Abstract: Apparatus (100) and accompanying methods for accurately simulating a digital facility, including impairments, in a public switched telephone network (PSTN). The invention is particularly suited for precisely emulating, as part of that facility, a line card within a channel bank including a hardware coder-decoder (CODEC) circuit contained therein, but without using an actual CODEC. Specifically, to properly emulate a digital facility, including such a line card, impairments, which need to be emulated, arise not only from the network facility itself but also from the line card. The former include robbed bit signaling (RBS), digital trunk loss and network delay. The latter include intermodulation distortion (IMD), analog loss and echo. To achieve very precise emulation, the simulator implements IMD and network impairments digitally but with the former impairment being processed at much higher, i.e., oversampled, (illustratively 32 kHz) sample rate relative to the sample rate for the latter impairments (e.g.