ABSTRACT
Driven by the need to maintain the reliability of the nation's critical communications infrastructure, the telecommunications industry has a long tradition of designing for high availability, sometimes referred to as “carrier grade availability” or loosely quantified as “five-nines availability” (Lancaster, 1986). Significant service disruptions must be reported to the FCC (Healy, 2016) by wireline, wireless, paging, cable, satellite, Voice over Internet Protocol (VoIP), and Signaling System 7 service providers. Communications providers must also report information regarding communications disruptions affecting Enhanced 9-1-1 facilities and airports that meet the thresholds in the FCC's rules. Detailed best practices for maintaining network continuity have been developed,* and switching, transport, and 68network infrastructures have been designed to meet these very stringent requirements. High availability has been attained using custom hardware, fault-tolerant software design, and extensive testing to ensure that the production software has few defects (Giloth, 1987). Excellent examples of highly reliable designs date back to digital switching systems such as the 4ESS (K. E. Martersteck, 1981) and 5ESS (Martersteck and Spencer, 1985), and networks have been designed with survivability as a core objective (Wu, 1992; Krishnan, 1994; Choudhury, 2004; Klincewicz, 2013; Ramesh Govindan, 2016).
