Freescale Semiconductor MPC8260 - Manual (Page 1036)

ATM AAL1 Circuit Emulation Service
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
31-20 Freescale Semiconductor
31.6 3-Step-SN Algorithm
The 3-step-SN algorithm is a fast and efficient state machine that has the ability to recover one lost or
misinserted cell. The 3-step-SN algorithm does not add significant delay to the 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.
31.6.1 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.
Figure 31-18. Recoverable Sync Fail sequence options
Insert dummy cell
One cell was lost
ESC = RSC
RSC = X
ESC = RSC–1
SYNC
Discard current cell
Misinserted cell
ESC = RSC
RSC = X
ESC = RSC+1
SYNC
Received current cell
SNP fault
ESC = RSC
RSC = X
ESC = RSC
SYNC
ESC: Expected sequence count.
RSC: Received sequence count.
RSN=X: Invalid cell received or, a cell that does not match to the ESC.