Cable modem termination system
Encyclopedia
A cable modem termination system or CMTS is a piece of equipment typically located in a cable
company's headend
or hubsite, and 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.
. On the subscriber side of the headend, the CMTS enables the communication with subscribers' cable modem
s. 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 (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 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 in MPEG transport stream packets. These MPEG 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 (data from cable modems to the headend or Internet) 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
(also known as the "T" channels), a much lower part of the frequency spectrum than the downstream signal usually 5-42Mhz in DOCSIS 2.0
.
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
(QOS) for the cable operator's own PacketCable
-based VOIP service. 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. However, a route could conceivably pass through a single CMTS.
)
ADC Telecommunications
(CMTS business acquired by BigBand Networks
)
BigBand Networks
(Exited CMTS business)
Cadant (Acquired by ARRIS)
Com21 (CMTS business acquired by ARRIS)
RiverDelta (Acquired by Motorola)
Terayon
(Exited CMTS business)
Pacific Broadband Communications
(Acquired by Juniper Networks
)
Juniper Networks
(Exited CMTS business)
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
company's headend
Cable television headend
A cable television headend is a master facility for receiving television signals for processing and distribution over a cable television system. The headend facility is normally unstaffed and surrounded by some type of security fencing and is typically a building or large shed housing electronic...
or hubsite, and used to provide high speed data services, such as cable Internet
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
or voice over Internet Protocol, to cable subscribers. A CMTS provides many of the same functions provided by the DSLAM
Digital subscriber line access multiplexer
A digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...
in a DSL system.
Connections
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 providerNetwork service provider
A network service provider is a business or organization that sells bandwidth or network access by providing direct backbone access to the Internet and usually access to its network access points...
. On the subscriber side of the headend, the CMTS enables the communication with subscribers' cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
s. 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
Hybrid fibre-coaxial
Hybrid fiber-coaxial is a telecommunications industry term for a broadband network which combines optical fibre and coaxial cable. It was commonly employed globally by cable television operators since the early 1990s.-Description:...
.
One way to think of a CMTS is to imagine a router with Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
interfaces (connections) on one side and coax RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
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
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
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
Hybrid fibre-coaxial
Hybrid fiber-coaxial is a telecommunications industry term for a broadband network which combines optical fibre and coaxial cable. It was commonly employed globally by cable television operators since the early 1990s.-Description:...
). 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
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
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 either 64-QAM or 256-QAM versions of quadrature amplitude modulation
Quadrature amplitude modulation
Quadrature amplitude modulation is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing the amplitudes of two carrier waves, using the amplitude-shift keying digital modulation scheme or amplitude modulation analog...
.
Upstream data (data from cable modems to the headend or Internet) is carried in Ethernet frames encapsulated inside DOCSIS frames modulated with QPSK, 16-QAM, 32-QAM, 64-QAM or 128-QAM using TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, ATDMA or S-CDMA frequency sharing mechanisms. This is done at the "subband" or "return" portion of the cable TV spectrum
North American cable television frequencies
In North American cable TV networks, the radio frequencies used to carry signals to the customer are allocated to standardarized channel numbers listed in the CEA standard 542. Cable channel frequencies are generally different from off-air broadcast frequencies...
(also known as the "T" channels), a much lower part of the frequency spectrum than the downstream signal usually 5-42Mhz in DOCSIS 2.0
DOCSIS
Data Over Cable Service Interface Specification is an international telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV system...
.
A typical CMTS allows a subscriber's computer to obtain an IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
by forwarding DHCP
Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
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
Traffic shaping
Traffic shaping is the control of computer network traffic in order to optimize or guarantee performance, improve latency, and/or increase usable bandwidth for some kinds of packets by delaying other kinds of packets that meet certain criteria...
is sometimes performed to prioritize application traffic, perhaps based upon subscribed plan or download usage and also to provide guaranteed Quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QOS) for the cable operator's own PacketCable
PacketCable
PacketCable is an industry consortium founded by CableLabs with the goal of defining standards for the cable television modem access industry....
-based VOIP service. 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
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
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.
Current CMTS Manufacturers
- ARRISARRISARRIS Group Inc. is a telecommunications equipment manufacturing company which provides cable operators with high-speed data, video and telephony systems for homes and businesses...
- Coaxial Networks Inc.
- Casa Systems
- Cisco SystemsCisco SystemsCisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
- MotorolaMotorolaMotorola, Inc. was an American multinational telecommunications company based in Schaumburg, Illinois, which was eventually divided into two independent public companies, Motorola Mobility and Motorola Solutions on January 4, 2011, after losing $4.3 billion from 2007 to 2009...
- BigBand
- Chongqing Jinghong
- WISI Communications GmbH
Historical CMTS Manufacturers
Broadband Access Systems (Acquired by ADC TelecommunicationsADC Telecommunications
ADC Telecommunications was a communications company located in Eden Prairie, Minnesota, a southwest suburb of Minneapolis. It was acquired by TE Connectivity in December 2010 and now ceases to exist as a separate entity...
)
ADC Telecommunications
ADC Telecommunications
ADC Telecommunications was a communications company located in Eden Prairie, Minnesota, a southwest suburb of Minneapolis. It was acquired by TE Connectivity in December 2010 and now ceases to exist as a separate entity...
(CMTS business acquired by BigBand Networks
BigBand Networks
BigBand Networks is a multinational corporation headquartered in Redwood City, California, United States. BigBand manufactures and sells digital video and data processing platforms and solutions in areas ranging from digital video to CMTS.-Locations:BigBand has 2 major development centers...
)
BigBand Networks
BigBand Networks
BigBand Networks is a multinational corporation headquartered in Redwood City, California, United States. BigBand manufactures and sells digital video and data processing platforms and solutions in areas ranging from digital video to CMTS.-Locations:BigBand has 2 major development centers...
(Exited CMTS business)
Cadant (Acquired by ARRIS)
Com21 (CMTS business acquired by ARRIS)
RiverDelta (Acquired by Motorola)
Terayon
Terayon
Terayon Communication Systems, Inc. was a company that vended equipment to broadband service providers for delivering broadband voice, video and data services to residential and business subscribers.-History:...
(Exited CMTS business)
Pacific Broadband Communications
Pacific Broadband Communications
Pacific Broadband Communications was a US-based company that was acquired by Juniper Networks in 2001 for US$ 200 million....
(Acquired by Juniper Networks
Juniper Networks
Juniper Networks is an information technology and computer networking products multinational company, founded in 1996. It is head quartered in Sunnyvale, California, USA. The company designs and sells high-performance Internet Protocol network products and services...
)
Juniper Networks
Juniper Networks
Juniper Networks is an information technology and computer networking products multinational company, founded in 1996. It is head quartered in Sunnyvale, California, USA. The company designs and sells high-performance Internet Protocol network products and services...
(Exited CMTS business)
External links
- DOCSISHelp.com CMTS & DOCSIS Support Community