Four-Building School Backbone: Fiber Type, Count, and Closeout
A school district is adding a 10G backbone from the MDF to four IDFs. Distances are 380 m, 620 m, 910 m, and 1.2 km. Each IDF has a 48-port access switch with two 10G SFP+ uplinks configured as an LACP pair. The district standard requires 50% fiber spare, 25-year life, and support for 40GBASE-SR4 or 100GBASE-LR4 in the future. The existing pathway is a 1 in conduit with two existing 12-strand OM1 cables; a new conduit would require a road cut and a 6-week permit. The project has 8 weeks to substantial completion. A vendor quotes 12-strand OM4 indoor/outdoor cable with a 6-week lead time; OS2 12-strand is in stock. The IT director wants OM4 because 'it is cheaper.' The PM asks you to define the fiber type and count and the documentation required for acceptance.
1. Determine the fiber type and strand count for each IDF route. Show the reach and count reasoning.
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Fiber type: OM4 cannot serve this backbone. 10GBASE-SR over OM4 is limited to 400 m, so the 620 m, 910 m, and 1.2 km IDFs fail immediately; only the 380 m IDF is within reach, and even that has no margin. 40GBASE-SR4 is worse, at 150 m over OM4. OS2 supports 10GBASE-LR to 10 km, 40GBASE-LR4 to 10 km, and 100GBASE-LR4 to 10 km, so OS2 meets both the current 10G requirement and the future 40G/100G requirement. Strand count: each IDF needs two 10G uplinks, and each duplex uplink uses two fibers, so 4 fibers are active. A 50% spare requires 2 more fibers, for a minimum of 6. The next standard count is 12 strands, which gives 4 active plus 8 spare, or 200% spare. A 12-strand OS2 cable per IDF is therefore the minimum sensible design; it also supports six future 100GBASE-LR4 duplex links or three 40GBASE-LR4 duplex links. Use LC UPC unless the selected optics require APC. Do not use OM4 merely because the cable price is lower; the cost of failing three of four routes is far greater than the cable delta.
2. The schedule is 8 weeks, but OM4 has a 6-week lead time and a new conduit needs a 6-week permit. How would you manage the procurement and pathway risk?
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Separate the technical decision from the schedule pressure. OS2 is in stock and is technically required, so release the OS2 order immediately and submit the optical specifications, connector type, and test plan for approval. Do not order OM4 to save a few dollars and then fail three routes. For the pathway, calculate the existing 1 in conduit fill with the two existing 12-strand OM1 cables plus the new 12-strand OS2. If fill, bend radius, or jam ratio is unacceptable, start the road-cut permit on day one as a parallel risk item, not after the cable arrives. Consider pulling the new OS2 through the existing conduit only if the fill and pull tension are acceptable; otherwise use the permit path or a directional bore. Sequence the work so the 380 m IDF, which could have used OM4, is still built as OS2 to keep one standard. Add a contingency in the schedule for the permit and for cable testing. Issue a written change order if the permit or pathway work changes cost or date. Hold a weekly procurement and construction coordination meeting, and do not let the 8-week target drive a non-compliant fiber choice.
3. What documentation and acceptance evidence should the district require before closeout?
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Require a closeout package that matches the design and supports future moves. It should include: approved submittals and a final as-built drawing showing each route, cable type, strand count, connector type, splice locations, and pathway; reel and lot numbers; TIA-606 labeling for cables, panels, ports, and IDF terminations; endface inspection and cleaning records for every connector; OLTS insertion-loss results at 1310 and 1550 nm for every fiber, with pass/fail against the 10GBASE-LR and future 100GBASE-LR4 budgets; bidirectional OTDR traces with launch and receive fibers for each route; splice loss records; and a test-equipment calibration record. The district should also receive the warranty, a spare-parts list, and a fiber assignment table showing active and spare strands. Acceptance should be against the agreed loss budget, not merely 'link lights are up.' Require a punch-list walk and a final test report signed by the installer and the district's representative. For a 25-year life, also require that the as-builts be delivered in both PDF and editable format so the district can maintain the records.