Availability in a segment of a communications network between two terminating devices periodically transmitting a data packet over the segment is determined. The two terminating devices receive transmitted data packets according to an expected arrival format for data packets. The segment is monitored over a monitoring time interval for continuity of the expected arrival format. An unavailable time after a change in arrival format of data packets from the expected arrival formal to a change in arrival format of data packets back to the expected arrival format is detected. An available time interval based on the monitoring time interval and the unavailable time interval is determined.