Abstract: A predetermined characteristic of each of incoming plesiochronous component signals in a time-division multiplexing equipment is derived at each stuffing time for transmission in the form of a stuffing bit in a respective stuffed signal to be multiplexed in a resulting signal. In a demultiplexing equipment receiving the resulting signal, the stuffing bit in a respective demultiplexed stuffed signal is sampled and simultaneously the predetermined characteristic of the plesiochronous signal outgoing from the demultiplexing equipment is derived at each unstuffing time. The sampled stuffing bit and the derived predetermined characteristic are compared in order to measure the transmission quality of a digital channel carrying the component signal through the two equipments. The predetermined characteristic is preferably the parity of the signal between two consecutive stuffing and unstuffing times.