Reliability of a Microprocessor-Based Protection Switching System

In high-capacity transmission systems, any failure may affect a large number of message circuits. Such systems usually include one or more hot spares to increase system reliability. When a regular transmission channel fails, its signal is rapidly transferred to the spare channel under the control of protection switching circuits so that there is little signal degradation or interruption. This paper studies the reliability of a microprocessor-based terminal protection switching system (TPSS). The specific transmission facility under consideration is the L5E coaxial cable analog system, which is an expanded version of the L5 system.1 The L5E multiplex equipment, or multimastergroup translators (MMGT), carry up to eight mastergroups, or 4800 telephone circuits. The TPSS will automatically switch into service a protection MMGT in the event of a failure of any one of up to 20 MMGTs. Reliability theory has been studied by numerous authors,2,3 and almost every Bell System transmission facility with automatic protection switching has been the subject of at least one reliability study.4,5 The present analysis was undertaken for several reasons. First, many simplifying assumptions were made in the previous studies. Not all the 2633