Hybrid fiber coax (HFC) is the term that describes the service delivery architecture used by cable operators and multi-system operators (MSO). The architecture includes a combination of fiber optic cabling and coaxial cabling to distribute video, data and voice content to/from the headend and the subscribers. Typically, the signals are transported from the headend through a hub, to within the last mile via fiber optic cable. As an example, for a service area ranging from 64 homes passed to 1,000* homes, the fiber optic cable ends in an HFC node. At this point, the optical signal is converted into a radio frequency (RF) signal and transmitted over coaxial cable to subscribers’ homes/businesses.
The coaxial cable that enters subscribers’ homes is a flexible, small “drop” cable that will connect directly to the cable modem, the set-top box or other consumer premise equipment. The RF signal on the coaxial cable is strong enough to allow signals to be split in different directions within the home. Sometimes the number of separate devices in the home is so large that amplification may be required. In this instance, a drop amp or house amp is used. Often, the splitters and amplifier are combined to reduce the number of connections.
The term HFC also implies the way in which signals are transported through the network. All HFC networks use frequency division multiplexing to pack the content into the spectrum slots of a cable plant. Spectrum in this case is typically referred to as the frequency bands that carry the content – 52MHz to 1004MHz for the forward, (headend to subscriber), and 5-42MHz for the reverse (subscriber to headend) in the US. Across the globe, spectrum allocations and split frequencies vary. Downstream and upstream are terms also used to describe these bands, respectively.
Signals that originate in the headend that must be transported to the subscriber are either analog or modulated with a scheme called quadrature amplitude modulation (QAM). QAM signals are generated by taking a digital representation of the original signal, whether it is an analog voice or video signal, and converting it by sampling and modulating a carrier. The resulting QAM signal is a high-capacity analog signal, which requires care in maintaining a high level of signal-to-noise ratio (SNR). This contrasts with a digital optical signal as used in GEPON or GPON (gigabit passive optical networks), for which SNR equivalent requirements are much simpler.
The standard that governs QAM transport is managed by CableLabs, a non-profit industry funded R&D organization, and is called Data Over Cable Service Interface Specification, or DOCSIS®. Currently, DOCSIS 3.0 is the most widely deployed. The newest version, DOCSIS 3.1, significantly improves the modulation rates and data throughput to subscribers, also expanding the downstream to 1200MHz and beyond, and the upstream to 85MHz and beyond.
So, how do MSOs seamlessly transition HFC to fiber-to-the-home? Stay tuned for another post on successful strategies.
*A significant consideration as to the size of the service area is the amount of bandwidth a subscriber consumes. As each HFC node has a direct connection back to the headend, smaller service areas get access to more data-per-home-passed delivered from the headend.
