The fibre in an optical cable is a small part of the cost of the route. Ducts, trenching, poles, permissions and reinstatement are most of it, and they are spent again if the cable laid has too few fibres. Yet cables are routinely specified at the count needed today, on the assumption that another cable can be blown in later. Sometimes it can. Often the duct is full, or the route was direct-buried, and the only answer is to dig. Planning the count at the start is the cheapest decision on the project.
Step 1: count the links actually needed
List every point-to-point connection the route must carry, and count two fibres for each one, since most transmission systems use a separate fibre for each direction. Add fibres for any service that runs on its own pair: SCADA and protection signalling in a substation, CCTV and access control on a campus, the telecom operator’s own network alongside a utility’s. That gives the working count.
Step 2: add spares for faults and moves
A fibre that is broken at a splice or damaged in the closure is repaired by moving to a spare, not by re-laying. Allowing a spare pair for every few working pairs is normal practice, and routes that pass through areas of frequent excavation deserve more. Keep spares as complete pairs, not odd single fibres.
Step 3: allow for growth over the life of the duct
The cable will be in service for decades, and the demand on it will not stay flat. A doubling of the working count over the life of the route is a conservative allowance for most enterprise and utility networks; an operator’s access network may need far more. Because the glass is cheap relative to the route, the difference between a 24-fibre and a 48-fibre cable is small in the purchase price and enormous in what it avoids later.
Step 4: match the count to a cable construction
The count then has to fit a construction that suits the route. For duct, aerial and short direct-buried runs at modest counts, a unitube cable is compact and economical, covering 1 to 48 fibres in a single central tube. For higher counts and long outside-plant routes, a multi-tube loose-tube cable spans 6 to 1152 fibres, with armouring options for direct burial and rodent exposure. Indoors, where the cable is terminated straight onto equipment, a breakout tight-buffer cable from 1 to 24 fibres avoids the splicing and fan-out work that a loose-tube cable would need. Standard counts step in multiples of the tube size, so a requirement for 40 fibres is normally met with a 48-fibre cable.
Step 5: think in routes, not cables
On a ring or a campus, the count on each segment is the sum of every link that passes through it, not just the links that terminate on it. The segment nearest the main equipment room carries the most and is usually where the count is under-estimated. Draw the route, mark every drop, and total the fibres per segment before choosing the cable. The fibre optic cable selection guide covers the choice of construction for each segment once the counts are known.