Availability is a measure of the probability that a system or network will perform as required at a given time. Ininformation technology and data transmission, the availability of data is an economic aspect that is related to the speed of availability. Availability is one of three goals for security. The other two are confidentiality and integrity.
Availability can apply equally to systems or networks and their functionality and to data or information. There is an interaction between availability and performance improvement. Since a high level of economic damage can occur if a system fails, data processing systems and communications networks should be designed with as much redundancy as possible.
Availability is determined by the Mean Time Between Failures( MTBF) and the Mean Time To Repair( MTTR). It results from the quotient of MTBF/(MTBF + MTTR) or from the ratio of uptime to the sum of downtime and uptime. It is divided into several classes ranging from simple availability to high availability to non-stop availability, with simple availability at 99.5%, high availability at 99.9% and higher, and non-stop availability at 100%. A high availability of 99.95% corresponds to a downtime of 4.38 hours per year.
In addition to this classification, there is also the evaluation by the Availability Level( AL). This characteristic value describes the restrictions in case of failure, abort, interruption of transactions and other impairments. There are four AL levels 1 to 4 in which the impairments are evaluated.
Availability includes hardware and software, networks and data transport, operating system and data storage, power supply and network components, security against unauthorized access and against sabotage, etc., so the first priority in availability is redundancy, which must be optimally organized. The network availability of carrier networks is distinguished by the carrier grade. This rating is linked to several specified rules, such as overall availability, which must be 99.999%, corresponding to a downtime of less than eight minutes per year, provision of new services within specified times, quality of service agreements( SLA), mechanical robustness, and much more.