SFP Fiber Module: Choosing 1G Optics Correctly
Quick answer / TL;DR: Short answer: a 1G SFP fiber module provides a gigabit optical interface. 1000BASE-SX uses 850 nm over multimode to about 550 metres, and 1000BASE-LX uses 1310 nm over single-mode to 10 kilometres. Both ends must use the same type and wavelength, and the module must match the switch's coding. LC connectors are standard, and DOM reports the optical power for diagnosis.
Fiber cords: two connector families, two fiber types, three polish options.A gigabit fibre link needs a module at each end that matches the fibre in the ground, the distance, and the switch's expectations. Three facts decide the purchase.
Buying on the phrase gigabit SFP alone produces modules that fit and do not link.
SX and LX
SX is the multimode option, cheap and short. LX is single-mode and reaches 10 kilometres, and some LX modules also work over multimode with a mode conditioning patch lead.
The fibre type decides which is legal. Putting an SX module on single-mode produces no link, and the reverse is equally true.
| Standard | Wavelength | Fibre | Reach | Connector |
|---|
| 1000BASE-SX | 850 nm | OM2 to OM4 | 550 m | LC duplex |
| 1000BASE-LX | 1310 nm | single-mode | 10 km | LC duplex |
| 1000BASE-LX on MMF | 1310 nm | OM3 with MCP | 550 m | LC duplex |
| 1000BASE-T | copper | Cat5e | 100 m | RJ45 |
Mode conditioning patch leads exist because launching 1310 nm into multimode without conditioning produces multiple propagation paths and a closed eye.
Coding and Vendor Compatibility
Some switch vendors require modules carrying their own coding, and reject generic ones at boot. The fix is a coded module or an unlocked port setting.
Record the module part numbers per port. In a mixed installation, that record turns a thirty minute fault-find into a five minute check.
- Check the switch compatibility list
- Vendor coding or unlocked mode
- Same module both ends
- Same wavelength both ends
- Spare module of each type
Keeping one spare of each type is worth the cost. A failed module in a link with no spare means a site visit.
Installation Practices
Keep the protective caps on until the moment of connection. A capped module is clean, and a module left open in a dusty cabinet is not.
Insert the module fully until it latches. A partially seated module sometimes links and then drops under vibration, which is difficult to diagnose.
Route fibre with a minimum bend radius of about ten times the cable diameter. A tight bend attenuates the signal without breaking the fibre, producing an intermittent link.
Label both ends of every fibre run with the same identifier. A patch panel with twenty unlabelled LC connectors is where fibre faults become expensive.
Where the link crosses between buildings, check that the fibre is single-mode before ordering LX modules. Multimode installed for a short internal run will not support the distance.
Copper Alternatives
For runs under 100 metres, 1000BASE-T modules over Cat5e are cheaper than optics and avoid the fibre termination work entirely.
They draw more power and run hotter than optics, which matters in a densely populated switch.
Where the run is short and inside a rack, a DAC cable avoids both modules and patch leads, and is the least expensive option of all.
Diagnostics
Digital optical monitoring reports transmit power, receive power and temperature. A link that fails with acceptable receive power points to a coding or speed problem rather than the fibre.
Clean the connectors before replacing anything. A fingerprint on a multimode ferrule can drop the receive level below the sensitivity threshold.
Record the commissioning levels. Comparing a later reading with the baseline identifies a degrading link before it fails completely.
Real-World Applications
Building to building at 1G
LX modules at each end over single-mode in the site duct, with the run length measured rather than estimated.
Rack to rack within a data hall
SX modules over OM3 patch leads, which is cheaper than single-mode for short distances.
Industrial cabinet link
SX over multimode with ruggedised leads where vibration is present.
Legacy switch with a fibre uplink
One module matching the existing coding, with a spare of the same type.
Action: Three Steps
First, identify the fibre type and the measured distance. Those two facts select the standard.
Second, check the switch's coding requirement before ordering.
Third, order a spare of the same type, since these are the parts that fail.
Related reading: 10G SFP+ modules — blogs/news/10g-sfp
Related reading: SFP28 versus SFP+ — blogs/news/sfp28-vs-sfp
Related reading: Multimode versus single-mode fibre — blogs/news/multimode-vs-singlemode-fiber
Standards and Verification
1000BASE-SX and 1000BASE-LX are defined by IEEE 802.3z. The SFP form factor and its electrical interface are defined by the SFF-8472 and INF-8074 specifications.
DOM is defined within SFF-8472 and is the mechanism by which receive power is reported. Not all switches expose it in the interface, though most do through the command line.
Verification: read the receive power and compare it with the module's sensitivity, then check the link comes up at 1G rather than at a lower negotiated speed. A link at 100 Mb indicates a duplex mismatch rather than an optical fault.
Frequently Asked Questions
SX or LX?
SX for multimode, LX for single-mode.
Can I mix types?
No. Both ends must match.
What connector?
LC duplex on almost all modules.
Will a generic module work?
Depends on the switch's coding.
Is DOM useful?
Yes. It shows receive power for diagnosis.
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