Understanding Fiber Technology

In FTTH, much of the alphabet soup of acronyms has to do with devices that convert electrical signals that travel in wires, to pulses of light and back again. Here's what you might want to know so you can understand the technical jargon. Let's start at the beginning of a fiber network.

OLT stands for Optical Line Terminal. OLTs put the pulses on the fiber in the first place. Because they are located in telephone exchanges and other network "central offices", residents and property owners rarely see them.

ONTs are Optical Network Terminals. They are also sometimes called ONUs, for Optical Network Units. They are the devices at the consumer end that turn the light pulses back into electrical signals. Usually, customers will have equipment such as computers that expect an Ethernet connection. This is a standard way of networking that's used around the world. Your computers, and perhaps your little home WiFi system, all use Ethernet. Ethernet connectors are built into virtually all computers that have been sold in this decade. So a typical ONT turns the light pulses into Ethernet signals.

In the United States the ONTs are typically inside cigar-box sized enclosures on the outside walls of houses or apartments. But they can be made smaller than a deck of cards, and can be used inside customer premises as well.

You'll also hear about the point of presence, or POP. That's the point at which the signal from multiple customers joins the rest of the extended network.

Hybrid fiber coax, or HFC, is the cable company's coax, with fiber bringing the signal fairly close to the building, or even into the basement or a central area on your property. A DOCSIS node, fed by fiber, then distributes the signal to individual households over coax. One node typically feeds 100 to 500 homes.

Pedestals and larger Fiber Distribution Hubs are enclosures close to the user premises. They can hold the beam splitters that take the signal from one fiber that extends back to the central office, and divides it (typically 8:1 up to 32:1 but as much as 64:1) among fibers that go to individual dwelling units. Pedestals and hubs can be below ground, above ground (they often look like short posts or squat air-conditioner-size boxes), or attached to buildings. Connections and splits can also be made in boxes hung under roof eaves, in attics or basements, on telephone poles, or on what look like power lines or phone lines. For best reliability, many contractors bring two fibers into each dwelling unit from the pedestal, not one. The fibers leading from the hub or pedestal to the user premises is called the drop cable.