Modelling Approach Event Scheduling –System modelled via characteristic events –Events have subroutines which update state variables. Process Orientation –Time oriented sequence of inter-related events that describes the experience of an entity as it flows through a system. –Overlay to an event scheduling system. –Approach adopted in most current software.
Common Features 1.Generating random numbers (i.e. ~U(0,1)) 2.Generating random variates from a specified probability distribution. 3.Advancing the simulation clock. 4.Determining the next event on the list event and passing control of to the appropriate piece of code. 5.Adding and deleting records from a list. 6.Collecting output statistics and reporting the results of the simulation run. 7.Trapping error conditions.
A Brief History of Simulation Simulation has been around for some time. Early simulations were event-driven (see Simscript MODSIM) and frequently military applications. In the 1960’s Geoffrey Gordon developed the transaction (process) based orientation that we are now familiar with. Gordon’s software was called General Purpose Simulation System (GPSS). GPSS was originally intended for analyzing time sharing options on mainframe computers. The software was included as a standard library on IBM 360s and its use was quite widespread.
SIMAN AND SLAM SIMAN is tailored for the PC market. SLAM remains focused on workstations. SIMAN introduces an animation package (CINEMA) about 1985 or so. The animation is an add on unit for the model. Originally it required specialized (& expensive) hardware. SLAM responds with a PC version of SLAM in the late 1980s (which also has animation). Both firms develop software to integrate factory scheduling into simulation runs.
Early PC Versions By the mid-80s the PC market is dominant. Mainframes are expensive. Cycles are expensive. Central IS groups are expensive. Engineers become computer experts. A lot of IE’s end up writing simulations. There are a number of issues: User development cycles are very long. Total memory (model, entities, etc) limited to about 32k by FORTRAN/C and early versions of DOS. Simulation language development tends to lag OS development.
SIMULATORS Advent of Windows 3.0/3.1. Mass penetration of PCs. Powerful hardware and software (especially OOC) becomes available. We start to see the creation of a variety of simulators. The simulators are usually graphically oriented (drag & drop model development),have integrated animation, and low purchase cost. Huge number of simulators come onto the market. These often lack statistical rigour.
Today’s Market There has been something of a rationalization in terms of the number of simulation languages/simulators available. See the May 1999 edition of IIE Solutions. The large simulation companies have all been bought or sold at least a couple of times in the past two – three years. SLAM Frontstep Systems (a logistics software supplier). Not in active development (last release ’99). SIMAN Rockwell Software (logistics & controls).
Trends Virtual reality animations. Advanced statistical functions Curve fitting for input data. Automatic detection of warm up Output analysis modules (including replication). Bolt on “Optimizers” – Tools to search for optimal settings of parameters.
Witness (Lanner Inc) Simple building block design Interactive Full range of logic and control options Elements for discrete manufacture, process industries, BPR, e-commerce, call centers, health, finance and government Statistical input and reports Link system to other software easily (CAD/Excel) Optional 3D/VR views Model Optimisation $13,000-$17,000 ($US)
ARENA Process hierarchy. Integrates with Microsoft desktop tools Spreadsheet interface Crystal reports Free runtime software. Fully graphical environment. No programming required. VBA embedded. Optimization with OptQuest for Arena. Builds reusable modules. $1,000 - $17,000 ($US). Various add-in modules available.
GPSS/H Successor to the “orginal” simulation language (GPSS). –Was freeware on IBM 360’s Makes use of common program blocks. Proven, reliable software. Extremely flexible. Extensive error checking routines. Post-process animations (Proof) can be built. ~$5,000 ($US)
Automod Combines Virtual Reality (VR) graphics with a discrete and continuous simulation environment. –Manufacturing operations –Material handling systems –Tanks and pipe networks –IC Manufacturing –Transportation and logistics systems $15,000 - $100,000 ($US)