Network reliability and availability
Network reliability and availability are often confused for many people. As an accurate definition, network reliability means how reliable a network is, which can be represented as a probability that the first failure occurs at a certain moment. In general, with time going on, the probability that the first failure occurs becomes higher and the reliability becomes lower, which means that a network tends to occur a failure.
In contrast, network availability is the possibility that a user can get its service or a network has no failure when the user wants to use the service. Different from network reliability, network availability considers network repair; that is, if a network fails, we can repair it and then use it; though the repair takes some time, after the network is repaired, it goes normal to provide services. For the time interval of network failure, there is a term called mean time to failure (MTTF), and for the time required for network failure repair, there is a termed called mean time to repair (MTTR). Based on these two terms, network availability is defined as MTTF/(MTTF+MTTR), which tells how much percent that a network is up for any moment that a user wants to access the network.
