Cable System Primer
How a Cable Plant Works
Cable television systems traditionally have delivered entertainment video to consumers' homes—with the emergence of fiber-optic technology and the convergence of video, voice and data, many cable systems provide extended services, such as video-on-demand, voice-over-Internet Protocol, and high-speed Internet access. While the technical capabilities of cable systems have been enhanced and expanded to support these services, the physical structure of cable systems has evolved into a hybrid-fiber coax (HFC) structure.
The cable television HFC architecture consists of five major components:
- the headend;
- optical fiber;
- feeder cable;
- drop cable; and
- terminal equipment.
Receiving and Transmitting Signals: The Headend
Program content is received and processed at the headend. A programmer usually transmits a television signal through the air from a satellite, microwave, or local television antenna to the headend. A programmer may also send the content by a direct fiber link from the studio to the headend. The content is modulated onto an electromagnetic carrier (a radio wave) and becomes a radio frequency (RF) signal that is part of a frequency spectrum. At the headend each signal is assigned a unique channel frequency. Each unique signal occupies a unique portion of the spectrum. The combined signals, carrying content from all providers, are then transmitted through the cable network to a subscriber's home.
System Architecture: the Trunk, the Feeder, the Drop and the HFC Evolution
In the early days of cable television, trunk, feeder, and drop were the coaxial cables that carried the signal from the headend through a series of distribution points to the terminal equipment in a subscriber's home. Large trunk cables run through the center of town passing near all neighborhoods. Smaller feeder cables connect to the trunk cable and branch off into individual neighborhoods. Amplifiers are placed along both the trunk and feeder cables to maintain adequate signal strength. When a customer purchases cable services, the cable operator runs a drop cable from a tap on a feeder cable into the subscriber's home where it is attached to terminal equipment.
In a modern HFC cable plant, the trunk cables and amplifiers are replaced by a single optical fiber. The fiber optics transmit RF energy as light reflecting down a glass fiber. This greatly reduces the noise and distortion in the cable plant because fiber uses no RF amplifiers whereas the trunk line needed an amplifier each half-mile or less. In the HFC architecture, a single fiber is run directly from the headend to an optical node in a neighborhood. The node converts the optical signal back to RF signals and the local neighborhood part of the cable plant distributes the RF signals the same as before. Nodes are spaced to serve a neighborhood with 500 to 1000 homes passed by the plant.
Another significant benefit of the HFC architecture is that it enables the cable plant to reliably deliver signals that originate in customers homes back to the headend. This two-way capability enables a plethora of interactive audio, video and data services.
Consumer Electronics: The Terminal Equipment
The drop cable is connected to consumer electronics equipment, called terminal equipment or customer premises equipment [CPE], inside the home. Terminal equipment processes the cable signals and enables subscribers to view, record, and interact with the services. Among the more common consumer electronics devices are television sets, set-top boxes, cable modems and personal video recorders.
Television sets have experienced significant changes over the last 10 years. Changes include the development of big-screen televisions that deliver a theater-like experience, flat panel plasma screens that are capable of displaying the latest high-definition programming, and digital receivers that can tune in ten or more razor-sharp channels where only one fit before. Televisions have become sophisticated electronic equipment.
Most cable subscribers do not have sophisticated television sets like those listed above. Cable system operators can provide set-top boxes that allow televisions to receive cable signals and provide advanced services on not-so-advanced television sets. Customers typically place the boxes on top of their television sets, hence the name.
Cable modems connect to the HFC plant to provide always-on, high-speed access to the Internet. CableLabs DOCSIS® (Data Over Cable Service Interface Specification) specifications for cable modems have enabled low cost, interoperable cable modems to be available at retail outlets.
Personal Video Recorders
Personal video recorders (PVRs) are similar to VCRs. PVRs record television data in a digital format such as Moving Picture Experts Group [MPEG]-1 or MPEG-2. The data is stored on a hard drive. PVRs have the same functionality as VCRs (recording, playback, fast forwarding, rewinding, and pausing) in addition to the ability to instantly move to any part of a program without having to rewind or fast-forward.
Today, an estimated 124.9 million American homes are passed by cable¹. Cable subscribers can now receive services such as high-speed data, voice over Internet Protocol, and video on demand, from their cable operator. As cable operators continue to invest in upgrading their cable systems with innovative technology, a new generation of integrated services will be delivered to homes and businesses around the world.
¹ Kagan, September 13, 2005.