Passive optical network

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A passive optical network (PON) is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32-128. A PON consists of an Optical Line Termination (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.

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

Upstream signals are combined using a multiple access protocol, invariably time division multiple access (TDMA). The OLTs "range" the ONUs in order to provide time slot assignments for upstream communication.

Contents

[edit] 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.
  • 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). In early 2008, Verizon is in the process of installing millions of lines, while British Telecom and AT&T are in advanced trials.
  • IEEE 802.3ah
    • EPON or GEPON (Ethernet PON) is an IEEE/EFM standard for using Ethernet for packet data.
  • IEEE 802.3av
    • 10GEPON (10 Gigabit Ethernet PON) is an IEEE Task Force for 10Gbit/s backwards compatible with 802.3ah EPON. 10GigEPON will likely multi-lamda downstream and continue to use a single lamda with ATDMA for upstream. It will also be WDM-PON compatible.

[edit] History

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Please help improve this article, or discuss the issue on its talk page. Editing help is available. (December 2007)

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) of downstream bandwidth and 155 Mbit/s of upstream traffic, although the standard accommodates higher rates.

The ITU-T G.984 (GPON) standard represents a boost 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 to allow for higher Quality of Service (QoS) for delay-sensitive traffic such as voice and video communications.

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. Recently, starting in early 2006, work began on a very high-speed 10 Gbit/s EPON (XEPON or 10-GEPON) standard (http://www.ieee802.org/3/av/).

A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic on a single Nonzero dispersion shifted fiber (ITU-T G.652). The specification calls for downstream traffic to be transmitted on the 1490 nanometer (nm) wavelength and upstream traffic to be transmitted at 1310 nm. The 1550 nm band is allocated 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 GPONm 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. The GPON address space accommodates service to up to 128 ONUs on a single fiber.

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). In apartment buildings, the ONT often separately connects via VDSL or Ethernet to the apartments, at speeds up to 100 megabits. An ONT is a single integrated electronics unit that terminates the PON and presents native service interfaces to the user. An ONU is the PON-side half of the ONT, terminating the PON, and may present one or more converged interfaces, such as xDSL or Ethernet, toward the user. An ONU typically requires a separate subscriber unit to provide native user services such as telephony, Ethernet data, or video. In practice, the difference between an ONT and ONU is frequently ignored, and either term is used generically to refer to both classes of equipment.

The OLT provides the interface between the PON and the backbone network. These typically include:

The ONT 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:

  • the ONU, which terminates the PON and presents a converged interface – such as xDSL 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 ONTs. Each ONT only reads the content of those packets that are addressed to it. Encryption is used to prevent eavesdropping on downstream traffic.

[edit] 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.)

There are basically 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.

[edit] Current Status of PON

[edit] TDM-PON

Both APON/BPON and EPON/GEPON have been deployed widely, but most networks designed in 2008 use GPON or GEPON. GPON is most common in the West, GEPON more common in Asia. Fiber to the home is expensive. France, DT, and AT&T are making standard for new builds, but only some carriers are fibering existing homes. NTT in Japan has passed 30M homes, and Verizon in the U.S. has passed 10M and continues at 3M per year.

The currently deployed PON (e.g., APON, GPON) is named as TDM-PON, in which the bandwidth is shared among the users in the manner of time domain multiplexing. This is different from the multiplexing manner discussed below, e.g., WDM-PON.

[edit] WDM-PON

Wavelength Division Multiplexing PON, or WDM-PON, is a type of passive optical networking, being pioneered by several companies, that uses multiple optical wavelengths to increase the upstream and/or downstream bandwidth available to end users. This technology looks forward to a day when optical technology is cheaper and easier to deploy, and end users demand higher bandwidth. WDM-PON can provide more bandwidth over longer distances by devoting more raw optical bandwidth to each user, and by increasing the link loss budget of each wavelength, making it less sensitive to the optical losses incurred at each optical splitter.

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. Samsung and Korea Telecom have done important research. Novera Optics Inc. has introduced first products, which supports the WDM PON deployment in Korea.

[edit] DOCSIS-PON

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 or GEPON) 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.

[edit] 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 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 may be provided by digitizing and transporting the upstream or return RF into the digital PON upstream overhead, or by the use a of a CWDM overlay. Implementations vary by vendor and there are no standards for the return portion of the RF overlay for PONs. 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.

[edit] Enabling technologies for PON

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 have may different amplitudes. In order to compensate the phase variation and amplitude variation in a short time (e.g., within 40 ns for GPON[1]), 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.

[edit] References

  1. ^ Rec. G.984, Gigabit-capable Passive Optical Networks (GPON), ITU-T, 2003.