S/UNI®-8x155 ASSP Telecom Standard Product Data Sheet
Released
Figure 5 Cell Delineation State Diagram
correct HCS
(byte by byte)
HUNT
Incorrect HCS
(cell by cell)
PRESYNC
ALPHA
consecutive
incorrect HCS's
(cell by cell)
DELTA
SYNC
consecutive
correct HCS's
(cell by cell)
The values of ALPHA and DELTA determine the robustness of the delineation process. ALPHA
determines the robustness against false misalignments due to bit errors. DELTA determines the
robustness against false delineation in the synchronization process. ALPHA is chosen to be 7
and DELTA is chosen to be 6. These values result in an average time to delineation of 32 µs for
the STS-3c/STM-1 rate.
10.6.2 Descrambler
The self-synchronous descrambler operates on the 48 byte cell payload only. The circuitry
43
descrambles the information field using the x + 1 polynomial. The descrambler is disabled
for the duration of the header and HCS fields and may optionally be disabled for the payload.
10.6.3 Cell Filter and HCS Verification
Cells are filtered (or dropped) based on HCS errors and/or a cell header pattern. Cell filtering is
optional and is enabled through the RXCP registers. Cells are passed to the receive FIFO while
the cell delineation state machine is in the SYNC state as described above. When both filtering
and HCS checking are enabled, cells are dropped if HCS errors are detected, or if the header
contents match the pattern contained in the RXCP Match Header Pattern and RXCP Match
Header Mask registers. Idle or unassigned cell filtering is accomplished by writing the
appropriate cell header pattern into the RXCP Match Header Pattern and RXCP Match Header
Mask registers. Idle/Unassigned cells are assumed to contain the all zeros pattern in the VCI
and VPI fields. The RXCP Match Header Pattern and RXCP Match Header Mask registers
allow filtering control over the contents of the GFC, PTI, and CLP fields of the header.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC- 2010299, Issue 2
76
PMC [ PMC-SIERRA, INC ]
