This counts AAL5 packets received on this port where the number of cells received, times 48 bytes, did not reconcile with the length value declared in the AAL5 trailer, specifically overshooting by more than 55 bytes or undershooting by more than 8 bytes, per the description's two conditions. This length mismatch on this specific port typically means either cells were lost mid-packet (undershoot) or extra spurious cells were inserted into the reassembly buffer (overshoot), making this counter useful for distinguishing reassembly corruption from a clean CRC failure that still has consistent length. A rising pportAAL5LengthError count on this port together with lost cells reported elsewhere on the same path would point to the undershoot condition, missing cells during reassembly, rather than extraneous cell insertion.