Abstract: Round trip absolute delays through a transmission system are measured. A modulation signal S1 of radian frequency .omega. is modulated onto a carrier. The modulation frequency is changed. While the change propagates through the system, the returned demodulated S2 signal remains at the old radian frequency .omega.. During that time, signal S1 advances in phase relative to signal S2. After the propagation time d which is equal to the network delay, the two signals stabilize at a fixed phase offset. The increase .theta..sub.o in the phase offset during the propagation time is determined. The delay d is then determined by dividing the phase offset increase .theta..sub.o by the difference between the two modulation frequencies.