A state where a (bidirectional or unidirectional) service is usable. Note: Each direction of a service is assumed to be in the available state unless a transition to the unavailable state is observed without a subsequent transition to the available state. In this standard the transitions between the available and unavailable states are: (a) transition to the unavailable state occurs at the beginning of 10 consecutive severely errored seconds (SES); (b) transition to the available state occurs at the beginning of 10 consecutive seconds none of which is an SES.