This counts seconds during which the emulated TDM circuit carried by this pseudowire was formally unavailable. UAS on this PW is the number that matters for an SLA on the emulated circuit, since it reflects seconds the TDM service was actually down, as opposed to merely having errored packets that the jitter buffer absorbed -- and this specific object covers a closed 15-minute history bin -- a fixed, final count for one completed interval rather than something still changing. For example, over that completed 15-minute bin, this pseudowire might show this counter at 2 -- a concrete count the admin can trend against neighboring bins to judge whether the condition it reflects is worsening.