A fiber optic connection does not need to occupy a dedicated fiber for every subscriber all the way to the central office. A passive optical network bundles multiple connections onto a shared optical feeder. For network operators, planners, and technical purchasers, it is crucial to understand how this distribution works and what requirements this imposes on components and operations.
What does "passive" mean in a fiber optic network?
"Passive" describes the optical distribution: between the central PON interface and the subscriber devices, signals are routed through fibers, connectors, splices, and optical splitters. These components require no electrical power supply to function. A splitter does not amplify the light, nor does it decide where a data packet goes based on a destination address.
In contrast, the active devices at both ends require power. Therefore, a PON is not a completely power-free network. The practical advantage is that neither an Ethernet switch nor its power supply and cooling are required at a passive distribution point for signal distribution.
PON structure: one central connection, multiple subscribers
The typical structure is point-to-multipoint: a central PON port serves multiple subscriber devices. From the central location, a common feeder fiber leads to a splitter. From there, individual fiber branches run to the connections. Splitters can also be arranged in multiple stages. Each complete path from the central device to the subscriber must meet the optical specifications.
OLT: the active central unit
The Optical Line Terminal (OLT) connects the PON to the higher-level network. It sends data to subscribers, receives their return traffic, and coordinates access to the shared transmission path. An OLT can have multiple PON ports; a port and the distribution tree connected to it form a shared capacity unit.
ODN: the passive optical distribution network
The Optical Distribution Network (ODN) comprises the optical path between the OLT and the subscriber devices. This includes the fiber optic paths, splitters, splices, and connectors. Housings, closures, and junction boxes protect and organize this infrastructure. The ODN is therefore more than just the splitter.
Fiber optic cables and optical splitters
For the Gigabit-capable Passive Optical Network (GPON) and 10-Gigabit-capable Symmetric Passive Optical Network (XGS-PON) systems considered here, single-mode fibers are used. Installation cables, feeders, and subscriber lines must be suitable for the installation route, environment, and optical specifications. A general category for fiber optics (LWL) may also include multimode products, which are not the right choice for these PON paths.
An optical splitter distributes the incoming optical power to multiple outputs. A 1:32 ratio, for example, denotes one input and 32 outputs. This neither guarantees a specific data rate per connection nor automatically implies the suitability of this split in a specific network. Loss increases with the number of outputs due to power distribution.
Splice and connection components
Splices connect fibers permanently; connectors allow for disconnectable transitions. Pigtails create a plug-in connection on a spliced fiber. Patch cables connect matching optical interfaces. Splice cassettes, couplings, and suitable housings keep fiber reserves organized and protect sensitive connection points. Cleanliness and correct fiber routing influence the quality of the entire transmission path.
ONU and ONT: the active subscriber side
Optical Network Unit (ONU) is the general term for an optical network unit on the subscriber side. An Optical Network Terminal (ONT) is an ONU that forms the network termination point for a single subscriber, such as in an apartment or office. Manufacturers sometimes use the terms interchangeably. Therefore, device function and interfaces are the decisive factors.
The subscriber device terminates the PON transmission and provides, for example, an Ethernet connection. Router functions or a wireless local area network (WLAN) may be integrated, but are not part of every ONT design. An ordinary Ethernet media converter does not replace an ONT compatible with the OLT.
How does a PON work?
In the systems described here, the transmit and receive directions use different optical wavelengths on the same fiber. This separation allows traffic in both directions. Access control additionally ensures that subscribers use the shared path in an orderly manner.
Downstream: from OLT to subscribers
The OLT sends a common data stream into the distribution tree. The splitter distributes the optical signal to the connected branches. The subscriber devices process their respectively assigned payload data. The splitter itself does not filter customer addresses. Mechanisms for subscriber isolation and encryption are part of the PON system and must be correctly configured during operation.
Upstream: from subscribers to OLT
In the opposite direction, signals from subscribers converge on the feeder fiber. To prevent them from overlapping, the OLT assigns transmission windows to the ONUs in time-division multiplexing-based PON systems. The devices send short data bursts within these time windows. Timing adjustment and dynamic bandwidth allocation help to serve connections with varying distances and changing data requirements.
Shared capacity does not mean a rigid split: A connection does not necessarily receive exactly one thirty-second of the port capacity at all times. Simultaneous usage, configured services, priorities, and available capacity are the decisive factors. For guaranteed business customer services, this layout must be explicitly verified.
Classifying GPON, EPON, XG-PON, and XGS-PON
PON denotes the network principle. A standard defines the concrete transmission. The International Telecommunication Union, Telecommunication Standardization Sector (ITU-T), describes GPON and XGS-PON, among others. Ethernet-based PON systems belong to the IEEE 802.3 family of the Institute of Electrical and Electronics Engineers (IEEE).
| System | Basis | Transmission and classification |
|---|---|---|
| GPON | ITU-T G.984; Gigabit-capable PON | Widely used version with approx. 2.49 Gbit/s downstream and 1.24 Gbit/s upstream; asymmetric. |
| XG-PON | ITU-T G.987; 10-Gigabit-capable PON | Refers to XG-PON1: approx. 10 Gbit/s downstream and 2.5 Gbit/s upstream; asymmetric. |
| XGS-PON | ITU-T G.9807.1 | Approx. 10 Gbit/s in both directions; symmetric. |
| EPON | IEEE 802.3; Ethernet PON | Own standard family. 1G-EPON and 10G-EPON differ; "EPON" alone does not define a clear speed level. |
The rounded ITU-T values denote the optical line rates of the shared PON port, not the usable data throughput per subscriber. Protocol overhead and sharing reduce the available payload capacity. Sources: ITU-T G.984.2, ITU-T G.987.2, ITU-T G.9807.1, and IEEE 802.3av.
Switching from GPON to XGS-PON is not merely a tariff change. The OLT and subscriber devices must support the target standard. Continued use of the ODN requires suitable components, appropriate wavelength ranges, and a sufficient attenuation budget. Parallel operation is possible with coexistence technology designed for this purpose, but it is not automatically possible in every existing network.
Comparing PON and AON
An Active Optical Network (AON) uses active network technology for distribution, frequently Ethernet switches. The following comparison considers a typical PON and an active Ethernet distribution. AON can also be built with dedicated fibers up to a central switch; a powered street cabinet is therefore not a mandatory feature of every AON architecture.
| Criterion | PON | AON / Active Ethernet |
|---|---|---|
| Network structure | Point-to-multipoint via splitter. | Typically dedicated subscriber connections to switch ports. |
| Distribution network | Passive optical splitting. | Active forwarding; switch central or decentralized. |
| Power supply | OLT and ONU/ONT require power; splitter does not. | Active switches and end devices require power. |
| Fiber requirements | Shared feeder fiber up to the splitter; then subscriber branches. | Own fiber connection per connection to the corresponding switch; uplinks can be shared. |
| Capacity | Port capacity shared within the PON tree. | Subscriber link dedicated; shared uplinks can also be limiting. |
| Scalability | Consider splitter ports, optical budget, OLT capacity, and system limits. | Consider switch ports, optics, fiber paths, and uplink capacity. |
| Maintenance | No active electronics at the splitter; optical path and end devices remain relevant for maintenance. | Additionally operation, power supply, and software of decentralized switches, if present. |
| Typical usage | FTTH access networks and suitable building networks. | Enterprise networks and access networks with dedicated subscriber links. |
The choice should be based on service requirements and existing infrastructure. A high proportion of simultaneously heavily utilized connections, individual port requirements, or specific operating models may favor dedicated Ethernet connections. Many distributed connections with sensibly shareable capacity, on the other hand, can fit well with PON.
Advantages, limitations, and important planning aspects
Where PON offers advantages
- Shared infrastructure: Multiple subscribers use one OLT port and the shared fiber section. This can reduce the need for feeder fibers and central ports.
- Passive distribution points: Power connection and active cooling for a distribution switch are not required at these points.
- Less decentralized electronics: Maintenance and replacement of active devices are concentrated at the endpoints.
- Network evolution: An appropriately designed ODN can support later PON generations. Its suitability must be verified before migration.
Whether this reduces investment or operating costs depends on routes, subscriber density, existing fibers, terminal devices, and redundancy requirements, among other things. Economic efficiency cannot be derived from the network name alone.
Attenuation budget instead of blanket range promises
Every optical path loses power: on the fiber, in the splitter, as well as at splices and connectors. The sum must remain within the permissible values of the optics used. The minimum attenuation or the overload limit of the receiver can also be relevant. Long distances and high split ratios cannot therefore be determined independently of one another.The planning approach is: fiber losses + splitter losses + connection losses + reserve must fit within the allowed optical budget. The specified limit values of the actual components and the relevant wavelengths are used. A reserve for aging, future repairs, and additional connections must be determined on a project-specific basis.
Calculation example, not a product specification: For an ideally even 1:N split, the pure split loss is 10 × log₁₀(N) dB. For 1:32, that is about 15.1 dB. Real splitters have additional losses; their data sheet is authoritative. Two series-connected splitters with 1:4 and 1:8 result in a total of 1:32. The losses of both components and all intermediate connections must be taken into account.
Capacity, availability, and compatibility
Plan for requirements during peak usage times, especially upstream for cloud applications, data backup, or video services. Determine the minimum capacity each connection should receive and how reserves will be made available. Also, check the OLT uplink and the downstream subscriber cabling for bottlenecks.
A failure of the common feeder fiber or an OLT port can affect multiple subscribers. For increased availability, separate paths, protection concepts, and power backup for active endpoints must be planned separately. Furthermore, the same PON standard does not automatically mean full interoperability: device approval, software version, registration, and service profiles must match.
Acceptance and documentation
Before installation, define measurement procedures, limit values, and handover documentation. This can include end-face inspection, attenuation measurement, wavelength-selective PON power measurement, and measurement with an optical time-domain reflectometer (OTDR). The OTDR locates events along the path; it does not replace every other acceptance test.
For branched PON paths, suitable devices, measurement parameters, and an appropriate evaluation method are required. Measurements in an active network require dedicated wavelengths and filters. Measurement cables and OTDR launch fibers are selected based on the fiber, connectors, measurement device, and measurement task. The length cannot be derived generally from the network name.
Where is PON used?
FTTH, FTTB, and residential complexes
Fiber to the Home (FTTH) refers to fiber optic access into the apartment or usage unit. Fiber to the Building (FTTB) runs fiber optics to the building; the further distribution is considered separately. Both belong to the collective term Fiber to the x (FTTx). These terms describe the reach of the fiber optic expansion, whereas PON describes the network architecture. An FTTH connection is therefore not automatically a PON connection.
In residential complexes, numerous units can be served via a common distribution tree. Handover points, building cabling, accessibility of distribution frames, and responsibilities for operation and troubleshooting must be clearly defined. The FTTx and data network solutions from abatec categorize suitable passive components and services.
Campus networks, hotels, clinics, and commercial sites
Passive optical distribution can also make sense within larger buildings or on a campus, for example for office spaces or hotel rooms. Subscriber devices provide the required local connections. Terminal devices powered via Ethernet still require appropriately supplied connection technology; fiber optics do not handle this electrical supply.
In clinics and industrial environments, suitability must be checked on an application-specific basis. Availability goals, latency behavior, network segmentation, environmental conditions, and the requirements of special devices are decisive. A PON suitable for office communication is not automatically suitable for time-critical machine controls. Likewise, a PON access network does not automatically replace the internal high-performance networking of a data center.
Which fiber optic components are required for a PON?
A functional bill of materials is helpful for purchasing. It should specify the passive path and the active endpoints separately, but consider their interfaces together.
- Cables: Define single-mode fiber type, fiber count, length, cable jacket, and installation environment. The Fiber optic cables and FO cables category serves as a starting point for the range; every design must be matched to the PON specification.
- Splitters: Specify the split ratio, wavelength range, insertion loss, return loss, connections, and mounting form. An optical power splitter is not equivalent to a simple connector coupling. A reliable data sheet is required for selection.
- Distribution frames: Select FO splice boxes and modules according to fiber count, splice tray, and configuration. For FO enclosures and connection boxes, additionally consider the mounting type, space, and environment. A DIN rail box is not automatically a suitable outdoor sleeve or FTTH subscriber wall outlet.
- Fiber terminations: Provide pigtails for splice boxes and FO patch cables with matching fibers and connectors. UPC (Ultra Physical Contact) and APC (Angled Physical Contact) are different polish types and must not be directly connected at a single junction.
- Installation and testing: Plan for splice protection, cleaning, inspection, and suitable measurement technology. The scope of tools for fusion splicing includes suitable FO fusion splicers. The splicing and measurement services from abatec are a complementary option for a coordinated scope of services.
- Active devices: Define OLT and ONU/ONT including standard, optical class, subscriber ports, management, and power supply. Their selection helps determine the requirements for the ODN.
Take a close look at patch cables: The typical PON subscriber connection uses a single fiber. The linked shop category currently features duplex patch cables. For a required simplex version, the specific connection solution and, if necessary, an individual FO cable assembly must be coordinated.
Once your specification is finalized, compare the suitable product groups in the network and telecommunications technology section of the abatec shop.
Frequently asked questions about passive optical networks
What is a PON explained simply?
A PON connects multiple fiber optic connections to a central PON interface via a passive optical splitter. The subscribers share its transmission capacity.
What does "passive" mean in a fiber optic network?
The signal distribution between the active terminal devices is passive: fibers, splitters, splices, and plug connections do not require power for this. This does not mean that the entire network operates without power.
What is the difference between PON and GPON?
PON denotes the general network principle. GPON is a specific implementation described in the ITU-T G.984 series. XGS-PON and EPON are other PON systems.
What is the difference between ONT and ONU?
ONU is the general term for a subscriber-side optical network unit. An ONT forms the network termination for a single subscriber. In product names, both terms are sometimes used interchangeably.
What is the task of an optical splitter?
It distributes the optical power of an input signal to several fiber branches and combines signals in the opposite direction. It requires no power, but it also does not amplify the signal.
What distinguishes PON and AON?
PON distributes the optical signal passively via splitters. AON uses active network technology, such as Ethernet switches. With AON, the switch can be central or decentralized; an active street cabinet is not strictly required.
Is PON used for FTTH?
Yes, PON can supply FTTH connections. However, FTTH only describes that the fiber optic cable reaches into the apartment or usage unit. Other architectures can also implement FTTH.
Does a PON require power?
Yes. The OLT and the ONU or ONT devices require power. Without a local power supply or a suitable backup supply, these active endpoints will not work. The passive splitter does not need a power connection.
From network concept to the suitable component
A PON can serve many fiber optic connections with a manageable number of active distribution components. Whether the concept fits the task is decided by capacity, optical budget, availability, and operational requirements. A coordinated component list and a verifiable acceptance test connect the planning with a reliable installation.
Are you planning an FTTx or fiber optic project? abatec supports you in the selection of passive FO components and individual cable assemblies. For coordination, information on the path, fiber type, connections, installation site, and desired splicing or measurement service is helpful. Coordinate components and project requirements with abatec.