Telecommunication operators are now deploying new optical transport and access networks capable of improving their services portfolio and satisfying high-demanding customer requirements. For transport, the operators have deployed high capacity transmission infrastructure, for example using optical fibre cables with multiple fibres per cable.
Point to point transport links permit the connection between the interfaces of two different equipments located at different Central Offices by means of one or two optical fibres 6. The usage of one or two optical fibres will depend on the transmission technology used: only one fibre will be used if different wavelengths are used for each direction (e.g. λE for communication from the first to the second Central Office and λW for communication from the second to the first Central Office), using Wavelength Division Multiplexing (WDM) techniques, or two fibres when the same wavelength (λC) is used for the transmission in both directions.
FIG. 1 shows the typical cross section of an optical fibre cable 1000 used by telecoms operators. The cable 1000 is protected by a coat 1004. If the cable needs an additional protection it has a shield 1005. Inside the cable 1000, the optical fibres 6, with an external jacket 1001 are grouped inside microtubes 1002, and there is an aramid (“aromatic polyamide”) core 1003 which provides traction resistance and strength to the whole cable.
Each fibre strand 6 is identified by the colour code used in the jacket 1001. This colour code allows identifying univocally each fibre 6. This colour code may be defined in a Telecommunications Standard as the TIA (Telecommunications Industry Association) Standard EIA-598-A where 12 different colours are defined. When there are multiple microtubes 1002, each one is also identified by a different colour.
For access networks, most operators are deploying point to multipoint passive optical access networks (they are called passive networks because they do not use any active device that must be remotely fed), also known as PON (Passive Optical
Networks). These PON networks used the same cables described in FIG. 1. International Telecommunications Union has standardized these optical access networks over fiber with a point to multipoint topology in different standards, for example, GPON (Gigabit capable Passive Optical Networks, ITU-T G.984.1), XG-GPON (10-Gigabit capable Passive Optical Networks, ITU-T G.987.1) and EPON (Ethernet Passive Optical Networks, ITU-T 802.3ah-2004 Part 3) and they are the most widespread solutions used to provide broadband access over fibre.
In the case of optical access, previously mentioned solutions have been designed to provide broadband access over a point to multipoint passive fibre infrastructure and have been chosen by operators because they provide high access rates without the investment required by point to point fibre access networks. FIG. 2 shows the topology of PON access networks used in these systems.
As it is shown in FIG. 2, MxN Customer Premises 2 are connected to a unique Optical Line Termination (OLT) 4 located in an OLT chassis 3 at the carrier's Central Office, CO, 1. The transmission medium, the fibre 6, connects the OLT 4 with MxN Optical Network Units (ONUs) 5 located at Customer Premises 2. To do so, fibre access infrastructure has a point to multipoint topology, using (passive) optical power splitters 7a (1:M) and 7b (1:N) to split the optical signal from the OLT 4 into the different ONUs 5. Optical splitting can be done at only one point, but for deployment reasons, optical power splitting is typically being done in two levels. For the first level is used only one power splitter 7a, with one input and M outputs and for the second splitting level, there are M optical power splitters 7b, each of them with one input and N outputs.
These new optical transmission and access infrastructures entail new challenges. As a relatively recent technology, fiber optical networks are still being deployed almost at the same time as their development is progressing. This leads to a continuous adaptation for managing bigger networks each time and so, the reliability of the optical fiber networks becomes a main issue.
As it is unavoidable that some faults appear, the main issue turns to the quick detection, location and repair of the fault to make as short as possible the time of service interruption, but supervising faults in a fibre infrastructure may lead to very high operational cost, especially behind branching elements, such as power splitters in the outside plant (OSP).
In case of optical transport, based on point to point optical links, there are solutions based on reflectometry techniques. These techniques permit the detection of failures (breaks, splices, bad contacts, bends . . . ) by sending optical pulses and measuring the delay of the received echo. This technique works, and it is extensively used by Telecommunications operators. But an optical cable used for high capacity transport links comprise several fibres. If one of those fibres fails, it is necessary to identify which one does, and that means to repeat the reflectometry test for each fibre in the optical cable. Once the failure has been identified, Telecommunications technicians have to register the colour code of the damaged fibre. It is a tedious process and there is no way to do it automatically.
In case of optical access, supervision problem in PON networks is even more complex. Traditional supervision methods based on classic reflectometry, which are suitable for point to point topologies, do not work properly in these type of networks due to their point to multipoint topology. Reflectometric methods consist of the emission of variable width pulses. The impairments (breaks, splices, bad contacts, . . . ) distributed along the transmission medium (copper, coaxial or fibre) create echoes, which are analysed at the emission point. The delay between the pulse and its echo permits to estimate the impairment (failure) location while the analysis of the echo waveform and spectrum helps to determine the type of impairment which has caused the echo. For this analysis, the system has a time domain/frequency domain analyzer.
These techniques are also applied in PON access networks, as it is shown in FIG. 3. There is an optical signal generator 300a which generates pulses at wavelength λ0 (said signal generator has a transmitter 310a and a time domain/frequency domain analyzer receiver 311a). These optical pulses are injected into the PON network using a circulator 320 and an Automated Optical Distribution Frame 8. The Automated Distribution Frame 8 permits that the same reflectometry based supervision system can be shared between the multiple PON trees deployed from the same Central Office 1. The received echoes are sent by the optical circulator 320 to the analyser 311a. 
But due to their point to multipoint topology, where the optical signal at wavelength λ0 emitted by the light source 300a reaches all the ONUs 5, it is not possible to univocally identify the point where the impairment is located. The received echoes generated by impairments in different PON branches can overlap if different impairments located at different branches are at the same distance from the CO 1. So, it is possible to determine the distance from the central office where the impairment is, but it is not possible to identify the branch of the PON network where this impairment is located.
Hence, with the existing supervision systems is not possible to unambiguously determine an impairment position, the supervision system does not help to reduce optical access network Operating Expenses.
Some solutions have been developed in the prior art for impairment location disambiguation:                Patent Application WO2012126738 discloses the usage of narrowband optical filters centred at different wavelengths for each customer. It is a complex approach because the operator needs to use a lot of number of narrowband optical filters, either Fibre Bragg Grating (FBG) or Thin Film Filters (TFF), each one centred at different wavelength, to identify each branch. So, as many different filters as branches are in the point to multipoint optical access network which imply a huge cost.        In other cases, solution consists of polarization makers situated in each branch of the point to multipoint tree, each one producing a unique Polarization Dependent Loss (PDL). So echoes generated by each marker will be different from the remainder allowing a univocal branch identification (as disclosed in Patent U.S. Pat. No. 6,396,575). This solution entails a detail analysis and inventory of the PDL introduced by each polarization markers used in the optical access network.        
Aforementioned solutions are complex because they entail a tight control by means of inventory of either the narrowband filters or the polarization markers assigned to each branch of each PON network. This tight inventory control entails the risk of monitoring and maintenance errors.
Hence there is a need for an efficient point to multipoint optic fibre supervision system which solve the drawback presented by the prior art systems.
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