Almost every system in a building sends its traffic through the backbone: the network, CCTV, access control, telephony and building management. A fiber optic backbone in buildings links telecom rooms across floors and between buildings at distances and speeds copper cannot reach, provided the fiber is chosen, installed and tested correctly.
This guide covers fiber types and cable construction, connectors and splicing, the Saudi Communications, Space and Technology Commission (CST) rules for infrastructure in new buildings, testing methods and loss budgets, documentation, and common mistakes.
Key takeaways
- Multimode fiber (OM3, OM4, OM5) suits short runs inside a building; singlemode (OS2) suits long backbones and links between buildings.
- The CST rules for new multi-dwelling buildings require at least four fiber connections per unit, SC/APC or LC/APC connectors, and documentation kept in the telecom room.
- Loss testing with an OLTS is the deciding acceptance test; the OTDR locates events and diagnoses faults.
- Inspecting and cleaning connector end faces before mating prevents a large share of loss and reflectance problems.
- Calculate the loss budget for each link from its length, connectors and splices, not as one figure for the whole project.
In this article
- Why fiber in the building backbone
- Fiber types
- Cable construction and selection
- Connectors and splicing
- CST requirements for new buildings
- Testing: Tier 1 and Tier 2
- Calculating the loss budget
- Documentation and handover
- Fiber optic backbone in buildings: common mistakes
- How AZEIDK can help
- Frequently asked questions
- Conclusion
Why fiber in the building backbone
Copper is limited to a 100 m channel, while fiber runs hundreds of metres in multimode and kilometres in singlemode. It is immune to electromagnetic interference and does not carry ground potential differences between buildings, which makes it the natural choice for links across compounds and industrial sites. On the other hand, fiber carries no power, so equipment at its ends needs a local supply. We compared copper and fiber in general in our structured cabling and networks guide; here we focus on fiber itself.
Fiber types
ISO/IEC 11801 classifies cabled optical fiber into categories. The most relevant for new projects are:
| Category | Type | Defining property per ISO/IEC 11801 | Typical use |
|---|---|---|---|
| OM3 | Multimode, 50 µm core | Minimum modal bandwidth 2000 MHz·km at 850 nm | In-building backbone over short distances |
| OM4 | Multimode, 50 µm core | 4700 MHz·km at 850 nm | Server rooms and longer in-building links |
| OM5 | Wideband multimode | Designed for multiple wavelengths between 850 and 953 nm | Data centres |
| OS1a | Singlemode | Maximum attenuation 1 dB/km at 1310, 1383 and 1550 nm | Indoor cables |
| OS2 | Singlemode | Maximum attenuation 0.4 dB/km at the same wavelengths | Long backbones and links between buildings |
For residential and office buildings connected to service providers, the CST rules require ITU-T G.657 to be followed for bend-insensitive singlemode fiber in distribution networks and customer premises, because it tolerates tighter bend radii inside units.
Cable construction and selection
- Tight-buffered cable: flexible and easy to terminate directly, suited to indoor routes.
- Loose-tube cable: protects fibers from tension, heat and moisture, suited to outdoor and long routes.
- Indoor/outdoor cable: runs from an external route into the building without an extra transition point.
- Armoured cable: for routes exposed to rodents or mechanical load in underground ducts.
Fire performance is set by the project specification, for example low-smoke zero-halogen cable, or riser and plenum ratings on projects that follow US specifications. Choose the fiber count to cover current needs with spare capacity: extra fibers cost far less at installation than a new cable later.
Connectors and splicing
The most common building connectors are LC and SC, with multi-fiber MPO in data centres. The polish is either UPC or angled APC, which reduces reflectance, and the two must never be mated with each other. The CST in-building rules for multi-dwelling units require SC/APC or LC/APC connectors.
Fibers are joined by fusion splicing a pigtail or by field-installed connectors. The same rules ask for splicing to be avoided where possible in the in-building infrastructure, for splice loss not to exceed 0.15 dB, and for splice return loss to be unmeasurable.
CST requirements for new buildings
By Board decision No. 6-134-2018 dated 12/9/1439H (27 May 2018), the Communications and Information Technology Commission, now the Communications, Space and Technology Commission, approved the Rules for ICT Infrastructure Provision and Deployment in New Developments. The rules require every new building to have high-speed-ready in-building physical infrastructure installed by the developer during construction, including lead-in ducts and cabling between the access point and the telecom room. Key points of the technical annex for multi-dwelling units published with the decision include:
- At least four fiber connections per unit, 15% spare capacity in the riser, and a minimum riser conduit diameter of 25 mm.
- A telecom room for buildings with more than 30 units, air-conditioned to 25 ± 10 °C with 40% to 60% humidity, and space for at least three providers at two square metres each.
- At least one network termination in each residential room other than wet rooms such as bathrooms and laundries, with a power outlet near the optical termination.
- Clear, unique labelling of all infrastructure components matching the as-built drawings, with records kept in the telecom room and updated after any change.
For the outside plant up to the access point, the rules require OTDR measurements on installation, recorded in the as-built documentation. Because CST has since published an update to the telecommunications infrastructure regulations for new development areas, check the version in force on the CST website, and the service provider’s requirements, before the design is approved.
Testing: Tier 1 and Tier 2
Tier 1 testing is the basis for acceptance in TIA practice: an optical loss test set (OLTS) measures total loss in dB, length and polarity, and the loss is compared with the link’s calculated budget. For multimode, current test practice recommends an encircled flux compliant launch so that results from different testers agree, with the reference set using the one-cord method, as Fluke Networks explains.
Tier 2 testing is usually optional: an OTDR trace shows the position, loss and reflectance of each connector, splice and bend, which helps locate faults and document long links. It does not replace loss testing for acceptance, because it can underestimate loss in multimode fiber by an unpredictable amount.
Inspect every connector end face under a microscope before mating or testing. IEC 61300-3-35 sets the inspection criteria; in its third edition (2022), only the core and cladding zones are graded, and measured optical performance takes precedence over a failed visual inspection. The working rule is: inspect, clean, and inspect again.
Calculating the loss budget
The budget is the sum of three elements: fiber attenuation multiplied by length, mated connector pair loss, and splice loss. According to The Fiber Optic Association (FOA), TIA-568 sets maximum values of 0.75 dB per connector pair and 0.3 dB per splice, which work as conservative planning figures before the actual product values are known. Take a 300 m OS2 backbone link with a connector pair at each end and two pigtail splices:
- Fiber: 0.3 km × 0.4 dB/km = 0.12 dB.
- Connectors: 2 × 0.75 = 1.5 dB.
- Splices: 2 × 0.3 = 0.6 dB.
- Budget: about 2.2 dB.
If measured loss clearly exceeds this, the cause is usually a dirty or damaged connector, a tight bend or a poor splice, and the OTDR will locate it. Also check that the budget is below the limit allowed by the application running on the link.
Documentation and handover
The handover file for each link should include the link ID under the agreed labelling scheme, wavelengths, test direction, reference method, calculated budget, measured result and OTDR traces where specified, together with as-built route drawings and labelling schedules. Keep a copy in the telecom room, as the CST rules require.
Fiber optic backbone in buildings: common mistakes
- Mating an APC connector with a UPC connector, which raises loss and reflectance.
- Connecting without inspecting and cleaning the end face.
- Exceeding the allowed bend radius in racks, enclosures and shafts.
- Relying on the OTDR alone to accept multimode links.
- Using indoor cable on an outdoor route exposed to moisture and heat.
- No spare fibers, so the first expansion needs a new cable.
- Missing labels, or labels that do not match the drawings, which makes links hard to trace during maintenance.
How AZEIDK can help
AZEIDK provides network and communication systems, including the fiber backbone that CCTV and access control systems rely on. We install and test the links through our installation and implementation service, deliver as-built drawings and test reports in the handover file, and support the system after commissioning under our annual maintenance contracts.
Frequently asked questions
When should I choose singlemode instead of multimode?
For long links, links between buildings and future speed upgrades. Multimode suits short runs inside the building and server rooms.
What is the difference between APC and UPC connectors?
An APC connector is polished at an angle to reduce reflectance and is usually green. The two types must not be mated.
Is an OTDR test enough for acceptance?
No. OLTS loss testing is the basis, and the OTDR complements it by locating events and documenting long links.
How many fibers does each flat need in a new building?
The CST rules for multi-dwelling buildings require at least four fiber connections per unit, plus spare capacity.
Why is loss high on a new link?
The most common causes are a dirty connector, a tight bend or a poor splice. Start by inspecting and cleaning the end faces, then use an OTDR to locate the problem.
Conclusion
A fiber optic backbone in buildings is a long-lived investment when the fiber type matches the distance and application, the cable suits the route, connectors are compatible and clean, CST requirements are applied in new buildings, and every link is tested against a calculated budget and documented. Then the backbone becomes a reliable foundation for every system in the building.
Need a fiber backbone designed or tested for a building or compound? Send us the drawings and telecom room locations, and we will propose the fiber and cable types and a testing and documentation plan. Talk to the AZEIDK team
