Fiber Optic Cable Planning: 22 km in Stock, No Way to Make 10 km

Here is a puzzle that fiber optic cable plants run into more often than they admit. The warehouse holds 22 km of fiber. A customer orders 10 km of cable. The stock report says yes. The shop floor says no.

The reason is simple once you see it: fiber is not a liquid you can pour from any container. It lives on individual spools, and a cable needs each fiber to run in one continuous piece from end to end. Two spools of 6 km each are not one 10 km spool. This article explains why, and how smart spool allocation turns "impossible" back into "shipped on time".

Why fiber can't simply be joined

In copper cable, a joint in a conductor is sometimes acceptable. In optical fiber, it almost never is inside a finished cable. Every splice adds signal loss (attenuation) and a mechanical weak point, and customers — telecom operators especially — specify continuous lengths and attenuation limits in their contracts. So for fiber optic cable manufacturing, the rule of thumb is: one fiber in the cable = one continuous piece from one spool.

That changes the planning question. It is no longer "how many kilometers do we have?" but "which spools can each individual fiber come from?"

A 24-fiber cable is really 24 material requirements

A cable with 24 fibers of 4 km does not need "96 km of fiber". It needs 24 separate continuous pieces, each slightly longer than 4 km to cover process allowances and the extra fiber length that sits loosely inside the tubes.

Say each piece must be 4.05 km, and your fiber arrives on 25 km spools. Each spool gives you six pieces (24.3 km) and leaves 0.7 km. Four spools cover all 24 fibers, with 2.8 km left over in total. Very efficient.

Now the customer asks for 4.3 km instead. Each 25 km spool now yields only five pieces (21.5 km) and leaves 3.5 km. Four spools give you 20 fibers; you need a fifth spool for the last four, which leaves another 7.8 km behind. A 6% longer cable needed 25% more spools and left piles of remnants.

Nothing about the fiber changed. Only the arithmetic of cutting it did.

The allocation puzzle: 11, 7 and 4 km

This is a classic problem that mathematicians call cutting stock — the same one a carpenter faces cutting shelves from boards. Here is a small version.

You have three spools: 11 km, 7 km and 4 km — 22 km in total. Two orders arrive: first one needing a 6 km continuous length, then one needing 10 km.

The intuitive way. The 6 km order arrives first, and the operator takes the biggest spool "to be safe": 11 km. Now you have 5, 7 and 4 km left. The 10 km order arrives — and no spool is long enough. You have 16 km of fiber and cannot make the cable.

The smart way. Give the 6 km order the shortest spool that fits: 7 km, leaving 1 km. Keep the 11 km spool for the 10 km order. Both orders ship, with 1 km + 1 km + 4 km left over.

Same stock, same orders, opposite outcome. The only difference was looking one order ahead.

Three rules planners use

1. Shortest spool that fits — as a starting point. It protects long spools for long orders. But it is a rule of thumb, not a law: fiber type, availability dates and the orders coming next week can justify a different choice.

2. Look at the order book, not just today's order. A long spool is valuable only if there is long-length demand coming. Before using one for a short order, check whether it is the only possible source for a firm order in the next weeks.

3. Treat remnants as a cost. A 1.2 km remnant is fine if you regularly sell 1 km patch lengths. It is near-scrap if your shortest standard order is 2 km. Review remnant stock against your real order mix, the same way you would review any slow-moving stock with safety stock and reorder point thinking.

Where the plan usually breaks

Most spool problems are not calculation problems. They are record problems:

• The system shows a spool as 7 km, but 0.4 km was cut for a test and never booked.
• A reserved spool was "borrowed" to keep a line running, and the order it was reserved for silently lost its only source.
• Jobs were combined into one production run, and the combined quantity hid the fact that each order still needs its own continuous lengths.

Each of these makes an order look feasible on paper right up until the colouring or stranding line stops. The cure is to track every spool as its own identity — its own batch, with its own remaining length and location — and to update it at every cut. The same discipline pays off when a customer asks which fiber lot went into which cable, as described in batch traceability.

How this fits the bigger cable picture

Spool allocation is one of three length-and-packaging constraints in cable production. The other two are drum capacity — covered in cable drums and containers — and the multi-stage flow of drawing, stranding, sheathing and testing, covered in cable manufacturing planning. They interact: the right fiber on the right spool still has to fit onto the right drum at the end.

How factory.online helps

In factory.online, each fiber spool can be tracked as a separate batch with its own remaining length and location, so a planner sees "11, 7 and 4 km" instead of "22 km". Material requirements from BOMs, stock movements and production confirmations keep those lengths current as fiber is cut, which is exactly what spool-by-spool allocation needs.

If your plant has ever had enough fiber and still missed a delivery, bring that order to a demo — it makes a far better test case than a clean example.