WCA September 2019

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. 3 Operation technology for optical cables quality and maintenance 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 (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 ❍ ❍ Figure 3 : Procedure of water penetration test with new system

connection quality deteriorates when the connector is penetrated for a long period of time. NTT executes optical pulse tests using OTM (Optical Test Module) [2] over a period of 90 days to search for water-penetrated closures and repair them. Figure 1 shows the underground optical cable facilities. OTMs are installed in a central office and water sensor modules [3] are embedded along a fibre in all underground closures for searching for water penetration. Water absorption of a water sensor module causes optical losses of a fibre. The loss in the OTDR waveform points to water penetration. 2.1 Manually operated periodical test The company has three maintenance issues for underground optical cables. Figure 2 shows the conventional maintenance work flow. Firstly, the monitoring operation takes a lot of effort with a periodical test of the huge volume of underground cables. To find a water-penetrated closure, OTMs execute a 90-day periodical OTDR test with the test wavelengths of 1.31, 1.51 and 1.61 µm. The testing of those three wavelengths allows operators to clearly recognise a bending loss by water absorption of the water sensor module. Since the test results are stored in the OTMs, engineers collect the results manually in most central offices where the OTMs have been installed. 2.2 Difficulty of finding location of penetrated closures The second issue is the difficulty in searching a water-penetrated closure. After collecting the test results, engineers search locations of water-penetrated closures by comparing OTDR waveforms with optical distribution diagrams. Finally, they identify a water-penetrated closure and MH. Since finding the location requires skill, sometimes errors occur because there is a fibre length difference between actual lengths shown in OTDR waveforms and those recorded in the facility information database. The difference derives from cable slack in the central office and outside. Considering the background, engineers find the water-penetrated closures according to their experience. However, they say that only 25 per cent of opened closures are the intended water-penetrated closure. That means that they will enter MHs and open closures on average four times 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, 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, even if the latest judgment still shows penetration possibility. meaning that this work is wasted. 2.3 Difficulty of the repair plan

Periodical Test

Sent email

System

System

Collect the test result

Test result flowchart

Pulse test

OTM status

Error

Distance difference from last time

Closure location progress

Not yet

Optical loss

Expire

Planning repair

Last time upgrade

OK

90

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Wire & Cable ASIA – September/October 2019