hen the primary unit fails, the secondary unit is powered up and goes into service until the primary unit is repaired. We call this a cold spare because the sec- ondary unit is not running. In the cold-spare scenario, we presume that the secondary unit does not fail while turned off, the 21 term in the reliability equation becomes just 1, and the outage time is about half that in the hot-spare case. This outage-time calculation does not count the time it takes to turn on and to warm up the secondary unit. 4.8.2 Real-world reliability issues The analysis in the preceding subsection is the tip of the reliability science iceberg. The model makes a number of assumptions, some of which we will list here. In this key concepts section we will not go into a detailed mathematical analysis of these issues. 10 The three equations for the three states are redundant; that is, any two are sufficient to de- fine the problem. Thus we picked the easiest two equations to use in solving the system. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The Base Station The Base Station Rather, we will identify the most important real-world issues that the model did not address. The model assumes that we can detect the redundant component failure even when there is no outage and that the repair in the ONE-WORKING case gets the same speed and enthusiasm as the repair of both components in the FAILED case. This requires moni- toring systems and standard operating procedures for repair. The mathematical model we used assumed that failures were independent of one an- other. However, when a single cause, such as a power surge or a flood, may disable mul- tiple components, a different model is required. Our models did not take all the components into account; for example, we did not ad- dress antenna failure or tower collapse. For relatively stable components, it is often b