Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
a run-completing cell are known. For N = 1, every cell completes a run. For N = 2, the cell with
the modulo lower Sequence Number (SN) is the run-completing cell. At that time it will be added
back onto the ring if there are more cells to send or if that cell was ONACKed, in which case that
cell can be resent.
The pointers for these cells are stored in two locations in the CCB. When starting from no cells in
the fabric, the first cell sent is always in POINTER0 and the second cell is always in POINTER1.
For multicast and unicast cells, use N = 2. The N = 1 setting is available for use, but has lower
utility than the N = 2 setting for virtually all situations.
2.6 Transmitter Operation
2.6.1 Transmit Queuing
Transmit cells are enqueued on a per-SC, per-VO basis. As there are 31 VOs, and 16 SCs per
VOs, there are a total of 496 queues. Singly linked lists are used to queue the cells. The head and
tail pointers are in internal RAM and the linked lists are in external RAM. Figure 40 shows an
example transmit per-SCQ linked list.
Per-SCQ Linked List
Link
Channel
Link
Link
Head
Tail
VO,
Figure 40. Transmit Per-SCQ Linked List
2.6.2 Transmit Congestion Management
A cell received from the switch fabric interface is queued by the transmit queue controller if it
passes ten buffer threshold checks: both maximum and congested thresholds for the device, VO,
SC, queue, and channel as shown in Figure 41 on page 36. The cell waits in the transmit cell
buffer DRAM until the transmit queue controller selects it for transmit multicast/header mapping.
The cell then exits the device through the UTOPIA interface.
A congestion hysteresis bit is kept for each threshold. This bit is set whenever the queue depth
exceeds the congestion limit for that threshold. This bit remains asserted until the queue depth
falls below one-half of the congestion threshold.
The congestion limits are kept in an exponential form. The interpretation of the limits is the same
for all measurements except the device limit. For the other measurements, the value of 0 causes
the measurement to always find congestion. The value of 1 may not be used. The value of F
h
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PMC [ PMC-SIERRA, INC ]
