现代语音和数据传输系统（如IP路由器或无线网络控制器）以及具有分层设计的移动性和有线通信网络故障的影响随着分层水平的增加而逐渐增加。影响加权平均无故障时间（IW-MTBF）——给定网络元件或网段的所有层次的MTBF值的组合，通过其对故障的影响对每个层次进行加权——被提议作为在设计阶段评估层次系统整体可靠性的方法。

The impact of failures in modern systems for voice and data transmission (e.g., IP routers or a Radio Network Controller) as well as mobility and wire-line communication networks with hierarchical design increases progressively with the hierarchical level. The Impact Weighted Mean Time Between Failure (IW-MTBF) - a combination of MTBF values for all hierarchical levels of a given network element or network segment weighted for each level by its respective impact on failures - is proposed as a method for evaluating overall reliability of the hierarchical system during the design phase.