Two design problems
An indoor fibre cable runs through risers, ceilings, trays and racks. It has to bend around corners, be terminated by an installer with connectors or pigtails, and meet building fire rules. An outdoor cable runs in ducts, on poles or in the ground. It has to resist water, ultraviolet light, temperature cycles, crushing and pulling tension over long lengths. Each design answers its own problem, and neither does the other's job well, which is why indoor/outdoor hybrid designs exist for the short run that crosses from one environment into the other.
The core difference is in how each fibre is protected, covered in detail in loose tube vs tight buffer: indoor cable uses tight buffers, outdoor cable uses loose tubes.
Construction side by side
The reference constructions below come from public datasheets: a 12-core tight-buffered distribution cable (GJFJV or GJFJH type) and a 24-core stranded loose tube cable (GYTS type). Weights vary widely between makers, so ranges are shown and the order follows the datasheet.
| Layer | Indoor: 12-core tight-buffered | Outdoor: GYTS 24-core loose tube |
|---|---|---|
| Fibre protection | 0.9 mm tight buffer on each fibre | Fibres loose in gel-filled tubes, stranded with fillers |
| Strength member | Aramid yarn | Central steel wire |
| Water blocking | None needed for dry indoor use | Tube gel plus water-blocking material in the core |
| Armour | None | Steel-polyethylene laminated tape (PSP); GYTA uses aluminium-polyethylene tape (APL) |
| Jacket | PVC or LSZH | PE sheath |
| Outer diameter | about 6.0 to 6.3 mm | about 9.7 to 10.5 mm |
| Weight | about 33 to 41 kg/km | 76 to 116 kg/km (per datasheet) |
| Core range in category | 2 to 144 cores | 2 to 288 cores |
| Typical drum | 1 km or 2 km | 2 km or 4 km |
| Reference requirements | Telcordia GR-409 | Telcordia GR-20 |
Jackets: fire versus weather
Indoor jackets are chosen for fire behaviour. In the EU, fibre cables permanently installed in buildings fall within EN 50575 under the Construction Products Regulation, so the reaction-to-fire class must be declared on the DoP. LSZH jackets are the usual route to the better classes and to low smoke and halogen acid gas results per test report to IEC 61034 and IEC 60754. In the US, indoor optical fibre cable carries an NEC Article 770 rating such as OFNR for risers or OFNP for plenum spaces, and US projects usually require listed products to satisfy the AHJ.
Outdoor jackets are chosen for weather. Black PE resists ultraviolet light and moisture far better than indoor compounds and gives a tough, low-friction surface for duct pulling, but ordinary PE is not a building fire compound. That is the reason an outdoor cable should not run a long distance inside a building: most projects require a transition to an indoor-rated cable within a short distance of the building entry, set by local rules and the project specification. Ask your project engineer or AHJ where the transition point must sit.
Water blocking and temperature
Water is the main long-term threat to an outdoor cable. If it travels along the core it can freeze, put stress on fibres and corrode metal elements. Outdoor designs block it in two places: gel inside each loose tube and water-blocking yarns, tapes or compound between the tubes and under the armour tape. The loose tube also gives each fibre some excess length, so the cable can stretch under tension or shrink in cold weather without straining the glass. See water blocking in cables for the materials used.
Indoor cables skip all of that. Tight buffers keep the cable thin and flexible and let the installer strip and connect each fibre directly, but they offer little room for expansion and no protection against standing water. An indoor cable left in a flooded duct can fail even if it looks intact.
Handling is different as well. Outdoor cables are pulled over long distances with winches, so the central strength member and the maximum tensile load on the datasheet set how long each pull can be and where intermediate pits or pulling points are needed. Indoor cables are pulled by hand over shorter runs, and the aramid yarn takes the load. Both have a minimum bending radius stated on the datasheet, one figure during installation and a smaller one once the cable is at rest, and both should be respected at tray corners, in cabinets and around drum flanges. Gel-filled cables also need cleaning before splicing, which adds time per joint and is worth allowing for in the installation plan.
- Indoor: small, flexible, fast to terminate, fire-rated jacket, short to medium lengths.
- Outdoor duct: loose tube, gel, PSP or APL tape, PE sheath, long drum lengths.
- Direct burial: armoured outdoor types; see armoured fibre cable.
- Aerial: self-supporting ADSS designs rated for span and weather, covered on the ADSS line.
Testing, drums and sourcing
The glass is usually the same: G.652.D single-mode or bend-insensitive G.657 for tight indoor routes, and OM3 or OM4 multimode for short building links. Fibre loss in both cable types is stated as cabled limits from the G.652 tables or the datasheet values, and older TIA-568 planning values give single-mode 1.0 dB/km indoors and 0.5 dB/km outdoors as a first estimate. Fibre colours follow TIA-598 so that splicers can identify each fibre and tube.
- Split the route into outdoor, entry and indoor sections with lengths.
- Choose the outdoor type by installation: duct, aerial or direct burial.
- Choose the indoor jacket by fire rating: CPR class or NEC 770 rating.
- Match core counts and colour codes across both cables.
- Set drum lengths and ask for per-drum fibre test records before shipment.
Outdoor fibre cable is sourced mainly from partner plants in the Wuhan Optics Valley and Qidu clusters, which have the loose tube and sheathing lines; indoor cable comes from the same clusters or from distribution cable specialists. Lead times are set by fibre and tube scheduling and confirmed in writing for each order, as explained on the MOQ and lead time page. The outdoor fibre cable line and indoor fibre cable line list the constructions the desk sources, and the quote form accepts a route schedule directly.