EuroWire July 2019

Technical Article

(b) Water sensor modules : The modules are installed in every underground optical closure and inserted to the maintenance-use optical fibres. The water sensor module causes optical loss of about 2.3 dB by submerging (c) OTMs : They are installed in a central office and are connected to the maintenance-use optical fibre. The OTMs are composed of ODTR, an optical switch and a controller unit. Periodical testing is carried out by the OTMs to detect penetration of the underground optical cables (d) Server : This is installed in a data centre and connected to all the OTMs and the operation terminals by DCN (Data Communication Network). Thanks to the nationwide DCN, it is possible to operate OTMs remotely from every service centre and the head office 3.2 Function for periodical test The conventional periodic test involves the following procedures. The OTMs automatically perform OTDR tests based on the set cycle (eg 90 days). The OTMs detect a bending loss of 2.3 dB or more and extract it as a candidate of water- penetrated cable. Engineers remotely operate the OTMs and monitor OTDR waveform to confirm water penetration. If the loss of 2.3 dB cannot be confirmed, the operator changes OTDR settings such as test wavelength, pulse width, etc, to clarify water-penetration. If there are many penetrated facilities, the next periodic test will start before the last repair work is completed, so the additional test results accumulate, and the data management becomes complicated. The newly developed system provides functions which summarise periodic tests and shows water-penetrated closures in real time. The summarised result is visually shown for easy understanding and engineers can obtain fresh information remotely and receive an alarm with automatically distributed email. Figure 4 shows the procedure of the water penetration test with the newly developed system. In this system, a test cycle is set up. This was set for every 30 days so that it could be repaired within 90 days. The result includes a bending loss by water sensor module and its location. The new system also pays attention to the fibre length. If the length is two per cent shorter or 0.2 km shorter than the last OTDR waveform, the system displays an alarm, and there is the possibility of a broken cable. There are also two points of logic to enhance the efficiency of penetrated facilities search. Engineers compare the optical distribution diagram with the OTDR waveform,

Estimate

OTDR result waveform

Linking distance

Route information

Distance correction function

Before

Repaired actual distance 800m

After

▲ ▲ Figure 4 : Function of distance correction

even if the latest judgment still shows penetration possibility. As an additional function, the proposed system includes the confirmation procedure of DCN and OTM status. In the conventional system, a network failure could not be detected without checking for OTM accidents. To detect those easily, the proposed system has a function to display the abnormality. From the functions, NTT has tried to improve the efficiency of periodic testing for water-penetration detection.

until finding closures which should be repaired. To confirm the penetration of the closure, engineers not only go into the MH and drain water, but also open the closure. If the opened closure is not water penetrated, it is necessary to replace a gasket and a sleeve in order to close the underground optical closure, meaning that this work is wasted. 2.3 Difficulty of the repair plan The third issue is progress management of repair works. After finding the location of the water penetration, the engineer considers the order of repair work. This is determined from the time of water penetration and the type of splice. Since long-term water penetration degrades the fibre, it is necessary to manage the deadline of the repair completion, but there is no effective tool. If the bending loss is detected at the same facilities as the last time, the previous test result will be ignored and only the last test result will be used as the repair target. In this case, every monthly procedure will be compared to the last estimate, and this will define the status of repair. If the status shows ‘confirmed last time’, then the estimated work can be skipped,

3 Operation tech-

nology for optical cables quality andmaintenance efficiency

3.1 Optical cable maintenance system The components of the optical cable maintenance system are listed below. (a) Maintenance-use optical fibres : One ribbon fibre out of every underground cable is used for maintenance purposes

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July 2019