A passive optical network (PON) is a telecommunications technology used to provide fiber to the end consumer, both domestic and commercial. A PON's distinguishing feature is that it implements a point-to-multipoint architecture, in which unpowered fiber optic splitters are used to enable a single optical fiber to serve multiple end-points. The end-points are often individual customers, rather than commercial. A PON does not have to provision individual fibers between the hub and customer. Passive optical networks are often referred to as the "last mile" between an ISP and customer.
A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single mode fiber (ITU-T G.652). BPON, EPON, GEPON, and GPON have the same basic wavelength plan and use the 1490 nanometer (nm) wavelength for downstream traffic and 1310 nm wavelength for upstream traffic. 1550 nm is reserved for optional overlay services, typically RF (analog) video.
As with bit rate, the standards describe several optical power budgets, most common is 28 dB of loss budget for both BPON and GPON, but products have been announced using less expensive optics as well. 28 dB corresponds to about 20 km with a 32-way split. Forward error correction (FEC) may provide for another 2–3 dB of loss budget on GPON systems. As optics improve, the 28 dB budget will likely increase. Although both the GPON and EPON protocols permit large split ratios (up to 128 subscribers for GPON, up to 32,768 for EPON), in practice most PONs are deployed with a split ratio of 1:32 or smaller.
A PON consists of a central office node, called an optical line terminal (OLT), one or more user nodes, called optical network units (ONUs) or optical network terminals (ONTs), and the fibers and splitters between them, called the optical distribution network (ODN). “ONT” is an ITU-T term to describe a single-tenant ONU. In multiple-tenant units, the ONU may be bridged to a customer premises device within the individual dwelling unit using technologies such as Ethernet over twisted pair, G.hn (a high-speed ITU-T standard that can operate over any existing home wiring - power lines, phone lines and coaxial cables) or DSL. An ONU is a device that terminates the PON and presents customer service interfaces to the user. Some ONUs implement a separate subscriber unit to provide services such as telephony, Ethernet data, or video.
An OLT provides the interface between a PON and a service provider's core network. These typically include:
IP traffic over Fast Ethernet, Gigabit Ethernet, or 10 Gigabit Ethernet;
Standard TDM interfaces such as SDH/SONET;
ATM UNI at 155–622 Mbit/s.
The ONT or ONU terminates the PON and presents the native service interfaces to the user. These services can include voice (plain old telephone service (POTS) or voice over IP (VoIP)), data (typically Ethernet or V.35), video, and/or telemetry (TTL, ECL, RS530, etc.) Often the ONU functions are separated into two parts:
The ONU, which terminates the PON and presents a converged interface—such as DSL, coaxial cable, or multiservice Ethernet—toward the user;
Network termination equipment (NTE), which inputs the converged interface and outputs native service interfaces to the user, such as Ethernet and POTS.
A PON is a shared network, in that the OLT sends a single stream of downstream traffic that is seen by all ONUs. Each ONU reads the content of only those packets that are addressed to it. Encryption is used to prevent eavesdropping on downstream traffic.
Upstream bandwidth allocation
The OLT is responsible for allocating upstream bandwidth to the ONUs. Because the optical distribution network (ODN) is shared, ONU upstream transmissions could collide if they were transmitted at random times. ONUs can lie at varying distances from the OLT, meaning that the transmission delay from each ONU is unique. The OLT measures delay and sets a register in each ONU via PLOAM (physical layer operations and maintenance) messages to equalize its delay with respect to all of the other ONUs on the PON.
Once the delay of all ONUs has been set, the OLT transmits so-called grants to the individual ONUs. A grant is permission to use a defined interval of time for upstream transmission. The grant map is dynamically re-calculated every few milliseconds. The map allocates bandwidth to all ONUs, such that each ONU receives timely bandwidth for its service needs.
Some services – POTS, for example – require essentially constant upstream bandwidth, and the OLT may provide a fixed bandwidth allocation to each such service that has been provisioned. DS1 and some classes of data service may also require constant upstream bit rate. But much data traffic, such as browsing web sites, is bursty and highly variable. Through dynamic bandwidth allocation (DBA), a PON can be oversubscribed for upstream traffic, according to the traffic engineering concepts of statistical multiplexing. (Downstream traffic can also be oversubscribed, in the same way that any LAN can be oversubscribed. The only special feature in the PON architecture for downstream oversubscription is the fact that the ONU must be able to accept completely arbitrary downstream time slots, both in time and in size.)
In GPON there are two forms of DBA, status-reporting (SR) and non-status reporting (NSR).
In NSR DBA, the OLT continuously allocates a small amount of extra bandwidth to each ONU. If the ONU has no traffic to send, it transmits idle frames during its excess allocation. If the OLT observes that a given ONU is not sending idle frames, it increases the bandwidth allocation to that ONU. Once the ONU's burst has been transferred, the OLT observes a large number of idle frames from the given ONU, and reduces its allocation accordingly. NSR DBA has the advantage that it imposes no requirements on the ONU, and the disadvantage that there is no way for the OLT to know how best to assign bandwidth across several ONUs that need more.
In SR DBA, the OLT polls ONUs for their backlogs. A given ONU may have several so-called transmission containers (T-CONTs), each with its own priority or traffic class. The ONU reports each T-CONT separately to the OLT. The report message contains a logarithmic measure of the backlog in the T-CONT queue. By knowledge of the service level agreement for each T-CONT across the entire PON, as well as the size of each T-CONT's backlog, the OLT can optimize allocation of the spare bandwidth on the PON.
EPON systems use a DBA mechanism equivalent to GPON's SR DBA solution. The OLT polls ONUs for their queue status and grants bandwidth using the MPCP GATE message, while ONUs report their status using the MPCP REPORT message.
APON/BPON, EPON and GPON have been widely deployed. In November 2014, EPON had approximately 40 million deployed ports and ranks first in deployments.
As of 2015, GPON had a smaller market share, but is anticipated to reach $10.5 billion US dollars by 2020.
For TDM-PON, a passive optical splitter is used in the optical distribution network. In the upstream direction, each ONU (optical network units) or ONT (optical network terminal) burst transmits for an assigned time-slot (multiplexed in the time domain). In this way, the OLT is receiving signals from only one ONU or ONT at any point in time. In the downstream direction, the OLT (usually) continuously transmits (or may burst transmit). ONUs or ONTs see their own data through the address labels embedded in the signal.
DOCSIS Provisioning of EPON or DPoE
Data Over Cable Service Interface Specification (DOCSIS) Provisioning of Ethernet Passive Optical Network, or DPoE, is a set of Cable Television Laboratory specifications that implement the DOCSIS service layer interface on existing Ethernet PON (EPON, GEPON or 10G-EPON) Media Access Control (MAC) and Physical layer (PHY) standards. In short it implements the DOCSIS Operations Administration Maintenance and Provisioning (OAMP) functionality on existing EPON equipment. It makes the EPON OLT look and act like a DOCSIS Cable Modem Termination Systems (CMTS) platform (which is called a DPoE System in DPoE terminology). In addition to offering the same IP service capabilities as a CMTS, DPoE supports Metro Ethernet Forum (MEF) 9 and 14 services for the delivery of Ethernet services for business customers.
Radio frequency over glass
Radio frequency over glass (RFoG) is a type of passive optical network that transports RF signals that were formerly transported over copper (principally over a hybrid fibre-coaxial cable) over PON. In the forward direction RFoG is either a stand-alone P2MP system or an optical overlay for existing PON such as GEPON/EPON. The overlay for RFoG is based on Wave Division Multiplexing (WDM) -- the passive combination of wavelengths on a single strand of glass. Reverse RF support is provided by transporting the upstream or return RF onto a separate wavelength from the PON return wavelength. The Society of Cable and Telecommunications Engineers (SCTE) Interface Practices Subcommittee (IPS) Work Group 5, is currently working on IPS 910 RF over Glass. RFoG offers backwards compatibility with existing RF modulation technology, but offers no additional bandwidth for RF based services. Although not yet completed, the RFoG standard is actually a collection of standardized options which are not compatible with each other (they cannot be mixed on the same PON). Some of the standards may interoperate with other PONs, others may not. It offers a means to support RF technologies in locations where only fiber is available or where copper is not permitted or feasible. This technology is targeted towards Cable TV operators and their existing HFC networks.
Wavelength Division Multiplexing PON, or WDM-PON, is a non-standard type of passive optical networking, being developed by some companies.
The multiple wavelengths of a WDM-PON can be used to separate Optical Network Units (ONUs) into several virtual PONs co-existing on the same physical infrastructure. Alternatively the wavelengths can be used collectively through statistical multiplexing to provide efficient wavelength utilization and lower delays experienced by the ONUs.
There is no common standard for WDM-PON nor any unanimously agreed upon definition of the term. By some definitions WDM-PON is a dedicated wavelength for each ONU. Other more liberal definitions suggest the use of more than one wavelength in any one direction on a PON is WDM-PON. It is difficult to point to an un-biased list of WDM-PON vendors when there is no such unanimous definition. PONs provide higher bandwidth than traditional copper based access networks. WDM-PON has better privacy and better scalability because of each ONU only receives its own wavelength.
Advantages: The MAC layer is simplified because the P2P connections between OLT and ONUs are realized in wavelength domain, so no P2MP media access control is needed. In WDM-PON each wavelength can run at a different speed and protocol so there is an easy pay-as-you-grow upgrade.
Challenges: High cost of initial set-up, the cost of the WDM components. Temperature control is another challenge because of how wavelengths tend to drift with environmental temperatures.
Time- and wavelength-division multiplexed passive optical network (TWDM-PON) is a primary solution for the next-generation passive optical network stage 2 (NG-PON2) by the full service access network (FSAN) in April 2012. TWDM-PON coexists with commercially deployed Gigabit PON (G-PON) and 10 Gigabit PON (XG-PON) systems.
Long-Reach Optical Access Networks
The concept of the Long-Reach Optical Access Network (LROAN) is to replace the optical/electrical/optical conversion that takes place at the local exchange with a continuous optical path that extends from the customer to the core of the network. Work by Davey and Payne at BT showed that significant cost savings could be made by reducing the electronic equipment and real-estate required at the local exchange or wire center. A proof of concept demonstrator showed that it was possible to serve 1024 users at 10Gbit/s with 100 km reach.
This technology has sometimes been termed Long-Reach PON, however, many argue that the term PON is no longer applicable as, in most instances, only the distribution remains passive.
Passive optical components
The drivers behind the modern passive optical network are high reliability, low cost, and passive functionality.
Single-mode, passive optical components include branching devices such as Wavelength-Division Multiplexer/Demultiplexers (WDMs), isolators, circulators, and filters. These components are used in interoffice, loop feeder, Fiber In The Loop (FITL), Hybrid Fiber-Coaxial Cable (HFC), Synchronous Optical Network (SONET), and Synchronous Digital Hierarchy (SDH) systems; and other telecommunications networks employing optical communications systems that utilize Optical Fiber Amplifiers (OFAs) and Dense Wavelength Division Multiplexer (DWDM) systems. Proposed requirements for these components were published in 2010 by Telcordia Technologies. 
The broad variety of passive optical components applications include multichannel transmission, distribution, optical taps for monitoring, pump combiners for fiber amplifiers, bit-rate limiters, optical connects, route diversity, polarization diversity, interferometers, and conherent communication.
WDMs are optical components in which power is split or combined based on the wavelength composition of the optical signal. Dense Wavelength Division Multiplexers (DWDMs) are optical components that split power over at least four wavelengths. Wavelength insensitive couplers are passive optical components in which power is split or combined independently of the wavelength composition of the optical signal. A given component may combine and divide optical signals simultaneously, as in bidirectional (duplex) transmission over a single fiber. Passive optical components are data format transparent, combining and dividing optical power in some predetermined ratio (coupling ratio) regardless of the information content of the signals. WDMs can be thought of as wavelength splitters and combiners. Wavelength insensitive couplers can be thought of as power splitters and combiners.
An optical isolator is a two-port passive component that allows light (in a given wavelength range) to pass through with low attenuation in one direction, while isolating (providing a high attenuation for) light propagating in the reverse direction. Isolators are used as both integral and in-line components in laser diode modules and optical amplifiers, and to reduce noise caused by multi-path reflection in high-bitrate and analog transmission systems.
An optical circulator operates in a similar way to an optical isolator, except that the reverse propagating lightwave is directed to a third port for output, instead of being lost. An optical circulator can be used for bidirectional transmission, as a type of branching component that distributes (and isolates) optical power among fibers, based on the direction of the lightwave propagation.
A fiber optic filter is a component with two or more ports that provides wavelength sensitive loss, isolation and/or return loss. Fiber optic filters are in-line, wavelength selective, components that allow a specific range of wavelengths to pass through (or reflect) with low attenuation for classification of filter types.