Abstract: In an ATM over STM transmission system, comprising a transmitter and a receiver, a local clock in the receiver has to be synchronized to a clock in the transmitter. To achieve said synchronization, the receiver comprises timing signal extraction means which generates a timing signal each time a first byte of an ATM cell is received in a first byte of an STM frame. Due to the periodically occurrence of the timing signal this is very suitable for synchronizing a local clock. In an advantageous embodiment of the invention, a timing reference value is transmitted by the transmitter to the receiver. This timing reference signal is temporarily stored in a buffer in the receiver. At the instant the timing signal is present at the output of the timing extraction means the content of the buffer is loaded into the clock counter.