Quick Answer / Summary: Match the module to the fiber and the distance: 1000BASE-LX or 10GBASE-LR at 1310 nm for a few kilometres, 10GBASE-ER or ZR at 1550 nm for longer runs. Check that the switch accepts the module’s coding, that both ends use the same wavelength and reach class, and that the optical budget covers fibre loss, connectors and splices with margin.
An SFP is the module that turns a switch port into a fiber port. With single mode fiber, the module choice sets the wavelength, the reach and the optical budget. Most link failures at this layer are mismatched modules or dirty connectors rather than bad fibre.
What the Module Contains
An SFP holds a laser transmitter, a photodiode receiver and a control chip that reports diagnostics. SFP+ runs at 10 Gbps in the same form factor. The module converts electrical signals to light and back, and it negotiates with the switch through an I2C interface that also exposes temperature, voltage and optical power.
Wavelength and Reach Codes
LX and LR use 1310 nm; ER and ZR use 1550 nm. The letters describe the reach class rather than a guarantee: LR is specified to 10 km, ER to 40 km, ZR to 80 km. A longer-reach module is not automatically better; it may overload a short link.
Coding and Vendor Lock
Some switches check a vendor code in the module’s EEPROM and refuse unknown parts. Compatible modules are coded for the switch family. This is a commercial restriction, not an optical one.
Pairing Modules Correctly
Both ends must use the same wavelength and compatible reach. A 1310 nm module will not talk to a 1550 nm module unless the pair is explicitly a bidirectional single-fiber design, which uses different wavelengths in each direction and is labelled as such.
Duplex and BiDi
Duplex modules use two strands, one per direction. BiDi modules use one strand with different wavelengths each way and must be purchased as a matched pair, typically 1310/1490 or 1310/1550.
Fiber Type
Single mode modules expect 9 micrometre core OS2 fibre. Connecting them to multimode fibre produces very high loss and usually no link at all.
Link Budget and Diagnostics
Every module has a launch power range and a receiver sensitivity. Subtract the receive sensitivity from the launch power to get the budget, then subtract fiber attenuation, connector pairs and splices. If the result is under about 3 dB, the link has no margin for ageing or repairs.
DOM Readings
Digital optical monitoring reports transmit and receive power. A receive level near the sensitivity floor explains intermittent drops; a receive level above the maximum overloads the receiver and causes errors on short links.
Attenuators
Long-reach modules on short runs often need an inline attenuator to bring the receive power into range. This is a common oversight when a 40 km module is used across a building.
Installation Practice
Clean both end faces, confirm the transmit strand at one end maps to the receive port at the other, and check the switch’s port status before assuming a module fault. A link that reports up but passes no traffic is often a duplex mismatch in the fiber pair rather than a dead optic.
Polarity
Duplex LC connectors are usually labelled A and B. Swapping them at one end is the single most common cause of a link that will not come up.
Temperature
Modules run warm, and some switches report thermal warnings in dense line cards. Ensure airflow and avoid stacking modules without spacing if the chassis allows it.
Single Mode SFP Reach Classes
| Standard | Speed | Wavelength | Rated reach | Typical use |
|---|---|---|---|---|
| 1000BASE-LX | 1 Gbps | 1310 nm | 10 km | Building and campus |
| 10GBASE-LR | 10 Gbps | 1310 nm | 10 km | Campus backbone |
| 10GBASE-ER | 10 Gbps | 1550 nm | 40 km | Metro and long campus |
| 10GBASE-ZR | 10 Gbps | 1550 nm | 80 km | Long haul |
| BiDi pair | 1 or 10 Gbps | 1310/1490 nm | 10 to 20 km | Single-strand fiber saving |
Practical Applications
Where single mode SFPs are used: linking two buildings across a campus, connecting a switch to an ISP handoff, extending a backbone between floors beyond copper limits, providing a long-reach uplink from a remote cabinet, and connecting a router to a PON OLT. In each case the module converts a copper port into an optical one at the distance the site requires.
Frequently Asked Questions
Can I use a 10G SFP+ module in a 1G SFP port?
Usually no. The port must support the speed and the module form factor. Some SFP+ ports accept 1G modules, but the reverse is rare.
Do SFP modules need to be the same brand at both ends?
No. They need matching wavelength, reach class and speed. Vendor coding affects whether the switch accepts the module, not whether the two ends talk.
Why does my link work at 100 m but not at 2 km?
Either the module is a short-reach type, the fiber has high loss or a bad connector, or the budget is exhausted. Check receive power with DOM.
What is DOM?
Digital optical monitoring. It reports transmit and receive power, temperature and voltage, which makes optical faults much easier to diagnose.
Can I mix single mode and multimode patch cords?
No. The core sizes and module types differ. Mixing them produces very high loss and normally no link.