Passive optical network

A Passive Optical Network (PON) is a fiber to the premises configuration 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 communication company s office and a number of Optical Network Units (ONUs) near end users. It is, in other words, a point-to-multipoint configuration, which reduces the amount of fiber required compared with point to point.

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).

= Standards =

*ITU-T G.983 (APON/BPON) *ITU-T G.984 (GPON) *IEEE 802.3ah (EPON) - part of Ethernet in the First Mile

= 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 most recent standard, ITU-T G.984, represents a significant boost in both the total bandwidth and in bandwidth efficiency through the use of larger, variable-length packets.

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 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, while the ONT provides the service interface to the end user. These services can include voice (plain old telephony service or voice over IP), data (typically Ethernet or V.35), video, and/or telemetry (TTL, ECL, RS530, etc.). 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.

= References =

PON Forum http://www.ponforum.org

FSAN: http://www.fsanweb.org

Recent Developments in Passive Optical Networking (FTTHBlog): http://www.ftthblog.com