Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
The controller has a scheduler that selects cells to be placed in pipelined, “ping-pong” buffers.
Once a cell is selected, it is placed in one of these buffers. Each of the four outputs to the switch
fabric has two buffers: while a cell in buffer A is being transmitted, another cell is selected and
placed into buffer B. On the subsequent switch fabric cell time, the buffers are “ping-ponged”,
and the cell in buffer B is sent. Meanwhile, another cell is selected for buffer A.
An exception to this process is when the controller receives a negative acknowledgment (NACK)
for transmission of a cell. There are two cases: the NACK is an MNACK, indicating cell trans-
mission failed due to collision in the middle of the network, or else the NACK is an ONACK,
indicating cell transmission failed due to collision at an output of the network. In the former case,
the cell’s switch fabric priority (assigned during VC setup) is compared with that of the cell (if
any) in the other ping-pong buffer. Call the first cell X, and the second cell Y. If the priority of cell
X is greater than or equal to that of cell Y, the buffers are not ping-ponged, and cell X will be
resent next time. If the priority of cell X is less than that of cell Y, cell X remains in its buffer, and
the buffers are ping-ponged as usual, with cell Y being sent next. In the latter case, the cell is
requeued at the head of its VC’s queue. Thus, the cell will be retransmitted, but at a later time than
if the cell was MNACKed.
The switch fabric has been specially designed to minimize the possibility of consecutive colli-
sions at the same place in the middle of the network, and thus a cell’s transmission that failed in
that manner stands a good probability of being successful in an immediately subsequent transmis-
sion attempt. Collisions at an output of the network are more likely to be recurring for a period of
time, and thus the next transmission attempt is delayed.
The scheduler that places cells in the ping-pong buffers operates as follows: The SCs are arranged
in a tabular fashion as seen in Figure 39. An SC is designated for either unicast or multicast traf-
fic. Additionally, an SC is designated as either strict priority SC1, strict priority SC2, or General
Purpose (GP). Associated with each SC is a weight of either 1, 4, 16, or 64. This information is
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PMC [ PMC-SIERRA, INC ]
