What is a Passive Optical Network (PON)?

The optical fiber and splitters are the truly “passive” building blocks of the PON, with no electrical powering required. Optical splitters are not wavelength selective and simply divide any optical wavelengths in the downstream direction. Of course, splitting of an optical signal does incur a power loss which depends on the number of ways a signal is split. Splitters require none of the cooling or other ongoing maintenance inherent to active network components (such as optical amplifiers) and can last for decades if left undisturbed. In addition to the passive components, active end devices are required to complete the PON network. 

The optical line terminal (OLT) is the starting point for the passive optical network. It is connected to a core switch through Ethernet pluggables. The primary function of the OLT is to convert, frame, and transmit signals for the PON network and to coordinate the optical network terminal (ONT) multiplexing for shared upstream transmission. The ONT is the powered device of the passive optical network system at the opposite (user) end of the network and includes Ethernet ports for in home device or network connectivity. 

These end-user devices are also referred to as optical network units (ONUs) by the IEEE, while the ITU-T typically uses the ONT acronym. This subtle difference in terminology also indicates which PON service and standard is being utilized (see below). 

PON networks adopt a point-to-multipoint (P2MP) architecture which utilizes optical splitters to divide the downstream signal from a single OLT into multiple downstream paths to the end users. The same splitters combine the multiple upstream paths from the end users back to the OLT. 

Point-to-multipoint was selected as the most viable PON architecture for optical access networks due to the inherent efficiencies of fiber sharing and low-power consumption. This architecture was standardized in 1998 via the ATM-PON G.983.1 specification. 

Today, the ITU-T G.984 standard for G-PON has supplanted the ATM standard, since Asynchronous Transfer Mode (ATM) is no longer utilized. 

A PON network starts with the optical line terminal (OLT) at the service provider source location typically known as a Local or Central Office, or sometimes referred to as an exchange or headend. From there, the fiber-optic feeder cable (or feeder fiber) is routed to a passive splitter, along with a backup fiber if one is used. Distribution fibers then connect from the splitter to a drop terminal, which can be located in a street cabinet, in a ruggedized housing mounted in a pit, on a telegraph pole, or even on the side of a building. Drop fibers then provide the final one-to-one connection from a drop terminal port to an end user ONT/ONU. In some cases, more than one splitter is used in series. This is referred to as a cascaded splitter architecture. 

PON Architecture

The signals carried on the feeder fiber can be split to provide service to as many as 256 users with an ONU or ONT converting the signals and providing users with internet access. The number of ways the downstream OLT signal is divided or split before reaching the end user is known as the splitter or split ratio (e.g. 1:32 or 1:64).  

In more complex configurations where RF video is being broadcast in parallel to the PON data service or additional PON services co-exist on the same PON network, passive multiplexers (MUX) at the central/local office merge the video overlay wavelength and additional PON service wavelengths onto the outbound OLT feeder fiber. 

Passive Optical Network Operation 

An innovation that is integral to PON operation is wave division multiplexing (WDM), used to separate data streams based on the wavelength (color) of the laser light. One wavelength can be used to transmit downstream data while another is used to carry upstream data. These dedicated wavelengths vary depending on the PON standard in use and can be present simultaneously on the same fiber.  

Time division multiple access (TDMA) technology managed by the OLT is used to allocate upstream bandwidth to each end user for a specific time period. This prevents wavelength/data collisions at the PON splitters or OLT due to multiple ONTs/ONUs transmitting data upstream at the same time. This is also referred to as burst-mode transmission for the PON upstream.  

Types of PON Service

Since its introduction in the 1990s, PON technology has continued to evolve and multiple iterations of the PON network topology have taken shape. The original passive optical network standards, APON and BPON, have gradually given way to the bandwidth and overall performance benefits of the newer versions. 

PON Wavelength Allocation and Coexistence

A PON is sometimes referred to as the “last mile” between the provider and user, or the Fiber to the X (FTTX) with “X” signifying the home (FTTH), building (FTTB), premises (FTTP) or other location, depending on where the optical fiber is terminated. Thus far, fiber-to-the home (FTTH) has been the main application for PON.  

The reduced cabling infrastructure (no active elements) and flexible media transmission of passive optical networks have made them an ideal fit for home internet, voice, and video applications. As PON technology has continued to improve, the potential applications have expanded as well. 

The rollout of 5G continues, and PON networks have found a new application in 5G fronthaul. The fronthaul is the connection between the baseband controller and the remote radio head at the cell site. Due to the bandwidth and latency demands imposed by 5G, utilizing PON networks to complete the fronthaul connections can reduce fiber count and improve efficiency without compromising performance.  

In much the same way the source signal is split between users for FTTH, signal from the baseband units can be distributed to an array of remote radio heads. The 25GS-PON Multi-Source Agreement (MSA) has brought major 5G operators and vendors together with the goal of providing symmetrical 25 Gbps service to meet the high speed requirements of 5G networks.  

Additional applications that are well suited to passive optical networks include college campuses and business environments. For campus applications, PON networks produce advantages with respect to speed, energy consumption, reliability, and access distances. The costs associated with build/deployment and on-going operation are also reduced.  

PON enables integration of campus functions such as building management, security and parking with reduced dedicated equipment, cabling, and management systems. Similarly, medium to large sized business complexes can reap immediate benefits from PON implementation, with the reduced installation and maintenance costs directly impacting the bottom line. 

Benefits of PON 

Limitations of PON 

Distance

Despite the numerous benefits, there are potential drawbacks to passive optical networks when compared to active optical networks. The range for PON is limited to between 20 to 40 km, while an active optical network may reach up to 100 km. 

Test Access

Troubleshooting can be challenging under some conditions, since test access can be forgotten or ignored when designing a PON and test tools must allow for in-service troubleshooting without disrupting service to other end users on the same PON. If test access exists, then tests can be performed with a portable or centralized test solution using out-of-band wavelengths such as 1650 nm to avoid clashing with existing PON wavelengths. Where test access is not planned, access must be gained from one of the endpoints at the OLT or ONT, or a section of the PON must be temporarily taken out of service. 

High Vulnerability to Breakdown in the Feeder Line or the OLT 

Due to the P2MP architecture, the feeder line and OLT service multiple end users (potentially up to 256). Since there is little redundancy in case of an accidental fiber cut or a faulty OLT, the service disruption can be extensive. 

Potential PON Faults and Level of Impact

Overall, the inherent benefits of passive optical networks substantially outweigh these limitations.