---
title: "OSNR and Optical Link Budget: A Working Engineer's Calculation Guide"
slug: "osnr-link-budget-practical-guide"
category: "Network Engineering"
tags: ["OSNR", "link budget", "optical power", "EDFA", "metro", "long-haul", "margin"]
seo_focus_keyword: "OSNR optical link budget calculation guide"
word_count_target: 1200
difficulty: advanced
---

Optical link budgets are one of those topics where the theoretical treatment in textbooks and the practical reality of commissioning a metro ring diverge significantly. The math isn't particularly difficult, but knowing which numbers to trust, which safety margins to apply, and where real systems consistently underperform their datasheet specifications takes experience that no textbook provides. This article walks through the calculations you actually need for metro and regional planning, with the margin tables that vendor application notes tend to omit.

**Starting with power budget: the basics**

For a passive point-to-point link (no amplification), the optical power budget is straightforward. Received power equals transmitted power minus all losses in the path:

P_received = P_transmit − IL_fiber − IL_connectors − IL_splices − IL_components

Where:
- P_transmit is the transceiver output power (in dBm, from the TX power specification)
- IL_fiber is insertion loss from fiber attenuation (typically 0.35 dB/km for SMF-28 at 1310nm, 0.20 dB/km at 1550nm)
- IL_connectors is connector pair insertion loss (budget 0.5 dB per mated pair, though good APC connectors achieve 0.2–0.3 dB)
- IL_splices is splice loss (0.1 dB per fusion splice is achievable; budget 0.2 dB for conservative planning)
- IL_components adds patch panels, WDM multiplexers, splitters, and any passive inline components

The received power must exceed the transceiver's receiver sensitivity by the required margin. A 100GBASE-LR4 transceiver (1310nm CWDM4 or LAN-WDM) typically specifies a minimum receiver sensitivity of -10.6 dBm and a maximum input of +4.5 dBm. The transmitter output is +4 to +4.5 dBm. A 10km link with good fiber and typical connectors consumes about 3–4 dB, leaving the received signal well above sensitivity.

The headroom between your calculated received power and the receiver sensitivity floor is your margin. You want at least 3 dB of margin for a stable link; 4–5 dB is better for long-term fiber plant aging and component degradation.

**Where passive budget calculations break down**

For spans beyond about 80km, passive loss exceeds what most transceiver receiver sensitivities can accommodate. A 100km SMF-28 run at 1550nm accumulates 20 dB of fiber loss alone. Add connectors and components, and you're at 22–25 dB. Standard coherent 400ZR transceivers have receive sensitivity around -21 dBm and transmit at 0 dBm, giving a 21 dB passive link budget — barely adequate for 100km with no margin.

This is where OSNR becomes the meaningful metric rather than raw optical power.

**OSNR: signal-to-noise in amplified links**

In amplified optical systems using EDFAs (Erbium-Doped Fiber Amplifiers), the limiting factor is not absolute received power but the ratio of signal power to accumulated amplified spontaneous emission (ASE) noise — the Optical Signal-to-Noise Ratio.

OSNR is defined as the ratio of signal power to noise power measured in a reference bandwidth (typically 12.5 GHz or 0.1 nm, the two are approximately equivalent in the C-band). It's expressed in dB:

OSNR (dB) = P_signal − P_noise

For a single EDFA span, the OSNR contribution is approximately:

OSNR_span = P_launch − NF_EDFA − 10×log10(h×ν×B_ref) − L_span

Where:
- P_launch is the signal power entering the amplifier (in dBm)
- NF_EDFA is the EDFA noise figure (typically 4–6 dB for modern inline amplifiers)
- h×ν×B_ref is the noise photon floor: at 1550nm in 12.5 GHz bandwidth, 10×log10(h×ν×B_ref) ≈ −58 dBm (a constant you can treat as a reference value)
- L_span is the span loss in dB

For a practical example: a 80km SMF span with 16 dB loss, EDFA with 5 dB noise figure, and +0 dBm launch power:

OSNR_span ≈ 0 − 5 − (−58) − 16 = 37 dB

That's the OSNR at the output of the first EDFA. Each additional span adds noise, and OSNR degrades approximately as 10×log10(N_spans) for equal-span, equal-amplifier systems. Four spans: −6 dB. Eight spans: −9 dB. For a 400G DP-16QAM signal, you need approximately 24–26 dB OSNR at the receiver (the FEC threshold). Work backwards from there to determine how many spans are feasible.

**Practical margin tables for metro and regional planning**

The following represents conservative real-world planning margins, not best-case datasheet values. Actual performance will typically exceed these — they're designed to survive six years of fiber aging, connection rematings, and EDFA gain drift.

| Application | Span Length | Fiber Loss | EDFA NF | Target OSNR | Margin |
|---|---|---|---|---|---|
| Metro DCI, 400ZR | 80 km | 16 dB | 5 dB | 26 dB | 4 dB |
| Metro ring, 100G | 60 km | 12 dB | 5 dB | 22 dB | 5 dB |
| Regional, 400G OpenZR+ | 200 km (3 spans) | 16 dB/span | 5 dB | 24 dB | 3 dB |
| Long-haul, 100G DP-QPSK | 600 km (8 spans) | 15 dB/span | 5 dB | 16 dB | 3 dB |
| Raman-boosted, 400G | 120 km | 24 dB | 4 dB (eff.) | 26 dB | 4 dB |

The margin column accounts for: connector aging (+0.5 dB over 5 years), splice point accumulation (+0.3 dB), EDFA gain flatness variation (±0.5 dB), chromatic dispersion compensation imperfection (+0.5 dB), and polarization-mode dispersion (PMD) margin (+0.5 dB). Add these, round up, and 3 dB is genuinely tight; 5 dB is comfortable.

**Chromatic dispersion: the other constraint**

High-speed coherent modulation formats are sensitive to chromatic dispersion (CD). Standard SMF-28 has approximately 17 ps/(nm·km) CD at 1550nm. For a 400G DP-16QAM signal with 60 GHz baud rate, the CD tolerance of a typical coherent DSP is ±80,000 ps/nm. That sounds large — it's enough for 4,700 km of SMF-28 without compensation. Modern coherent DSPs (Acacia Pico, Marvell Canopus, Ciena WaveLogic 5) compensate dispersion digitally, eliminating the need for dispersion compensation fiber (DCF) that was mandatory in 10G-era deployments.

For 10G direct-detect transceivers (10GBASE-ER, 10GBASE-ZR), dispersion remains a real constraint. 10GBASE-ER at 1550nm specifies a maximum of 1,600 ps/nm CD tolerance. At 17 ps/(nm·km), that's about 94km before dispersion compensation is needed. This is why 10G long-haul deployments either use 1310nm (near zero dispersion wavelength, approximately 3 ps/(nm·km)) or require inline dispersion compensation.

**Common planning mistakes**

Trusting vendor datasheet OSNR sensitivity without applying a real-world penalty is the most common error. Datasheet values are typically measured with back-to-back configurations, calibrated test equipment, and ideal polarization conditions. Real links accumulate 1–2 dB of effective OSNR penalty from PDL (polarization-dependent loss), filter narrowing through cascaded ROADMs, and nonlinear optical effects at higher launch powers. Apply a 2 dB system penalty to any coherent link with ROADMs in the path.

ROADM filtering deserves special attention. Each ROADM passthrough adds approximately 0.5–1.0 dB of effective OSNR penalty due to filter bandwidth narrowing. A signal traversing eight cascaded ROADMs accumulates 4–8 dB of filtering penalty that must be included in the budget. Coherent DSPs compensate some of this through adaptive equalization, but not all.

Launch power optimization is often overlooked. Increasing launch power improves OSNR linearly — until nonlinear effects (self-phase modulation, cross-phase modulation, four-wave mixing) kick in and degrade it. The optimal launch power for a typical SMF-28 100km span is typically +0 to +2 dBm for 100G coherent. Above +4 dBm, nonlinear penalties start exceeding the OSNR improvement. The sweet spot depends on channel count, baud rate, and fiber type — this is worth computing explicitly rather than defaulting to maximum launch power.

Good link budgeting is iterative. Start with the margin tables, apply real-world penalties, check both power budget and OSNR, and revisit if the margin is below 3 dB. If you're within 1 dB of the OSNR threshold, you're operating in the territory where normal day-to-day variation in EDFA gain, fiber temperature, and connector condition can push you into errors.
