EuroWire July 2019

Technical Article

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

is continuously required. Furthermore, it is planning to continue using legacy copper cables for providing the IP service, so reducing the maintenance and management cost of copper cables is an important issue. It has constructed optical fibre networks, overlaid on copper cable networks, and is facing the task of realising efficient maintenance for the telecommunications equipment. It has intensively developed operation systems for optical network construction, while on the other hand, the maintenance operation system has not proved efficient enough. This paper describes the maintenance system which promotes efficiency of operation for the water penetration in the underground optical closure. underground optical cable maintenance NTT has taken measures against water penetration by installing optical cables and closures with water-blocking functions. Whilst using several solutions for water penetration, ageing degradation and cable creeping can cause water penetration into the closures. In some underground sections, it has adopted an MT (Mechanically Transferable) connector for switching cables easily. Periodic water penetration detection is necessary, because the 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 2 Issues for

▼ ▼ Figure 3 : Procedure of water penetration test with new system

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

Optical loss

Not yet

Expire

Planning repair

Last time upgrade

OK

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July 2019