The invention relates to novel guanidine derivatives of the formula . In turn, the service node includes a switch coupled to the xDSL lines, a gateway coupled to the Internet, a router which couples the switch to the gateway and a bandwidth measurement server coupled to the gateway. The offset threshold is then compared to a detected signal from the photodetection element when the light emission element emits light, thereby determining the presence of a through hole between the light emission element and the photodetection element through which hole light passes without being influenced by external light.