4.3 可恃性配當

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4.3 Dependability Allocation
可恃性配當
Defining the dependability requirements for sub-systems is an essential part of the system design work.  The objective of this task is to find the most effective system architecture to achieve the dependability requirements (and thus contribute to the feasibility study).  As dependabilty is the collective term for reliability, availability and maintainability, an allocation for each of these characteristecs is necessary.  However as allocation techniques for all three characteristics are similar, the collective term dependabilty is used in this instance.  

The first step is to allocate the dependability requirements of the overall system to sub-systems, depending on the complexity of these sub-systems based on experience with comparable sub-systems.  If the requirements are not met by the initial design, allocation and/or design shall be repeated.  Allocation is also often made on the basis of considerations such as complexity, criticality, operational profile and environmental conditions.  

Since dependability allocation is normally required at an early stage when little or no information is available, the allocation should be updated periodically.  

Allocation, sometimes called apportionment, of system dependability to the sub-system and assembly levels is necessary early in the product definition phase in order to
- check the feasibility of dependabilty requirements for the system,
- establish realistic dependability design requirements at lower levels,
- establish clear and verifiable dependability requirements for sub-suppliers.

When accomplishing dependability allocation, the following steps are needed:
- Analyse the system and identify areas where design is known and information concerning values of dependability characteristics is available or can be readily assessed.  
- Assign the appropriate weights and determine their contribution to the top-level system dependabilty requirement.  The difference constitutes the portion of the dependabilty requirement that can be allocated to the other areas.  

Dependabilty allocation has the following benefits:  
- It provides a way for the product development to progress and to understand the dependability goals relationships between system and their items (e.g., sub-systems, equipment, components).  
- It considers dependability equally with other design parameters such as cost and performance characteristics.  
- It provides specific dependability goals for the suppliers to meet for their deliveries, which, in turn, leads to improved design and procurement procedures.
- It may lead to optimum system dependabilty because it considers such factors as complexity, criticality and effect of operational environment.  

On the other hand, some limitations should be noted:  
- Assumption is often made that the items of a system are independent, i.e., failure of one item does not affect others.  Since this assumption is often not valid, this limitation reduces the benefits of the method.  
- Allocation of redundant systems is more complex.  In these cases, it is appropriate to use an iterative method to check whether dependabilty goals for the system can be reached, for example the fault tree method.