A renewal process in which the process alternates between states A and B. Let the average length of a period in state A be μ_A and the average length of a period in state B be μ_B. The long-run proportion of the time that the system is in state A isAn alternating renewal process is a special case of a semi-Markov process. When the states are ‘working’ and ‘under repair’, the probability that the system is working at time t is called the availability.