Cable modem termination system

A cable modem termination system or CMTS is a piece of equipment, typically located in a cable company's headend or hubsite, which is used to provide high speed data services, such as cable Internet or Voice over Internet Protocol, to cable subscribers. A CMTS provides many of the same functions provided by the DSLAM in a DSL system.

In order to provide high speed data services, a cable company will connect its headend to the Internet via very high capacity data links to a network service provider. On the subscriber side of the headend, the CMTS enables the communication with subscribers' cable modems. Different CMTSs are capable of serving different cable modem population sizes—ranging from 4,000 cable modems to 150,000 or more, depending in part on traffic. A given headend may have between 1-12 CMTSs to service the cable modem population served by that headend or HFC hub.

One way to think of a CMTS is to imagine a router with Ethernet interfaces (connections) on one side and coaxial cable RF interfaces on the other side. The RF/coax interfaces carry RF signals to and from the subscriber's cable modem.

In fact, most CMTSs have both Ethernet interfaces (or other more traditional high-speed data interfaces like SONET) as well as RF interfaces. In this way, traffic that is coming from the Internet can be routed (or bridged) through the Ethernet interface, through the CMTS and then onto the RF interfaces that are connected to the cable company's hybrid fiber coax (HFC). The traffic winds its way through the HFC to end up at the cable modem in the subscriber's home. Traffic from a subscriber's home system goes through the cable modem and out to the Internet in the opposite direction.

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