This counts how many filler 'dummy' cells the AAL1 reassembly process has substituted in place of cells that were actually lost in transit, the mechanism that keeps a CBR circuit's timing intact despite cell loss. A rising count on a voice or leased-line-emulation circuit tells the admin real cell loss is occurring upstream even if the circuit still looks up, since each dummy cell represents a small gap in the reconstructed bit stream. For example, a count that jumps from 0 to 400 in an hour on a DS1 CBR circuit points to a period of ATM cell loss a listener on that emulated line would have heard as clicks.