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HFC (Hybrid Fiber/Coax)
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
An HFC network is a cable (CATV) network that includes a combination of fiber-optic and coaxial cable, with fiber-optic cable running from the cable company's facility to a location near a home and coaxial cable running from there into the home. The fiber cable provides high bandwidth to multiple users in a single neighborhood. It forms what is called the "trunk line" that stretches from cable office to neighborhoods. The coaxial cable is called the "feeder circuit." An upgrade to an HFC system usually requires replacement of existing coaxial trunk lines with fiber trunk lines. In addition, equipment is needed at the neighborhood junction to join the coaxial and fiber cables.
The original CATV systems were designed to support only downstream broadcasts to subscriber television sets. To provide full Internet access, service providers must also support upstream traffic (or else upstream traffic can travel across a separate phone line, which is inconvenient). Bidirectional traffic requires that carriers upgrade amplifiers and related equipment. Subscribers install cable modems to access the data signals on the cable.
The data signals for Internet access are transmitted on the cable using frequency division multiplexing. The downstream signal from the providers occupies 450 MHz to 750 MHz, while the upstream signal occupies 5 MHz to 50 MHz. Normal television signals fall between these bands. The bandwidth is allocated to subscribers by using time division multiplexing. More recently, Terayon has pioneered a form of CDMA (Code Division Multiple Access) called S-CDMA (Synchronous CDMA) to support spread spectrum signaling. S-CDMA can support upstream data rates of up to 30 Mbits/sec.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.