G.652.D vs G.657.A1 vs G.657.A2: which fiber for FTTH?
What separates G.652.D from bend-insensitive G.657.A1, A2 and B3 fiber, where each belongs in an FTTH network, and why they splice together without trouble.
What is the difference?
All four fibers are single-mode fibers with the same 125 µm glass and a mode-field diameter close to 9 µm. The difference is how much light escapes when the fiber is bent.
- G.652.D is the standard single-mode fiber with low water peak. It is cheap, proven and ideal where cable is laid straight: backbone, feeder and most duct cable.
- G.657.A1 adds bend tolerance down to a 10 mm radius while still meeting the G.652.D specification.
- G.657.A2 goes further, to a 7.5 mm radius, and is the usual choice for FTTH drops and wall outlets.
- G.657.B3 tolerates a 5 mm radius for very tight corners and stapled indoor routes.
Where each belongs in an FTTH network
| Network section | Recommended fiber | Why |
|---|---|---|
| Backbone and feeder | G.652.D (or G.657.A1) | Long, straight routes; lowest cost per km |
| Distribution | G.657.A1 | Closures and cabinets bend fibers more often |
| Drop to the home | G.657.A2 | Corners, clips and wall entries |
| Indoor and riser | G.657.A2 or B3 | Tight bends behind furniture and in trunking |
Why bend tolerance matters
Macro-bend loss grows fast at longer wavelengths. GPON upstream runs at 1310 nm, downstream at 1490 nm, and live-fiber OTDR testing at 1625 nm, where a fiber bent too tightly loses most. A drop cable stapled around a door frame can lose enough light to break the link. Bend-insensitive fiber keeps that loss small.
Splicing and connecting mixed fibers
G.657.A fibers are designed to be fully compatible with G.652.D, so a G.657.A2 drop spliced to a G.652.D feeder behaves like any other splice. When you specify cable, name the fiber per section rather than one fiber for the whole network.
What to write in your BOM
State the fiber type next to every cable line: for example “24F duct cable, G.652.D” and “1F flat drop, G.657.A2, LSZH”. Our guide on sending an FTTH BOM lists the other fields that let us quote without back-and-forth.
Next guide PLC splitter ratios and the GPON loss budget: a worked example