 a network could imply denial of all those vital services and applications. CHAPTER 5 NETWORK ROADMAP EVOLUTION J.P.G. Sterbenz et al. define resilience for networks as follows: [We define] resilience as the ability of the network to provide and maintain an acceptable level of service in the face of various faults and challenges to normal operation. Resilience and Survivability in Communication Networks: Strategies, Principles, and Survey of Disciplines -J.P.G. Sterbenz et al. (2010) Although resilience is defined based on a network's ability to provide and maintain an acceptable level of service, survivability of the network is also an equally important aspect and is defined as follows by P. Cholda et al.: [Network survivability is the] Quantified ability of a system, subsystem, equipment, process, or procedure to continue to function during and after a natural or man-made disturbance. A Survey of Resilience Differentiation Frameworks in Communication Networks - P. Cholda et al. So what are the disciplines on which network resilience can be analyzed? The fundamental concept that governs the ideas within the resilience domain are driven by the fault error failure chain, as shown in Figure 5-1. Fault could be any inherent Fault weakness in the system, accidental or by design Error is any observable difference Error in the value of a state, different from the expected state, visible in the system due to triggering of a fault Failure is the result of an error where the Failure expected service is not provided by the system not working as per expectation Figure 5-1. The fault error failure link CHAPTER 5 NETWORK ROADMAP EVOLUTION The rate at which dormant faults are exploited to become errors and observable errors become failures in the system can be limited by defenses built into the network. Also, by systematically improving the complete communication channel, the ability of the network to provide end-to-end connectivity to users is improved. This is disruption tolerance, also defined in following way: Disruption t