A cable modem termination system (CMTS) is a device that acts as a translator between cable modems and the cable provider’s head end. It routes internet traffic and offers other services such as DHCP and security.
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 header, which is the main hub where all cable signals for Internet traffic and cable television are sent and received by subscribers. Cable modems cannot communicate directly with each other and must go through the cable modem’s termination system.
The primary purpose of a cable modem termination system is to act as a translator. Typically, a cable provider’s head end is where signals for cable television or the Internet are sent to you. The head-end is where the cable modem termination system is stored, as well as devices to receive the various signals, such as a router for Internet connection and satellites for television signals. The cables used to connect to the subscriber are the same for both Internet and television traffic, but the cable modem termination system typically uses only one or two channels to communicate with cable modems.
A typical cable modem termination system is configured to have several interfaces, similar to a router. Basic CMTSs will have an Ethernet connection on one end, the type used to transmit Internet Protocol (IP) packets, and radio frequency (RF) connectors on the other. The signal comes from the Internet in IP packets, passes through the cable modem termination system, and then exits the RF connector. The RF signals are then routed through hybrid fiber coaxial (HFC) cable to the subscriber’s cable modem.
Downstream traffic, or traffic 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 modulated to travel over 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 television satellites is not routed through the CMTS, as it carries only IP traffic.
In addition to providing a way to route Internet traffic, a cable modem termination system will offer other services to the subscriber. At a minimum, it will route 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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