PM7340 S/UNI-IMA-8
PRELIMINARY
INVERSE MULTIPLEXING OVER ATM
DATA SHEET
PMC-2001723
ISSUE 3
INVERSE MULTIPLEXING OVER ATM
10.4 TC Layer
10.4.1 TX TC Layer (TTTC)
The TX TC layer (TTTC) performs the TC layer functions. These functions consist
of optional HEC generation, optional payload scrambling, and cell-rate
decoupling through physical layer (idle) cell insertion.
This function removes data byte by byte from the per-link FIFOs as required to
provide data to the physical layer function. When the physical layer function
needs a byte of data, it will request data from the (TTTC). The TTTC will then
read a byte of data from the per-link FIFO. If that byte is the first byte of a cell
and the logical channel FIFO is empty, the TTTC will format the next 53 bytes as
a physical layer (idle) cell. If the byte is the fifth byte of a cell, the byte is
optionally overwritten by the CRC-8 calculation over the previous four bytes for
that logical channel. The sixth through 53rd bytes may be scrambled by a x43+1
self-synchronous scrambler.
Note: Since the Link FIFOs are cell based, an underrun cannot happen in the
middle of a cell.
10.4.2 Rx TC Layer (RTTC)
The Rx TC (RTTC) layer 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.
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
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
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PMC [ PMC-SIERRA, INC ]
