Table of Contents - G.681 (11/2025) - Distributed fibre optic sensing system for terrestrial optical transmission systems

1	Scope
2	References
3	Definitions
	3.1	Terms defined elsewhere
	3.2	Terms defined in this Recommendation
4	Abbreviations and acronyms
5	Conventions
6	Overview of distributed fibre optic sensing system and interfaces
	6.1	Backscattering type sensing
	6.2	Wavelength allocation
7	Parameter definition
	7.1	Fibre type
	7.2	Allowable sensing distance
	7.3	Allowable optical loss budget
	7.4	Minimum and maximum DFOS signal peak power
	7.5	Operating wavelength band
	7.6	Minimum and maximum central frequency
	7.7	Maximum DFOS signal spectral excursion
	7.8	Duration of DFOS signal on-time
	7.9	Mean power of DFOS signal during off-time
	7.10	Maximum impact on the DWDM channel required OSNR in the presence of DFOS signal
	7.11	Maximum impact on the DWDM channel receiver sensitivity in the presence of DFOS signal
8	Parameter values
9	Other requirements
	9.1	Optical safety consideration
Appendix I – System configuration for feedforward type sensing
	I.1	Out-of-band sensing wavelength, bidirectional DWDM transmission
	I.2	Out-of-band sensing wavelength, unidirectional DWDM transmission
	I.3	In-band sensing wavelength, bidirectional DWDM transmission
	I.4	In-band sensing wavelength, unidirectional DWDM transmission
Appendix II – Case study for backscattering type sensing
	II.1	Test conditions
	II.2	Experimental tests and numerical simulation results
Appendix III – Operating examples of sensing distance and optical loss budge
Bibliography