Delivering services over a cable network requires different radio frequencies. Downstream frequencies are in the 50 to 860 MHz range, and the upstream frequencies are in the 5 to 42 MHz range.
Two types of equipment are required to send digital modem signals upstream and downstream on a cable system:
- Cable Modem Termination System (CMTS) at the headend of the cable operator
- Cable Modem (CM) on the subscriber end
Click the highlighted components in the figure for more information about how each device contributes to communication.
A headend CMTS communicates with CMs located in subscriber homes. The headend is actually a router with databases for providing Internet services to cable subscribers. The architecture is relatively simple, using a mixed optical-coaxial network in which optical fiber replaces the lower bandwidth coaxial cable.
A web of fiber trunk cables connects the headend to the nodes where optical-to-RF signal conversion takes place. The fiber carries the same broadband content for Internet connections, telephone service, and streaming video as the coaxial cable carries. Coaxial feeder cables originate connect the node to the subscribers and carries RF signals.
In a modern HFC network, typically 500 to 2,000 active data subscribers are connected to a cable network segment, all sharing the upstream and downstream bandwidth. The actual bandwidth for Internet service over a CATV line can be up to 160 Mb/s downstream with the latest iteration of DOCSIS, and up to 120 Mb/s upstream.
When high usage causes congestion, the cable operator can add additional bandwidth for data services by allocating an additional TV channel for high-speed data. This addition may effectively double the downstream bandwidth that is available to subscribers. Another option is to reduce the number of subscribers served by each network segment. To reduce the number of subscribers, the cable operator further subdivides the network by laying the fiber-optic connections deeper into the neighborhoods.