What Is Passive Optical Networking (PON)?
Passive optical networking (PON), like active optical networking, uses fiber-optic cabling to provide Ethernet connectivity from a main data source to endpoints.
While there are many subtle differences, a clear distinction between active optical networking and PON topology is PON's use of a technique that distributes a single signal to multiple branches through unpowered devices called optical beam splitters.
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What are the benefits of PON?
PON, developed in the mid-1990s, was originally designed to allow internet service providers (ISPs) to deliver broadband triple-play services (data, voice, and video) to residential users.
Its purpose was to reduce the number of fiber runs needed to reach multiple end-user locations and to eliminate the need to provide power to transmission devices between the central office (also called the head end) and the end user. Both of these issues had hindered deployment of fiber to the premises (FTTP) services at the time.
While PON was initially focused on fiber connectivity to the home, other types of network users–such as hotels, hospitals, and high-density residential buildings–are now seeing similar advantages in "last mile" power distribution and fiber efficiency by deploying this technology.
How does PON work?
OLTs and ONTs
In a PON network, a device called an optical line terminal (OLT) is placed at the head end of the network. A single fiber-optic cable runs from the OLT to a nonpowered (passive) optical beam splitter, which multiplies the signal and relays it to many optical network terminals (ONTs). End-user devices such as PCs and telephones are connected to the ONTs.
Since the splitting function is a one-to-many broadcast of the same data stream, the ONTs are responsible for filtering packets meant for the various connected endpoint devices. Encryption ensures that each ONT reads only the contents addressed to the endpoints connected to it.
Passive optical splitting
Optical splitters take a single light source (a single fiber-optic strand) and refract and duplicate it multiple times to "outbound" fibers. In its simplest form, an optical beam splitter splits a light source in two by using two back-to-back prisms.
For the XGS-PON standard, up to 256 splits are supported on an XGS-PON port. Splits may be centralized where the splitter is directly connected to an OLT in the central office by a single fiber and to the customer ONT, or cascaded where a stage 1 splitter is connected to an OLT in the central office by a single fiber and to other splitters that connect to the customer ONT. In centralized deployments, the typical split ratio is 1:32, meaning 1 OLT port connects to 32 customer ONT. With cascaded deployments, the typical split ratio at stages 1 and 2 is 1:8, meaning 1 OLT port can connect to 64 customer ONT.
WDM and TDM
Because PON uses the same strand of fiber to send and receive data, the passive optical splitter also acts as an optical combiner receiving data traffic from the same connected end devices. To achieve this, PON takes advantage of two distinct types of long-established telephony multiplexing concepts: wavelength division and time division.
Wavelength-division multiplexing (WDM) allows bidirectional traffic across a single fiber by using a different wavelength for each direction of traffic:
XGS-PON: 1577-nanometer (nm) wavelength for downstream traffic and 1270-nm wavelength for upstream traffic.
The 1550-nm wavelength is reserved for optional overlay services, typically RF (analog) video.
Future iterations of the PON standard will define separate wavelengths for backward compatibility.
Time-division multiplexing (TDM) allows multiple end devices to transmit and receive independent signals across a single fiber by reserving time slots in a stream of data. PON uses two such technologies: TDM for downstream traffic and time-division multiple access (TDMA) for upstream traffic.
As a passive device, the splitter acts as distribution point, with the single feed of downstream data broadcast to all connected ONT endpoints. The ONT accepts packets assigned to its TDM channel (frame time slot). It filters and discards packets meant for other ONTs.
TDMA enables multiple transmitters to be connected to one receiver. For PON, TDMA is used to recombine the multiple upstream feeds at the coupler. A splitter and a coupler are often found in one device.
Standards
XGS-PON is defined by the ITU-T G.9807.1 standard for a 10 Gbps symmetric passive optical network in an optical access network with the latest revision in 2023 related to out-of-band noise limits.