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SCOOT USER GUIDE
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independence. They differ in that when OPNI is set to NO, SCOOT will continue
to calculate the values used by the cycle time independence triggers and
NEVER stops these calculations. This can improve the performance of the
triggers if OPNI is switched between NO and AUTO by time of day. (When
reading the SCOOT Traffic Handbook, setting OPNI to NEVER is equivalent to
setting the internal 'allow independence' flags to NO.)
When a node is running independently the cycle time will be the minimum
practical cycle time (MPCY) of the node.
Node independence is only permitted for nodes that are not in a sub-region.
It is an important point to note - a node or sub-region can only become
independent if it will run a cycle time lower than the region - or more precisely
the MPCY of the node is lower than the highest MPCY of all the nodes in the
region. A typical use of Node Transfer is to move a node that is holding the region
at an artificially high cycle time into its own region; trying to replace this use of
Node Transfer by setting the OPNI to YES for the critical node will not work.
Instead, the other nodes in region would need to be in a sub-region, and cycle time
independence enabled for the sub-region.
Sub-Region Independence
A sub-region can be selected for cycle time independence. The SCOOT
CHAN/RUBA parameter OPSI (Operator Sub-Region Independence) selects the
cycle time independence mode. It takes the same values as OPNI (see above).
A sub-region's independent cycle time is calculated in a similar way to the region
cycle time - it is the highest MPCY of all the nodes in the sub-region.
Cycle Time Independence Triggers
When OPNI/OPSI are set to AUTO, SCOOT switches between non-independence
and independence by working out the delay ratio - the ratio of the predicted delays
when the node/sub-region is running at the region cycle time and at the
independent cycle time. When the delay ratio > 1.0 then this indicates that
independence will yield less delays; when the delay ratio < 1.0 then this indicates
that non-independence is the better choice.
SCOOT has two mechanisms for calculating the delay ratio - complex and simple.
The simple method only appears to be of benefit in being less CPU intensive than
the complex method but at the cost of less accuracy. For PC SCOOT, no CPU
loading issues have been seen with using the complex method so its use is
recommended and it is the default.
The delay ratio can be seen on the C52/C53 messages. The independence and
non-independent threshold parameters (INTH, NITH) specify the thresholds
above which or below which the node/sub-region independence status is changed.
The recommended values and defaults are 115% (or 1.15) and 95% (or 0.95); i.e.
when the delay ratio is > 1.15 then node/sub-region independence will be enabled.
When the delay ratio drops below 0.95 then cycle time independence is stopped.
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