used to transfer optic signals at 1625 1550 1490 and 1310 nm channel 19 is used to transfer optic signals at 1650 nm therefore during formal data exchange operation optic data circulates solely on channel 18 which allows wavelengths of 1550 1490 and 1310 nm if a first test laser impulse is send at a wavelength of 1625 nm this impulse is also sent into channel 18 as far as the subscriber if a fault is detected by the otdr system such as shown in fig1 a second laser impulse is sent at a wavelength of 1650 nm this second impulse is guided into channel 19 via demultiplexer 14 therefore the laser impulse at 1650 nm propagates along a length of fibre greater than the length on which the first impulse propagated at 1625 nm for example a fault detected during the first measurement gives rise to transitory discontinuity in the monotonicity of the backscattered optical power tracing this discontinuity being positioned at a length of 500 m this first measurement indicates the position of the fault on the faulty line during the second measurement using a wavelength of 1650 nm this discontinuity in monotonicity moves by a length of 15 m and is therefore positioned at 515 m this length of 15 m corresponds to an overlength associated with a particular fully identified line therefore each of overlengths 15 being different for each of modules 17 measurement of the movement of the discontinuity associated with the.