Match five scenarios to the right optic

short · 25 min · Objective 1.5

Task

Choose the correct transceiver and connector for five link requirements, then check your answers against the arithmetic. Selection questions are most of what objective 1.4 asks, and the selection is driven by three numbers: distance, speed, and fibre type.

Steps

  1. Write your answer for each scenario before checking anything. Name the form factor, the standard, and the connector.
  2. A. 40 m between two switches in the same room, 10 Gbps, cheapest option.
  3. B. 2 km across a campus, 1 Gbps, existing single-mode fibre.
  4. C. 300 m between two floors, 10 Gbps, existing OM3 multimode.
  5. D. 25 m to a top-of-rack switch, 10 Gbps, inside one cabinet.
  6. E. 80 km to a second site, 10 Gbps, single-mode.
  7. For each, state which of the three numbers ruled out the alternatives.
  8. Note the connector each answer implies: LC for most modern optics, SC on older equipment, MPO/MTP for parallel 40/100 Gbps.
  9. Now state, for each, what would happen if you fitted the wrong one — it fits the cage in every case, which is the point.

Verify

python3 -c "
ans = {
 'A': '10GBASE-SR SFP+ over OM3/OM4 multimode, LC -- 40 m is inside the 300 m limit',
 'B': '1000BASE-LX SFP over single-mode, LC -- 1 Gbps, 10 km reach',
 'C': '10GBASE-SR SFP+ over OM3, LC -- 300 m is exactly the OM3 limit; OM4 gives margin',
 'D': 'Direct attach copper (DAC) SFP+ twinax -- no optic at all, cheapest inside a rack',
 'E': '10GBASE-ZR SFP+ single-mode, LC -- ER reaches 40 km, ZR reaches 80 km',
}
for k, v in ans.items():
    print(k, v)
"

Compare line by line. Scenario D is the one most people miss: inside a single cabinet the right answer is not an optic at all.

Notes

The failure mode worth remembering is that a module that fits is not a module that works. Mode mismatch, wavelength mismatch and vendor lock all present as a dead link with no cable fault, and lesson 40 shows you the digital diagnostic readings that tell them apart.

OM3 at 300 m and OM4 at 400 m for 10GBASE-SR are the two distance numbers most worth committing to memory.