- Transport Network Failures and Their Impacts
- Survivability Principles from the Ground Up
- Physical Layer Survivability Measures
- Survivability at the Transmission System Layer
- Logical Layer Survivability Schemes
- Service Layer Survivability Schemes
- Comparative Advantages of Different Layers for Survivability
- Measures of Outage and Survivability Performance
- Measures of Network Survivability
- Network Reliability
- Expected Loss of Traffic and of Connectivity
3.9 Measures of Network Survivability
Measures of outage are different than measures of survivability. An outage is an event that arises from a failure that has actually occurred. Survivability is, however, the ability of a network to continue to provide service in the event of a failure that might arise. Survivability is, thus, an inherent attribute of the network design or technology employed regardless of if, or how often, failures actually occur. The term survivability itself is not usually given quantitative meaning but is used in general to refer to the quality of being able to keep on functioning in the face of either internal failures or externally imposed damage. The quantitative measures of survivability are necessarily more specific.
One class of such measures are called conditional or Given Occurrence of Failure (GOF) models. In these measures, each relevant failure scenario is first postulated to have occurred, then an assessment of survivability is made. These tend to be design-oriented measures since they reflect the merit of a survivable design over a pre-specified set of failure scenarios against which coverage is to be ensured but do not depend on knowledge or assumptions about how often such failures may actually occur. They deal with questions such as, "If failure x occurs, how well are network services protected from it?"
The other general class of survivability measures aim to take into account the probability of failure onset as well as the survivability response or capability of the network. These are called Random Occurrence of Failure (ROF) models. In contrast to the GOF orientation, ROF measures typically ask questions such as: "How likely is it that a path between nodes has an outage over x minutes in any given year?" ROF models are usually based on the assumption that failures can be characterized by random variables with given probability distribution functions and are thus closely related to the fields of reliability and availability which we will review.