MPC8280 datasheet(1080/1386 Pages) FREESCALE

1080 / 1386 page

ATM AAL1 Circuit Emulation Service

MPC8280 PowerQUICC II Family Reference Manual, Rev. 1

32-20

Freescale Semiconductor

32.6

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.

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.

Figure 32-18. 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.

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