Hybrid fiber-coaxial

Hybrid fiber–coaxial (HFC) is a broadband telecommunications 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, television channels are sent from the cable system's distribution facility, the headend, to local communities through optical fiber subscriber lines. At the local community, a fiber media converter translates the signal from a light beam to radio frequency current (RF), and sends it over coaxial cable lines for distribution to subscriber residences. The fiber optic trunk lines provide enough bandwidth to allow additional bandwidth-intensive services such as cable internet access through DOCSIS. Bandwidth is shared among users of an HFC. Encryption is used to prevent eavesdropping. Customers are grouped into service groups, which are groups of customers that share bandwidth among each other since they use the same RF channels to communicate with the company.

Description

The fiber optic network extends from the cable operators' master headend, sometimes to regional headends, and out to a neighborhood's hubsite, and finally to an optical to coaxial cable node which typically serves 25 to 2000 homes. A master headend will usually have satellite dishes for reception of distant video signals as well as IP aggregation routers. Some master headends also house telephony equipment (such as automatic telephone exchanges) for providing telecommunications services to the community. In an HFC network telephony is provided using PacketCable. A regional or area headend/hub will receive the video signal from the master headend 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, along with internet from a CMTS (an Integrated CMTS, which includes all parts required for operation), or a CCAP which provides both internet and video. Separate Edge QAMs can be used to provide QAM modulated video suitable for transmission in a coaxial cable network, from digital video sources. Edge QAMs can also be connected to a CMTS to provide internet data instead of video, in a modular CMTS architecture. CCAPs aim to replace the conventional, integrated CMTS which only provides data and Edge QAMs used for video which are separate pieces of equipment. Video can be encoded according to standards such as NTSC, MPEG-2, DVB-C or the QAM standard and data according to DOCSIS, analog video can be scrambled, signals can be modulated by analog or digital video modulators including QAM modulators or edge QAMs for video and/or data depending on whether a modular CMTS is used, at the CMTS for data only, or at the CCAP for video and data, and upconverted onto RF carriers in this equipment. The various services from CMTSs, CCAPs, Edge QAMs and QAM modulators are combined onto a single RF electrical signal using headend RF management modules such as splitters and combiners and the resulting signals are inserted into a broadband optical transmitter which in practice is a transmitter module in an "optics platform" or headend platform such as an Arris CH3000, Scientific Atlanta Prisma, or a Cisco Prisma II.

These platforms host several transmitters and receivers the latter of which can be used for cable internet, and can also host Erbium-Doped Fiber Amplifiers (EDFAs) to extend the reach of the optical signals in fiber optics. Each transmitter and receiver can service one optical node. It is common to see more reception cables than transmission cables in the CCAP/CMTS and associated optical receivers. The reception cables carry fewer channels due to lower bandwidth, and having more cables for the upstream than the downstream reduces noise caused by noise funneling. Thus, it is common for a single downstream cable to be shared among several optical nodes via splitters, and for each upstream cable to be shared among fewer or even a single optical node, in a ratio of 4 to 12 upstream channels for every downstream channel. Having more upstream cables than downstream cables alleviates noise funneling at the receiver, the CMTS, which is not a problem for the downstream at the cable modem because it is on the receiving end of a broadcast point to multipoint architecture. This optical transmitter converts the RF electrical signal to a downstream optically modulated signal that is sent to the nodes. Fiber optic cables connect the headend or hub to the optical nodes in a point-to-point or star topology, or in some cases, in a protected ring topology. Each node can be connected via its own dedicated fiber, so fiber optic cables laid outdoors in the outside plant can have several dozen to several hundred or even thousands of fibers, an extreme example being 6912 fibers.

Fiber optic nodes A fiber optic node has a broadband optical receiver, which converts the downstream optically modulated signal coming from the headend or hub to an electrical signal going to the customers. As of 2015, the downstream signal is a RF modulated signal that typically begins at 50 MHz and ranges from 550 to 1000 MHz on the upper end. The fiber optic node also contains a reverse- or return-path transmitter that sends communication from customers back to the headend. 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. This electrical signal is then outputted through coaxial cable to form a coaxial trunk. 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 1550 nm and the return signal could be on a wavelength at 1310 nm.

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