This counts how many times the AAL5 reassembly timer expired on this port before a complete packet could be reassembled from its constituent cells. A reassembly timeout on this specific port means cells belonging to a packet stopped arriving partway through, typically because later cells were lost or badly delayed, so this counter isolates incomplete-arrival failures distinctly from packets that arrived complete but failed a length or CRC check. A burst of pportAAL5ReassemblyTimerError events on this port coinciding with a period of high pportInDiscards would confirm that upstream congestion, not corruption, was the cause of the incomplete AAL5 packets.