OTUk_UAS is the unavailable seconds of the optical transmission segment. Unavailable seconds refer to severe error seconds that last for more than 10 seconds. In this case, the unavailable time starts from the 11th second, but the severe error seconds of these 10 seconds are also counted into the unavailable time. When severe error seconds disappear for more than 10 seconds, the available time starts from the 11th second, and the 10 seconds without severe error seconds are also counted into the available time.