A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters utilizing Brewster's angle principles are used to enable a single optical fiber to serve multiple premises, typically 32-128. A PON consists of an optical line terminal (OLT) at the service provider's central office and a number of optical network units (ONUs) near end users. A PON configuration reduces the amount of fiber and central office equipment required compared with point to point architectures.
Upstream signals are combined using a multiple access protocol, usually time division multiple access (TDMA). The OLTs "range" the ONUs in order to provide time slot assignments for upstream communication.
Early work on efficient fiber to the home architectures was done in the 1990s by the Full Service Access Network (FSAN) working group, formed by major telecommunications service providers and system vendors. The International Telecommunications Union (ITU) did further work, and has since standardized on two generations of PON. The older ITU-T G.983 standard is based on Asynchronous Transfer Mode (ATM), and has therefore been referred to as APON (ATM PON). Further improvements to the original APON standard – as well as the gradual falling out of favor of ATM as a protocol – led to the full, final version of ITU-T G.983 being referred to more often as broadband PON, or BPON. A typical APON/BPON provides 622 megabits per second (Mbit/s) (OC-12) of downstream bandwidth and 155 Mbit/s (OC-3) of upstream traffic, although the standard accommodates higher rates.
The ITU-T G.984 (GPON) standard represents a boost, compared to BPON, in both the total bandwidth and bandwidth efficiency through the use of larger, variable-length packets. Again, the standards permit several choices of bit rate, but the industry has converged on 2.488 gigabits per second (Gbit/s) of downstream bandwidth, and 1.244 Gbit/s of upstream bandwidth. GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic with frame segmentation.
The IEEE 802.3 Ethernet PON (EPON or GEPON) standard was completed in 2004 (http://www.ieee802.org/3/), as part of the Ethernet First Mile project. EPON uses 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. 10Gbit/s EPON or 10G-EPON was ratified as an amendment IEEE 802.3av to IEEE 802.3. 10G-EPON supports 10Gbps/1Gbps. The downstream wavelength plan support simultaneous operation of 10Gbps on one wavelength and 1Gbps on a separate wavelength for operation of IEEE 802.3av and IEEE 802.3ah on the same PON concurrently. The upstream channel can support simultaneous operation of IEEE 802.3av and 1Gbps 802.3ah simultaneously on a single shared (1310nm) channel.
A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single [Non-dispersion shifted 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 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 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 1x32 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, whereas ONU is an IEEE term. In Multiple Tenant Units, the ONT may be bridged to a customer premise 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 ONT 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.
The OLT provides the interface between the PON and the service providers network services. These typically include:
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 ONT functions are separated into two parts:
A PON is a shared network, in that the OLT sends a single stream of downstream traffic that is seen by all ONTs. Each ONT only reads the content of those packets that are addressed to it. Encryption is used to prevent eavesdropping on downstream traffic.
The OLT is responsible for allocating upstream bandwidth to the ONTs. Because the optical distribution network (ODN) is shared, ONT upstream transmissions could collide if they were transmitted at random times. ONTs can lie at varying distances from the OLT, meaning that the transmission delay from each ONT is unique. The OLT measures delay and sets a register in each ONT via PLOAM (physical layer operations and maintenance) messages to equalize its delay with respect to all of the other ONTs on the PON.
Once the delay of all ONTs has been set, the OLT transmits so-called grants to the individual ONTs. 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 ONTs, such that each ONT 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 – internet surfing, for example – 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 ONT 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 ONT. If the ONT has no traffic to send, it transmits idle frames during its excess allocation. If the OLT observes that a given ONT is not sending idle frames, it increases the bandwidth allocation to that ONT. Once the ONT's burst has been transferred, the OLT observes a large number of idle frames from the given ONT, and reduces its allocation accordingly. NSR DBA has the advantage that it imposes no requirements on the ONT, and the disadvantage that there is no way for the OLT to know how best to assign bandwidth across several ONTs that need more.
In SR DBA, the OLT polls ONTs for their backlogs. A given ONT may have several so-called traffic containers (T-CONTs), each with its own priority or traffic class. The ONT 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.
Both APON/BPON and EPON/GEPON have been deployed widely, but most networks designed in 2008 use GPON or GEPON. GPON has less than 2 million installed ports. GEPON has approximately 15 million deployed ports.
Data Over Cable Service Interface Specification (DOCSIS) PON, or D-PON/DPON, is a type of passive optical networking, being proposed by several companies, that implements 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. Some DPON systems may optionally support the Metro Ethernet Forum (MEF) 9 and 14 specifications for the delivery of Ethernet Transport services including Ethernet LANs (ELAN), Ethernet Virtual Private Line (EVPL), and point to point Ethernet Transport (ELINE) services. In these instances the DPON system also acts as an IP/MPLS Provider Edge (PE) Router.
Radio Frequency PON (RF-PON) or Radio Frequency over Glass (RFOG) or Hybrid-Fiber-Coax PON (HFC-PON) or Cable PON, is a type of passive optical networking, that proposes to transport RF signals that are now transported over copper (principally over a hybrid fiber and coaxial cable) over PON. In the forward direction RF-PON is either a stand alone P2MP system or an optical overlay for existing PON such as GPON or GEPON/EPON. The overlay for RF-PON works in the same way that some CWDM PON or potential WDM-PON overlays work. Reverse RF support is provided by transporting the upstream or return RF into on a separate lamda from the PON return wavelength. Implementations vary by vendor there because the standard is not yet complete. The Society of Cable and Telecommunications Engineers (SCTE) Interface Practices Subcomittee (IPS) Work Group 5, is currently working on IPS 910 RF over Glass. RF-PON offers backwards compatibility with existing RF modulation technology, but offers no additional bandwidth for RF based services. 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 not 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
Due to the topology of PON, the transmission modes for downstream (i.e., from OLT to ONU) and upstream (i.e., from ONU to OLT) are different. For the downstream transmission, the OLT broadcasts optical signal to all the ONUs in continuous mode (CM), i.e., the downstream channel always has optical data signal. However, in the upstream channel, ONUs can not transmit optical data signal in CM. Use of CM would result in all of the signals transmitted from the ONUs converging (with attenuation) into one fiber by the power splitter (serving as power coupler), and overlapping. To solve this problem, burst mode (BM) transmission is adopted for upstream channel. The given ONU only transmits optical packet when it is allocated a time slot and it needs to transmit, and all the ONUs share the upstream channel in the time division multiplexing (TDM) mode. The phases of the BM optical packets received by the OLT are different from packet to packet, since the ONUs are not synchronized to transmit optical packet in the same phase, and the distance between OLT and given ONU are random. Since the distance between the OLT and ONUs are not uniform, the optical packets received by the OLT may have different amplitudes. In order to compensate the phase variation and amplitude variation in a short time (e.g., within 40 ns for GPON), burst mode clock and data recovery (BM-CDR) and burst mode amplifier (e.g., burst mode TIA) need to be employed, respectively. Furthermore, the BM transmission mode requires the transmitter to work in burst mode. Such a burst mode transmitter is able to turn on and off in short time. The above three kinds of circuitries in PON are quite different from their counterparts in the point-to-point continuous mode optical communication link.
Passive optical networks do not use electrically powered components to split the signal. Instead, the signal is distributed using beam splitters. Each splitter typically splits the signal from a single fiber into 16, 32, or 64 fibers, depending on the manufacturer, and several splitters can be aggregated in a single cabinet. A beam splitter cannot provide any switching or buffering capabilities; the resulting connection is called a point-to-multipoint link. For such a connection, the optical network terminals on the customer's end must perform some special functions which would not otherwise be required. For example, due to the absence of switching capabilities, each signal leaving the central office must be broadcast to all users served by that splitter (including to those for whom the signal is not intended). It is therefore up to the optical network terminal to filter out any signals intended for other customers. In addition, since beam splitters cannot perform buffering, each individual optical network terminal must be coordinated in a multiplexing scheme to prevent signals leaving the customer from colliding at the intersection. Two types of multiplexing are possible for achieving this: wavelength-division multiplexing and time-division multiplexing. With wavelength-division multiplexing, each customer transmits their signal using a unique wavelength. With time-division multiplexing (TDM), the customers "take turns" transmitting information. TDM equipment has been on the market longest; WDM-PON equipment became available in 2005 .
Passive optical networks have both advantages and disadvantages over active networks. They avoid the complexities involved in keeping electronic equipment operating outdoors. They also allow for analog broadcasts, which can simplify the delivery of analog television. However, because each signal must be pushed out to everyone served by the splitter (rather than to just a single switching device), the central office must be equipped with a particularly powerful piece of transmitting equipment called an optical line terminal (OLT). In addition, because each customer's optical network terminal must transmit all the way to the central office (rather than to just the nearest switching device), customers can't be as far from the central office as is possible with active optical networks.