Abstract: Telecommunications networks require a variety of tests to monitor their proper operations. Performance monitoring (and fault detection) of the physical layer of a transmission system is one of them and is usually accomplished by using a bit error rate test (BERT). The BERT of the invention uses modified ATM idle cells to conduct one-way, bidirectional and loopback BERT. ATM idle cells are loaded with BERT data at the transmit end and recovered at the receive end. The BERT data are processed according to the different data rates of the ATM transmission system. Performance parameters are derived from the received BERT data. The disclosure describes a method as well as a system for performing such a test.