Almost every transceiver name includes a two-letter reach code — SR, LR, ER, or ZR — tacked onto the speed and form factor. These codes tell you the maximum distance the module is designed to transmit over, and mixing them up is one of the most common ordering mistakes.
The Reach Codes, Roughly Ranked
| Code | Meaning | Typical Reach | Fiber Type |
|---|---|---|---|
| SR | Short Reach | Up to ~300–400m | Multi-mode |
| LR | Long Reach | Up to ~10km | Single-mode |
| ER | Extended Reach | Up to ~40km | Single-mode |
| ZR | Extra-Long Reach | Up to ~80km | Single-mode |
Exact maximum distances vary somewhat by manufacturer and specific product line, so always check the datasheet for the precise figure on the module you’re considering — the table above is a general guide to the tiers, not a fixed spec.
Why You Can’t Just “Round Up” to Be Safe
It’s tempting to assume a longer-reach module is simply the safer choice, but that’s not quite right. Longer-reach modules are tuned for lower-loss, longer runs, and can actually perform worse or fail to establish a link on very short runs due to receiver overload — the signal arrives too strong rather than too weak. Match the rating to the actual distance, not to “more is safer.”
Can I use an LR transceiver on a short in-rack connection?
Sometimes, but not always safely — on very short links, a long-reach module’s signal can be too strong for the receiver, causing errors. It’s better to match the reach rating to the actual distance.
Do SR modules work over single-mode fiber?
No. SR modules are designed for multi-mode fiber. Running one over single-mode fiber (or the reverse) will not work correctly regardless of distance.
Is there anything beyond ZR?
Yes, some manufacturers offer even longer-reach options (sometimes with amplification), but ZR covers the vast majority of standard long-haul enterprise and data center interconnect needs.
Not sure which reach rating your link actually needs? Tell us the distance you’re covering and we’ll point you to the right module.
