OFC link budget: how to know your fibre run will work before you blow the cable
Optical fibre link budget math, without the RF-physics pain. Transmitter power, receiver sensitivity, connector loss, splice loss, and the safety margin you actually need. With a worked 12 km example and the OTDR trace you'd expect to see.
The link budget formula
Three numbers you need from the SFP spec sheet:
- Tx power — how much light the transmitter sends out (dBm, usually negative) - Rx sensitivity — the minimum power the receiver can hear (dBm, very negative) - Budget — Tx minus Rx, the total loss the link can tolerate
Add to that the losses you'll incur along the cable:
- Fibre attenuation — typically 0.35 dB/km at 1310nm, 0.25 dB/km at 1550nm - Connector pairs — each pair (one at each end of a patch panel) is 0.3–0.5 dB - Fusion splices — 0.02–0.05 dB each - Mechanical splices — 0.1–0.3 dB each (use only for emergency repairs) - Bend loss — variable, can be huge if the cable is bent too tightly
Total loss < Budget − Margin is the rule. The margin you want is 3 dB minimum, more for long runs or critical links.
A worked example
A 12 km OFC run between two buildings, with one splice joint in the middle (because the cable is supplied in 6 km drums).
SFP: 1G LX, 1310nm - Tx power: -9 to -3 dBm (worst case -9) - Rx sensitivity: -20 dBm - Budget: 11 dB worst case
Losses: - Fibre attenuation: 12 km × 0.35 dB/km = 4.2 dB - Connector pairs: 4 pairs (one at each end of the patch panel, plus the joint) × 0.5 dB = 2.0 dB - Fusion splices: 1 × 0.05 dB = 0.05 dB - Total: 6.25 dB
Margin: 11 − 6.25 = 4.75 dB
That's healthy. We typically spec 3 dB minimum, so we're 1.75 dB above the floor. The link will work.
What if the budget is tight
If your margin comes out under 3 dB, before you worry, check the actual losses with an OTDR or a light source + power meter. Specs are worst-case; real fibres often do better.
But if it really is tight, in order:
1. Use 1550nm instead of 1310nm. Fibre attenuation drops from 0.35 to 0.25 dB/km. Over 12 km that's a 1.2 dB saving. 2. Add fusion splices instead of mechanical splices. 0.2 dB per splice improvement × 3 splices = 0.6 dB. 3. Use a higher-power SFP (LX+ or "extended"). Some vendors have "LX+" modules with -3 dBm Tx instead of -9, gaining 6 dB. 4. Use a 10G ZR SFP at 1G. The 10G ZR has a much higher budget (and a much higher price). Works at 1G if the rate-select is right. 5. Move the splice joint to a more accessible location. Sometimes the joint is in a manhole that's prone to flooding, and you're losing 0.5 dB to water ingress. Fix the splice housing, gain 0.5 dB back.
What an OTDR trace tells you
When we commission an OFC run, the OTDR (Optical Time Domain Reflectometer) sends a pulse down the fibre and measures the backscatter. The trace shows:
- The first connector (a sharp event near the start) - The fibre itself (a gentle slope, 0.35 dB/km at 1310nm) - The mid-run splice (a small step) - The far end (a sharp event)
What you want to see:
- Slope of the fibre section matches the spec - Each connector event is 0.3–0.5 dB - Each splice event is 0.02–0.1 dB - No "ghost" events that don't correspond to physical splices (these are usually reflection problems — bad connector)
What you don't want to see:
- A slope that goes up at the end of the run (this is the far-end reflection from a dirty connector) - A step event that's larger than 0.5 dB (bad splice) - A sudden drop in the middle of the run (fibre break)
Field checklist before you blow
1. Get the SFP spec — Tx power, Rx sensitivity, wavelength 2. Measure the run length — not the straight-line distance, the actual cable route, including vertical risers 3. Count connectors and splices — don't forget the patch panel at each end 4. Add the losses — fibre + connectors + splices 5. Subtract from the budget — that's your margin 6. If margin < 3 dB, plan for fixes — wavelength change, better splices, higher-power SFP
Get this right on paper and you won't have to climb a tower at midnight to swap an SFP.
When to call us
OFC link budget calculations, OTDR trace interpretation, splice joint design — these are our bread and butter. Send us the SFP model, the run length, and the connector count, and we'll give you a verdict in an hour. Toll-free 1800-5GHZ-NET, or raise a ticket.
Frequently asked
What is a typical OFC link budget for a 10 km run?
For a 1G LX SFP at 1310nm, transmitter power is -9 to -3 dBm, receiver sensitivity is -20 dBm, total budget is 11–17 dB. After 10 km of fibre at 0.35 dB/km (3.5 dB), 4 connector pairs at 0.5 dB each (2 dB), and 2 fusion splices at 0.05 dB each (0.1 dB), you have 5.6 dB of total loss, leaving 5.4–11.4 dB of margin. Plenty.
How do I know if my fibre run will work at 10G?
10G LR (1310nm) is similar to 1G LX. 10G ER (1550nm) gives more budget for longer runs. 10G ZR (1550nm) is for 80 km. The same loss budget applies — just check the spec sheet for the specific SFP you plan to use.
What is the difference between fusion splice and mechanical splice loss?
Fusion splice is permanent, welded with an electric arc, typically 0.02–0.05 dB loss. Mechanical splice uses a gel and a guide, typically 0.1–0.3 dB. Always use fusion splices in the field; mechanical splices are for emergency repairs only.
Still need a hand?
If the article didn't fix it, raise a ticket and we'll get on it. Most P1 calls land within an hour.