Page 458 - ExtendSim User Guide

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432 Miscellaneous
Precision
This chapter covers:
• Theprecisionofcalculations
• Usingbiastogivepreferencetosomecomponentorportionofamodelandhowthataffects effective rates
• GlobalandadvancedoptionsintheExecutive
• Valueconnectorabbreviationsandmeanings
• Thedifferenttypesofinformationthatanimationdisplays
Precision
An LP area is made up of one or more rate sections; it encompasses all the rate sections for which the Executive block has been notified that effective rates might change. The LP area has a linear program (LP) that is responsible for calculating an effective rate for each section con- tained within that area. (The LP area and LP calculations are discussed fully in “LP technol- ogy” on page 448.)
The maximum mathematical precision for an LP area is 12 digits. Because one LP can be responsible for calculating multiple effective rates for its rate sections, and because LP preci- sion is limited to 12 digits, precision can become an issue not only for the individual effective rates but also for the effective rates calculated for the entire LP area. For example, if an LP area contains two rate sections where the first rate section's effective rate was 1,000,000 flow units per time unit (FPT), the effective rate for the second section could be no smaller than 0.0001 FPT.
☞ To preserve adequate precision for all rate sections, don’t separate any two effective rates within an LP area by more than 12 digits of precision.
Biasing flow
The discrete rate architecture includes a feature called bias – a method for stating a preference that flow travel one route rather than another.
☞ Bias is only relevant when the flow is merged or diverged.
The advantages of the bias concept are that it:
1) Givesyouawaytospecifypreferencesforhowflowcirculatesinonepartofthemodel compared to other parts.
2) Providesflexibilityinresolvingconflictsforhowflowshouldbedistributedamongcom- peting branches.
Bias is present in a model whenever you use one or more of the following blocks:
• ABiasblockinamodelthatcontainsMergeorDivergeblockssettoanon-fixedmode(dis- cussed in “Merge and Diverge blocks” on page 434).
• AMergeorDivergeblock(Distributional,Priority,orSensingmodesonly)
Because bias can skew the way flow is distributed, it is taken into consideration by the global LP calculation and can thus have an effect on effective rates. (For specific information on how bias is used in the calculation of effective rates, see the advanced topic “LP technology” on page 448.)
☞ We suggest that you read the chapters “Rates, Constraints, and Movement” and “Merging, Diverging, and Routing Flow” before the Bias section.
Discrete Rate
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