Optical cable construction and types

The fibre carries the light; the cable carries the fibre through ducts, soil, air, buildings and data-centre trays without letting it feel strain, water, rodents or fire. A cable is a set of layers, each solving one problem, and the designs differ in how they combine those layers. This page describes the layers, the main cable families and where each is used, the fire ratings that govern indoor cable, the fibre and tube colour codes used to identify fibres, and how to read a cable data sheet so the cable, the route and the link budget agree.

The layers

Layer	What it is	What it does
Coated fibre	glass 125 µm with dual acrylate coating to 250 or 200 µm, coloured	the optical path (How fibre is made)
Buffer	loose tube — 12 or 24 fibres float in a 2–3 mm plastic tube with gel or dry water-blocking yarn; tight buffer — 900 µm plastic layer on each fibre	loose tube decouples fibre from cable strain and temperature (0.1–0.5 % excess fibre length); tight buffer makes fibres handleable indoors
Ribbon	4–24 fibres bonded side by side in a matrix; rollable ribbons are bonded intermittently	mass fusion splicing, extreme density
Central strength member	fibreglass rod (FRP/GRP) or steel wire	anti-buckling, takes compression and part of the tension
Stranding	tubes and fillers stranded around the core, usually in reversing SZ pattern	mid-span access to any tube without cutting the others
Peripheral strength members	aramid yarns, fibreglass yarns, steel wires	take the installation tension so the fibre does not
Water blocking	petroleum-based gel in tubes and core, or dry super-absorbent yarns and tapes	stops water travelling along the cable and freezing on the fibres
Armour	corrugated steel tape, steel wire armour, or dielectric fibreglass yarns	crush, rodents, direct burial; dielectric where lightning or induced voltage rule out metal
Jacket (sheath)	polyethylene (HDPE/MDPE) outdoors, LSZH or PVC indoors, polyurethane or nylon for special environments	mechanical, UV and chemical protection; fire behaviour indoors
Ripcords, markings	nylon cords under the jacket; printed metre marks, type, count, date	opening without a knife; identification and length accounting

Cable families

Family	Construction	Fibre count	Where
Stranded loose tube	6–12 tubes × 12 fibres SZ-stranded around a central member, gel or dry, single or double jacket	12–288, up to 432+ with 24-fibre tubes	the default outdoor cable: ducts, direct burial with armour, aerial with messenger
Central (uni-) tube	one large tube with all fibres, strength members in the jacket	2–96	small counts, drop and distribution, cheap
Ribbon loose tube	stacks of 12-fibre ribbons in tubes or a central tube	144–1 728	metro backbones, hyperscale interconnect
Rollable ribbon	intermittently bonded ribbons rolled into tubes; 200 µm fibres	1 728–6 912 in 30–40 mm	data-centre interconnect, dense ducts
Micro-cable	small-diameter loose tube or ribbon with 200 µm fibres and a thin jacket	12–288 in 4–10 mm, 432 in ~12 mm	blown into microducts (5/3.5 to 16/12 mm); fill 55–80 % of the duct bore (Installation)
Tight-buffered distribution	900 µm fibres with aramid under one LSZH jacket	2–144	indoor backbones, risers, patch panels; needs breakout kits or pigtails
Breakout (fan-out)	each fibre in its own 2–3 mm sub-cable	2–24	direct connectorisation, harsh indoor, short runs
Indoor/outdoor	loose tube with LSZH jacket and dry core	12–144	one cable from the manhole to the rack without a transition splice
ADSS (all-dielectric self-supporting)	loose tube core, aramid yarns sized for the span, track-resistant jacket near HV	12–144	power-line corridors; spans 50–1 500 m; no metal anywhere
OPGW (optical ground wire)	fibres in a stainless tube inside aluminium-clad steel strands	12–144	replaces the HV line's lightning shield wire; utility backbones
Figure-8	cable moulded to a steel messenger	12–144	fast aerial deployment on poles
FTTH drop	flat 2 × 5 mm with two FRP rods, or round 3–5 mm; G.657 fibres	1–4 (12)	pole or duct to the home; indoor drop 2–3 mm; "invisible" 0.9 mm inside flats
Submarine	steel-tube fibre unit, steel wire vault, copper power conductor, polyethylene insulation; armoured near shore	4–24 fibre pairs	intercontinental and coastal systems
Hybrid / composite	fibres plus copper power or coax	any	remote radio heads, cameras, PoE extenders
Tactical / deployable	ruggedised polyurethane, 2–12 fibres, expanded-beam or hardened LC connectors	2–12	broadcast, military, temporary events
Pre-terminated trunk	MPO/MTP-terminated 12–144 fibres with pulling grips, factory tested	12–144 (bundles to 3 456)	data centres — install in hours, no field splicing (Fibre network topologies)

Indoor fire ratings

Regime	Ratings	Meaning
USA / NEC	OFNP (plenum, NFPA 262), OFNR (riser, UL 1666), OFNG/OFN (general, UL 1685); OFC when conductive (metal inside)	where the cable may run: air-handling spaces, vertical shafts, general
IEC / LSZH	IEC 60332-1-2 (single cable flame), 60332-3 (bunched cables), 61034 (smoke density), 60754 (halogen acid gas)	low smoke, zero halogen — the default indoor jacket outside North America
EU / CPR (EN 50575)	Euroclasses Aca, B1ca, B2ca, Cca, Dca, Eca, Fca plus s1–s3 smoke, d0–d2 droplets, a1–a3 acidity	mandatory declaration for permanently installed cable since 2017; typical requirements Cca-s1b,d1,a1 or Dca
Russia / EAEU	GOST 31565-2012 designations: нг(A), нг(A)-LS (low smoke), нг(A)-HF (halogen-free ≈ LSZH), нг(A)-FRLS / FRHF (fire resistant), LTx (low toxicity)	which type is allowed in which building category

Outdoor polyethylene jackets are flammable and must stop at the building entrance (typically within 15 m or at the first splice point) unless the cable is dual-rated indoor/outdoor.

Fibre and tube colour codes

Position	TIA-598-C / IEC 60304 colour	Position	Colour
1	blue	7	red
2	orange	8	black
3	green	9	yellow
4	brown	10	violet
5	slate (grey)	11	rose (pink)
6	white	12	aqua

Fibres 13–24 in a 24-fibre tube repeat the sequence with a black tracer stripe (yellow tracer on the black fibre). Tubes follow the same sequence; fillers are natural or grey. Fibre number in the cable = (tube number − 1) × 12 + fibre position. Other schemes exist — the German DIN VDE 0888 sequence (red, green, blue, yellow, white, grey, brown, violet, turquoise, black, orange, pink) and manufacturer-specific sets with uncoloured fibres in Russian and Asian cables — so the splice plan must state which scheme the cable uses (Documentation & labelling).

Jacket colours: yellow single-mode, orange OM1/OM2, aqua OM3/OM4, lime OM5, black outdoor (any fibre); indoor colours are conventions, not standards (Labels & colour codes).

Reading a cable data sheet

Parameter	Typical values (outdoor loose tube)	Why it matters
Fibre type and count	G.652.D / G.657.A1, 12–288; OM4 indoors	link budget and connector plan (Fibre types)
Cabled attenuation	≤ 0.36 dB/km at 1310, ≤ 0.22 dB/km at 1550 (max); design with these, not the bare-fibre values	the fibre in a cable at −40 °C is not the fibre on a spool (Link budget)
Tensile strength	installation 1 500–2 700 N (ADSS 10–30 kN by span); long-term 600–1 000 N	pulling and blowing limits; fibre strain stays below 0.2 % long-term (Reliability & ageing)
Crush resistance	1 000–3 000 N per 100 mm (IEC 60794-1-21 E3)	direct burial, trays, clamps
Impact, torsion, repeated bending	pass per IEC 60794-1-2	handling on site
Minimum bend radius	20 × OD under tension, 10 × OD installed (15 × / 10 × for some)	duct bends, closure entry, tray corners
Temperature	installation −10 … +50 °C; operation −40 … +70 °C (−60 °C variants for northern climates); storage −40 … +70 °C	cold installation cracks jackets; cold operation adds microbend loss
Water penetration	1 m head, 24 h, no leakage over 3 m (IEC 60794-1-22 F5)	closures still must be sealed — the cable blocks longitudinal flow only
Diameter and weight	8–20 mm, 60–300 kg/km	duct fill, reel sizes, pole loads
Armour and jacket	CST, SWA, dielectric; HDPE, LSZH, nylon anti-termite	environment (Installation)
Fire rating	see above	indoor route
Reel length	2–6 km typical, up to 12 km for micro-cables	number of splice points on the route
Standards	IEC 60794-3 (outdoor), 60794-2 (indoor), 60794-4 (aerial along power lines), 60794-5 (microduct); Telcordia GR-20; GOST R 52266 in Russia	what to write in the tender (Standards map)

Choosing the cable

Situation	Choose	Avoid
Duct, long route, growth expected	stranded loose tube 144–288, or microducts with blown micro-cables	tight-buffered indoor cable outdoors
Direct burial	armoured loose tube (CST or dielectric), double jacket	unarmoured cable in rodent country
Along or on HV lines	ADSS sized for span and electric field, or OPGW at the shield-wire position	any metallic cable near HV
Poles, fast deployment	figure-8 or ADSS with short spans	lashing indoor cable to a messenger
Building entrance to rack	indoor/outdoor dry LSZH loose tube	outdoor PE cable running 50 m through a building
Riser and horizontal indoors	tight-buffered distribution LSZH/OFNR, plenum-rated where required	gel-filled cable vertically (gel migrates)
Data centre between rows	pre-terminated MPO trunks, rollable-ribbon high-count between halls	field splicing hundreds of fibres
FTTH last 50 m	flat or round drop with G.657.A2/B3	G.652 in tight indoor bends
Harsh temporary	tactical polyurethane with hardened connectors	standard patch cords outdoors

In CodingBox

Nothing in a module's memory describes the cable, but two of its fields interact with cable choice: the fibre-length ratings assume the cabled attenuation of a standard cable, and the connector code must match the trunk termination — MPO for parallel optics, LC for duplex. CodingBox shows both so the cable, the panel and the module are ordered as one system (Check transceiver, Breakout & MPO cabling).