*2.3.1 Maintenance activity*

Lalli et al. [8] said that maintenance is divided into two parts, which include the replacement of defective elements. Moreover, preventive maintenance, which includes scheduled maintenance in the design phase, separates maintenance activity into three phases:

**In Phase I:** This includes planning and designing maintenance capability, which is part of a design process that involves the construction of elements that are easy to maintain and repair. In analyzing the measuring capability, the number of errors is often used from predictable times to maintain N's maintenance. Secondly, the expected time to maintain n is the expected maintenance with the program, as well as the expected time to repair and replace items that have a limited life span and a scheduled change program. Suppose the system is shut down during maintenance, then we can measure its availability.

**In Phase II:** This includes handling failure and replacing items that are shortlived as well as preventive maintenance operations.

**In Phase III:** This involves handling fault elements that specify whether it should be repaired or replaced.
