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What is a Cable Modem Termination System?

By Solomon Branch
Updated May 16, 2024
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A cable modem termination system (CMTS) is a device used in a cable network to communicate with cable modems using a digital signal. It is usually found in a cable provider’s head-end, which is the main hub where all the cable signals for Internet traffic and cable television get sent to and are received from subscribers. Cable modems cannot communicate directly with each other and must go through the cable modem terminations system.

The main purpose of a cable modem termination system is to act as a translator. Typically, the head-end of a cable provider is where the signals for cable television or the Internet are sent to the user. The head-end is where the cable modem termination system is stored, as well as the devices to receive the various signals, such as a router for Internet connection and satellites for television signals. Cables used to connect to the subscriber are the same for both Internet and television traffic, but the cable modem termination system typically only uses one or two channels to communicate with the cable modems.

A typical cable modem termination system is set up to have several interfaces, similar to a router. Basic CMTSs will have an Ethernet connection on one end, the type used to transmit the Internet Protocol (IP) packets, and radio frequency (RF) connectors on the other end. The signal comes from the Internet in IP packets, goes through the cable modem terminations system, and then out the RF connector. RF signals are then routed through hybrid fiber coax (HFC) cable to the subscriber’s cable modem.

Downstream traffic, or traffic that is routed through a cable modem termination system to a cable modem, is the same IP protocol used on the Internet, but it is encapsulated in another special type of packet that is modulated to travel on a cable television frequency. When traffic comes from the cable modem to the cable modem termination system, called upstream traffic, it is encapsulated in a different type of packet that is in a lower frequency range than downstream packets. Traffic from the television satellites is not routed through the CMTS, as it only carries IP traffic.

In addition to providing a way for Internet traffic to be routed, a cable modem termination system will offer other services for the subscriber. At the very least, it will route the dynamic host configuration protocol (DHCP) traffic, which is the protocol used to obtain an IP address and other pertinent information. It can also provide some security, blocking potential attacks and other unauthorized traffic.

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