Hybrid Fibre-Coaxial

Hybrid fiber-coaxial (HFC) is a telecommunications industry term for a broadband network that combines optical fiber and coaxial cable. It has been commonly employed globally by cable television operators since the early 1990s.

In a hybrid fiber-coaxial cable system, the television channels are sent from the cable system’s distribution facility, the head end, to local communities through optical fiber trunk lines.

At the local community, a box called an optical node translates the signal from a light beam to electrical signal, and sends it over coaxial cable lines for distribution to subscriber residences. The fiber optic trunk lines provide adequate bandwidth to allow future expansion and new bandwidth-intensive services.

The fiber optic network extends from the cable operators’ master head end, sometimes to regional head ends, and out to a neighborhood’s hub site, and finally to a coaxial cable node which serves anywhere from 25 to 2000 homes. A master head end will usually have satellite dishes for reception of distant video signals as well as IP aggregation routers. Some master head ends also house telephony equipment for providing telecommunications services to the community.

A regional or area head end/hub will receive the video signal from the master head end and add to it the public, educational, and government access (PEG) cable TV channels as required by local franchising authorities or insert targeted advertising that would appeal to a local area.

The various services are encoded, modulated and up converted onto radio frequency (RF) carriers, combined onto a single electrical signal and inserted into a broadband optical transmitter.

This optical transmitter converts the electrical signal to a downstream optically modulated signal that is sent to the nodes. Fiber optic cables connect the headend or hub to optical nodes in a point-to-point or star topology, or in some cases, in a protected ring topology.

A fiber optic node has a broadband optical receiver, which converts the downstream optically modulated signal coming from the head end or hub to an electrical signal going to the homes. As of 2015, the downstream signal is a RF modulated signal that typically begins at 50 MHz and ranges from 550–1000 MHz on the upper end.

The fiber optic node also contains a reverse- or return-path transmitter that sends communication from the home back to the head end. In North America, this reverse signal is a modulated RF ranging from 5–42 MHz while in other parts of the world, the range is 5–65 MHz. The optical coupler combined with the optical receiver forms a node.[clarification needed]

The optical portion of the network provides a large amount of flexibility. If there are not many fiber-optic cables to the node, wavelength division multiplexing can be used to combine multiple optical signals onto the same fiber. Optical filters are used to combine and split optical wavelengths onto the single fiber. For example, the downstream signal could be on a wavelength at 1490 nm and the return signal could be on a wavelength at 1310 nm.

The coaxial portion of the network connects 25–2000 homes (500 is typical) in a tree-and-branch configuration off of the node. RF amplifiers are used at intervals to overcome cable attenuation and passive losses of the electrical signals caused by splitting or “tapping” the coaxial cable.

Trunk coaxial cables are connected to the optical node and form a coaxial backbone to which smaller distribution cables connect. Trunk cables also carry AC power which is added to the cable line at usually either 60 or 90 V by a power supply (with a lead acid backup battery inside) and a power inserter.

The power is added to the cable line so that optical nodes, trunk and distribution amplifiers do not need an individual, external power source. The power supply might have a power meter next to it depending on local power company regulations.

From the trunk cables, smaller distribution cables are connected to a port of the trunk amplifier to carry the RF signal and the AC power down individual streets. If needed, line extenders, which are smaller distribution amplifiers, boost the signals to keep the power of the television signal at a level that the TV can accept. The distribution line is then “tapped” into and used to connect the individual drops to customer homes.