Abstract: A method for monitoring the quality of a digital signal in sections of a data transmission circuit wherein the quality of the transmitted digital signals is monitored in a synchronous network by digital signal distributors arranged along the circuit and the acquired data is forwarded to a centrally controlled emergency network switching system. A BIP-8 code word (bit-interleaved parity-8 code) in a B3 time slot for parity monitoring is transmitted from the source to a sink in the path overhead of a virtual container (VC-41) in a STM-1 signal. The code word is also transmitted as B4 monitoring in a Z time slot (Z4) of the path overhead which is available, and it is evaluated in every digital signal distributor and is re-supplied into the outgoing line of the signal distributors. Since every modification in the virtual container (VC-4) falsifies the B3 monitoring, a correction code word which cancels the modification by the BIP-8 code word in the first Z time slot (Z4) is inserted in a further Z time slot (Z5).