availability

1. The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Note 1: The conditions determining operability and committability must be specified. Note 2: Expressed mathematically, availability is 1 minus the unavailability. 2. The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week. Note 2: Typical availability objectives are specified in decimal fractions, such as 0.9998. 3. Timely, reliable access to data and information services for authorized users. [INFOSEC-99]4.The prevention of denial of service [CESG].5. The property of being accessible and useable upon demand by an authorized entity [7498-2].6. The prevention of the unauthorized withholding of information or resources [ITSEC].7. The property of an object being accessible and usable upon demand by an authorized subject [POSIX.6].