runk, and its association with a single line for a call, is no longer the right concept. 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 Key Radio Concepts 4.8 Component Reliability Reliable base stations are key to service availability and quality. In this section we will look at both the mathematical modeling and the practical engineering and manage- ment issues related to designing, installing, and maintaining reliable base stations. 4.8.1 A simple reliability model Base-station equipment can fail. Power supplies can go out, controllers can stop work- ing, and amplifiers can burn out. Any one of these three failures will take a significant area out of service. Therefore, let us take a look at reliability as an issue of base- station design. Let us use a simple model of a base station as three separate components, power supply, controller, and amplifier, components A, B, and C. The base station is working only when components A, B, and C are all working. Like an electric circuit, we call this a series system in reliability theory, as shown in Fig. 4.5. When the components them- selves are reliable, we can simplify the mathematics and approximate the outage time for the series system as the sum of the outage times for the components. If components A, B, and C are each working 99 percent of the time, then we might say that com- ponents A, B, and C are 99 percent reliable and that the entire system is 97 percent reliable. We can evaluate the reliability of each component by considering it to be in one of two states, WORKING or FAILED, as shown in Fig. 4.6. (These states are similar to the call states in Chap. 16.) We presume that the component has a failure rate 1 and, once broken, some rate of repair . These are average rates rather than a specified schedule. Consider a component that breaks down an average of once per month