A cable modem termination system or CMTS

A cable modem termination system or CMTS is equipment typically found in a cable company's headend, or at cable company hubsite and is used to provide high speed data services, such as cable internet or Voice over IP, to cable subscribers.

In order to provide these 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 half a dozen to a dozen or more 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 coax 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) 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 going from a subscriber's home systems go through the cable modem and out to the Internet in the opposite direction.

CMTSs typically carry only IP traffic. Traffic destined for the cable modem from the Internet, known as downstream traffic, is carried in IP packets encapsulated in MPEG transport stream packets. These MPEG packets are carried on data streams that are typically modulated onto a TV channel using Quadrature Amplitude Modulation.

Upstream data (data from cable modems to the headend or Internet) is carried in Ethernet frames modulated with QPSK, 16-QAM, 32-QAM, 64-QAM, or S-CDMA. This is done at the "subband" portion of the cable TV spectrum (also known as the "T" channels), a much lower part of the frequency spectrum than the downstream signal.

A typical CMTS allows a subscriber's computer to obtain an IP address by forwarding DHCP requests to the relevant servers. This DHCP server returns, for the most part, what looks like a typical response including an assigned IP address for the computer, gateway/router addresses to use, DNS servers, etc.

The CMTS may also implement some basic filtering to protect against unauthorized users and various attacks. Traffic shaping is sometimes performed to prioritize application traffic, perhaps based upon subscribed plan or download usage. However, the function of traffic shaping is more likely done by a Policy Traffic Switch. A CMTS may also act as a bridge or router.

A customer's cable modem cannot communicate directly with other modems on the line. In general, cable modem traffic is routed to other cable modems or to the Internet through a series of CMTSs and traditional routers. A route could conceivably pass through a single CMTS.

A CMTS provides many of the same functions provided by the DSLAM in a DSL system.

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