Pattern FO-A

Teleprotection between two substations

The optical budget and the delay budget are calculated together, because the link is part of the protection.

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Area: Fiber & OT
Environments: 4
Products used: 12

How it is built

Data path
Path that does not exist

The link is part of the protection, not a service to it. Delay and symmetry decide.

Used in these environments

Substation with teleprotectionDelay and channel asymmetry decide whether the protection trips correctly — the link is part of the protection scheme, not a service to it.
Railway signalling installationVibration and a 40-year installation lifetime set the requirements, not the data rate.
Link between buildings over public landThe operator’s later cross-connects eat your budget after handover, which is why the acceptance measurement matters.
Wind farmLong loops and lightning exposure, with maintenance access measured in hours of travel.

How it works

A protection relay at each end, a converter at each end, and a fiber path between them. The link is part of the protection scheme, not a service to it.

Delay and symmetry are different requirements

Transfer tripping needs low delay. Line differential protection additionally needs the delay to be the same in both directions. The relay estimates one-way delay as half the round trip — it assumes symmetry rather than measuring it, so asymmetry never appears as a fault. It appears as a phase error, and the protection makes a wrong decision with no alarm.

Where the asymmetry comes from

A transport network that protection-switches one direction onto a longer path. The switch is a success from the network’s point of view and a fault from the protection’s. This is why a dark fiber pair with a known, symmetrical route is worth more here than bandwidth.

The budget has to be calculated

Span attenuation, connector pairs, splices, an ageing margin and a repair margin. A quoted “typical distance” is not an engineering figure, and the difference between 1310 nm and 1550 nm is about 26 dB over 200 km.

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