Faults, maintenance and upgrades — Fibre Optics Practice

Sample questions

1. 01A link carries traffic but the equipment logs errors, and the loss test result looks normal. Which group of causes does that pattern point to?

   • ADirt, a tight bend, a poor splice or a small mismatch
   • BReflections, or too much or too little receive power
   • CA break, a disconnected fibre or a very large mismatch
   • DDamage done to the whole cable during the original pull

   Why: A link that runs with errors while its loss looks fine points to reflections, or too much or too little power at the receiver. Polarity is tempting, but uncrossed pairs give a different picture: the light gets through and the loss is fine, yet the link never comes up at all. Dirt, bends, poor splices and small mismatches show up as high loss, which this link does not have. A break, a disconnected fibre or a very large mismatch gives no light at all, and damage to the whole cable shows as high loss or breaks on every fibre.

2. 02An old OM1 plant has 62.5 µm fibre. A technician patches in a 50 µm cord, so light passes from the 62.5 µm fibre into the 50 µm cord at one joint. How much loss can that joint add?

   • AUp to about 4 dB at that single joint
   • BAround 20 dB, which is most of the light
   • CAround 0.3 dB, the same as a normal pair
   • DNone, as both fibres share a 125 µm cladding

   Why: Going from 62.5 µm into 50 µm fibre can cost up to about 4 dB per joint, which is why an OM1 plant needs 62.5 µm cords and reference cords. Around 20 dB is the figure for a joint between singlemode and multimode. A matched mated pair is the 0.3 dB case, and sharing a 125 µm cladding does not make the cores match.

3. 03To bridge a gap, an installer tries to join two backbone trunks through an MPO adapter. Both trunks end in pinned MPO connectors. What will happen?

   • AThey will not mate, as one side has to be unpinned
   • BThey will mate, provided the two ferrules are both APC
   • CThey will mate, but the joint will lose about 20 dB
   • DThey will mate, as long as both are the same polarity type

   Why: A mated MPO pair needs pins on one connector and holes on the other. Installed trunks normally end in pinned connectors and patch cords have none, so two pinned trunks will not go together at all. Polarity type decides which positions the light reaches, not whether the connectors fit, and neither the polish nor the 20 dB singlemode to multimode figure has anything to do with pins.