Introduction: Indoor and Outdoor Optical Cable
Optical cable serves as the foundation of fiber‑optic communication lines. When designing a network, one of the first tasks is to determine where the route will run: inside a building or outdoors. This determines the choice between indoor and outdoor cable versions. The difference lies in the sheath materials, mechanical protection and resistance to external influences.
An indoor optical cable is intended for laying in premises: risers, cable ducts, false ceilings, trays. Its sheath is made of materials that do not propagate combustion, with low smoke emission. Typical marking is LSZH (Low Smoke Zero Halogen) or LSOH. An outdoor optical cable, on the contrary, is created for an aggressive environment: ultraviolet radiation, precipitation, temperature fluctuations, mechanical loads. Its sheath is reinforced polyethylene, often with armour of steel wire or tape. More details about fiber types and cable designs can be found in the material details about optical cable.
What both types have in common is the transmission of a light signal over single‑mode fiber 9/125 or multimode fiber 50/125. The number of fibers varies: 2 fibers, 4 fibers, 8 fibers, 16 fibers and more. But the operating conditions dictate completely different requirements for the protective coverings.
Comparative Table of Characteristics
| Parameter | Indoor optical cable | Outdoor optical cable |
| Outer sheath material | LSZH, LSOH, sometimes PVC of reduced flammability | Reinforced polyethylene (PE, HDPE), often with UV stabilization |
| Fire safety | Non‑propagation of combustion, low smoke formation, halogen‑free | Not regulated for premises; the sheath is combustible, emits smoke |
| Resistance to ultraviolet | Low, not designed for direct sunlight | High, the material is resistant to degradation from UV |
| Moisture resistance | Limited, only moisture from the air; no longitudinal sealing | High, often filled with hydrophobic gel, has water‑blocking tapes |
| Operating temperature range | From –10 to +50 °С | From –40 to +70 °С |
| Mechanical protection | Minimal, rarely armoured; only the sheath and aramid yarns | Armouring with steel tape or wire, fiberglass rods for self‑supporting designs |
| Permissible tensile force | Small, not standardized for heavy loads | High, up to several kilonewtons, especially for self‑supporting ones |
| Number of fibers | 2, 4, 8, 16, up to 64 | 2, 4, 8, 16, 32, 64 and above |
| Types of fibers | Single‑mode 9/125 and multimode 50/125, 62.5/125 | Mainly single‑mode 9/125, less often multimode for short inter‑building links |
| Bending radius | Smaller, up to 10 cable diameters | Larger, up to 20 diameters, especially for armoured ones |
| Cost (per metre) | Lower | Higher due to the reinforced design and gel filling |
| Installation complexity | Lower, termination is simpler, no sealing of ends required | Higher, moisture sealing is needed, careful handling of armour, sometimes splicing at height |
| Service life | 25 years indoors | 25–30 years for outdoor installation |
Key Differences in Sheath and Design
The main difference between indoor and outdoor optical cables is the polymer of the outer sheath. The indoor LSZH sheath contains no halogens, and when burning it emits little smoke and toxic substances. This is critical for escape routes, offices and residential buildings. An outdoor optical cable is covered with polyethylene, which is resistant to solar radiation, abrasion and moisture, but is combustible and in a fire gives black smoke with drops of melt. Therefore, laying an outdoor cable indoors is prohibited by fire safety standards — it will not pass inspection according to SP 5.13130 and Federal Law No. 123‑FZ.
Another important difference is protection against moisture. An outdoor cable often contains a hydrophobic gel that fills the voids around the fibers and under the sheath, as well as water‑blocking tapes. This prevents longitudinal water penetration if the sheath is damaged. An indoor cable does not contain gel, since it is laid in dry heated premises. Laying standards taking into account bending radii and protection against rodents are discussed in the article optical cable laying.
Armouring is another attribute of an outdoor cable. For laying in the ground, on gantries, underwater, armour made of steel wire or corrugated steel tape is used. An indoor cable rarely needs armour; its mechanical protection is limited to aramid yarns that hold the tension when pulling into ducts.
Types of Fibers and Count: Single‑mode, Multimode, 2, 4, 8, 16 Fibers
Both types of cable support single‑mode and multimode fibers. For backbone outdoor lines over long distances, single‑mode fiber 9/125 is always chosen. Multimode fibers 50/125 are used for indoor data center networks and local connections over short distances — up to 400 metres. The differences between fiber types are discussed in detail in the article about single‑mode and multimode.
The number of fibers is selected for a specific task. To connect the internet to a private house, 2 or 4 fibers are sufficient: one working, one backup. In an office distribution network, 8 fibers are laid, and in backbones between buildings — 16 fibers and more. An indoor cable usually contains up to 16 fibers, an outdoor one — up to 64 and above. Small numbers 2, 4, 8 are widely used in subscriber lines.
When an Indoor Optical Cable Is Needed
- Laying inside office and residential buildings, in cable ducts, above suspended ceilings, in risers. Fire safety requirements oblige the use of a cable with an LSZH sheath.
- Connection of workstations, IP cameras, access points indoors. A small number of fibers — 2 or 4 — is sufficient.
- Server and cross‑connect rooms — for patching between racks, an indoor multimode or single‑mode cable is used.
- Indoor distribution networks where there is no ultraviolet radiation and precipitation, and enhanced mechanical protection is not needed.
When an Outdoor Optical Cable Is Needed
- Laying between buildings: overhead on poles, in cable ducts, in the ground. The sheath withstands ultraviolet radiation, rain, frost.
- Backbone communication lines, including urban networks of telecom operators. An outdoor cable with armour is protected from rodents and mechanical damage.
- Optical entry into a building. From the pole to the service entrance panel, an outdoor self‑supporting cable runs, which then transitions to an indoor one through a splice closure.
- Infrastructure objects: bridges, tunnels, gantries, where the impact of the external environment is combined with vibrations and temperature fluctuations.
- Video surveillance systems in open areas, where the optical cable is laid along a fence or on a facade.
Recommendations for Use Scenarios
| Scenario | Cable type | Rationale |
| Apartment, internet from the stairwell | Indoor 2–4 fibers | Short route, flexibility and LSZH are needed |
| Office floor, horizontal distribution | Indoor 8–16 fibers | Fire safety, compactness |
| Cable between factory buildings across the territory | Outdoor armoured 8–16 fibers | Soil, possible mechanical impacts |
| Suspension on poles along a street | Outdoor self‑supporting 4–8 fibers | UV, wind, ice, high tensile forces |
| Entry to a private house from a pole | Outdoor 2–4 fibers + indoor 2 fibers with a splice closure | Outside — UV‑resistant, inside — fire‑safe |
| Server room in a data center | Indoor multimode 50/125 or single‑mode 9/125, up to 16 fibers | Short links, high density |
Typical Mistakes in Selection
- Laying an outdoor polyethylene cable inside a building. The sheath is combustible, does not comply with fire safety standards. The facility will not be accepted during inspection.
- Using an indoor cable outdoors. Ultraviolet radiation and moisture will quickly destroy the sheath, and the fibers will lose transparency due to microcracks.
- Ordering a cable without hydrophobic filling for underground laying. Moisture will penetrate along the fiber and cause an increase in attenuation.
- Incorrect choice of the number of fibers “for growth”. An excess of fibers increases the price, a shortage forces a new cable to be pulled. For an indoor network per floor, 8–12 fibers are usually enough, for a backbone — 16–24.
- Neglecting the bending radius when installing an outdoor armoured cable. This leads to microcracks and an increase in attenuation on the installed line.
- Using a cable without an LSZH sheath in hospitals, schools, shopping centres — a direct path to prescriptions from the Ministry of Emergency Situations.
Frequently Asked Questions
What is the difference between an indoor and an outdoor optical cable?
An indoor one has an LSZH sheath, does not propagate combustion, is not designed for ultraviolet radiation and precipitation. An outdoor one has a polyethylene sheath that is weather‑resistant and often has armour and gel filling.
Can an outdoor cable be used inside a building?
No, if its sheath is not certified as LSZH. Polyethylene burns, emits dense smoke, which is prohibited by fire safety standards. An exception is special types with an LSZH sheath that are suitable for both outdoors and indoors, but they are rare and expensive.
How many fibers should be chosen for internet to an apartment?
2 fibers are enough: one for reception, one backup. Sometimes 4 are laid if several services from different providers are planned.
What is LSZH and where is it used?
LSZH — Low Smoke Zero Halogen, a halogen‑free sheath with low smoke and gas emission. It is used for indoor optical cables in buildings with mass occupancy.
What type of fiber is needed for outdoor installation — single‑mode or multimode?
For distances over 400–500 metres — only single‑mode 9/125. For short inter‑building links up to 300 m, multimode 50/125 is acceptable, but economically single‑mode is more often chosen, as it is more versatile.
What is the difference between 50/125 and 9/125 cable?
The first number is the core diameter in microns. 9/125 — single‑mode fiber for long routes, 50/125 — multimode for short ones. More details — in the article about single‑mode and multimode.
Conclusion
The choice between an indoor and an outdoor optical cable is dictated by the place of installation. Indoor — for premises, with an emphasis on fire safety and compactness. Outdoor — for the outside, with protection from weather, ultraviolet radiation and mechanical loads. A mistake in this choice leads either to supervisory prescriptions or to cable failure within a few seasons. The range of indoor and outdoor optical cables, including single‑mode and multimode with the number of fibers 2, 4, 8, 16, is presented in the catalogue optical cable.
