Campus 10G Backbone: Singlemode vs Legacy Multimode
A regional university wants a 10GBASE-LR backbone linking its data center to three campus buildings. The longest route, data center to the science building, is 2.4 km of outside plant; the other two are 0.9 km and 1.6 km. The existing 100 mm conduit has two spare 32 mm innerducts, but one is occupied by a legacy 12-fiber OM2 cable that terminates in each building's telecom room. The university's standard is 12 duplex fibers per building plus 30 percent spare, all terminated in LC patch panels. Facilities will not allow aerial construction, and the only new conduit path crosses a steam tunnel that requires a confined-space permit. The IT director wants a single fiber type across the whole backbone to simplify spares. Your firm must deliver a design and loss-budget justification before the capital request is due in six weeks.
1. Compute the worst-case optical loss for the 2.4 km data center-to-science-building link and state whether 10GBASE-LR is supported. Assume three fusion splices, one mated LC pair at each end, and TIA-568 values of 0.4 dB/km at 1310 nm, 0.1 dB per splice, and 0.75 dB per mated connector pair.
Read the worked answer
Fiber loss: 2.4 km × 0.4 dB/km = 0.96 dB. Splices: 3 × 0.1 dB = 0.30 dB. Connectors: 2 mated pairs × 0.75 dB = 1.50 dB. Subtotal: 2.76 dB. Adding a 3.0 dB design margin gives 5.76 dB. 10GBASE-LR has a 6.2 dB power budget, so the link passes by only 0.44 dB. That is technically compliant but operationally thin: any extra patch-panel pair, a poor splice, or a dirty connector can consume the margin. The design should specify a measured OLTS result, end-face inspection, and a maximum of two mated pairs per link. Because the route is 2.4 km, distance is not the limiting factor; connector and splice discipline is.
2. Recommend a fiber type for the entire backbone and explain how the existing OM2 cable should be handled. Justify the choice with reach and bandwidth reasoning.
Read the worked answer
Use OS2 singlemode everywhere. 10GBASE-SR over OM4 is limited to about 400 m, so it cannot serve the 2.4 km span; OM2 is limited to roughly 82 m at 10GBASE-SR and cannot be reused for the new backbone. Singlemode with 10GBASE-LR gives 10 km reach and supports later 10GBASE-ER or coherent upgrades. Do not mix the legacy OM2 into the new 10G channels. Mode-field mismatch and differential modal delay make mixing multimode grades unpredictable, and the OM2 cable has low modal bandwidth. Leave it in place as a separate legacy asset, label it clearly, and use it only for existing low-speed services. If the university wants one spares kit, standardize on LC singlemode connectors, OS2 patch cords, and 1310/1550 nm test equipment.
3. Determine the fiber count per building and describe how the steam-tunnel permit affects the design and schedule.
Read the worked answer
Required duplex channels: 12 × 1.30 = 15.6, rounded up to 16 duplex channels, or 32 fibers per building. A 36-fiber cable provides 18 duplex channels and is a practical standard sheath; a 48-fiber cable is better if the capital request can absorb it, because it covers future building growth without a second pull. The steam tunnel is both a design constraint and a schedule risk. A confined-space permit typically requires a hazard review, atmospheric testing plan, and an escort, and the tunnel may prohibit cable that is not plenum- or riser-rated for the environment. Design a route that avoids the tunnel if a viable duct path exists; if not, specify a suitable jacket and include the permit lead time in the schedule. Submit the permit application immediately, because a four-to-six-week approval can sit on the critical path and block the capital request.