Passive optical network (PON) is a fiber-optic telecommunications technology for delivering broadband network access to end customers.
Its architecture implements a point-to-multipoint topology. A single optical fiber serves multiple endpoints by using passive fiber optic splitters. It divides the fiber bandwidth among multiple access points.
Passive optical networks are often known as the “last mile” between an Internet service provider (ISP) and its customers.
Downstream traffic in active (top) vs. passive optical network
Passive optical network consists of an optical line terminal (OLT) at the service provider’s central office (hub).
Number of optical network units (ONUs) or optical network terminals (ONTs), near end users.
Different from point-to-point architectures, PON reduces the amount of fiber and central office equipment. A passive optical network is a form of fiber-optic access network.
In most cases, downstream signals are broadcast to all premises sharing multiple fibers. Encryption can prevent eavesdropping.
Upstream signals combine using a multiple access protocol, usually time division multiple access (TDMA).
They develop standards along with a number of other industry organizations. The Society of Cable Telecommunications Engineers (SCTE) also specified radio frequency over glass for carrying signals over a passive optical network.
FSAN and ITU
Since 1995, the Full Service Access Network (FSAN) working group, comprised of major telecommunications service providers and system vendors, began developing FTTH architectures. The International Telecommunications Union (ITU) continued this work and standardized on two generations of PON.
ITU-T G.983 standard
The older ITU-T G.983 standard, also known as APON (ATM PON), was based on Asynchronous Transfer Mode (ATM). However, the industry has gradually fallen out of favor with ATM as a protocol, and improvements to the original APON standard have led to the final version of ITU-T G.983 being referred to more often as broadband PON, or BPON. Although the standard can accommodate higher rates, a typical APON/BPON provides 622 megabits per second (Mbit/s) of downstream bandwidth and 155 Mbit/s (OC-3) of upstream traffic.
Compared to BPON, the ITU-T G.984 Gigabit-capable Passive Optical Networks (GPON) standard represents an increase in both total bandwidth and bandwidth efficiency through the use of larger, variable-length packets. Although the standards permit several choices of bit rate, the industry has converged on 2.488 gigabits per second (Gbit/s) of downstream bandwidth and 1.244 Gbit/s of upstream bandwidth. The GPON Encapsulation Method (GEM) allows for efficient packaging of user traffic with frame segmentation.
By mid-2008, Verizon had installed over 800,000 lines, and British Telecom, BSNL, Saudi Telecom Company, Etisalat, and AT&T were conducting advanced trials in Britain, India, Saudi Arabia, the UAE, and the US, respectively. GPON networks have been deployed in numerous networks worldwide, and trends indicate higher growth in GPON than other PON technologies.
Finally, G.987 defined 10G-PON with 10 Gbit/s downstream and 2.5 Gbit/s upstream, which is designed to coexist with GPON devices on the same network with similar framing.
In the 2009 Cable Manufacturing Business to meet SIPRNet requirements of the US Air Force. Secure passive optical network (SPON) integrates gigabit passive optical network (GPON) technology and protective distribution system (PDS).
Changes to the NSTISSI 7003 requirements for PDS and the mandate by the US federal government for GREEN technologies allowed the US federal government consideration of the two technologies as an alternative to active Ethernet and encryption devices.
The chief information officer of the United States Department of the Army issued a directive to adopt the technology by fiscal year 2013. Companies such as Telos Corporation market it to the US military.
Ethernet in the first mile project of the IEEE 802.3 ratified the Ethernet PON (EPON or GEPON) standard 802.3ah-2004. EPON is a “short haul” network that uses ethernet packets, fiber optic cables, and a single protocol layer. This technology employs standard 802.3 Ethernet frames with symmetric 1 gigabit per second upstream and downstream rates. EPON is applicable for data-centric networks, as well as full-service voice, data, and video networks.
The IEEE 802.3 ratified an amendment to the standard in 2009, known as 10G-EPON or 10Gbit/s EPON. 10G-EPON supports 10/1 Gbit/s, and the downstream wavelength plan enables simultaneous operation of 10 Gbit/s on one wavelength and 1 Gbit/s on a separate wavelength. This allows for concurrent operation of IEEE 802.3av and IEEE 802.3ah on the same PON. The upstream channel can support simultaneous operation of IEEE 802.3av and 1 Gbit/s 802.3ah on a single shared (1310 nm) channel.
EPON is the most widely deployed PON technology globally, with over 40 million installed EPON ports in 2014. Additionally, cable operators’ business services rely on EPON as part of the DOCSIS Provisioning of EPON (DPoE) specifications.
Furthermore, 10G EPON is fully compatible with other Ethernet standards and requires no conversion or encapsulation to connect to Ethernet-based networks on either the upstream or downstream end. This technology connects seamlessly with any type of IP-based or packetized communications. Due to the ubiquity of Ethernet installations in homes, workplaces, and elsewhere, implementing EPON is generally very inexpensive.
What is PON
Wavelength division multiplexing (WDM) enables a PON to use one wavelength for upstream traffic and another for downstream traffic on a single mode fiber (ITU-T G.652). BPON, EPON, GEPON, and GPON follow the same basic wavelength plan, where the 1490 nanometer (nm) wavelength is for downstream traffic and the 1310 nm wavelength is for upstream traffic. The 1550 nm wavelength is for optional overlay services, usually RF (analog) video.
The standards describe several optical power budgets, with the most common being a 28 dB loss budget for both BPON and GPON. However, products using less expensive optics have also been announced. A loss budget of 28 dB corresponds to about 20 km with a 32-way split. GPON systems may also provide for another 2-3 dB of loss budget through the use of Forward Error Correction (FEC). As optics improve, the 28 dB budget is likely to increase. Although GPON and EPON protocols permit large split ratios of up to 128 subscribers for GPON and up to 32,768 for EPON, in practice, most PONs are deployed with a split ratio of 1:32 or smaller.
A PON includes an optical line terminal (OLT) as a central office node, one or more optical network units (ONUs) or optical network terminals (ONTs) as user nodes, and the optical distribution network (ODN), which consists of fibers and splitters between them. An ONT is an ITU-T term used 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, or DSL. An ONU 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.
OLT and ONU
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 receives the converged interface and outputs native service interfaces to the user, such as Ethernet and POTS.
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 address to it. Encryption mean to prevent eavesdropping on downstream traffic.
Upstream bandwidth allocation
In a Passive Optical Network (PON), the Optical Line Terminal (OLT) distributes upstream bandwidth to Optical Network Units (ONUs).
To avoid transmission collisions, the OLT measures delays and sets registers in each ONU using PLOAM messages. The OLT then assigns specific time intervals for upstream transmissions via grants, which are dynamically re-calculated every few milliseconds to allocate bandwidth fairly among all ONUs.
Dynamic Bandwidth Allocation (DBA) oversubscribes PON for upstream traffic by using statistical multiplexing. There are two types of DBA in GPON: Non-Status Reporting (NSR) and Status-Reporting (SR).
NSR allocates a small amount of extra bandwidth to each ONU continuously, while SR polls ONUs for their backlogs, and ONUs report their status using a logarithmic measure of backlog in Transmission Containers (T-CONTs).
Based on the service level agreement for each T-CONT and the size of its backlog, the OLT optimizes allocation of spare bandwidth on the PON. In EPON systems, 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, which is equivalent to GPON’s SR DBA mechanism.
Based on market data, as of 2015, EPON had the largest deployment with around 40 million ports, and although GPON had a smaller market share at the time, it is expected to reach a value of $10.5 billion USD by 2020.
In TDM-PON, a passive optical splitter is utilized in the optical distribution network, which allows each ONU or ONT to burst transmit in an assigned time-slot in the upstream direction.
This results in the OLT receiving signals from only one ONU or ONT at a time. In the downstream direction, the OLT typically transmits continuously or may burst transmit, with ONUs or ONTs identifying their own data using 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
DOCSIS Provisioning of Ethernet Passive Optical Network (DPoE) is a collection of specifications created by Cable Television Laboratory.
These specs enable the implementation of DOCSIS service layer interface on existing Ethernet PON standards such as EPON, GEPON, and 10G-EPON MAC and PHY. Simply put, it incorporates DOCSIS Operations Administration Maintenance and Provisioning (OAMP) features on EPON equipment, making the EPON OLT behave like a DOCSIS Cable Modem Termination System (CMTS) platform, referred to as DPoE System.
DPoE offers the same IP services as a CMTS and also supports Metro Ethernet Forum (MEF) 9 and 14 services, providing Ethernet services to business customers.
Wavelength Division Multiplexing PON, or WDM-PON, is a non-standard passive optical networking technology that is being developed by some companies. With WDM-PON, multiple wavelengths can be used to separate Optical Network Units (ONUs) into several virtual PONs that co-exist 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.
However, there is no common standard for WDM-PON, nor any unanimously agreed-upon definition of the term. Some definitions suggest using a dedicated wavelength for each ONU, while others define WDM-PON as the use of more than one wavelength in any one direction on a PON. This lack of consensus makes it difficult to identify unbiased lists of WDM-PON vendors.
WDM-PON offers several advantages over traditional copper-based access networks, such as higher bandwidth, improved privacy, and better scalability, since each ONU only receives its own wavelength. Moreover, the MAC layer is simplified because the point-to-point connections between the OLT and ONUs are realized in the wavelength domain, eliminating the need for P2MP media access control. Additionally, each wavelength can run at a different speed and protocol, allowing for an easy pay-as-you-grow upgrade.
There are some challenges associated with WDM-PON, such as the high cost of initial set-up due to the cost of WDM components. Temperature control is another challenge since wavelengths tend to drift with environmental temperatures.
TWDM-PON is an advanced solution for the next-generation of passive optical networking, specifically designed for stage 2 of NG-PON2 as recommended by the Full-Service Access Network (FSAN) in April 2012. Unlike its predecessors, TWDM-PON utilizes both time and wavelength-division multiplexing to significantly increase the capacity and efficiency of the network. This innovative technology is designed to coexist with already deployed Gigabit PON (G-PON) and 10 Gigabit PON (XG-PON) systems, making it a versatile solution for network providers.
Long-Reach Optical Access Networks
The Long-Reach Optical Access Network (LROAN) aims to eliminate the need for optical/electrical/optical conversion at the local exchange by creating a continuous optical path from the customer to the core of the network.
Studies have demonstrated significant cost savings through reducing the amount of electronic equipment and real estate required at the local exchange or wire center. A proof of concept demonstrator successfully served 1024 users at 10Gbit/s with a reach of up to 100 km.
While sometimes referred to as Long-Reach PON, the term PON may no longer be applicable since, in most instances, only the distribution remains passive.
Passive optical networks (PONs) use different transmission modes for downstream and upstream communication. The OLT broadcasts an optical signal in continuous mode (CM) for downstream transmission, which provides an optical data signal to all ONUs at all times.
However, the upstream transmission mode is burst mode (BM) because transmitting optical data signals in CM would result in signal overlapping and attenuation at the power splitter. In BM, the ONU transmits optical packets only when allocated a time slot, and all ONUs share the upstream channel using time-division multiplexing (TDM).
The phases and amplitudes of the BM optical packets received by the OLT may vary, and to compensate for this, burst mode clock and data recovery (BM-CDR) and burst mode amplifiers, such as burst mode TIA, are employed. In BM, the transmitter must work in burst mode, allowing it to turn on and off quickly. The circuitries used in PONs are different from those used in point-to-point continuous mode optical communication links.
Fiber to the premises
Main article: Fiber to the x
Passive optical networks use beam splitters to distribute the signal without any switching or buffering capabilities. The resulting connection is a point-to-multipoint link, which requires optical network terminals on the customer end to filter out signals intended for other customers and coordinate in a multiplexing scheme to prevent collisions. Two types of multiplexing are possible: wavelength-division multiplexing and time-division multiplexing. Passive optical networks avoid the complexities of keeping electronic equipment outdoors, allow for analog broadcasts, and simplify the delivery of analog television, but require a powerful transmitting equipment and reach extenders for long distances. Alternatively, optical distribution networks can use a point-to-point homerun topology with splitters and/or active networking located at the central office.
Passive optical components
Passive optical networks are driven by high reliability, low cost, and passive functionality.
These networks use single-mode, passive optical components like branching devices, which include Wavelength-Division Multiplexer/Demultiplexers (WDMs), isolators, circulators, and filters. These components are used in various telecommunications networks, including Fiber In The Loop (FITL), Hybrid Fiber-Coaxial Cable (HFC), Synchronous Optical Network (SONET), and Synchronous Digital Hierarchy (SDH) systems that employ optical communications systems. They also utilize Optical Fiber Amplifiers (OFAs) and Dense Wavelength Division Multiplexer (DWDM) systems. Telcordia Technologies published the proposed requirements for these components in 2010.
Passive optical components have a broad range of applications that include multichannel transmission, distribution, optical taps for monitoring, pump combiners for fiber amplifiers, bit-rate limiters, optical connects, route diversity, polarization diversity, interferometers, and coherent communication.
WDMs are optical components that split or combine power 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 that split or combine power independently of the wavelength composition of the optical signal. A single component may combine and divide optical signals simultaneously, such as in bidirectional (duplex) transmission over a single fiber. Passive optical components are data format transparent, dividing and combining optical power in a predetermined ratio (coupling ratio) regardless of the signals’ information content. WDMs act as wavelength splitters and combiners, while wavelength insensitive couplers act as power splitters and combiners.
An optical isolator is a two-port passive component that allows light to pass through with low attenuation in one direction, while isolating light propagating in the reverse direction (by providing high attenuation). Isolators are used as 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 similarly to an optical isolator, but instead of losing the reverse propagating lightwave, it directs it to a third port for output. An optical circulator can be used for bidirectional transmission, distributing (and isolating) 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.