Page 128 - ExtendSim User Guide
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102 Introduction
Terminology and architecture
Items and informational values
The basic units that are passed between discrete event blocks are items. An item is an individ- ual entity that represents an element of the system being modeled; it can only be in one place at a time. Items have a life cycle in which they are created, transformed, and eventually destroyed. They change state (physically move, are delayed, or have their properties altered) when events occur, such as a part being assembled, a customer arriving, and so on. In manufac- turing models, items may be parts on an assembly line; in network models, an item would be a packet of information; in business models, items may be invoices or people. Items are passed from block to block through item connectors. The Create block can generate items with a ran- dom distribution, at a constant rate of arrival, at a fixed schedule, or on demand. The Resource Item block provides a finite pool of items.
Items can have properties — different pieces of information attached to an item that make the item unique. Item properties include attributes, priorities, and quantities, as discussed in “Item properties” on page 102.
Values provide information about items and about model conditions. Values tell you the num- ber of customers in queue, how many parts have been shipped, and how frequently telephone calls occur. Values also report processing time, utilization, and cycle time. These informational values are passed through value connectors. When you use a plotter in a discrete event model you are plotting information about items, not the items themselves. For example, when the top output of an Exit block (total exited) is connected to a plotter, it displays the time that each item left the model and the number of items that have exited.
Item properties
A property is a characteristic of an item that stays with the item as it moves through the simu- lation. Item properties include attributes, priorities, and quantities.
Attributes
Attributes are an important part of a discrete event simulation because they provide informa- tion about items. Each attribute consists of a name that characterizes the item and a value that indicates some dimension of the named characteristic. For example, an item’s attribute name might be “color” and its value could be “1” (for “red”). Or the attribute name might be “Pro- cessTime” and its value “4.76”. Attributes are often used for routing instructions, operation times, or part quality in statistical process control; they are discussed fully on page 125.
Priorities
Priorities allow you to specify the importance of an item. For instance, there might be a step in a manufacturing process where a worker looks at all the pending job orders and chooses the one that is most urgent. Each item can only have one priority. The top priority has the lowest value, including negative values (that is, an item with a priority of “-1” has a higher priority than an item with a priority of “2”). Priorities are discussed fully on page 133.
Quantities
Each item can be a single entity or a group of duplicates. If the quantity of an item is 1, it rep- resents one item; if it is greater than 1, it represents a group. By default, items have a quantity of 1. The quantity can be changed by a block like the Set block. For more information, see “Quantities” on page 134.
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