Motorola PowerQUICC II MPC8280 Series Reference Manual page 1142

3-Step-SN Algorithm
AAL1 CES reassembly process due to the fact that the decision to accept a cell is taken
immediately after cell arrival. If a lost cell is detected, the receiver will insert a dummy cell.
If a misinserted cell is detected, the receiver will discard the cell received after the
misinserted cell. If more than one successive cell was lost or misinserted, the 3-step-SN
algorithm will switch to hunt mode, where it stays until a cell with a valid SN field is
received.
32.6.1 The Three States of the Algorithm
The 3-step-SN algorithm has three states:
1. Hunt—The Hunt state is the initial state of the algorithm or the first state after the
receiver loses its synchronization. In this state no cell is delivered to the Rx buffer.
When a valid cell is detected, the algorithm switches to the Sync state and delivers
the current cell to the Rx buffer.
2. Sync—The Sync state is the steady state of the algorithm. In this state any received
cell is automatically passed to the Rx buffer. The algorithm will switch from this
state when an invalid cell is received (SNP error), or it receives a cell that does not
sequence with the last valid cell (SC error).
3. Sync Fail—The Sync Fail state is a transient state. In this state the receiver checks
the difference between the received sequence count (RSC) and the expected
sequence count (ESC). If the difference between the two (ESC-RSC) is -1, 0, or 1,
the receiver switches to sync mode and either discards the current cell, receives the
current cell, or inserts a dummy cell, correspondingly. In any other cases the
receiver will switch to hunt mode and discard the current cell.
SYNC
ESC = RSC
RSC = X
ESC = RSC–1
Insert dummy cell
One cell was lost
ESC: Expected sequence count.
RSC: Received sequence count.
RSN=X: Invalid cell received or, a cell that does not match to the ESC.
Figure 32-18. Recoverable Sync Fail Sequence Options
32-22
Freescale Semiconductor, Inc.
SYNC
ESC = RSC
RSC = X
ESC = RSC+1
Discard current cell
Misinserted cell
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SYNC
ESC = RSC
RSC = X
ESC = RSC
Received current cell
SNP fault
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