PMC-Sierra, Inc.
PRELIMINARY
PM5381 S/UNI-2488
DATASHEET
PMC-2000489
ISSUE 1
SATURN USER NETWORK INTERFACE FOR 2488 MBIT/S
10.7.1 ATM Cell Delineation
Cell Delineation is the process of framing to ATM cell boundaries using the header check
sequence (HCS) field found in the cell header. The HCS is a CRC-8 calculation over the first 4
octets of the ATM cell header. When performing delineation, correct HCS calculations are
assumed to indicate cell boundaries. Cells are assumed to be byte-aligned to the synchronous
payload envelope. The cell delineation algorithm searches the 53 possible cell boundary
candidates individually to determine the valid cell boundary location. While searching for the cell
boundary location, the cell delineation circuit is in the HUNT state. When a correct HCS is found,
the cell delineation state machine locks on the particular cell boundary, corresponding to the
correct HCS, and enters the PRESYNC state. The PRESYNC state validates the cell boundary
location. If the cell boundary is invalid, an incorrect HCS will be received within the next DELTA
cells, at which time a transition back to the HUNT state is executed. If no HCS errors are
detected in this PRESYNC period, the SYNC state is entered. While in the SYNC state,
synchronization is maintained until ALPHA consecutive incorrect HCS patterns are detected. In
such an event a transition is made back to the HUNT state. The state diagram of the delineation
process is shown in below.
Figure 12: 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
consecutive
SYNC
correct HCS's
(cell by cell)
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