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Passive optical network

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This is an old revision of this page, as edited by 84.246.54.230 (talk) at 13:01, 4 November 2010 (ITU-T T-REC-G.984.1-200803 par. 3.2.8, 3.2.9 & 5.1 and ITU-T T-REC-G.984.3-200803 par. 3.2.8, 3.2.9 & 5.1: ONT is a special, single-user case of an ONU.). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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. A passive optical network is a form of fiber-optic access network.

Downstream signals are broadcast to each premises sharing a single fiber. Encryption is used to prevent eavesdropping.

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.

Standards

  • ITU-T G.983
    • APON (ATM Passive Optical Network). This was the first Passive optical network standard. It was used primarily for business applications, and was based on ATM.
    • BPON (Broadband PON) is a standard based on APON. It adds support for WDM, dynamic and higher upstream bandwidth allocation, and survivability. It also created a standard management interface, called OMCI, between the OLT and ONU/ONT, enabling mixed-vendor networks.
  • IEEE 802.3ah
    • EPON or GEPON (Ethernet PON) is an IEEE/EFM standard for using Ethernet for packet data. 802.3ah is now part of the IEEE 802.3 standard. There are currently over 25 million installed EPON subscribers. Commercial upgrade capability to 10G EPON will become available in 2010 (see IEEE 802.3av, 10G-EPON).
  • ITU-T G.984
    • GPON (Gigabit PON) is an evolution of the BPON standard. It supports higher rates, enhanced security, and choice of Layer 2 protocol (ATM, GEM, Ethernet). By mid-2008, Verizon had installed over 800 thousand lines. British Telecom, Mobily-SaudiArabia, Etisalat-UAE, and AT&T are in advanced trials. It is the successor to G.983.
  • ITU-T G.987
    • 10G-PON has 10Gbps downstream and 2.5Gps upstream – framing is ”G-PON like” and designed to coexist with GPON devices on the same network.[1]
  • IEEE 802.3av
    • 10G-EPON (10 Gigabit Ethernet PON) is an IEEE Task Force for 10Gbit/s, backward compatible with 802.3ah EPON. 10GEPON will use separate wavelengths for 10G and 1G downstream. 802.3av will continue to use a single wavelength for both 10G and 1G upstream with TDMA separation. The 802.3av task force has concluded with the .3av inclusion in the IEEE 802.3 standard. Commercial 10G EPON equipment is expected in 2010.
  • SCTE IPS910
    • RFoG (RFoverGlass) is an SCTE Interface Practices Subcomittee standard in development for Point to Multipoint (P2MP) operations that has a proposed wavelength plan compatible with data PON solutions including EPON,GEPON and 10G-EPON. RFoG offers an FTTH PON like architecture for MSOs without having to select or deploy a PON technology.

History

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 (1310 nm) channel.

Network elements

A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single nondispersion-shifted fiber (ITU-T G.652). BPON, EPON, GEPON, and GPON have the same basic wavelength plan[contradictory] 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 to describe a special, single-user case of an ONU. In Multiple Tenant Units, the ONU 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 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.

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 ONU functions are separated into two parts:

  • the ONU, which terminates the PON and presents a converged interface – such as xDSL, coax, or multiservice Ethernet – toward the user, and
  • network termination equipment (NTE), which provides the separate, native service interfaces directly to the user

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 only reads the content of 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 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.

Current status

TDM-PON

Both APON/BPON and EPON/GEPON have been deployed widely, but most networks designed in 2008 use GPON or GEPON. GPON has fewer than 2 million installed ports. GEPON has approximately 30 million deployed ports. For TDM-PON, a passive power splitter is used as the remote terminal. Each ONUs (Optical network units) signals are multiplexed in the time domain. ONUs see their own data through the address labels embedded in the signal.

DOCSIS over EPON or DPON

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.

RF-PON or RFoG

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 lambda 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.

WDM-PON

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 a 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.

Enabling technologies

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[2]), 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.

Fiber to the premises

Main article: Fiber to the x

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 [3].

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.

Passive optical components

The drivers behind the modern passive optical network are the optical components that enable Quality of Service (QoS).

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. Industry proposed requirements for these components are detailed in GR-1209, Generic Requirements for Passive Optical Components.

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 highbit-rate 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).

GR-1221-CORE, Generic Reliability Assurance Requirements for Passive Optical Components, addresses the long-term reliability of passive optical components.

References

  1. ^ http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.987.1-201001-I!!PDF-E&type=items
  2. ^ Rec. G.984, Gigabit-capable Passive Optical Networks (GPON), ITU-T, 2003.
  3. ^ Novera's Got a New PON Spin from Light Reading, retrieved on 2009-09-02.
Notes
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