fig1 shows a passive optical network i which comprises a tree - like operator network 2 as a first section . the operator network 2 has a main fiber line 3 being located between an optical line termination ( olt ) 4 and a 1 × n passive optical distribution network ( splitter ) 5 connecting the main line 3 with a number n of branches , three of which ( 6 . 1 to 6 . 3 ) are shown in fig1 . the branches 6 . 1 to 6 . 3 of the operator network 2 are continued in a user network 7 which constitutes a second section of the optical network 1 and which comprises optical network terminations ( onts ) 8 . 1 to 8 . 3 , one for each of the branches 6 . 1 to 6 . 3 . a so - called demarcation point 9 . 1 to 9 . 3 marks the limit between the operator network 2 and the user network 7 in each of the branches 6 . 1 to 6 . 3 . in the operator network 2 , monitoring units dpm 1 to dpm 3 are arranged adjacent to each of the demarcation points 9 . 1 to 9 . 3 , which , due to their position in the optical network 1 are also referred to as a demarcation point monitors ( dpms ) in the following . the person skilled in the art will appreciate that the pon 1 of in fig1 only shows the basic structure of pons and that real pons are usually much more complex . as the operator is responsible for faults occurring in the operator network 2 , e . g . due to fiber breaks , the operator has to monitor the network 1 continuously to spot possible defects . due to the large distances which are covered by the fiber links in the network 1 , the operator generally has only access to the network 1 via the olt 4 , also referred to as central office , and has to check the correct operability of the network 1 from this site all the way down to the demarcation points 9 . 1 to 9 . 3 . for monitoring the network 1 , the olt 4 generates a wake - up signal 10 which is transmitted downstream through the branches 6 . 1 to 6 . 3 of the operator network 2 to the monitoring units dpm 1 to dpm 3 . the wake - up signal 10 is an easily detectable signal , for example a sine wave signal with a frequency of 10 khz which is not modulated such that the wake - up signal 10 cannot address a particular one of the monitoring units dpm 1 to dpm 3 . the wake - up signal 10 is only transmitted during a short time interval , the time duration of which is chosen to be sufficiently long for allowing detection of the wake - up signal 10 in the monitoring units dpm 1 to dpm 3 , which will be described in the following with reference to fig2 showing the first one ( dpm 1 ) of the monitoring units dpm 1 to dpm 3 . the first monitoring unit dpm 1 has a tap coupler 11 which branches part ( about 5 % of the total intensity ) of the downstream signal from the branch 6 . 1 to an optical receiver 12 comprising a photodiode which is used as an o / e - conversion element . a detector 13 implemented in a microprocessor ( not shown ) is provided in the first monitoring unit dpm 1 for detecting the sine wave wake - up signal 10 in the downstream transmission from the olt 4 to the onts 8 . 1 to 8 . 3 . after the detector 13 , a delay generator 14 is arranged in the signal path of the first monitoring unit dpm 1 , the delay generator 14 being implemented e . g . as a counter of the microprocessor for generating a pre - determined time delay δt a before the generation of a response signal a in a response signal generator 18 of the monitoring unit dpm 1 . a transmitter 15 is used to perform e / o - conversion of the delayed response signal a which is then transmitted in upstream via a further tap coupler 17 , the upstream transmission wavelength of the response signal a corresponding to the downstream transmission wavelength of the wake - up signal 10 . the response signal generator 18 may generate a simple sine wave signal with constant frequency , or information about the monitoring unit dpm 1 , e . g . about its temperature , may be added to the response signal a , e . g . using amplitude modulation . it is understood that other modulation formats such as frequency - shift keying may be used as well for the addition of information to the response signal . alternatively , it is possible to generate a binary signal , e . g . at 100 khz , which may also include a header sequence , in the response signal generator 18 . in any case , after the detection of the wake - up signal 10 , the transmission of the response signal a is delayed by the pre - defined time delay δt a , the absolute value of the time delay δt a being stored e . g . in a memory of the microprocessor of the monitoring unit dpm 1 . in the embodiment of the monitoring unit dpm 1 shown in fig2 , a switching unit 19 is provided in the detector 13 for switching the monitoring unit dpm 1 from a first operation mode with low power consumption to a second operation mode with higher power consumption . for this purpose , the switching unit 19 sends a trigger signal 19 a to the transmitter 15 when the wake - up signal 10 is detected , which causes the transmitter 15 to switch from a standby mode to a mode which allows the generation and transmission of the response signal a . the time delay of the delay generator 14 is chosen sufficiently long for the transmitter 15 to have time to switch from the first to the second operation mode . the monitoring unit dpm 1 may be powered by a battery ( not shown ) which is only slowly discharged in the first operation mode having low power consumption . alternatively or in addition , the monitoring unit dpm 1 may be powered by an external power source or by using the power of the o / e - converted part of the downstream signals during time intervals in which no wake - up signal 10 is detected in the detector 13 . in the latter case , a rechargeable energy storage such as a capacitor may be provided in the monitoring unit dpm 1 . the monitoring unit dpm 1 transmits a response signal a with a pre - defined time delay δt a with respect to the detection of the wake - up signal 10 , the time delay δt a being essentially determined by the delay generator 13 . as each of the monitoring units dpm 1 to dpm 3 of fig1 has an individual time delay δt a , δt b , δt c , by adjusting these pre - defined time delays δt a , δt b , δt c in an appropriate way , the superposition of their response signals a , b , c on the main line 3 generates a superimposed signal in which the three response signals a , b , c are delayed with respect to each other , such that they may be received at three different receiving times r 1 to r 3 . it is understood that the time delay of the response signals a , b , c on the main line 3 is not only due to the time delays δt a , δt b , δt c of the monitoring units dpm 1 to dpm 3 , but also due to the different signal propagation delays of the response signals a , b , c which are caused by the different lengths of the branches 6 . 1 to 6 . 3 connecting the monitoring units dpm 1 to dpm 3 to the main line 3 . consequently , the signal propagation delays have to be taken into account for generating a desired delay of the response signals a , b , c on the main line 3 , which may be done by measurements of the signal propagation times within the fibers . in the example of fig1 , the propagation delays have been neglected , such that the time delays δt a , δt b , δt c on the main line 3 coincide with the time delays δt a , δt b , δt c of the monitoring units dpm 1 to dpm 3 . due to the characteristic delay of the response signals a , b , c , the olt 4 can perform a simple mapping for checking the availability of the monitoring units dpm 1 to dpm 3 . in case that no data is contained in the response signals a , b , c , a simple envelope detection is sufficient for checking the availability of the corresponding monitoring units dpm 1 to dpm 3 . such an envelope detection may be performed also in case that data is encoded in the response signals a , b , c if a fast detection of the availability of the monitoring units dpm 1 to dpm 3 is desired . by adjusting the time delays of a plurality of monitoring units dpm 1 to dpmn in an appropriate way , it is also possible to allocate each of the response signals a , b , . . . of the monitoring units dpm 1 to dpmn to one of a plurality of detection intervals w 1 to wn represented in fig3 a , the detection intervals w 1 to wn alternating with gaps g 1 , g 2 , . . . during which no response signals are received . during the gaps g 1 , g 2 , . . . an evaluation of the data contained in the response signals a , b , . . . may be performed when these are received in the olt 4 , or the olt 4 may use the gaps g 1 , g 2 , . . . to transfer the extracted information to a network control unit ( not shown ), it also being possible for the olt 4 to perform odtr measurements during the gaps g 1 , g 2 , . . . . for this purpose , the time duration t , of the detection intervals w 1 to wn may be chosen much smaller than the time duration t 2 of the gaps g 1 , g 2 , . . . . in a typical example , t 1 may be set to a value of 10 msec , whereas t 2 may be set to 2 sec . in the example shown in fig3 a , each of the response signals is allocated to only one detection interval w 1 to wn . however , an allocation may also be chosen in which response signals from a plurality of monitoring units dpm 1 , dpm 2 , . . . are allocated to a first detection interval w 1 , whereas response signals from a second plurality of monitoring units dpmn , dpmn + 1 , . . . are allocated to a second detection interval w 2 , as shown in fig3 b . in such a way , the time duration t 1 of the detection intervals w 1 , w 2 , . . . increases , the time duration t 2 of the gaps g 1 , g 2 may be decreased in this case . in summary , in the way described above , a simple and cost - effective scheme for monitoring of passive optical networks is provided . in particular , power consumption of the monitoring units can be reduced by provision of a standby mode . the monitoring units can be connected easily within the network and the demarcation point can be moved with the network extension . moreover , the solution is transparent , as the data channels are not influenced , provided that the upstream transmission wavelengths of the data channels are chosen to be different from the upstream transmission wavelength of the monitoring units . the above description of the preferred embodiments has been given by way of example . from the disclosure given , those skilled in the art will not only understand the present invention and its attendant advantages , but will also find apparent various changes and modifications to the structures and methods disclosed . the applicant seeks , therefore , to cover all such changes and modifications as fall within the spirit and scope of the invention , as defined by the appended claims , and equivalents thereof .