Editing Cable television (section)

==Principle of operation==
[[File:HFC Network Diagram.svg|thumb|upright=1.8|Diagram of a modern [[hybrid fiber-coaxial]] cable television system. At the regional ''[[cable television headend|headend]]'', the TV channels are sent multiplexed on a light beam which travels through [[optical fiber]] trunklines, which fan out from ''distribution hubs'' to ''optical nodes'' in local communities. Here the light signal from the fiber is translated to a [[radio frequency]] electrical signal, which is distributed through [[coaxial cable]] to individual subscriber homes.]]

In the most common system, multiple television channels (as many as 500, although this varies depending on the provider's available channel capacity) are distributed to subscriber residences through a [[coaxial cable]], which comes from a trunkline supported on [[utility pole]]s originating at the cable company's local distribution facility, called the [[cable television headend|headend]]. Many channels can be transmitted through one coaxial cable by a technique called [[frequency division multiplexing]]. At the headend, each television channel is translated to a different [[frequency]]. By giving each channel a different frequency ''slot'' on the cable, the separate television signals do not interfere with each other. At an [[Cable box (outside)|outdoor cable box]] on the subscriber's residence, the company's service drop cable is connected to cables distributing the signal to different rooms in the building. At each television, the subscriber's television or a set-top box provided by the cable company translates the desired channel back to its original frequency ([[baseband]]), and it is displayed onscreen. Due to widespread [[cable television piracy|cable theft]] in earlier analog systems, the signals are typically [[encrypted]] on modern digital cable systems, and the set-top box must be activated by an [[activation code]] sent by the cable company before it will function, which is only sent after the subscriber signs up. If the subscriber fails to pay their bill, the cable company can send a signal to deactivate the subscriber's box, preventing reception.

There are also usually [[upstream (networking)|upstream]] channels on the cable to send data from the customer box to the cable headend, for advanced features such as requesting [[pay-per-view]] shows or movies, [[cable internet access]], and [[cable telephone service]]. The [[downstream (networking)|downstream]] channels occupy a band of [[frequencies]] from approximately 50&nbsp;MHz to 1&nbsp;GHz, while the upstream channels occupy frequencies of 5 to 42&nbsp;MHz. Subscribers pay with a monthly fee. Subscribers can choose from several levels of service, with premium packages including more channels but costing a higher rate. At the local headend, the feed signals from the individual television channels are received by [[parabolic dish|dish antenna]]s from [[communication satellite]]s. Additional local channels, such as local [[terrestrial television|broadcast television]] stations, [[educational television|educational channel]]s from local colleges, and [[community access channel]]s devoted to local governments ([[public, educational, and government access|PEG]] channels) are usually included on the cable service. [[television advertisement|Commercial]] advertisements for local business are also [[Local insertion#Cable television|inserted in the programming]] at the headend (the individual channels, which are distributed nationally, also have their own nationally oriented commercials).

===Hybrid fiber-coaxial===
{{Main|Hybrid fibre-coaxial}}

Modern cable systems are large, with a single network and headend often serving an entire [[metropolitan area]]. Most systems use [[hybrid fiber-coaxial]] (HFC) distribution; this means the trunklines that carry the signal from the headend to local neighborhoods are [[optical fiber]] to provide greater bandwidth and also extra capacity for future expansion. At the headend, the electrical signal is translated into an optical signal and sent through the fiber. The fiber trunkline goes to several ''distribution hubs'', from which multiple fibers fan out to carry the signal to boxes called ''optical nodes'' in local communities. At the optical node, the optical signal is translated back into an electrical signal and carried by [[coaxial cable]] distribution lines on utility poles, from which cables branch out to a series of signal amplifiers and line extenders. These devices carry the signal to customers via passive RF devices called taps.