Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
2.4.2.1.3 Direct Status Indication Using Four Cell Available Signals
When configuring the device, setting the MSP mode bit implicitly turns on direct status indica-
tion, since it is a subset of the implemented MSP method.
2.4.2.2 Priority Encoding and TDM Table
The Transmit UTOPIA selects PHY devices for service based upon:
•
•
•
•
the assigned UT PRIORITY for the PHY(refer to “UT_PRIORITY” on page 119).
the configuration of the TDM (Time Division Multiplex) table
per VO presence of the cells in the QEngine
cell available assertions received from the PHYs.
The use of priority servicing is beneficial when using multi-phy configurations and the UTOPIA
bandwidth is nearly fully subscribed.
2.4.2.2.1 Basic 2 Level Priority Algorithm
When TDM is disabled (refer to section 7.2.10 UTOPIA_CONFIG) a PHY device is assigned
either a high or low UTOPIA priority based of the bandwidth of the PHY device. Within a priority
level (high or low), further control over the service algorithm can be implemented by assigning
the lowest numbered PHY addresses to the highest bandwidth PHYs. The general algorithm for
deciding which PHY to service is as follows:
1. The High priority encoder has highest service priority. From the high priority PHYs, the lowest address
PHY that has indicated it can accept a cell (and for which a cell is present in the QEngine) is selected. If
no high priority PHY is selected, then the low priority set is considered next.
2. The Low priority encoder has the next highest service priority. The lowest address PHY that has indi-
cated it can accept a cell (and for which a cell is present in the QEngine) is selected. If no low priority
PHY is selected then the cell time is wasted unless the Watchdog is configured for operation, in which
case the stale priority set is considered next. The Watchdog is only available on the Transmit side.
3. The Transmit Stale priority encoder has the lowest priority and is created for the PHY devices that the
Watchdog deems stale. The lowest address PHY that has been detected dead or "stale" by the Watch-
Dog (and for which a cell is present in the Qengine) is selected. The cell is played out on the interface in
order to relieve VO queue depth congestion. The Watchdog plays the role of making a best effort deliv-
ery, even though the PHY is considered dead.
Caveat: Service selection is performed each cell time with the CLAV information gathered from
the previous cell time. This is particularly important, when the standard polling method is used
and not all phy's can be polled in a single cell time. In this mode, UTOPIA Priorities have relative
meaning within 4 address groups of 8 (0to7, 8to15, 16to24 and 25to31). For example a high prior-
ity phy of address=1 will compete for service with a low priority phy of address=7, but will not
compete for service against a low priority phy of address=10 since they are in different groups. It
is conceivable that a low priority phy can receive as much service as a high priority phy. This
could be the case if the phy at address=10 is the only phy in its address group. It will get the
entire cell time bandwidth simply because there are no other phys to compete with.
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PMC [ PMC-SIERRA, INC ]
