According to the present invention there is provided methods of performing a distributing sensing measurement, comprising the steps of, modulating the frequency of one or more light signals output from one or more light sources, using one or more multi-level sequence of bits so that the one or more light signals are frequency modulated; using the one or more frequency modulated light signals to provide a pump signal and a probe signal; propagating the pump and probe signals along an optical fiber; using interactions between the pump and probe signal to perform a distributed sensing measurement. There is further provided corresponding sensing systems.