Campus Backbone Design/Build: 25-Year Life, 16-Week Lead Time, Late Change

Six strands is a first-cost design, not a 25-year design. Two strands light a 10G link, leaving four spares per building, which sounds adequate until you count a fiber failure, a re-route, and the 100G upgrade. 100GBASE-LR4 uses four wavelengths over a single pair, so OS2 at 1310 nm comfortably reaches 10 km and does not force a re-pull, but 100GBASE-SR4-class parallel optics would need eight strands per link, and a future parallel or redundant topology would consume the spares quickly. I would specify 24 strands per building: 12 lit and 12 spare, giving room for redundancy, testing, and future links without another construction project. The star topology from the core is also a single point of failure: a cut on one duct run takes a building offline. Recommend route diversity or dual homing for at least the two critical buildings, even if it means using both spare conduits for fiber and finding another path for the medium-voltage feeder. Never share a conduit with a medium-voltage feeder. Present the strand-count change as a cost comparison against a future re-pull, which is the number that actually wins the argument.

Put the long-lead item on the critical path at day one. Cable ordered at notice to proceed arrives at week 16, so it must be released with a letter of intent before final submittals are approved, or the project cannot finish. Duct repair runs weeks 0 to 3 and is off the cable critical path but becomes the constraint for pulling, so it must be complete and inspected by week 16. Pull, splice, terminate, and test occupy roughly weeks 16 to 20, acceptance and closeout weeks 20 to 22, which leaves zero float. That is not a schedule I would sign without mitigation. Options to present: shop pre-terminated or pre-connectorized assemblies to shorten field work, phase the pulls building by building so testing starts earlier, and confirm which conduit the collapsed section affects, because if it is the one selected for fiber the repair becomes critical immediately. Identify the 16-week cable as the single schedule risk and require weekly supplier confirmation. If the owner will not accept zero float, the honest answer is to move substantial completion or accept a temporary link for the first buildings.

Technically the distance is not the problem: 2.4 km is well inside the 10 km specification for 10GBASE-LR, and a loss check at 0.4 dB/km gives about 0.96 dB of fiber loss plus connectors and splices, which leaves ample margin, and 100GBASE-LR4 would also work later. The real work is elsewhere. A new 2.4 km route needs a route survey, right-of-way or easement confirmation, and permitting, which is frequently the true schedule driver, not the cable. It changes the strand count, the duct capacity calculation, the splice closure count, and possibly the topology if the building should be dual-homed. Price it as a formal change with a design review rather than accepting a verbal add, and state the schedule impact in weeks. Check whether the new route can share the existing duct bank and whether the spare conduit reserved for medium voltage is affected. Finally, re-run the loss budgets and the acceptance criteria for the added link and issue a revised as-built scope, so the owner is not accepting a design that was never reviewed.