A device an method for detecting digital impairments affecting an upstream pulse code modulation (PCM) channel in a digital communication network (36), involves: receiving, by a digital PCM modem (38) interconnected to the digital communication network (36), a random sequence of digital values selected from a constellation of digital values transmitted over the upstream PCM channel of the digital communication network (36); establishing distributions of the received digital values, each distribution corresponding to one of a plurality of time intervals; and deriving from the distributions the types of robbed bit signaling and digital loss affecting the upstream PCM channel of the digital communication network (36) for each time interval.