Two recommendations, one family
ITU-T G.652 describes standard single-mode fibre and cable; category D is the low water peak version that is now the default for outdoor and backbone cable. ITU-T G.657 describes bending-loss insensitive single-mode fibre. Its A categories are designed to be fully compliant with G.652.D, so an A1 or A2 fibre can be spliced and connected to G.652.D in the same link. Its B categories push bend performance further and are aimed at the very last metres into a building or a home, with compatibility to G.652.D confirmed per datasheet rather than assumed.
In geometry the fibres look the same: 125 um cladding (125.0 plus or minus 0.7 um is a typical manufacturer tolerance) and a 242 um coating, about 250 um once coloured. A 200 um reduced-coating version exists for high-count and micro cables. The difference is inside the glass, in the refractive index profile that keeps light in the core when the fibre is bent tightly.
Bend radius: the number that separates them
The headline figure is the minimum design bend radius. ITU material on G.657 gives 10 mm for A1, 7.5 mm for A2 and B2, and 5 mm for B3. Standard G.652.D fibre is normally handled with a larger bend radius, and the cable maker's datasheet states the cable bend limit, usually as a multiple of the cable diameter.
| Grade | Minimum design bend radius | Compatibility with G.652.D | Typical place in the network |
|---|---|---|---|
| G.652.D | Per cable datasheet | Reference grade | Outdoor duct, aerial and buried cable, backbones |
| G.657.A1 | 10 mm | Designed to comply | Distribution cable, closures, general FTTH |
| G.657.A2 | 7.5 mm | Designed to comply | Drop cable, patch cords, crowded cabinets |
| G.657.B3 | 5 mm | Confirm per datasheet | In-home wiring with very tight routing |
Fibre bend radius is not the same as cable bend radius. A G.657.A2 fibre inside a 6 mm indoor cable still needs the cable's own bend limit respected during pulling, because the jacket, buffers and aramid yarn can be damaged even when the glass would survive. The cable bending radius guide covers the installation side.
Mode field diameter and splicing
Mode field diameter (MFD) is the effective width of the light path in the core. Datasheets typically give 8.6 to 9.2 um at 1310 nm, plus or minus 0.4 um, and different fibre makers sit at different points in that range; treat it as a per-datasheet value rather than one standard number. Bend-insensitive designs often sit lower in the range than standard G.652.D.
The practical effect appears in testing. When two fibres with different MFD are spliced, an OTDR shot in one direction can show an apparent gain at the splice and the opposite direction an exaggerated loss. Neither reading is the true splice loss. The usual practice is to test from both ends and average the two readings, which is worth writing into the acceptance plan for any route that mixes G.652.D trunk cable with G.657 drop or distribution cable. The OTDR test report guide explains what a two-way report should contain.
Loss: the same limits, different behaviour in bends
On a straight run, G.652.D and G.657.A fibres behave alike. The cabled fibre limits in G.652 tables, 0.4 dB/km from 1310 to 1625 nm and 0.3 dB/km at 1550 nm, are the reference ceiling for both; the actual values for a given product are on its datasheet and on the drum test report. The difference is macrobend loss, the extra loss when the fibre is coiled tightly in a closure, a wall box or a cabinet. Bend losses rise faster at longer wavelengths, so a fibre that looks fine at 1310 nm can show extra loss at 1550 or 1625 nm when coiled. That matters on PON networks that use 1490 or 1550 nm downstream and on routes monitored at 1625 nm. G.657 grades were written for exactly that problem.
Coating size and the 200 um option
Bend-insensitive fibre is also what made reduced-coating fibre practical. A 200 um coated fibre has a noticeably smaller cross-section than a 242 um fibre, which lets a plant put more fibres into the same tube or build a smaller micro cable for blown installation into small ducts. The trade-off sits with the installer: fusion splicer holders, strippers and fan-out kits must suit the coating size, and some field crews only stock 250 um tools. Mixed coating sizes on one route are workable, but the splicing team should know in advance.
For most buyers the decision follows the cable type. Standard duct, aerial and drop cables use 242 um fibre, coloured to about 250 um. High-count ribbon or micro cables may use 200 um fibre, and the datasheet will say so. If the quote does not state the coating, ask; a cable that arrives with a different coating from the one the contractor planned for can slow a splicing crew for days.
What to put on the spec sheet
- Fibre grade, for example G.657.A1, and the fibre product named by the plant.
- Coating diameter, 242 um or 200 um, and colour scheme to TIA-598.
- Attenuation at each wavelength the network uses, per fibre, on the drum report.
- Cable bend radius for installation and in service, from the cable datasheet.
Which one to buy
The desk's default recommendations, to be confirmed by the network designer:
- Outdoor duct, aerial and buried cable: G.652.D is the normal choice and the widest supply base. G.657.A1 is increasingly used in the same cables at little or no premium, which helps where the cable ends in crowded closures.
- Distribution and riser cable: G.657.A1 or A2, because these cables are routed through buildings and cabinets.
- FTTH drop cable and patch cords: G.657.A2 as standard; the reference flat drop on our FTTH drop and patch cords page is offered with A1 or A2 fibre.
- In-home micro wiring: B-class grades only when the route needs it and the operator has approved them.
Mixing grades on one network is normal. What causes trouble is mixing them unknowingly. Writing the grade into the spec sheet for each cable type, and checking it on the drum report, is what the desk does on telecom and FTTH projects. The optical fibre page shows the reference construction, and the quote form asks for the fibre grade as a separate field.