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Concept
Cable modem termination system
Summary
A cable modem termination system (CMTS, also called a CMTS Edge Router) 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 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, although it is recommended for an I-CMTS to service, for example, 30,000 subscribers (cable modems). 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 Ethernet side is known as the Network Side Interface or NSI.
A CMTS has separate RF interfaces and connectors for downlink and uplink signals. The RF/coax interfaces carry RF signals to and from coaxial "trunks" connected to subscribers' cable modems, using one pair of connectors per trunk, one for downlink and the other for uplink. In other words, there can be a pair of RF connectors for every service group, although it is possible to configure a network with different numbers of connectors that service a set of service groups, based on the number of downstream and upstream channels the cable modems in every service group use. Every connector has a finite number of channels it can carry, such as 16 channels per downstream connector, and 4 channels per upstream connector, depending on the CMTS.
Official source
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