A communications network element (10, 30, 40, 50, 60, 80) comprising: an input arranged to receive an input optical signal having an input spectral property and carrying input traffic; an output arranged to output an output optical signal; a monitoring port (16, 32) arranged to output an optical monitoring signal; optical signal processing apparatus arranged to receive the input optical signal and to form the output optical signal, the output optical signal having an output spectral property and carrying output traffic; an optical splitter (20, 34) arranged to tap off a part of one of the input optical signal and the output optical signal to form a tapped signal having a spectral property comprising a respective one of the input spectral property and the output spectral property and carrying traffic comprising a respective one of the input traffic and the output traffic, the element characterised in comprising: optical signal transforming apparatus (22, 36, 42, 44, 52, 54, 82) arranged to receive the tapped signal and to apply an optical transfer function to the tapped signal to form an optical monitoring signal, the optical transfer function being arranged to preserve the spectral property of the tapped signal and to apply a time-domain obfuscation to the tapped signal, and the optical signal transforming apparatus being further arranged to provide the optical monitoring signal to the monitoring port. an input arranged to receive an input optical signal having an input spectral property and carrying input traffic; an output arranged to output an output optical signal; a monitoring port (16, 32) arranged to output an optical monitoring signal; optical signal processing apparatus arranged to receive the input optical signal and to form the output optical signal, the output optical signal having an output spectral property and carrying output traffic; an optical splitter (20, 34) arranged to tap off a part of one of the input optical signal and the output optical signal to form a tapped signal having a spectral property comprising a respective one of the input spectral property and the output spectral property and carrying traffic comprising a respective one of the input traffic and the output traffic, the element characterised in comprising: optical signal transforming apparatus (22, 36, 42, 44, 52, 54, 82) arranged to receive the tapped signal and to apply an optical transfer function to the tapped signal to form an optical monitoring signal, the optical transfer function being arranged to preserve the spectral property of the tapped signal and to apply a time-domain obfuscation to the tapped signal, and the optical signal transforming apparatus being further arranged to provide the optical monitoring signal to the monitoring port. optical signal transforming apparatus (22, 36, 42, 44, 52, 54, 82) arranged to receive the tapped signal and to apply an optical transfer function to the tapped signal to form an optical monitoring signal, the optical transfer function being arranged to preserve the spectral property of the tapped signal and to apply a time-domain obfuscation to the tapped signal, and the optical signal transforming apparatus being further arranged to provide the optical monitoring signal to the monitoring port.