This blog post is part of a series called “CommScope Definitions,” in which we will explain common terms in communications network infrastructure.
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’
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.
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.