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. 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.

1 Standards
2 History
3 Dynamic Bandwidth Allocation
4 Current Status of PON
5 BPON Equipment Manufacturers
6 GPON Equipment Manufacturers
7 EPON Equipment Manufacturers
8 References

[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.
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). It also created a standard management interface, called OMCI, between the OLT and ONU/ONT, enabling mixed-vendor networks.
IEEE 802.3ah
- EPON (Ethernet PON) is an IEEE/EFM standard for using Ethernet for packet data.

[edit] 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) 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. A GPON network delivers up to 2,488 Mbits per second (Mbit/s) of downstream bandwidth, and 1,244 Mbit/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 a symmetrical 1 Gbps 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 Gigabit/second EPON (XEPON or 10-GEPON or ) standard (http://grouper.ieee.org/groups/802/3/10GEPON_study/index.html).

A PON takes advantage of wavelength division multiplexing (WDM), using one wavelength for downstream traffic and another for upstream traffic. This allows for two-way traffic on a single fiber optic cable. The latest specification calls for downstream traffic to be transmitted on the 1490 nanometer (nm) wavelength and the upstream traffic to be transmitted at 1310 nm. The 1550 nm band is purposely left open in case the service provider wishes to share the PON fiber with a hybrid fiber-coax (HFC) network, which is the traditional Cable TV architecture.

A PON consists of a central office node, called an optical line terminal (OLT), one or more user nodes, called optical network units (ONU) or optical network terminals (ONT), and the fibers and splitters between them, called the optical distribution network (ODN).

The OLT provides the interface between the PON and the backbone network. These typically include: - standard time division multiplexed (TDM) interfaces such as SONET/SDH or PDH at various rates - Internet Protocol (IP) traffic over Gigabit or 100 Mbit/s Ethernet - ATM UNI at 155-622 Mbit/s

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), data (typically Ethernet or V.35), video, and/or telemetry (TTL, ECL, RS530, etc.). Often, the ONT functions are separated into two parts: (1) the ONU, which terminates the PON and presents a converged interface - such as xDSL or multiservice Ethernet - toward the user, and (2) network termination equipment (NTE) which provides the separate, native service interfaces directly to the user

A PON is a converged network, in that all of these services are converted and encapsulated in a single packet type for transmission over the PON fiber.

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 unauthorized snooping of downstream traffic. The OLT also communicates with each ONT in order to allocate upstream bandwidth to each node. When an ONT has traffic to send, the OLT assigns a timeslot in which the ONT can send its packets. Because bandwidth is not explicitly reserved for each ONT but allocated dynamically, a PON allows statistical multiplexing and over-subscription of both upstream and downstream bandwidth. This gives PON yet another advantage over point-to-point networks, in that not only the fiber but also the bandwidth can be shared across a large group of users, without sacrificing security.

[edit] Dynamic Bandwidth Allocation

Dynamic bandwidth allocation (DBA) for PONs is an active research topic. DBA is a term that refers to the management of the upstream (the path from ONUs to OLT) bandwidth resources. DBA has two primary goals: 1) minimize the delay experienced by packets sitting at an ONU waiting for transmission, and 2) maximize the utilization of the bandwidth resources. These goals often compete with each other. The idea is to create the appropriate balance between the two as well as to provide differential treatment for different types of traffic (real-time and non real-time traffic). For a discussion of the early DBA research work generated for Ethernet PONs (EPONs), please see http://www.public.asu.edu/~mmcgarry/CommMag_Aug2004.pdf

[edit] Current Status of PON

Both APON/BPON and EPON/GEPON have been deployed on a large scale in many networks around the world (see list, below). The ITU G.984 GPON standard has been finalized more recently, and is in lab and field trials in the US, Europe, and Asia-Pacific regions, with mass deployment scheduled to begin in 2007 by large service providers such as British Telecom, Verizon, as well as many others. Deployment of PON -- or any fundamentally new infrastructure -- has been constrained by the pace of protocol standardization, equipment available and cost, conservatism in moving to new technology, but most importantly by the cost of installing new fiber all the way to customer sites. Some service providers are deploying PON ONUs as outside plant equipment in existing or new outside plant enclosures, using asymmetrical digital subscriber line (ADSL) or very high speed DSL (VDSL2) links over existing copper twisted pairs to the subscriber premises to avoid the cost of the last leg. This strategy provides high bandwidth services, but does not maximize the operational cost savings that are possible with a 100% passive outside plant.

Telecommunications technology never stops progressing. As can be seen by the progress of standards from APON to BPON to GPON, and from EPON to GEPON, the industry is looking at ways to deliver even more bandwidth over longer distances than ever before. Two ways of doing this are by increasing the number of optical wavelengths being used on the PON fiber, and by increasing the bandwidth and bandwidth efficiency of each wavelength.

[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. An excellent article on this subject can be found at http://networks.cs.ucdavis.edu/publications/2005_amitabha_2005-11-19_05_24_32.pdf#search=%22WDM%20PON%22.

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. For an article discussing bandwidth management for multiwavelength PONs in the context of EPON upstream transmission, please see http://www.public.asu.edu/~mmcgarry/JON_Sep2006.pdf