A method in a first network node for determining a latency of a transmission link between the first network node and a second network node in a communications network. The first network node transmits an electromagnetic signal to the second network node over the transmission link as a first Frequency-Division Multiplexed (FDM) signal comprised in a first electromagnetic carrier also comprising payload data. The first network node receives the electromagnetic signal from the second network node over the transmission link, wherein the electromagnetic signal has been transmitted from the second network node as a second FDM signal comprised in a second electromagnetic carrier also comprising payload data. The first network node determines the latency of the transmission link based on a phase difference between the transmitted electromagnetic signal and the received electromagnetic signal.