HFC is used by multi-system operators (MSO) and cable operators as a service delivery architecture. It uses coaxial and fiber optic cabling to distribute voice content, data and video to and from the headend and subscribers. Signals are typically sent from the headend via a hub to within a mile of the final destination using a fiber optic cable. For a service area that ranges from 64 homes to 1,000 subscribers, the fiber optic cable is terminated in an HFC node. Optical signals are then converted to radio frequency (RF) signals and transmitted via coaxial cable to subscriber's businesses or homes.
The size of the service area that can be handled is determined by the amount of bandwidth subscribers consumes. As each HFC node is connected directly back to the headend, more data can be delivered per subscriber from the headend for smaller service areas.
The coaxial cable used at the subscriber is a small, flexible cable that can be connected directly to the set-top box, cable modem, or other equipment. The RF signal on this cable is strong enough so that signals can be split within the premises. If there are many separate devices at the premises, amplification may be needed. This is normally done with a house amp or drop amp. Amplifier and splitters may be pooled to decrease the number of connections.
Frequency division multiplexing is used in HFC networks to package content into a cable plant’s spectrum slots. In this case, spectrum is the frequency bands that are used for the content. In USA, this is 5 to 42 MHz for subscriber to headend (reverse) and 52 to 1004 MHz for headend to subscriber (forward). Split frequencies and spectrum allocations do however differ internationally.
Signals that start in the headend and need to be sent to subscribers can be analog or digital. In the latter case, the signal is modulated with quadrature amplitude modulation (QAM). These signals are produced by using digital representations of the original signal, be it a video or analog voice signal, and then converting it through sampling and modulation of a carrier. The resultant signal is an analog signal with high-capacity. It should be noted that this signal needs to maintain a high signal-to-noise ratio (SNR). In contrast, a digital optical signal used in GPON or GEPON (gigabit passive optical networks), has much simpler SNR requirements.
CableLabs, a non-profit R&D organization funded by industry, manages the standard governing the QAM transport. This standard is known as Data Over Cable Service Interface Specification (DOCSIS). DOCSIS 3.0 is deployed the most widely. DOCSIS 3.1, the latest version, improves the data throughput to subscribers and modulation rates significantly, while at the same time expands the upstream to 85 MHz and more and the downstream to 1200 MHz and more.
Share this post