RELEASED
PM7350 S/UNI DUPLEX
DATA SHEET
PMC-1980581
ISSUE 8
DUAL SERIAL LINK PHY MULTIPLEXER
9.2.1 Upstream Functions
In the upstream direction (input from the modems), the S/UNI-DUPLEX provides
2 inputs (clock and data) per channel and implements HCS cell delineation,
payload descrambling, idle cell filtering and header error detection to recover
valid ATM cells. These functions are performed in accordance with ITU-T
Recommendation I.432.1.
When configuring the S/UNI-DUPLEX in clocked serial data mode the user must
modify the Transmit Logical Channel FIFO Depth register (0x5E) accordingly.
See the register descriptions for details.
Cell delineation is the process of framing to ATM cell boundaries using the
header check sequence (HCS) field found in the ATM cell header. The HCS is a
CRC-8 (x8 + x2 + x + 1) calculation over the first 4 octets of the ATM cell header.
In accordance with ITU-T Recommendation I.432.1, the coset polynomial
x6 + x4 + x2 + 1 is added (modulo 2) to the received HCS octet before
comparison with the calculated result. 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. In such
an event a transition is made back to the HUNT state. The state diagram of the
cell delineation process is shown in Fig. 8.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
60
PMC [ PMC-SIERRA, INC ]
