. FIFO underflow and overflow events are detected and path AIS is optionally inserted
in the outgoing data stream for three frames to alert downstream elements of data corruption.
Pointer Generator
The Pointer Generator generates the H1 and H2 bytes in order to identify the location of the
path overhead byte (J1) and all the SPE bytes of the constituent STS-1/3c/12c/48c/192c
(VC3/4/4-4c/4-16c/4-64c) payloads. The pointer generator is a time multiplexed finite state
machine that can process any mix of STS-1/3c/12c/48c/192c (AU3/4/4-4c/4-16c/4-64c)
pointers. Within the pointer generator algorithm, five states are defined as shown below:
NORM_state (NORM)
AIS_state (AIS)
NDF_state (NDF)