CATV was developed in the 1940s and 1950s to deliver TV to homes via a coaxial cable, rather than the usual arials. Early systems used one "communal" antenna to pick up the TV signal at a "headend", and then distribute it via cable to all the subscribers in the area. However, it wasn't long before people realised that CATV could be used to send TV channels in its own right, broadcasting directly into the headend.
Modern CATV systems do not use coax for the whole route, from headend to subscriber, but instead use a system called HFC in which optic fibres are used to carry the signal most of the way, from the headend to local distribution nodes from where coaxial cable carries it the rest of the way to each subscriber's premises.
Each cable TV channel takes up about 6MHz (North America) or 8MHz (Europe) on a cable that has a data band from about 5MHz – 850MHz. I'll leave you to do the maths and work out how many TV channels that could be fitted on to the CATV system.
When the modern CATV system was developed, it was decided that some of the bandwidth would be reserved for an upstream channel. This would be used to allow subscribers to send information back to the headend for limited interactivity, but would later be used as an upstream channel for data transfer.
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