Optical fibre
The cost of an optical link is decided in the trench, not in the fibre.
- Parent division
- Networks and communications
The problem
Fibre is cheap; the route it takes is not.
Optical cable is a modest share of the cost of a link. The spend sits in the civil works, the wayleaves, the road crossings, the chambers and the connection points.
That is what makes the route survey decisive. A route settled without a ground survey meets an obstacle that was not on the plan, and the rework costs more than the survey that was saved.
Then comes acceptance. A link is accepted by measurement, with the records handed to the client. Without them, a link that works on the day it is laid and degrades within six months can be attributed to nobody, because nothing records its initial state.
What decides a link’s cost and its initial state, schematic
- 01Route survey
- On the ground, before any order. A route settled on plans meets the obstacle that was not on them.
- 02Permissions
- Wayleaves and road crossings. They govern the schedule far more than the installation does.
- 03Civil works
- Trenching, ducts, draw chambers, reinstatement. This is where the spend sits.
- 04Pulling and splicing
- Slack coiled at jointing points, optical distribution frames, systematic labelling of every strand.
- 05Optical acceptance
- OTDR measurement in both directions. Without it, no initial state exists.
- 06Allocation plan
- Strands in use, spare strands, fault location procedure and response time.
Scope
What the service covers
- Route survey
- Ground survey, underground or aerial decided, crossings and obstacles identified, predicted optical budget, and the wayleave application file.
- Civil works
- Trenching, ducts, draw chambers, road crossings, backfill and reinstatement, warning tape and marking of the works.
- Installation and splicing
- Cable pulling or blowing, slack coiled at jointing points, fusion splicing, optical distribution frames, systematic labelling of every strand at both ends.
- Optical acceptance
- OTDR measurement in both directions, power budget, every splice and connector checked, and handover of records and traces.
- Campus and site interconnection
- Links between buildings and between sites, connection to active equipment, optical modules chosen from the distance and the measured optical budget.
- Documentation and operation
- Optical network plan, strand allocation, spare strands, fault location procedure, and an agreed response time.
Situations
The most frequent situations
- Link between buildings on one site
- Distances of a few hundred metres, internal civil works constraints, and often existing ducts whose real occupancy is unknown.
- Connecting a remote site
- Long distance, exposed route, multiple permissions. The optical budget and the choice of regeneration points govern feasibility.
- Extending a carrier network
- Installation along an existing route, compliance with the host network’s engineering rules, acceptance to the operator’s requirements.
- Recovering a degraded link
- The link works with a rising error rate. OTDR locates the splice or connector at fault, which avoids replacing the whole run.
Requirements
What to require, of us as of anyone
These requirements hold whichever supplier is appointed. Written into a tender, they filter out the responses that will not hold.
- A ground survey before the route is settled, rather than a route settled on plans alone.
- The predicted optical budget, then the measured one, compared at acceptance.
- OTDR traces in both directions, handed to the client and kept as the link’s initial state.
- Strand labelling at both ends and the allocation plan, without which any later intervention begins with identification work.
- Spare strands, sized for foreseeable needs rather than for the need of the day.
- Reinstatement of roads and ground to their previous state, accepted separately.
- The fault location procedure and the response time, written before the link enters service.
Pitfalls
Common mistakes, and what they cost
- Laying only the strands strictly needed
- The saving falls on the cable, which is the cheapest part. The day one more strand is needed, it is the trench that has to be reopened.
- Accepting without measurement
- A link that “works” may have no optical margin left. It will fail at the first ageing splice, and with no initial state, nobody will be able to say what changed.
- Ignoring bend radius
- Fibre pinched in a chamber or behind a frame loses power and eventually breaks. The defect is invisible to the eye and perfectly visible on an OTDR trace.
- Leaving permissions until last
- Road crossings and wayleaves govern the schedule far more than the installation does. Started late, they hold up a project that is otherwise ready.
Questions
Questions asked before consulting
Underground or aerial?
Underground protects better and lasts longer, at the price of civil works that dominate the budget. Aerial costs less and goes in faster, but is exposed to weather, vehicles and third parties working on the poles. The choice is made section by section, from the real exposure and the poles available.
Single-mode or multimode?
Single-mode covers long distances and places no practical ceiling on future bandwidth; it has become the norm between buildings and between sites. Multimode retains some value over very short runs inside a technical room. For infrastructure meant to last, single-mode avoids having to decide again in ten years.
What does an OTDR measure?
It sends a light pulse and analyses what returns, which locates every splice, connector and fault to within a few metres, together with the loss each introduces. It is run in both directions, because a splice can read as sound from one end and faulty from the other. It is the only measurement that documents a link’s initial state.
How long does an optical link last?
The cable itself is measured in decades. What ages is the termination: dirty connectors, frames handled repeatedly, splices redone after an accidental break. Effective life therefore rests on the quality of the civil works and on operating discipline, more than on the fibre.
Can existing ducts be used?
Often, provided their occupancy and continuity are verified by rodding before the route is settled. A duct believed clear and blocked halfway turns a simple installation into a civil works project, discovered at the worst moment.
How long to restore a cut link?
It depends on the location, on access to the break point and on the availability of a splicing team. It is contracted before the link enters service, with the matching means: measurement equipment available, compatible cable in stock, and a named on-call contact.
Neighbouring subjects
Solutions concerned
Consultations · Pre-qualifications · Partnerships
Let us discuss the actual case.
Describe the site, the dominant constraint and the deadline. We will say what requires a preliminary study and what can be committed directly.