Infrastructure Engineering · Lesson 21 of 24 · 25 minutes

Separate physical routes, optical budgets and fabric capacity

Two fibres are not automatically two independent services, and a low optical loss is not automatically an application pass. Separate the physical administration route from the logical fabric, then calculate a narrow loss budget and post-uplink-loss capacity. The lesson adds communications reasoning without pretending that the existing electrical simulator already models packets.

Learning objectives

• Distinguish distribution spaces from switching functions.
• Calculate optical loss plus an explicit engineering margin.
• Evaluate post-failure uplink capacity and common route dependencies.

Predict → run → explain

1. Read the declared boundary and units. Predict the selected result before calculating.
2. Run the starting case and inspect the ledger, assumptions and model limits.
3. Apply the challenge, run again, and explain the difference. Export only the last completed inputs and results.

Physical dB = length m/1,000 × dB/km + connectors × connector dB + splices × splice dB; allocation dB = physical dB + engineering margin; remaining dB = budget − allocation; ratio = downlink Gb/s / active uplink Gb/s. With zero active uplinks, the model is disconnected and ratio is unavailable.

Declared starting inputs

{
  "lengthM": 100,
  "dbPerKm": 3.5,
  "connectors": 4,
  "connectorDB": 0.35,
  "splices": 2,
  "spliceDB": 0.1,
  "engineeringMarginDB": 0.5,
  "budgetDB": 2.6,
  "downlinks": 48,
  "downlinkGbps": 25,
  "uplinks": 6,
  "uplinkGbps": 100,
  "lostUplinks": 0
}

Challenge

Does one uplink failure change the optical budget of the remaining channel?

{
  "lostUplinks": 1
}

The line-rate ratio becomes 2.4:1. The separately declared optical channel stays at 2.45 dB allocation and 0.15 dB remaining margin.

A physical route beneath the logical fabric — static equivalent

1. Reveal entrance → distribution spaces → racks on a physical floor plane.
2. Lift the logical leaf–spine fabric above it without merging the identities.
3. Inspect each optical loss event and a separate margin reservation.
4. Remove one uplink and recalculate the capacity ratio; expose shared trays separately.

Separate physical-route and logical-link tables plus optical-loss and line-rate ledgers.

Toggle physical/logical layers, remove uplinks with keyboard controls and inspect each loss term.

Narrative steps are not physical seconds. Essential states and quantities remain available without animation.

Assessment and answer criteria