We can use these service and performance requirements to distinguish between applications that need predictable or guaranteed service and those that can use besteffort service. Based on service and performance requirements, we type applications as missioncritical, ratecritical, or realtime, where • Missioncritical applications have predictable, guaranteed, and highperformance RMA requirements • Ratecritical applications have predictable, guaranteed, and highperformance capacity requirements • Realtime and interactive applications have predictable, guaranteed, and highperformance delay requirements These application types are described by their requirements and service metrics. 68 CHAPTER 2 Requirements Analysis: Concepts RMA Let’s first look at RMA, consisting of reliability, maintainability, and availability. Reliability is a statistical measure of the frequency of failure of the network and its components and represents the unscheduled outages of service. Maintainability is a statistical measure of the time to restore the system to fully operational status, once it has experienced a fault. Availability is a measure of the relationship between the frequency of missioncritical failures and the time to restore service. How do these measures relate to the applications that will use the network? RMA requirements can be subjective. Many users argue that their applications require a high degree of reliability, maintainability, and availability from the network, but there are some applications that must maintain high degrees of RMA in order to function. A loss of any part of RMA in such applications may be serious or disastrous, such as: • Loss of revenue or customers. Examples include applications that handle lots of transactions and money, such as investment banking, airline reservation, or credit card processing applications. • Unrecoverable information or situation. Telemetry processing and teleconferencing applications are good examples of this type of reliability. • Loss of sensitive data. Examples include customer ID and intelligencegathering applications. • Loss of life. Examples include transportation or healthcare monitoring applications. In these situations either the system is not available to process user requests or the system is not available to complete the transactions that are in progress. For applications such as these, a network that offers only besteffort service is not likely to be adequate, owing to its unpredictable and unreliable behavior. These applications require predictable or guaranteed reliability, maintainability, and availability, which may take the form of a predictable or bounded RMA, or a high degree of RMA, or both. Applications that require predictable or high RMA are termed here missioncritical applications. Capacity In terms of capacity, there are some applications that require a predictable, bounded, or high degree of capacity.