PM73487 datasheet(44/251 Pages) PMC

44 / 251 page

PM73487 QRT

PMC-Sierra, Inc.

PMC-980618

Issue 3

622 Mbps ATM Traffic Management Device

Released

Datasheet

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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