The invention refers to synchronizing a client clock of a network node to a reference clock, wherein the network node performs the steps of receiving from the reference clock synchronization data, determining a comparison value indicative of a degree of deviation of the client clock with respect to the reference clock, and determining a synchronization state (S1, S2) out of a free running state (S1) and a locked state (S2), based on the comparison value, a first threshold (TH1), and a second threshold (TH2). The invention further refers to a telecommunication node (21) and a computer program.