
MT90520
Data Sheet
77
Zarlink Semiconductor Inc.
Here are some examples of incoming cell streams containing errors and the corresponding corrective actions taken
by the SDT RX_SAR.
4.6.1.5 Received Cell Processing
The RX_SARs are responsible for extracting data from received cells and transferring that data to the TDM module,
for eventual transmission onto the TDM output bus. In addition, these modules are responsible for:
performing slip checking and RX_SAR write-pointer adjustment prior to transferring data to the TDM module
generating dummy cells in the case of sequence errors such as lost or misinserted cells
SDT pointer processing
extracting CAS from SDT cells and transferring it to the TDM module
out_of_seq
invalid sequence number
- discard cell
start
received cell is in sequence with last “good”
cell (i.e., cell misinsertion)
- discard cell (because misinserted
cell was accepted when it shouldn’t
have been)
- declare
misinserted_cell_error
sync
received cell is in sequence with last received
cell (i.e., cell loss)
- calculate number of lost cells (up
to 6)
- SDT RX_SAR inserts dummy
cell(s)
- after dummy cell insertion(s),
accept received cell
- declare
lost_cell_error
sync
received cell has a valid sequence number
that is two greater than the last in-sequence
cell (i.e., sequence number protection failure)
- accept received cell
sync
received cell has valid sequence number, but
doesn’t meet any of the 3 previous criteria
Table 17 - Operation of SDT Fast Sequence Number Processing State Machine
- discard cell
out_of_sync
Examples of Operation of SDT Fast Sequence Number Processing State Machine
Description of Error
Sample Incoming Cell
Stream
Cells Used for
Reassembly
Errors Declared
Sequence number protection
failure (looks like single cell
loss)
1 - 3* - 3 - 4 - 5
1 - 3* - 3 - 4 - 5
1 AAL1 Sequence Error
Sequence number protection
failure (looks like multiple cell
loss)
1 - 4* - 3 - 4 - 5
1 - 4* - 3 - 4 - 5
1 AAL1 Sequence Error
Single lost cell
1 - 2 - 4 - 5
1 - 2 - 4 - D
3
- 5
1 AAL1 Sequence Error
1 Lost Cell
Multiple lost cells (from 2 to 6
lost cells)
1 - 2 - 6 - 7
1 - 2 - 6 - D
3
- D
4
- D
5
- 7
1 AAL1 Sequence Error
3 Lost Cells
Misinserted (swapped) cell
1 - 2 - 4 - 3 - 5 - 6
1 - 2 - 4 - 5 - D
4
- 6
2 AAL1 Sequence Errors
1 Misinserted Cell
1 Lost Cell
NOTE:
D indicates the insertion of a dummy cell containing user-programmable data.
Current State
Transition Event
Action Taken
Next State