Design a one-mile interbuilding example

What you should be able to do

Evaluate a fictional one-mile interbuilding link using a published channel-loss limit, a separate receiver-power check and documented project assumptions.

Service And Route

This is a separate 1 Gb/s exercise, not a replacement for lesson 323's 10 Gb/s requirement. Two buildings need a duplex Gigabit Ethernet connection. One international mile is 5,280 ft or 1.609344 km. Assume a complete 1.700 km channel in each direction after additional routing, slack and patch cords. The additional 90.656 m is an invented exercise input, not a prescribed allowance.

Consider Cisco GLC-LH-SMD modules with compatible host ports and G.652 single-mode fiber. Actual host and software support remain to be confirmed. The two fibers provide opposite transmission directions; they are not two physically diverse routes.

Documented Optical Limits

Cisco's Gigabit SFP data sheet lists LX/LH reach of 10 km on single-mode fiber and a 6 dB maximum channel insertion-loss entry for G.652. It separately lists transmit power from -9.5 to -3 dBm and receive power from -20 to -3 dBm. Both channel-loss and receiver specifications matter.

Subtracting minimum receive power from minimum transmit power gives 10.5 dB. That number does not replace the published 6 dB channel entry. The sheet notes that additional loss may be allowed at shorter distances; this exercise does not invoke that possibility or invent an alternate approved limit.

Assumed Loss Inventory

Use these classroom values at 1310 nm: Fiber: 1.700 km at 0.40 dB/km. Mated connections: four at 0.50 dB each. Splices: two at 0.10 dB each. Additional design reserve: 3.00 dB.

A mated connection is counted once, not once for each connector half. Confirm the inventory and reference boundaries in a real channel. These allowances are not universal component limits or measured acceptance results.

Worked through

Fiber loss = 1.700 × 0.40 = 0.68 dB. Connection loss = 4 × 0.50 = 2.00 dB. Splice loss = 2 × 0.10 = 0.20 dB. Estimated physical loss = 0.68 + 2.00 + 0.20 = 2.88 dB. Loss plus reserve = 2.88 + 3.00 = 5.88 dB. Remaining space below the selected 6.00 dB ceiling = 0.12 dB.

The assumed length is below the documented reach and the calculation fits the selected ceiling. The remaining space after reserve is small. Preserve the reserve as a declared design assumption; do not describe it as physical loss already measured in the cable.

Receiver Screening

At minimum transmit power with the estimated physical loss: -9.5 dBm - 2.88 dB = -12.38 dBm. If the entire reserve becomes additional loss: -9.5 dBm - 5.88 dB = -15.38 dBm. Both values are above the stated minimum receive level.

For the high-power side, this module's maximum launch power equals its maximum receive input. A passive nonnegative channel loss does not raise received power above that launch value. Verify the complete specifications and actual receive levels rather than copying this conclusion to other optics.

Original Change Case

The route gains one additional mated connection at the assumed 0.50 dB: 2.88 + 0.50 + 3.00 = 6.38 dB. The result exceeds the selected ceiling by 0.38 dB.

Flag the change for design review. A raw 10.5 dB power difference is not permission to accept it. A revised design could change the topology or use evidence-supported component specifications, but the apprentice should not simply erase the reserve or substitute a favorable measurement from another link.

Interbuilding Checklist

Before approving construction, the responsible team must address:

• Exact service rate and compatible hosts, ports and software.
• Appropriate optical interfaces, connector arrangements and polarity.
• Complete route and channel-length evidence.
• Outdoor cable suitability and indoor rating/transition details.
• Entrance sealing and any required firestop system.
• Conductive components requiring bonding review.
• Route rights, permissions, access and restoration responsibilities.
• Required tests, limits, measurement records and acceptance authority.

This checklist identifies coordination topics; it is not a complete installation procedure. A successful optical calculation does not approve a trench, building penetration or work on another owner's facilities.

Knowledge Check

Where beginners go wrong

Paper Exercise

Recalculate the one-mile case after two additional mated connections at 0.50 dB each, retaining the full 3.00 dB reserve. State the physical estimate, design total and disposition.

Answer: Physical loss becomes 3.88 dB; the total becomes 6.88 dB, 0.88 dB over the selected ceiling. Refer the revised design for review rather than deleting reserve.

Sources