EB632 datasheet(45/112 Pages) FREESCALE

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CPM

Functional Differences Between MSC8101 (Mask 2K42A) and MSC8103 (Mask 2K87M), Rev. 2

Freescale Semiconductor

14.4.5 Receive ATM Cell Functions

The ATM receive cell functions block (RCF) performs the receive functions of the TC block. It performs cell

delineation, cell payload descrambling, HEC verification and correction, and idle/unassigned cell filtering.

Cell delineation is the process of framing data to ATM cell boundaries using the header error check (HEC) received

in the ATM cell header. The HEC is a CRC-8 calculation over the first four octets of the ATM cell header. The cell

delineation algorithm assumes that repetitive correct HEC calculations over consecutive cells indicate valid ATM

cell boundaries.

The RCF performs a sequential bit-by-bit hunt for a correct HEC sequence, and the cell delineation state machine

is in HUNT state. When a correct HEC is found, the RCF locks on the particular cell boundary and enters the

PRESYNCH state, which indicates that the previously detected HEC pattern is not a false indication. If a correct

HEC pattern is false, an incorrect HEC is received within the next DELTA cells. If an incorrect cell is detected,

there is a transition to the HUNT state. If an incorrect HEC is not detected in the PRESYNCH state, there is a

transition to the SYNCH state. In the 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 16. 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.

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, x8 + x2 + x + 1. The coset polynomial x6 +x4 + x2 + 1 is added

(modulo 2) to the received HEC octet before comparison with the calculated result (programmable).

Figure 16. TC Cell Delineation State Machine

HUNT

PRESYNCH

SYNCH

DELTA Consecutive

Correct HEC

Incorrect HEC

ALPHA

Incorrect HEC

Consecutive

Bit-by-Bit

Cell-by-Cell

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