A cable modem termination system or CMTS 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. Connections
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 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 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 on one side and coaxial cable 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 as well as RF interfaces. In this way, traffic that is coming from
the Internet can be routed through the Ethernet interface, through the CMTS and
then onto the RF interfaces that are connected to the cable company's hybrid
fiber coax. The traffic winds its way through the HFC to end up at the cable
modem in the subscriber's home. Traffic from a subscriber's home system goes
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 according to DOCSIS standard. These packets are carried on
data streams that are typically modulated onto a TV channel using either
64-QAM or 256-QAM versions of quadrature amplitude modulation.
Upstream data is carried in Ethernet frames encapsulated inside DOCSIS frames
modulated with QPSK, 16-QAM, 32-QAM, 64-QAM or 128-QAM using TDMA, ATDMA or
S-CDMA frequency sharing mechanisms. This is done at the "subband" or
"return" portion of the cable TV spectrum, a much lower part of the
frequency spectrum than the downstream signal, usually 5 - 42 MHz in DOCSIS 2.0
or 5 - 60 MHz in EuroDOCSIS. 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 and also to provide
guaranteed Quality of service for the cable operator's own PacketCable-based
VOIP service. However, the function of traffic shaping is more likely done by a
Cable Modem or 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. However,
a route could conceivably pass through a single CMTS.
Architectures A CMTS can be broken down into two
different architectures, Integrated CMTS or Modular. There are both pros and cons
to each type of architecture. = Integrated CMTS =
The I-CMTS architecture consists of all components housed in a single chassis.
The RF interface and IP Networking components are all integrated in a
single device. This makes for much simpler RF combining in the headend. The
benefits of an all-in-one solution are less single points of failure, lower
costs and ease of deployment. = Modular CMTS =
In an M-CMTS solution the architecture is broken up into two components. The
first part is the Physical Downstream component which is known as the Edge
QAM. The second part is the IP networking and DOCSIS MAC Component
which is referred to as the M-CMTS Core. There are also several new protocols and
components introduced with this type of architecture. One is the DOCSIS Timing
Interface, which provides a reference frequency between the EQAM and M-CMTS
Core via a DTI Server. The second is the Downstream External PHY Interface. The
DEPI protocol controls the delivery of DOCSIS frames from the M-CMTS Core to
the EQAM devices Some of the challenges that entail an M-CMTS platform are
increased complexity in RF combining and an increase in the number of failure
points. One of benefits of an M-CMTS architecture is that is extremely
scalable to larger numbers of downstream channels.
Manufacturers = Current =
ARRIS Group C9 Networks
Coaxial Networks Inc. Casa Systems
Cisco Systems Chongqing Jinghong
WISI Communications GmbH Kathrein
Suma Scientific Huawei Technologies
Harmonic Inc. = Historical =
3COM Broadband Access Systems
ADC Telecommunications BigBand Networks
Cadant Com21
RiverDelta Terayon
Pacific Broadband Communications Juniper Networks
Motorola See also
DOCSIS References
External links DOCSISHelp.com CMTS & DOCSIS Support
Community