PM7346 S/UNI-QJET
DATASHEET
PMC-960835
ISSUE 6
SATURN QUAD USER NETWORK INTERFACE FOR J2, E3, T3
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.
The ATMF performs a sequential bit-by-bit, a nibble-by-nibble (DS-3 direct
mapped), or a byte-by-byte (J2 and E3 direct-mapped) hunt for a correct HCS
sequence. This state is referred to as the HUNT state. When receiving a bit
serial cell stream from an upstream transmission system sublayer entity, the bit,
nibble, or byte boundaries are determined from the location of the overhead.
When a correct HCS is found, the ATMF locks on the particular cell boundary
and assumes the PRESYNC state. 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.
At that point a transition back to the HUNT state is executed. If an incorrect HCS
is not found in this PRESYNC period then a transition to the SYNC state is
made. In this state synchronization is not relinquished until ALPHA consecutive
incorrect HCS patterns are found. In such an event a transition is made back to
the HUNT state. The state diagram of the cell delineation process is shown in
Figure 10.
Figure 10
- Cell delineation State Diagram
Correct HCS
(bit by bit)
PRESYNC
HUNT
Incorrect HCS
(cell by cell)
ALPHA consecutive
incorrect HCS's
(cell by cell)
DELTA consecutive
correct HCS's
(cell by cell)
SYNC
The values of ALPHA and DELTA determine the robustness of the delineation
method. ALPHA determines the robustness against false misalignments due to
bit errors. DELTA determines the robustness against false delineation in the
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