Certified Security Management Professional Module 5 Security Design, Evaluation and Survey

Transcription

1 Background Note 5.4 Detailed Instructions on How to Create Adversary Path/Task Diagrams Using the Logic Diagram Model Source and Copyright: Garcia, Mary Lynn (2007) Design and Evaluation of Physical Protection Systems, 2 nd Edition. Elsevier Science. Logic Diagrams The logic diagram is a useful tool for determining the potential theft and sabotage targets for a complex facility. One type of logic diagram, called a fault tree (Fussell, 1976), graphically represents the combinations of component and subsystem events that can result in a specified undesired state. A simple logic diagram for penetrating the outer boundary of a site with some physical protection components present is shown in Figure 4.2. The following discussion on logic diagrams borrows heavily from the notation used in digital electronics (Putman, 1986). Figure 4.2 Simple Logic Diagram. The diagram develops all the ways to penetrate the outer boundary of a facility. 1 P a g e

2 In this example, the undesired event is defeat of the boundary, which can be accomplished by defeating the personnel or vehicle portals, by passing over or under the boundary, or by defeating the fence. Further elaboration of the actions required to defeat the personnel and vehicle portals, as well as defeating the fence, is also shown. In a more complex example, one undesired consequence (or event) for a dam is the uncontrolled release of large quantities of water as a result of sabotage of critical components. The PPS is intended to prevent sabotage of these components. Logic diagrams that are intended to identify the sets of components an adversary would have to sabotage to cause the consequence are called sabotage fault trees and are used for vital area analysis. They describe the actions an adversary must accomplish to cause sabotage and can be used to identify the areas (locations) to be protected in order to prevent sabotage. Figure 4.3 illustrates the symbols that are used in logic diagrams. The logic diagram shown represents relationships between events. Each event will have a written description in the large rectangle in the logic diagram. A smaller rectangle placed immediately under the description will show the event name or label. Event names should be brief and may be formed from combinations of letters and numbers. Figure 4.3 Logic Diagram Symbols. The logic diagram is a graphical representation of combinations of events that can result in a specified state or event. Each symbol has a specific meaning. The symbols of the logic diagram shown in Figure 4.3 will be discussed in detail. These include symbols for logic gates, events, and transfer operations. Two kinds of logic gates, the AND gate and the OR gate, are used in the logic diagrams. Gates have inputs and an output. Inputs enter the bottom of the gate; outputs exit the top of the description rectangle above the gate. 2 P a g e

3 AND Gate The shape of the AND gate is a round arch with a flat bottom (see Figure 4.4). For the undesired event described above for the AND gate to occur, all the events that have an input into the AND gate must occur. Thus, if any one of the input events can be prevented, the event described above the AND gate will be prevented. For example, assume Event E-AND is generated by an AND gate whose inputs are Events 1-1, 1-2, and 1-3. Event E-AND will occur if, and only if, Events 1-1, 1-2, and 1-3 all occur. Figure 4.4 Example of an AND Gate. All inputs must occur for the output to occur. OR Gate The shape of the OR gate is a pointed arch with a curved bottom (see Figure 4.5). For the undesired event described above the OR gate to occur, any one (or more) of the events that input to the OR gate must occur. All the input events must be prevented in order to prevent the event described above the OR gate. For example, in Figure 4.3, Event E-OR is defined by an OR gate whose inputs are Events 1-1, 1-2, and 1-3. Event E-OR occurs if one or more of Events 1-1, 1-2, or 1-3 occur. 3 P a g e

4 Figure 4.5 Example of an OR Gate. Any one of the inputs must occur for the output to occur. Events There are several types of events in logic diagrams. They include end events, intermediate events, and primary events. If an event is not used as input to another gate, it is called an end event. Logic diagrams have only one end event, the topmost event of the tree. In Figure 4.3, Event 1 is the end event. Sometimes this event is also called the treetop. Events that have both inputs and outputs are called intermediate events. In Figure 4.3, Event 1-2 is an intermediate event. Primary Events are events that do not have an input. They represent the start of actions that ultimately generate the end event. Two types of primary events are distinguished by the symbol that appears immediately below the name of the primary event: the basic event and the undeveloped event. The basic event is symbolized by a circle below the rectangle, as shown in Figure 4.6. A basic event can be understood and evaluated qualitatively or quantitatively, depending on the purpose of the analysis, without further development of the event into causes or specific cases. In Figure 4.3, Events 1-1 and 1-3 are basic events. 4 P a g e

5 Figure 4.6 Basic Event. These are the starting events that lead to the end event. Figure 4.7 shows an undeveloped event, symbolized by a diamond below the rectangle. An undeveloped event is an event whose causes are insufficiently understood to be included in the logic diagram. For the purpose of evaluation, the undeveloped event is treated as a basic event. The conclusions drawn from the analysis of a tree that contains an undeveloped event are tentative and subject to revision if the event is better characterized. In Figure 4.3, Event 2-2 is an undeveloped event. Figure 4.7 Undeveloped Event. These are events where causes are not sufficiently understood to be included in the logic diagram. 5 P a g e

6 Transfer Operation The transfer operation is represented by an upright triangle. The transfer operation is used to make the graphic display of the logic tree more compact and readable. Because many logic diagrams, as they are developed, occupy a wide left-to-right space across a page, it might be necessary to disconnect the development of an event and place it at a more convenient position on the page or on another page. To connect the event and its development without drawing a line between separate figures the transfer symbol is used. An example of the transfer symbol is shown in Figure 4.8. The diagram shown contains one transfer symbol. The transfer operation is shown at the bottom as a separate diagram. The development of Event 1-2 is transferred. Event 1-2 is shown twice: once in the diagram whose development is truncated by the transfer and once at the top of the subdiagram that develops Event 1-2. In general, an event may occur at several places in a logic diagram and the common development of that event may be transferred. The development will appear only once on the page. The A which appears within the transfer symbol to the left of Event 1-2 is the name of the event for which Event 1-2 is an input. In general, there will be a list of every event to which the transferred event is an input. Figure 4.8 Example of the Transfer Operation. Event A has been developed in a different location on the diagram. Transfer operations make the logic diagram more compact and readable. 6 P a g e