The cable system uses a coaxial cable that carries radio frequency (RF) signals across the network. Coaxial cable is the primary medium used to build cable TV systems.
Modern cable systems provide two-way communication between subscribers and the cable operator. Cable operators now offer customers advanced telecommunications services, including high-speed Internet access, digital cable television, and residential telephone service. Cable operators typically deploy hybrid fiber-coaxial (HFC) networks to enable high-speed transmission of data to cable modems located in a SOHO.
The cable TV industry uses a portion of the RF electromagnetic spectrum. Within the cable, different frequencies carry TV channels and data. At the subscriber end, equipment such as TVs, Blu-ray players, DVRs, and HDTV set-top boxes tune to certain frequencies that allow the user to view the channel or use a cable modem to receive high-speed Internet access.
DOCSIS (Data-over-Cable Service Interface Specification) defines the communications and operation support interface requirements for a data-over-cable system, and permits the addition of high-speed data transfer to an existing CATV system. Cable operators employ DOCSIS to provide Internet access over their existing HFC infrastructure.
DOCSIS specifies the OSI Layer 1 and Layer 2 requirements:
- Physical layer – For data signals that the cable operator can use, DOCSIS specifies the channel widths, or bandwidths of each channel, as 200 kHz, 400 kHz, 800 kHz, 1.6 MHz, 3.2 MHz, and 6.4 MHz. DOCSIS also specifies modulation technique, which is how to use the RF signal to convey digital data.
- MAC layer – Defines a deterministic access method, Time-Division Multiple Access (TDMA), or Synchronous Code Division Multiple Access method (S-CDMA).
Cable Connection Components
Delivering services over a cable network requires different radio frequencies. 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
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.