Design a five-mile backbone example

What you should be able to do

Evaluate a five-mile route, identify a reserve shortfall and develop a conditional revision without hiding the assumptions that make it pass.

Project Inputs

This independent classroom scenario requires 1 Gb/s duplex Ethernet. The route is five international miles: 5 × 1.609344 = 8.04672 km. Assume a full 8.300 km optical channel in each direction. The additional 253.28 m represents invented routing, slack and patch-cord contributions, not a standard allowance.

Consider GLC-LH-SMD modules on compatible hosts with G.652 single-mode fiber. Cisco documents 10 km LX/LH reach and a 6 dB SMF channel insertion-loss entry. The table separately gives transmit power from -9.5 to -3 dBm and receive power from -20 to -3 dBm. The manufacturer requires receiver and channel-loss specifications to be satisfied. This exercise uses the published 6 dB ceiling without asserting any shorter-distance exception.

Initial Design Assumptions

At 1310 nm, assume:

• 8.300 km of fiber at 0.40 dB/km.
• Four mated connections at 0.50 dB each.
• Four splices at 0.10 dB each.
• A project-required 2.00 dB reserve.

All values are classroom inputs. The reserve is not physical attenuation already installed. A mated connection is counted once, not once per connector half.

Fiber loss: 8.300 × 0.40 = 3.32 dB. Connection loss: 4 × 0.50 = 2.00 dB. Splice loss: 4 × 0.10 = 0.40 dB. Estimated physical loss: 3.32 + 2.00 + 0.40 = 5.72 dB. Including reserve: 5.72 + 2.00 = 7.72 dB.

The length is within the documented reach. The estimated physical loss is below 6 dB, but the plan cannot retain its full required reserve within that ceiling. The shortfall is 7.72 - 6.00 = 1.72 dB. That is a design-reserve failure, not proof that the unbuilt link cannot ever pass traffic.

Worked through

Suppose the approved physical layout can remove unnecessary cross-connects and intermediate splice points. The proposed revision assumes two mated connections, each with a supported maximum allowance of 0.20 dB, and two splices at 0.05 dB each. The fiber allowance remains unchanged.

Revised estimated loss: 3.32 + (2 × 0.20) + (2 × 0.05) = 3.82 dB. Including the unchanged reserve: 3.82 + 2.00 = 5.82 dB. Remaining space below the ceiling: 6.00 - 5.82 = 0.18 dB.

This is a conditional design candidate. Confirm that the construction plan can achieve those counts and that product specifications, installation methods and acceptance criteria support the tighter allowances. A splicer's estimated loss alone does not substantiate a completed channel. If the assumptions cannot be supported, this revision is not available merely because its arithmetic works.

Receiver Screening

For the revised physical estimate at minimum transmit power: -9.5 - 3.82 = -13.32 dBm. If the entire reserve becomes additional attenuation: -9.5 - 5.82 = -15.32 dBm. Both are above the stated -20 dBm minimum receive level.

At the other end of the range, the specified maximum launch and maximum receive input are both -3 dBm for this example. Nonnegative passive loss does not raise power above the launch value. Confirm actual equipment and measured levels; different optics can require attenuation. Raw power subtraction does not waive the channel specification.

Original Change Case

The final route adds a required splice enclosure containing one additional splice in each working fiber. Under the revised 0.05 dB per-splice assumption, the per-direction total with reserve becomes 5.87 dB. Remaining space becomes 0.13 dB.

Update the inventory and review the available reserve and evidence. Do not keep the old count because the new total still fits. If another design change creates an unsupported assumption, reopen the review.

Before Construction And Acceptance

Confirm service rate, host/software compatibility, fiber and connector interfaces, polarity and environmental suitability. Resolve cable ratings, entrances, pathway permissions and any conductive-component review. Verify the full optical length and inventory. Establish the required insertion-loss, polarity and other project tests, preserve results and obtain the responsible technical review.

If the conditional redesign is not feasible, evaluate another documented optical solution or an approved topology change. Recheck all limits; do not simply buy a stronger transmitter. This lesson does not authorize either procurement or field changes.

Knowledge Check

Where beginners go wrong

Paper Exercise

Starting with the conditional 5.82 dB total, add two separate splices per direction at 0.05 dB each. Keep the reserve and explain whether this approves construction.

Answer: 5.82 + 0.10 = 5.92 dB, leaving 0.08 dB below 6.00 dB. Only the arithmetic fits; supported allowances, compatibility and the complete design still require review.

Sources