Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
Figure 43 illustrates the steps that are taken when playing out a cell.
Run Queue Service Algorithm to
determine the Service Class (SC)
Find pointer to cell
from linked list
Determine VO
Read OUTCHAN
from cell buffer
Play out cell and
update channel state
Fetch cell
Figure 43. Cell Playout Steps
2.6.4 Transmit Resequencing Algorithm
To guarantee the FIFO delivery of cells, the QRT supports an algorithm to make sure the cells can
be put back into order. The algorithm it supports is a classic window algorithm where only N cells
are allowed to be outstanding without acknowledgment. In the QRT, N is either 1 or 2. This limits
the data rate of an individual connection to approximately 155 Mbps. The transmit end reorders
the cells according to their SN.
The resequencing of one algorithm ignores the incoming SN and accepts all cells as if their SN
were correct. This can be used for multicast cells as the QSE delivers them in FIFO order.
The resequencing of two algorithms inspects an incoming cell to determine if it has the expected
SN, e. If it does, the cell is immediately processed. If it has SN e + 1, then it is stored to await the
run-completing cell (that is, the cell with the original expected SN, e). If it has neither SN e, nor
SN e + 1, a recovery algorithm is started which gets the channel back into sequence. This is
described in “Transmit Recovery Algorithm” on page 40.
Cell ONACKed
Cells arrive at receive UTOPIA
Cells are numbered and sent
across the fabric
4
5
4
6
3
Cells are sent from transmit UTOPIA
Figure 44. Transmit Resequencing Operation
The resequencing of two algorithms interacts with EPD. When a cell is missing, the algorithm
cannot determine if the missing cell is an End-Of-Frame (EOF) cell. It is then necessary to defer
the choice of whether or not to send both cells until the run-completing cell is received. The
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PMC [ PMC-SIERRA, INC ]
