Freescale Semiconductor MPC8260 - Manual (Page 1183)

ATM Transmission Convergence Layer
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 34-5
SYNCH state, the TC is assumed to be synchronized so that other functions can be applied to the received
cell. A transition back to the HUNT state is made only after ALPHA consecutive incorrect HEC patterns
are detected.
The cell delineation state machine is shown in Figure 34-3 . The ALPHA and DELTA parameters
determine the robustness of the delineation method. ALPHA determines the robustness against false
misalignment due to bit errors. DELTA determines the robustness against false delineation in the
synchronization. Both parameters are programmable for each TC block and are provided to help tune the
system according to the line error characteristics of a specific application.
Figure 34-3. TC Cell Delineation State Machine
The RCF descrambles (programmable) the cell payload using the self-synchronizing descrambler with a
polynomial of x
43
+ 1.
The HEC calculation is a CRC-8 calculation over the first four octets of the ATM cell header. The RCF
verifies the received HEC using the accumulation polynomial, x
8
+ x
2
+ x + 1. The coset polynomial
x
6
+ x
4
+ x
2
+ 1 is added (modulo 2) to the received HEC octet before comparison with the calculated
result (programmable).
The RCF can perform single bit error correction on the header. If multiple bit errors are found in the HEC,
the cell is discarded. If the single bit error correction mode is not enabled (TCMODE[SBC] = 1), the cell
is also discarded when a single bit error is found in the header.
When the cell delineation state machine is in the SYNCH state, the HEC verification state machine
(see
Figure 34-4 ) implements the correction algorithm. This state machine makes sure that a single cell
header is corrected at a time. If consecutive cells are detected with single bit errors in their headers, only
the first cell error is corrected and the rest are discarded. This state machine reduces the probability of the
delivery of cells with errored headers under bursty error conditions.
HUNT
PRESYNCH
SYNCH
DELTA Consecutive
Correct HEC
Correct HEC
Incorrect HEC
ALPHA
Incorrect HEC
Consecutive
Bit-by-Bit
Cell-by-Cell
Cell-by-Cell