Equipment
A DOCSIS architecture includes two primary components: a cable modem (CM) located at the customer premises, and a cable modem termination system (CMTS) located at the CATV headend. Cable systems supporting on-demand programming use a hybrid fiber-coaxial system. Fiber optic lines bring digital signals to nodes in the system where they are converted into RF channels and modem signals on coaxial trunk lines.
A typical CMTS is a device which hosts downstream and upstream ports (it is functionally similar to the DSLAM used in DSL systems). While downstream and upstream communications travel on a shared coax line in the customer premises, and connect to a single F connector on the cable modem, it is typical for the CMTS to have separate F connectors for downstream and for upstream communication. This allows flexibility for the cable operator. Because of the noise in the return (upstream) path, an upstream port is usually connected to a single neighborhood (fiber node), whereas a downstream port is usually shared across a small number of neighborhoods. Thus, there are generally more upstream ports than downstream ports on a CMTS. A typical CMTS has 4 or 6 upstream ports per downstream port.
Before a cable company can deploy DOCSIS 1.1 or above, it must upgrade its Hybrid fiber-coaxial (HFC) network to support a return path for upstream traffic. Without a return path, the old DOCSIS 1.0 standard still allows use of data over cable system, by implementing the return path over regular phone lines, e.g. "plain old telephone service" (POTS). If the HFC is already 'two-way' or "interactive", chances are high that DOCSIS 1.1 or higher can be implemented
The customer PC and associated peripherals are termed Customer-premises equipment (CPE). The CPE are connected to the cable modem, which is in turn connected through the HFC network to the CMTS. The CMTS then routes traffic between the HFC and the Internet. Using the CMTS, the cable operator (or Multiple Service Operators - MSO) exercises full control over the cable modem's configuration; the CM configuration is changed to adjust for varying line conditions and customer service requirements.
DOCSIS 2.0 is also used over microwave frequencies (10 GHz) in Ireland by Digiweb, using dedicated wireless links rather than HFC network. At each subscriber premises the ordinary CM is connected to an antenna box which converts to/from microwave frequencies and transmits/receives on 10 GHz. Each customer has a dedicated link but the transmitter mast must be in line of sight (most sites are hilltop, ).
DOCSIS 1.x, 2.0, and 3.0 architecture is also used for fixed wireless with equipment utilizing the 2.5 - 2.7 GHz MMDS microwave band in the U.S.
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