How does a fiber optic connection work?

Introduction

Have you seen technicians digging trenches in your street, installing green enclosures on building facades, or running cables along poles? These are the signs that a fiber optic connection is being deployed in your neighbourhood.

A fiber optic connection is a team effort that brings together civil engineering specialists, cable layers, fiber splicers, network technicians, and service activation agents. It can take from a few weeks to several months depending on the size of the project.

In this course, we will follow the complete path of the fibre, from the operator’s optical connection node (NRO) all the way to the wall socket at the subscriber’s premises.

Figure 1 — Complete FTTH network architecture: NRO → SRO → PBO → PTO

---

1. Overview of the FTTH network

1.1 The four network nodes

An FTTH network consists of four successive levels, each with a precise role:

Acronym	Full name	Role	Location
NRO	Optical Connection Node	Signal source, OLT installed	Operator building, air-conditioned
SRO	Optical Sub-Distribution Node	Redistribution to neighbourhoods	Street cabinet, 200–500 m from NRO
PBO	Optical Branching Point	Last distribution level	Enclosure on façade or pole
PTO	Optical Terminal Socket	Subscriber-end connection	Interior wall socket

1.2 The tree logic

Think of a tree: the NRO is the trunk, the SRO is a large branch, the PBO is a small branch, and the PTO is the leaf that reaches each dwelling. The fibre splits progressively through passive splitters (dividers that require no electricity) to serve more and more subscribers.

Concrete example: one NRO can feed several dozen SROs. Each SRO serves several hundred PBOs. Each PBO typically serves 4 to 16 subscribers. A single NRO can therefore cover several thousand homes.

---

2. Step 1: Civil engineering works

2.1 Planning and authorisations

Before anything is dug, the operator or its subcontractor must:

• Study the maps of existing underground networks (water, gas, electricity, telecoms)
• Submit a notice of intention to commence works to identify all underground networks
• Obtain road occupation permits from the municipality or local authority
• Notify residents (mandatory notice in Côte d’Ivoire)

Key takeaway: never dig without first verifying the presence of underground networks. Accidentally striking an electricity cable or a gas pipe can cause a serious accident.

2.2 Trench excavation

For underground cables, trenches are excavated according to precise rules:

• Width: between 20 and 40 cm depending on the number of conduits
• Depth: at least 50 cm in pavements, 60 cm in roadways
• Signage: cones, safety barriers, and DANGER signs are mandatory

2.3 HDPE conduits

Cables are not buried directly in the trenches. Conduits (HDPE — High-Density Polyethylene tubes) are installed first:

• Blue colour for telecommunications (CI standard)
• Internal diameter of 40 mm to 63 mm depending on the zone
• A warning tape in red-brown is laid 30 cm above the conduits

2.4 Pull pits (draw pits)

At regular intervals (every 50 to 100 metres), pull pits (inspection chambers) are installed. They allow technicians to access the conduits in order to:

• Pull cables through successive sections
• Perform joints if necessary
• Inspect and maintain the network

2.5 Façade and aerial installation

In certain areas, particularly in older neighbourhoods or rural zones, cables are not buried but are instead installed:

• On facades: fixed with clamps or collars on building walls
• Aerially: suspended between poles, with a metallic messenger wire to support the weight

---

3. Step 2: The distribution cable (NRO → SRO)

3.1 Characteristics of the distribution cable

The cable linking the NRO to the various SROs is the largest cable in the network. It contains a large number of fibres because it must feed many neighbourhoods:

Capacity	Typical use
48 to 72 fibres	Small residential neighbourhood
96 to 144 fibres	Dense neighbourhood, medium-sized city
288 to 432 fibres	Major urban artery, main road

This cable is armoured: it contains steel wires or fibreglass reinforcement rods to withstand mechanical stress during pulling and throughout its service life on the ground (20 to 30 years).

3.2 Cable pulling

Pulling a cable through a conduit is done using a motorised or manual cable puller. The technique involves:

1. First passing a pulling rod (thin, strong wire) through the conduit
2. Attaching the cable to the pulling rod
3. Pulling progressively, lubricating the cable if necessary
4. Respecting the minimum bend radius (approximately 30 cm) to avoid breaking fibres

3.3 Splices at the NRO

Once the cable arrives at the NRO, the fibres are fusion-spliced to the ODF (Optical Distribution Frame) fibres. The ODF is the optical patching panel that organises connections between the active equipment (OLT) and the distribution cables.

Each splice must have a loss below 0.05 dB (fusion). Poor-quality splices are the primary cause of network faults.

3.4 The passive splitter at the SRO

At the SRO level, a passive splitter (also called an optical coupler) divides the signal into several sub-signals. For example, a 1:8 splitter transforms 1 incoming fibre into 8 outgoing fibres, each directed toward a different sector.

Splitter type	Ratio	Optical loss
1:2 splitter	1 → 2 fibres	~3.5 dB
1:4 splitter	1 → 4 fibres	~7 dB
1:8 splitter	1 → 8 fibres	~10.5 dB
1:16 splitter	1 → 16 fibres	~13.5 dB
1:32 splitter	1 → 32 fibres	~16.5 dB

---

4. Step 3: The branching cable (SRO → PBO)

4.1 From the SRO to the PBO enclosure

The branching cable links the SRO to the various PBOs (Optical Branching Points) distributed throughout the neighbourhood. It is a more flexible, less bulky cable (generally 12 to 24 fibres) than the distribution cable.

4.2 Installing the PBO enclosure

The PBO is the enclosure commonly seen fixed to building facades, on poles, or sometimes in basement niches. It contains:

• A second level of splitter (1:4, 1:8, or 1:16)
• Trays for organising fibres
• SC/APC connectors for plugging in subscriber cables

Key takeaway: the PBO is the fibre “delivery point” for a building or group of houses. It is from this enclosure that the technician individually connects each subscriber.

4.3 Continuity check

After each section of the network, a continuity test is performed using an optical network detector (light tester). This simple test verifies that the fibre path is clear and that there are no breaks.

---

5. Step 4: The subscriber cable (PBO → PTO)

5.1 The drop cable

The cable linking the PBO to the terminal socket at the subscriber’s premises is called the drop cable or subscriber connection cable. Its characteristics:

• 2 fibres (one active, one spare)
• Flexible and lightweight
• UV-resistant outer jacket (for installation on facades)
• Typical length: 10 to 100 metres

5.2 Routing through the building

The cable can be installed in several ways depending on the building configuration:

• On the facade: fixed with clips or an external cable trunking up to the window or technical duct
• In the technical duct: in buildings with an existing telecoms duct
• Under skirting board: inside the dwelling, concealed under plastic skirting board

5.3 The optical terminal socket (PTO)

The PTO is installed inside the dwelling or office. It is a small wall box with:

• An SC/APC connector (green colour) to receive the patch cord toward the ONT
• A dust protection cover
• An engraved reference to identify the fibre

Important: the PTO must never be opened by the subscriber. It is the property of the operator.

---

6. Step 5: Service activation

6.1 Optical power measurement

Before activating the service, the technician measures the optical signal power at the PTO using an OPM (Optical Power Meter).

Power level	Interpretation
-5 to -25 dBm	Excellent signal, perfect connection
-25 to -30 dBm	Acceptable signal, check connectors
Below -30 dBm	Insufficient signal, locate and correct the fault
No signal	Fibre break or incorrect connection

6.2 OTDR measurement (optional)

For complex installations or in the event of a problem, an OTDR (Optical Time-Domain Reflectometer) can pinpoint exactly where a fault is located in the fibre. This measurement is more advanced and will be covered in a dedicated course.

6.3 Connecting the ONT

Once the optical power has been validated, the technician connects the ONT (Optical Network Terminal — also called the fiber box):

1. An SC/APC patch cord links the PTO to the ONT
2. The ONT connects to the Wi-Fi router via an RJ45 Ethernet cable
3. Activation is triggered remotely by the operator (within a few minutes)

6.4 Service tests

At the end of the installation, the technician performs the final checks:

• Speed test: verification that the promised throughput is achieved (e.g. 100 Mbit/s, 1 Gbit/s)
• Latency test: verification that the response time is acceptable (< 10 ms on fibre)
• Phone test: if included in the service offer
• TV test: if included in the service offer
• Document handover: service activation form signed by the subscriber

---

7. Duration and cost of a connection

7.1 Typical duration per phase

Phase	Average duration
Civil engineering (trenches, conduits)	1 to 3 weeks depending on length
Distribution cable installation and splicing	1 to 2 weeks
PBO enclosure installation	3 to 5 days
Subscriber connection	2 to 4 hours per dwelling
Service activation	30 to 60 minutes per subscriber

7.2 The teams involved

An FTTH connection mobilises several specialist teams:

• Civil engineering team: excavation, conduits, backfilling
• Cable pulling team: threading cables through conduits
• Splicing team: fibre fusion splices at the NRO, SRO, and PBO
• Subscriber connection team: drop cable, PTO, ONT
• Service activation team: testing, activation, documentation

---

8. Recap quiz

1. What does PBO stand for?

• a) Passive Base Optic
• b) Optical Branching Point
• c) Port of Base Optics
• d) Optical Patching Board

2. What is the minimum trench depth for fibre in a pavement?

• a) 20 cm
• b) 30 cm
• c) 50 cm
• d) 100 cm

3. A 1:8 splitter produces how many output fibres?

• a) 1
• b) 4
• c) 8
• d) 16

4. Which measurement is used to check signal power at the terminal socket?

• a) OTDR
• b) OPM (Optical Power Meter)
• c) VFL (Visual Fault Locator)
• d) BERT

Answers: 1-b, 2-c, 3-c, 4-b

---

Conclusion

A fiber optic connection is not a simple cable plug-in: it is a chain of precise operations requiring varied skills and rigorous organisation. Every step is critical — a poor-quality splice, a dirty connector, or an over-bent cable can degrade service quality for dozens of subscribers.

This is why technician training is essential. In Côte d’Ivoire, the fiber optics sector is actively recruiting to support the ongoing deployment in Abidjan and other major cities.

Estimated duration: 40 minutes · Level: beginner

Want to learn how to carry out a fiber optic connection yourself, from splicing to OTDR testing? Discover the certified fiber optic training courses offered by KMC in Abidjan.