FTTH basics: OLT, ONU and splitters

What FTTH is and how it delivers internet

FTTH (Fiber to the Home) is a network architecture that runs fiber optic cable from the ISP's central site all the way to each subscriber's premises, replacing copper (DSL) or coaxial cable for the last mile. DataRadius is ISP billing and management software that monitors OLTs and ONUs, giving fiber providers signal-level visibility alongside subscriber billing in one place.

Unlike DSL, which sends electrical signals over copper telephone lines, or cable, which sends RF signals over coaxial cable, FTTH uses pulses of light traveling through glass fiber. Light does not degrade over distance the way electrical signals do — a fiber strand can carry data 20 km with minimal signal loss, while copper struggles beyond 5 km. Light also carries far more data: a single fiber strand can transport hundreds of gigabits per second, while the best copper technologies top out in the tens of megabits.

For an ISP, FTTH means two things: higher speeds per subscriber and longer reach without active amplification between the central site and the subscriber. The trade-off is higher upfront cost — fiber cable, splicing equipment, and subscriber-side devices — but lower ongoing maintenance because fiber does not corrode, does not pick up electrical interference, and does not degrade in wet conditions the way copper does.

An FTTH network has three main components: the OLT at the provider's site, splitters in the field, and an ONU at each subscriber's premises. A single strand of fiber leaves the OLT, passes through one or more splitters that divide the light signal into multiple copies, and each copy reaches a different subscriber's ONU. This point-to-multipoint architecture is what makes FTTH economical — one OLT port and one fiber strand can serve 32 or 64 subscribers.

The OLT: the ISP's fiber hub

The OLT (Optical Line Terminal) is the device at the ISP's central site — the headend, data center, or point of presence — that manages all fiber connections to subscribers.

Think of the OLT as the equivalent of the router in a copper network, but purpose-built for fiber optics. It converts Ethernet traffic from the ISP's upstream network into optical signals that travel down fiber to subscribers, and it converts the optical signals coming back from subscribers into Ethernet traffic headed upstream.

An OLT performs four main functions:

Aggregation. The OLT connects to the ISP's core network via 10 Gbps or 25 Gbps Ethernet uplinks and aggregates traffic from all PON ports. A typical chassis has 8 to 16 PON ports. A 16-port OLT with 1:32 splits serves 512 subscribers from a single device.

Scheduling. Multiple ONUs share the same upstream fiber strand, so the OLT coordinates transmit timing using TDMA (time-division multiple access). Each ONU transmits only during its assigned time slot, preventing signal collisions.

Management. The OLT tracks all connected ONUs — serial numbers, status, optical signal levels, configuration. Through the OMCI protocol (ITU-T G.988), the OLT remotely configures each ONU: VLAN tags, speed limits, port settings, and firmware versions.

Authentication. New ONUs register with the OLT using their serial number. The OLT accepts or rejects based on its policy — auto-accept, whitelist, or manual approval — preventing unauthorized devices from joining.

OLTs from vendors like Huawei, ZTE, Nokia, Calix, and FiberHome all follow the ITU-T G.984 standard, making the core GPON protocol interoperable across brands.

Splitters: dividing light without electricity

A splitter is a passive optical component that takes a single fiber input and divides the light signal into multiple outputs — without any power supply, without any electronics, and without any signal processing.

A 1:32 splitter takes one fiber strand carrying an optical signal and produces 32 identical (but weaker) copies, each going to a different subscriber's ONU. The splitter does not amplify, regenerate, or modify the signal. It simply divides the optical power equally among its output ports.

Splitters come in standard ratios:

Split ratio	Output ports	Signal loss per port	Typical use case
1:2	2	3.5 dB	Redundancy, backbone splitting
1:4	4	7.0 dB	Small MDU, cabinet distribution
1:8	8	10.5 dB	Building distribution, rural
1:16	16	13.0 dB	Dense residential, urban cabinets
1:32	32	17.0 dB	Standard urban deployment
1:64	64	20.0 dB	High-density deployment, short distance

The signal loss column is critical. Every time light passes through a splitter, it gets weaker. A 1:32 splitter loses approximately 17 dB of optical power — the light coming out of each output port is about 1/50th as strong as the light going in. This loss is unavoidable because the splitter divides the total energy among all outputs.

ISPs deploy splitters in two patterns:

Single-stage splitting. One splitter between the OLT and the ONUs — for example, a 1:32 splitter in a street cabinet fanning out to 32 drop cables. Simple to design and troubleshoot.

Cascaded (two-stage) splitting. A first-level splitter feeds second-level splitters closer to subscribers. A 1:4 at a hub feeding four 1:8 splitters in buildings gives an effective 1:32 split, using less trunk cable at the cost of slightly higher total insertion loss.

Dense urban areas favor cascaded splitting. Spread-out suburban or rural areas favor single-stage. Because splitters are passive — no power, no firmware — they are the most reliable component in an FTTH network, with the most common failure being physical damage to fiber pigtails.

The ONU: light to Ethernet at the subscriber

The ONU (Optical Network Unit), sometimes called an ONT (Optical Network Terminal), is the device at the subscriber's premises that converts the optical signal from the fiber into Ethernet that the subscriber's router and devices can use.

An ONU is a small box — typically wall-mounted or placed on a shelf — with a fiber input on one side and one or more Ethernet ports on the other. Some ONUs also include Wi-Fi radios, phone ports (for VoIP), and USB ports, functioning as a combined fiber modem and home gateway. Others are simple fiber-to-Ethernet bridges with a single port, leaving routing and Wi-Fi to a separate device.

The ONU performs several functions:

Optical-to-electrical conversion. The ONU receives downstream light pulses at 1490 nm and converts them to electrical signals. Upstream, it converts electrical signals to light at 1310 nm and transmits during its assigned time slot.

Frame filtering. The OLT broadcasts downstream data to all ONUs on a PON port. Each ONU filters out traffic not addressed to it using GEM port IDs, passing only its own data to the Ethernet ports. Other subscribers' data is discarded at the ONU and never reaches the subscriber's router.

Traffic classification. The ONU can tag Ethernet frames with VLAN IDs and apply QoS markings based on rules pushed from the OLT via OMCI.

Status reporting. The ONU reports its optical signal level, temperature, and link status to the OLT. A gradual decline in signal level — from -18 dBm to -24 dBm over months — indicates a degrading connection that should be investigated before it fails.

Bridged vs routed mode

ONUs operate in one of two modes:

Bridge mode. The ONU acts as a transparent bridge — it converts fiber to Ethernet and passes everything through. The subscriber's separate router handles PPPoE authentication, DHCP, NAT, and Wi-Fi. This mode is preferred by ISPs who want to control the subscriber's IP assignment via RADIUS and use the ONU purely as a media converter.

Router mode. The ONU handles PPPoE authentication, NAT, DHCP for the local network, and Wi-Fi. The subscriber connects devices directly to the ONU without needing a separate router. This mode reduces the number of devices at the subscriber's premises but gives the ISP less flexibility — the ONU's routing capabilities are typically more limited than a dedicated router.

Most ISPs deploying FTTH to residential subscribers start with router mode for simplicity (one device at the premises instead of two) and switch high-demand subscribers or business accounts to bridge mode with a dedicated router for more control.

GPON: the standard behind most FTTH

GPON (Gigabit Passive Optical Network) is the ITU-T standard (G.984 series) that defines how OLTs, ONUs, and the fiber between them communicate. It is the most widely deployed FTTH technology globally.

GPON specifies:

Downstream bandwidth: 2.488 Gbps. The OLT broadcasts data downstream at 2.488 Gbps to all ONUs on a PON port. This bandwidth is shared — if 32 ONUs are connected, the average available per subscriber is about 77 Mbps if all are active simultaneously. In practice, not all subscribers are active at once, so individual throughput is higher.

Upstream bandwidth: 1.244 Gbps. ONUs transmit upstream at 1.244 Gbps in time-division slots. This asymmetry (2:1 downstream to upstream) matches typical residential usage patterns where subscribers download more than they upload.

Wavelengths. Downstream uses 1490 nm. Upstream uses 1310 nm. Video overlay (where deployed) uses 1550 nm. Using different wavelengths for different directions allows simultaneous two-way communication over a single fiber strand.

Maximum split ratio: 1:128. The standard allows up to 128 ONUs per PON port, though practical deployments rarely exceed 1:64 due to power budget constraints.

Maximum reach: 20 km (differential distance). The farthest ONU on a PON port can be up to 20 km farther from the OLT than the nearest ONU. The absolute maximum logical reach is 60 km, but power budget typically limits practical range to 10-20 km with standard optics.

GPON vs EPON

EPON (Ethernet PON), standardized by IEEE as 802.3ah, is the alternative to GPON. The core difference is the encapsulation: GPON uses GEM frames (optimized for carrying multiple service types), while EPON uses standard Ethernet frames.

Feature	GPON (ITU-T G.984)	EPON (IEEE 802.3ah)
Downstream speed	2.488 Gbps	1.25 Gbps
Upstream speed	1.244 Gbps	1.25 Gbps
Encapsulation	GEM frames	Ethernet frames
Max split ratio	1:128	1:32
Typical deployment	EMEA, Africa, Asia	Japan, Korea, parts of Asia
Management	OMCI (ITU-T G.988)	OAM + vendor SNMP

GPON offers higher downstream bandwidth and a more mature management interface (OMCI). EPON offers symmetric speeds and simpler Ethernet-native encapsulation. For most ISPs deploying new FTTH networks, GPON is the default choice due to wider vendor support and higher downstream capacity. The successor standards — XGS-PON (10 Gbps symmetric, ITU-T G.9807.1) and 10G-EPON (IEEE 802.3av) — extend both architectures to 10 Gbps.

Power budget: the math that limits your network

The power budget is the total optical signal loss allowed between the OLT's transmitter and the ONU's receiver. If the total loss exceeds the power budget, the ONU cannot detect the signal, and the link fails.

Understanding power budget matters because it determines two things every ISP cares about: how far fiber can run and how many subscribers can share a single port.

What contributes to signal loss

Every component and connection in the fiber path introduces loss, measured in decibels (dB):

Fiber attenuation. Glass fiber loses about 0.35 dB per kilometer at 1310 nm (upstream wavelength) and 0.25 dB per kilometer at 1490 nm (downstream wavelength). A 10 km fiber run loses roughly 2.5 to 3.5 dB from the fiber itself.

Splitter loss. The largest single source of loss. A 1:32 splitter introduces approximately 17 dB of loss. A 1:64 splitter introduces approximately 20 dB. This is inherent to splitting the light — dividing one signal into 32 copies means each copy has 1/32nd of the original power.

Connector loss. Each connector pair (where two fiber ends are mechanically joined) introduces 0.3 to 0.5 dB of loss. A typical path might have 4 to 8 connector pairs between OLT and ONU — at the OLT port, at the splitter input, at the splitter output, at patch panels, and at the ONU.

Splice loss. Each fusion splice (where two fiber ends are permanently joined by melting them together) introduces 0.05 to 0.1 dB of loss. Splices are more common in long runs where individual cable segments are joined.

Bend loss. Fiber bent below its minimum bend radius loses light. Macro-bends (visible curves) can add 0.5 dB or more. Micro-bends from crushing or improper cable routing add smaller but cumulative losses.

Calculating a power budget

GPON defines optical power classes for the OLT and ONU transceivers:

Class	OLT transmit power	ONU receiver sensitivity	Power budget
Class B	+1.5 to +5 dBm	-25 dBm	25 dB
Class B+	+1.5 to +5 dBm	-28 dBm	28 dB
Class C+	+3 to +7 dBm	-32 dBm	32 dB

A Class B+ system has a 28 dB power budget. Here is a sample calculation for a 15 km deployment with a 1:32 splitter:

Loss source	Value
Fiber (15 km × 0.35 dB/km)	5.25 dB
1:32 splitter	17.0 dB
6 connectors (× 0.5 dB)	3.0 dB
2 splices (× 0.1 dB)	0.2 dB
Safety margin	1.0 dB
Total	26.45 dB

At 26.45 dB total loss against a 28 dB budget, this deployment works with 1.55 dB of headroom. That headroom accounts for connector aging, temperature variations, and minor fiber degradation over time.

If the same deployment used a 1:64 splitter (20 dB loss instead of 17 dB), the total would be 29.45 dB — exceeding the 28 dB Class B+ budget. The ISP would need to either reduce the fiber distance, use Class C+ optics, or reduce the split ratio.

Power budget is the constraint that links fiber distance, splitter ratio, and subscriber count. An ISP cannot maximize all three independently — more splitter outputs or longer fiber runs both increase loss. Practical planning starts by picking the optics class (usually Class B+), subtracting a 1–3 dB safety margin, then allocating the remaining budget between distance and split ratio. For urban deployments under 10 km with 1:32 or 1:64 splits, Class B+ is sufficient. Longer rural runs benefit from Class C+ optics, which add 4 dB of headroom at higher per-unit cost.

Designing an FTTH network: end-to-end overview

An FTTH network has four zones:

Core / headend. The OLT sits here, connected to the ISP's upstream network. Needs reliable power, cooling, and physical security.

Feeder network. Trunk fiber cables (12 to 96 strands) run from the headend to distribution points. This is the most expensive part to build and should include spare strands for growth.

Distribution network. Splitters sit at cabinets, poles, or building basements, dividing feeder fibers into drop fibers heading to subscribers.

Drop network. The final cable from the last splitter to the subscriber's premises, where the ONU is installed.

Each zone is independent — an ISP adds subscribers by connecting drop cables to existing splitter ports without touching feeder or headend infrastructure.