A network node of a synchronous network, wherein said network node comprises a timing circuit which recovers a reference clock from a reception signal received by said network node from an upstream network node of said synchronous network and uses the recovered reference clock for a transmission signal transmitted by said network node to a downstream network node of said synchronous network; and a clock stability monitoring circuit which monitors internal control parameters (CP) of said timing circuit to detect an instability of the reference clock distributed within said synchronous network.