This counter sums invalid PDUs, those with incorrect length, undefined type, or bad alignment, together with PDUs that triggered MAA error codes and were discarded. It catches malformed or protocol-violating signaling messages at the SSCOP layer itself, before they're even interpreted as call-control messages, so a rising count points to bit-level or framing corruption on the signaling channel rather than a legitimate call being rejected for a real reason. A signaling link with a marginal physical layer might show this counter climbing even while very few calls are actually being attempted.