S/UNI-IMA-4 Telecom Standard Product Data Sheet
Released
Cell delineation is the process of framing to ATM cell boundaries using the header check
8
2
sequence (HCS) field found in the ATM cell header. The HCS is a CRC-8 (x + x + x + 1)
calculation over the first 4 octets of the ATM cell header. In accordance with ITU-T
6
4
2
Recommendation I.432.1, the coset polynomial x + x + x + 1 is added (modulo 2) to the
received HCS octet before comparison with the calculated result (coset addition is not
configurable). When performing delineation, correct HCS calculations are assumed to indicate
cell boundaries. The cell delineation circuitry performs a sequential bit-by-bit hunt for a correct
HCS sequence. This state is referred to as the HUNT state. When a correct HCS is found, a
particular cell boundary is assumed and the PRESYNC state is entered. This state verifies that
the previously detected HCS pattern was not a false indication. If the HCS pattern was a false
indication, then an incorrect HCS should be received within the next DELTA cells and the
delineation state machine falls back to the HUNT state. If an incorrect HCS is not found in this
PRESYNC period, then a transition to the SYNC state is made, cell delineation is declared, and
all non-idle cells with a correct HCS are passed on. In the SYNC state, synchronization is not
relinquished until ALPHA consecutive incorrect HCS patterns are found. If this happens, a
transition is made back to the HUNT state. The state diagram of the cell delineation process is
shown in Figure 26.
Figure 26 Cell delineation State Diagram
correct HCS
(bit by bit)
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 cell delineation method.
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.
Proprietary and Confidential to PMC-Sierra, Inc., and for its customers’ internal use.
Document No.: PMC-2020889, Issue 2
82
PMC [ PMC-SIERRA, INC ]
