GPON Networks: Architecture, Operation and FTTH Deployment

GPON (Gigabit Passive Optical Network) is the technology that underpins virtually all FTTH deployments in France and across Europe. Standardised by the ITU-T under the reference G.984, it allows a single central device to serve up to 128 subscribers via a network of fibres and passive splitters. Understanding its architecture and operation is essential for any professional involved in the design, deployment or maintenance of fibre optic networks.

Definition and standards of GPON

GPON is defined by the ITU-T G.984 standards series (G.984.1 to G.984.6), published between 2003 and 2008. Its full name — Gigabit-capable Passive Optical Network — indicates two fundamental properties: throughput in the order of Gigabits, and a fully passive distribution infrastructure (without electrical power between the central office and the subscriber).

Its main transmission parameters:

• Downstream rate (OLT → ONU): 2.488 Gbps shared among all subscribers of a PON port
• Upstream rate (ONU → OLT): 1.244 Gbps shared
• Wavelengths: 1490 nm downstream / 1310 nm upstream (+ 1550 nm for optional analogue CATV television)
• Maximum reach: 20 km between the OLT and the most distant ONU
• Maximum split ratio: 1:128 (typically 1:32 to 1:64 in real deployments)

By way of comparison, the next generation — XGS-PON (G.9807.1) — multiplies by 4: 10 Gbps symmetrical downstream and upstream, on the same fibre infrastructure and the same splitters.

Architecture of a GPON network: the 3 segments

A GPON network breaks down into three successive segments, each with a specific role and equipment:

Segment 1 — Transport network (Feeder)

The transport fibre connects the OLT (Optical Line Terminal), located in the operator's NRO (Optical Distribution Node), to the first level of splitter in the street or building. This segment typically uses high-density fibre cables (48 to 288 fibres) laid in underground conduit. Fibre here is precious — every transport fibre serves dozens of subscribers downstream.

Segment 2 — Distribution network

This segment runs from the first-level splitters (1:4 or 1:8) to the second-level splitters (1:8 or 1:16) located at the foot of the building or in street cabinets (PBO). It is the passive core of the network — no active equipment, only fibre and optical splitters. Maintenance of this segment is virtually nil over 25 to 30 years.

Segment 3 — Drop network

This last segment connects the PBO to the subscriber's home or premises — typically an individual fibre of 50 to 200 m laid on the facade or underground. It terminates at the wall-mounted PTO (Optical Terminal Socket) in the home, where the subscriber connects their patch cord to the ONU or operator box.

How GPON works: GEM, TDMA and DBA

The technical peculiarity of GPON lies in its transport protocol and its method of accessing the shared medium. Three mechanisms must be understood:

GEM (GPON Encapsulation Method)

GPON does not use native Ethernet to encapsulate data (unlike EPON). It uses its own transport protocol, GEM, which encapsulates all Ethernet, IP, VoIP and IPTV frames in variable-size GEM frames. Each service flow (Internet, voice, television) is identified by a unique GEM Port-ID — which allows clean separation of services without complex VLANs.

TDMA — Time Division Multiple Access

In the downstream direction, the OLT broadcasts continuously to all ONUs — each ONU reads only the frames bearing its identifier (PLOAM). In the upstream direction, several ONUs share the same fibre to the OLT: to avoid collisions, the OLT assigns each ONU precise time slots (to the microsecond) during which it is authorised to transmit. This is the TDMA principle.

DBA — Dynamic Bandwidth Allocation

TDMA allocation is not fixed: DBA dynamically adjusts the bandwidth granted to each ONU based on its real-time traffic. An inactive subscriber frees up throughput for a neighbour who is downloading. DBA algorithms are defined in T-CONT profiles (Traffic Container) — each T-CONT corresponds to a level of service guarantee (Best Effort, Committed Information Rate, etc.).

Security and encryption in GPON networks

A GPON network is a shared network: in the downstream direction, all frames sent by the OLT are received by all ONUs on the same PON port. Without protection, a malicious ONU could read the traffic of its neighbours.

GPON incorporates two security mechanisms:

• AES-128 encryption of downstream traffic — each ONU has a unique AES key negotiated with the OLT during registration. A subscriber's traffic is unreadable by their neighbours, even if they physically capture the signal.
• ONU authentication by serial number — each ONU has a unique GPON Serial Number (8 hexadecimal characters). The OLT only validates ONUs whose SN is on its whitelist, preventing unauthorised connections.

In enterprise deployments, an additional layer of security is often added via per-subscriber VLANs and MAC filtering rules on the OLT side — even if an unauthenticated ONU managed to register, it would only see its own isolated VLAN.

Residential, enterprise and campus GPON

While GPON was born for residential FTTH, its qualities — high throughput, passive infrastructure, long reach, multi-service — have established it in much broader environments:

Residential FTTH — the original use case. A central OLT serves a building or an entire neighbourhood. Each home has an ONU or a box integrating a GPON ONT. French operators (Orange, SFR, Bouygues) operate millions of GPON ports.

Enterprise and campus network — GPON advantageously replaces copper Ethernet cabling for large buildings. An OLT in the central technical rack serves ONUs in open-spaces, meeting rooms and floors via passive fibres. Zero intermediate switches, zero power supply in technical risers — maintenance is reduced to a single intervention on the OLT.

Hospitality and serviced residences — GPON is ideal for hotels and managed residences: each room has its ONU (Internet, telephony, IPTV), centralised management is done via the OLT's EMS software. See our Hotenet case study for a concrete example of an all-GPON hotel installation.

Rural areas and public initiative networks (RIP) — GPON's 20 km reach makes it possible to serve remote hamlets from a single NRO, without multiplying intermediate active equipment.

Planning a GPON deployment

The design of a GPON network is based on four interdependent parameters:

1. Number of subscribers and split ratio

The total split ratio (product of all splitter levels on the OLT → ONU path) determines how many subscribers share a PON port. A 1:4 × 1:16 = 1:64 ratio is the most common in residential deployment. Below 1:32, each subscriber has comfortable bandwidth even at peak hours.

2. Optical budget

The sum of all losses on the path (cable, connectors, splitters) must remain below the optical budget of the equipment class:

• Class B+: budget 28 dB — standard for urban deployments up to 20 km
• Class C+: budget 32 dB — for long distances or high split ratios
• Class C++: budget 35 dB — for ultra-dense deployments or extended rural areas

3. Splitter architecture

Two approaches: centralised split (single 1:64 splitter at the NRO, dedicated fibre to each subscriber) or distributed cascaded split (1:4 at the NRO + 1:16 at the PBO). Centralised split simplifies maintenance but consumes more transport fibres. Distributed split optimises cable utilisation at the cost of greater complexity at intermediate nodes.

4. OLT capacity and scalability

A 4-port PON OLT manages up to 256 subscribers (1:64 ratio). Plan a margin of 20 to 30% for future expansion, and choose an OLT whose line cards are replaceable for a GPON → XGS-PON migration without chassis replacement.

Migration from GPON to XGS-PON 10G

Standard GPON (2.5G/1.25G) is reaching its limits in the face of intensive cloud uses and the proliferation of connected devices. Migration to XGS-PON (10G/10G) is underway at all European operators.

The good news: XGS-PON and GPON coexist on the same infrastructure. The two technologies use different wavelengths (1270/1577 nm for XGS-PON vs 1310/1490 nm for GPON), which allows them to coexist via WDM filters on the same cables and splitters. Migration is therefore done ONU by ONU — the subscriber equipment is replaced during a field intervention, without touching the passive distribution network.

On the OLT side, modern equipment supports GPON and XGS-PON line cards in the same chassis. A PON port can even serve GPON ONUs (legacy subscribers) and XGS-PON ONUs (new subscribers) simultaneously with a COMBO OLT.

Frequently asked questions — GPON networks