This read-only counter tallies frames that could not be completely reassembled before a reassembly timeout expired, and were discarded before being transmitted on this connection. Because AAL5 frames arriving as ATM cells have to be reassembled before being forwarded onto the frame-relay side, a rising count means cells belonging to a frame are being lost or delayed somewhere upstream on the ATM side, leaving reassembly waiting for cells that never arrive until it gives up. Cell loss on a congested upstream ATM trunk feeding this interworking connection would show up here as reassembly timeouts well before any complete-frame counters show a drop.