The Cable Plant

CATV was initially deployed to bring nearby broadcast television stations to areas where reception was poor or non-existent. In this manner it served subscribers as a unidirectional (one-way) video distribution mechanism, wherein television broadcasts were transmitted downstream from the CATV video distribution point (known as the headend) to the subscriber. This CATV distribution network, often referred to as a cable plant, traditionally consists of branched metallic (aluminum and copper) cabling with amplifiers spaced throughout the network to improve signal strength. Larger coaxial cable, called trunk cable, transmits the signal long distances where it is later branched out onto thinner cables that are sometimes called express cables. Amplifiers exist on both trunk and express cables. At the subscriber premises, a strand of thin cable, called feeder or drop cable, connects the trunk cable to the customer premises equipment (CPE). Figure 13-1 shows a traditional branched coaxial network architecture with drop, express, and feeder cables.

For subscribers distant from the cable headend, a branched all-metallic design can mean many amplifiers between the subscriber and the headend. Amplifiers of older designs were often manufactured as unidirectional devices because of the then unidirectional design of the CATV network.

Although purely metallic coaxial-based networks can still be found in some areas, the constraints of coaxial networks and the huge advances in analog fiber optics have lead to significant improvements in overall cable network design.

One limitation of all-coaxial cable networks is signal attenuation. Signals that travel along a metallic cable need relatively frequent amplification, particularly signals at higher frequencies. Cable modem technologies, by definition, involve the modulation and demodulation of an analog signal, and when the analog signal has been amplified sequentially several times, issues with signal quality often arise. The frequent amplification of an analog signal not only amplifies transmitted signals, but also amplifies any associated noise and interference, which reduces signal quality. A signal can only be amplified a finite number of times before signal quality becomes unacceptable. Modern CATV networks limit the number of sequential amplifiers (called amplifier cascades). Although older network designs had 20 or even 80 amplifiers in a cascade, newer designs typically have only five to seven amplifiers to optimize signal quality and increase reliability. These amplification constraints that are inherent in metallic coaxial-based CATV networks limit the distance that a subscriber can be from the cable headend and the overall size of the cable plant.

Additional amplification concerns in the cable network relate to the fundamental ways in which frequencies are differentially attenuated when traversing the network. Higher frequencies attenuate more rapidly than lower frequencies. This results in what is often called a slope or tilt effect in signal strength. To mitigate the effects of differential attenuation, amplifiers are often installed in conjunction with equalizers. Equalizers are engineered with a frequency response, which mirrors the cable span that precedes the amplifier and, when used in conjunction with an amplifier, produces a level signal to compensate for differential attenuation. Although with equalizers and amplifiers signal levels can be maintained for all frequencies, one result of differential attenuation is that high frequencies require greater or more frequent amplification than others. This results in compounding noise, distortion, and interference issues on higher frequencies.

Figure 13-1 Traditional Branched Coaxial Cable Network

Feeder Cable

Figure 13-1 Traditional Branched Coaxial Cable Network

Feeder Cable

A modern CATV network today includes network segments of both optical fiber and coaxial cabling and can serve millions of subscribers in numerous geographic locations. Television content is transmitted to MSOs through cables, broadcast transmissions, and satellite transmissions. Old unidirectional amplifiers have been replaced with two-way amplifiers to allow for both downstream and upstream signals, and the overall numbers of amplifiers in cascades are being reduced. Optical fiber is combined with coaxial in the modern cable plant to provide the connectivity of copper with the benefits of fiber.

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