SR vs LR vs ER vs ZR: How to Choose the Right Optical Reach
SR, LR, ER and ZR describe how far an optical transceiver can transmit a usable signal — not how fast, and not how "good" the optic is. Specifying the wrong reach code is one of the most common causes of a link that won't come up, or one that comes up and then fails intermittently once power margins are tight.
| SR~400 m · MMF | LR~10 km · SMF | ER~40 km · SMF | ZR~80 km · SMF |
What the reach code actually tells you
The reach suffix on a transceiver part number (e.g. -SR, -LR, -ER, -ZR) defines the target optical link budget the transceiver is engineered for: a combination of launch power, receiver sensitivity, wavelength and expected fibre attenuation. It does not by itself guarantee a working link — actual reach also depends on fibre quality, connector loss, splices and any passive components (splitters, CWDM/DWDM muxes) in the path.
Reach comparison, to scale
Illustrative, log-scale reach comparison based on typical industry reference values for standard point-to-point links on good-quality fibre with negligible passive loss. A 15 km link (dashed marker) already falls outside SR and LR's typical envelope and is comfortably inside ER — exactly the kind of check to run before ordering, not after.
Full specification comparison
| Reach class | Typical wavelength | Fibre type | Typical max reach* | Common use case |
|---|---|---|---|---|
| SR | ~850 nm | Multimode (OM3/OM4) | Up to ~300–400 m | In-rack, in-row, same-room data centre links |
| LR | ~1310 nm | Single-mode (OS2) | Up to ~10 km | Building-to-building, campus and metro edge |
| ER | ~1550 nm | Single-mode (OS2) | Up to ~40 km | Metro aggregation, longer campus/inter-site links |
| ZR | ~1550 nm | Single-mode (OS2) | Up to ~80 km | Long-haul metro, carrier and ISP backbone links |
*Figures are typical industry reference values. Exact guaranteed reach varies by transceiver model and vendor datasheet — flagged for technical reviewer to confirm/adjust against current product datasheets before publish.
Will a reach class actually close your link?
Reach figures assume a "typical" loss budget. For anything other than a short, clean run, it's worth running a basic optical power budget check before ordering:
Available budget (dB) = Tx power (dBm) − Rx sensitivity (dBm) Estimated loss (dB) ≈ (length km × dB/km) + (connectors × loss) + (splices × loss)
If estimated loss is close to or exceeds the available budget, the link is marginal — move up a reach class, or plan for optical amplification, rather than installing and hoping.
This is exactly the check worth running before ordering optics for a new long-haul or metro segment, and before troubleshooting a link that's "up" but flapping.
Common specification mistakes
- Ordering by port speed alone. A 10G port can take an SR, LR, ER or ZR optic — speed and reach are independent decisions.
- Mixing SR (multimode) with single-mode fibre, or vice versa. Wavelength and fibre type must match on both ends of the link; a mismatched pair will not link, or will only appear to link with a heavily degraded signal.
- Under-specifying reach on a route with passive components. Every CWDM/DWDM mux, splitter or patch panel adds loss — factor this in before choosing LR over ER, or ER over ZR.
- Assuming both ends need to be identical brand. Both ends need to be the same reach class, wavelength and fibre type — not necessarily the same manufacturer, provided each end is correctly coded for its respective platform.
Quick decision guide
- Under ~400 m, multimode fibre in place → SR
- Up to ~10 km, single-mode fibre → LR
- 10–40 km, single-mode, minimal passive loss → ER
- 40–80 km, or a route with meaningful passive loss → ZR, and run the power budget check above
For families like SFP, SFP+, SFP28 and QSFP — and how form factor interacts with reach selection — see our companion guide on transceiver form factors.
Next step
If you're specifying optics for a new or upgraded link, it's worth confirming compatibility and reach against your exact equipment model and firmware before ordering.
Check compatibility with your equipment →FAQs
- Can I use a ZR optic on a short link instead of SR or LR?
- Technically often yes for the up-link side, but it's rarely cost-effective, and some ZR/DWDM-class optics have minimum-loss or attenuation requirements at very short distances — check the datasheet rather than assuming "longer reach is always safe."
- Does reach class affect latency?
- Not meaningfully at these distances — reach class affects power budget and link integrity, not processing latency.
- Are BiDi, CWDM and DWDM optics a fifth reach category?
- No — those describe wavelength multiplexing approaches rather than reach classes; a CWDM or DWDM optic still has an associated reach spec (often ER or ZR-equivalent). This is covered in our dedicated BiDi/CWDM/DWDM guide.