Bandwidth problems have long restricted the ability of cable television systems to provide information services to subscribers. Although a coaxial cable system may permit a cable system operator to provide, for example, 50 television channels, each 6 MHz wide, with a total bandwidth of 300 MHz, this total bandwidth is insufficient to permit an arrangement wherein each subscriber may have, in addition to these 50 channels, an interactive information service that functions independently of interactive information services to all other subscribers and provides full color video, motion typical of movies or television, and sound.
The reason for the insufficiency in bandwidth is apparent on a consideration of the demands on the system. Typically a subscriber on a cable system obtains information services over a communication path that starts at the headend, proceeds over one of typically a number of trunks, and then over one of a number of feeders, and then over one of a number of taps. Each feeder may have, for example, fifty or more subscribers, and each trunk might serve a hundred or more feeders. The result is that 5000 subscribers per trunk is not atypical. Thus merely to provide a private one-way information service, and nothing else, to each of these 5000 subscribers would require the trunk to carry 5000 different signals, each using about 6 MHz of bandwidth, and would alone require a trunk bandwidth of 30 GHz, which is nearly two orders of magnitude greater than provided by a typical coaxial cable system.
The use of fiber optic trunks can assist in providing additional bandwidth, but to the extent that coaxial cable secondary trunks and feeders are used in a hybrid fiber-cable system, bandwidth limitations may continue to pose problems. While video compression schemes may assist in bringing the bandwidth requirements within more practical limits, each subscriber would then need to be provided with his own decompression unit.
Another problem lies in how to handle the switching and computing demands on the headend to provide separate and private information service to potentially hundreds of thousands of subscribers simultaneously.
In one paper, it has been suggested that a portion of cable system bandwidth be used to provide the most popular channels universally to all subscribers and remaining services be delivered to individual busses on a demand basis only. Large, D., "Tapped Fiber Vs Fiber-Reinforced Coaxial CATV Systems: A comparison of Evolutionary Paths," Draft Paper, Aug. 4, 1989, at pages 16 et seq. A three level distributed switching system was proposed, with one switch at the headend to switch among hubs, one at each hub to switch among distribution lines, and a third level "interdiction circuit" to select the service for each dwelling. No architecture for such a scheme was proposed, and the author noted that "a significant development effort will be required". Id., at page 19. Moreover, the author notes that his scheme poses a problem for the subscriber in using the system, because most channels will be accessed in the normal way using the television tuner while switched services must be accessed by first tuning to an available switch channel, then using an auxiliary communications device to control that channel. "Given that customers have historically resisted any complications created by cable companies in accessing services, this may be a potential problem." Id., at 20.