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(Cable Modem Termination System) A computer-based device that enables cable modems to send and receive packets over the Internet. It inserts IP packets from the Internet into MPEG frames and transmits them to the cable modems via an RF signal. It reverses the process coming from the cable modems.

A DOCSIS-compliant CMTS enables the customer's router or PC to dynamically obtain an IP address from the cable company by acting as a proxy and forwarding requests to DHCP servers. The CMTS may include a variety of functions, including filtering to protect against theft of service and denial of service attacks as well as preventing hackers from breaking into the cable system. It may also provide traffic shaping, which guarantees quality of service (QoS) to selected customers, and it may provide bridging or routing capabilities. See cable modem.

Cable Modem Infrastructure
A CMTS performs packet format conversion and IP address assignment (see DHCP). It may also provide routing, bridging, filtering and traffic shaping. The combiner merges the TV programming feeds with the RF data from the CMTS.

A Small CMTS
This CMTS supports approximately 1,300 users. It provides one downstream port of 40 Mbps and up to 6 upstream ports of 10 Mbps each. (Image courtesy of ADC Telecommunication, Inc.)