Two ways to protect a 250 um fibre
A coloured fibre is only about 250 um across, with a 242 um coating under the colour layer. On its own it can be handled, but not pulled through a duct or left in the weather. Cable designs add protection in one of two ways.
In a loose tube construction, several fibres sit loosely inside a plastic tube, usually PBT, filled with a water-blocking gel. The tube is larger than the bundle, so the fibres have room to move when the cable stretches, shrinks or changes temperature. The reference GYTS outdoor cable on our outdoor fibre cables page uses tubes of about 2.1 mm outside diameter with a wall of about 0.3 mm, carrying 2 to 12 fibres each, stranded around a central strength member.
In a tight buffer construction, each fibre gets its own thermoplastic layer, typically 0.9 mm across, applied directly over the coating. The buffered fibres are then bundled with aramid yarn under a jacket. The reference 12-core distribution cable on our indoor fibre cables page carries twelve 0.9 mm buffers under a PVC or LSZH jacket, with an outside diameter of about 6.0 to 6.3 mm.
Side by side
| Item | Loose tube | Tight buffer |
|---|---|---|
| Fibre protection | Gel-filled PBT tube, 2 to 12 fibres per tube | 0.9 mm buffer on each fibre |
| Strength | Central steel wire or FRP, plus armour or aramid as designed | Aramid yarn, sometimes with an FRP centre in higher counts |
| Water blocking | Tube gel plus water-blocking compound, yarn or tape | Usually none; designed for dry indoor routes |
| Typical sheath | Black MDPE or HDPE | PVC or LSZH, flame-retardant |
| Temperature and strain behaviour | Fibres decoupled from cable strain | Fibres follow the cable more closely |
| Termination | Splice to pigtails in a closure or panel | Direct connectorisation of the 0.9 mm buffer |
| Core counts | 2 to 288 in common ranges | 2 to 144 in common ranges |
| Where it goes | Ducts, aerial, burial when armoured | Risers, trays, racks, office floors |
Why outdoor cable is loose tube
Outdoor cable lives with temperature swings, wind, ice, long pulls and moisture. Datasheets for outdoor loose tube cables commonly state an operating range of about -40 to +60 C and tensile ratings such as 600 N long-term and 1,500 N short-term for lower counts, with higher counts rated higher. Those figures are per datasheet, but the principle is constant: the tube lets the cable carry the load while the fibre inside stays relaxed. The fibre is laid in the tube with a little excess length, so the cable can stretch before the glass does.
The gel matters as much as the tube. Water that enters a damaged sheath can travel along an unfilled cable for long distances; gel in the tubes and a water-blocking compound, yarn or tape in the cable core stop it. The water blocking guide covers the materials, and the GYTS vs GYTA guide explains the armour choice that usually comes next.
Why indoor cable is tight buffered
Inside a building, the priorities change. Routes are short, dry and crowded; the cable has to be flexible, fire-rated and quick to terminate. A 0.9 mm buffer can be stripped and fitted with a connector directly, with no fan-out kit and no gel to clean, which saves time on every port in a patch panel. Tight buffered cable also tolerates being handled in a rack in a way that a gel-filled cable does not.
The trade-off is mechanical. Indoor distribution datasheets commonly state tensile ratings of around 200 N long-term and 600 N short-term, and static and dynamic bend radii of 10 and 20 times the cable diameter. Those are adequate for a riser and too low for a long duct pull. Indoor cable is also governed by fire rules: in the EU, cable for permanent installation in buildings falls under EN 50575 with a CPR class declared on the DoP, and in the US under NEC Article 770 ratings such as OFNR and OFNP. Ordering "LSZH" alone is not enough for a European project; the CPR class should be on the order.
Termination and labour cost
Construction choice moves cost into the field. Loose tube fibres must be spliced to factory-made pigtails in a splice closure, an optical distribution frame or a wall box, which needs a fusion splicer, trained staff and time to clean the gel. Tight buffered fibres can be terminated directly with field connectors, or bought pre-terminated as assemblies. For a long route with few access points, loose tube is cheaper overall; for a building with hundreds of short links, tight buffer usually wins.
Freight follows the same logic. Weight per kilometre varies widely between plants for the same nominal construction; twelve-core indoor distribution cables in public datasheets range from roughly 33 to 41 kg/km, and 24-core GYTS outdoor cables from roughly 76 to 116 kg/km. Use the chosen plant's datasheet, not a generic figure, when estimating drum weights and shipping cost.
Many networks use both: loose tube cable between buildings, a closure or panel at the entry point, and tight buffered cable inside. The transition point should be designed, not improvised, because outdoor cable usually cannot run far inside a building under fire rules. Local codes and the authority having jurisdiction set the allowed distance, so confirm with the project engineer.
What to ask on the datasheet
- Fibre grade and count: G.652.D, G.657 or OM grade, and the number of fibres.
- For loose tube: number of tubes, fibres per tube, fillers, tube colours to TIA-598 and the water-blocking method.
- For tight buffer: buffer diameter and material, aramid quantity and jacket material.
- Mechanical ratings: tensile long and short term, crush, bend radius static and dynamic, temperature range.
- Fire or outdoor ratings: CPR class or NEC rating for indoor cable; Telcordia GR-20 or GR-409 compliance if the buyer's network asks for it.
- Packing: drum length, commonly 2 km or 4 km for outdoor cable and 1 km or 2 km for indoor cable.
Lead time for both constructions depends on fibre allocation and tube or buffer scheduling and is confirmed in writing for each order; the MOQ and lead time page explains how the desk sets those terms.