A cable modem termination system (CMTS, also called a CMTS Edge Router) [1] is a piece of equipment, typically located in a cable company's headend or hubsite, which is used to provide data services, such as cable Internet or Voice over IP, 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). [2] 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. [3] [4]
A service group is a group of customers that share communication channels and thus bandwidth. 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. [2] For example, if the cable modems on every service group use 24 channels for downstream, and 2 channels for upstream, then 3 downstream connectors can service the cable modems on two service groups, and be serviced by 1 upstream connector. [5] A service group may serve up to 500 households. A service group has channels, whose bandwidth is shared among all members of the service group. [6] [7] [8] [9] [10] [11] The channels are later regrouped at the cable headend or distribution hub and serviced by CMTSs and other equipment such as Edge QAMs.
The RF signals from a CMTS, are connected via coaxial cable to headend RF management modules for RF splitting and combining, with other equipment such as other CMTSs so that several CMTS can service one service group, [2] [3] and then to an "optics platform" or headend platform, which has transmitter and receiver modules that turn the RF signals into light pulses for delivery over fiber optics through an HFC network. [12] [13] Examples of optics platforms are the Arris CH3000 and Cisco Prisma II. At the other end of the network, an optical node converts the light pulses into RF signals again and sends them through a coaxial cable "trunk". The trunk has one or more amplifiers along its length, and on the trunk there are distribution "taps" to which customers' modems are connected via coaxial cable.
In fact, most CMTSs have both Ethernet interfaces (or other more traditional high-speed data interfaces like SONET) 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 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 (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 usually 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 5–42 MHz in DOCSIS 2.0 or 5–65 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 (QoS) 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.
A CCAP (Converged Cable Access Platform) combines CMTS and Edge QAM functionality in a single device so that it can provide both data (internet) with CMTS functionality, and video (TV channels) with Edge QAM functionality. [14] [15] Edge QAM (Quadrature Amplitude Modulator/Modulation) converts video sent via IP (internet protocol) or otherwise, into a QAM signal for delivery over a cable network. Edge QAMs are normally standalone devices placed at the "edge" of a network. They can also be connected to a CMTS core, to make up an M-CMTS system which is more scalable. A CMTS core is normally a conventional or I-CMTS that supports operation as a CMTS core in an M-CMTS system. [16] [17] [18]
A CMTS can be broken down into several different architectures, Integrated CMTS (I-CMTS), Modular (M-CMTS), Virtual CMTS (vCMTS) and remote CMTS. An I-CMTS incorporates into a single unit all components necessary for its operation. [19] There are both pros and cons to each type of architecture.
In a M-CMTS solution the architecture of an I-CMTS is broken up into two components. The first part is the Physical Downstream component (PHY) which is known as the Edge QAM (EQAM). 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 (DEPI). The DEPI protocol controls the delivery of DOCSIS frames from the M-CMTS Core to the EQAM devices [20] 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 the benefits of an M-CMTS architecture is that it is extremely scalable to larger numbers of downstream channels. [21]
Virtual CCAPs (vCCAPs) or virtual CMTSs (vCMTSs) are implemented on commercial off the shelf x86-based servers with specialized software, [22] and can be used to increase service capacity without purchasing new CMTS/CCAP chassis, or add features to the CMTS/CCAP more quickly. [23]
Remote CMTS/Remote CCAP moves all CMTS/CCAP functionality to the outside plant, in stark contrast to conventional CMTSs or CCAPs which are installed at a service provider location. [24] [25]
Cable television is a system of delivering television programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This contrasts with broadcast television, in which the television signal is transmitted over-the-air by radio waves and received by a television antenna, or satellite television, in which the television signal is transmitted over-the-air by radio waves from a communications satellite and received by a satellite dish on the roof. FM radio programming, high-speed Internet, telephone services, and similar non-television services may also be provided through these cables. Analog television was standard in the 20th century, but since the 2000s, cable systems have been upgraded to digital cable operation.
A cable modem is a type of network bridge that provides bi-directional data communication via radio frequency channels on a hybrid fibre-coaxial (HFC), radio frequency over glass (RFoG) and coaxial cable infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high bandwidth of a HFC and RFoG network. They are commonly deployed in the Americas, Asia, Australia, and Europe.
A digital subscriber line access multiplexer is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques. Its cable internet (DOCSIS) counterpart is the cable modem termination system.
Data Over Cable Service Interface Specification (DOCSIS) is an international telecommunications standard that permits the addition of high-bandwidth data transfer to an existing cable television (CATV) system. It is used by many cable television operators to provide cable Internet access over their existing hybrid fiber-coaxial (HFC) infrastructure. Bandwidth is shared among users of an HFC, within service groups which are groups of customers that share RF channels. DOCSIS is used within service groups to provide internet access.
A cable television headend is a master facility for receiving television signals for processing and distribution over a cable television system. A headend facility may be staffed or unstaffed and is typically surrounded by some type of security fencing. The building is typically sturdy and purpose-built to provide security, cooling, and easy access for the electronic equipment used to receive and re-transmit video over the local cable infrastructure. One can also find head ends in power-line communication (PLC) substations and Internet communications networks.
Hybrid fiber-coaxial (HFC) is a broadband telecommunications network that combines optical fiber and coaxial cable. It has been commonly employed globally by cable television operators since the early 1990s.
A passive optical network (PON) is a fiber-optic telecommunications network that uses only unpowered devices to carry signals, as opposed to electronic equipment. In practice, PONs are typically used for the last mile between Internet service providers (ISP) and their customers. In this use, a PON has a point-to-multipoint topology in which an ISP uses a single device to serve many end-user sites using a system such as 10G-PON or GPON. In this one-to-many topology, a single fiber serving many sites branches into multiple fibers through a passive splitter, and those fibers can each serve multiple sites through further splitters. The light from the ISP is divided through the splitters to reach all the customer sites, and light from the customer sites is combined into the single fiber. Many fiber ISPs prefer this system.
PacketCable network is a technology specification defined by the industry consortium CableLabs for using Internet Protocol (IP) networks to deliver multimedia services, such as IP telephony, conferencing, and interactive gaming on a cable television infrastructure.
DOCSIS Set-top Gateway is a specification describing how out-of-band data is delivered to a cable set-top box. Cable set-top boxes need a reliable source of out of band data for information such as program guides, channel lineups, and updated code images.
In telecommunications, cable Internet access, shortened to cable Internet, is a form of broadband internet access which uses the same infrastructure as cable television. Like digital subscriber line and fiber to the premises services, cable Internet access provides network edge connectivity from the Internet service provider to an end user. It is integrated into the cable television infrastructure analogously to DSL which uses the existing telephone network. Cable TV networks and telecommunications networks are the two predominant forms of residential Internet access. Recently, both have seen increased competition from fiber deployments, wireless, mobile networks and satellite internet access.
Switched video or switched digital video (SDV), sometimes referred to as switched broadcast (SWB), is a telecommunications industry term for a network scheme for distributing digital video via a cable. Switched video sends the digital video more efficiently freeing bandwidth. The scheme applies to digital video distribution both on typical cable TV systems using QAM channels, or on IPTV systems.
Cable Television Laboratories, Inc. (CableLabs) is a nonprofit corporation promoting innovation as a research and development lab founded in 1988 by American cable operators. System operators from around the world are eligible to be members.
Addressability is the ability of a digital device to individually respond to a message sent to many similar devices. Examples include pagers, mobile phones and set-top boxes for pay TV. Computer networks are also addressable via the MAC address on Ethernet network cards, and similar networking protocols like Bluetooth. This allows data to be sent in cases where it is impractical to control exactly where or to which devices the message is physically sent.
A cable converter box or television converter box is an electronic tuning device that transposes/converts channels from a cable television service to an analog RF signal on a single channel, usually VHF channel 3 or 4, or to a different output for digital televisions such as HDMI.
Zenith Cable Modem was one of the first proprietary cable modems. The two basic models are one operating at 500 kilobits per second (Kbit/s), and the other at four megabits per second (Mbit/s) with BPSK and approximately a 25% alpha.
In telecommunications, radio frequency over glass (RFoG) is a deep-fiber network design in which the coax portion of the hybrid fiber coax (HFC) network is replaced by a single-fiber passive optical network (PON). Downstream and return-path transmission use different wavelengths to share the same fiber. The return-path wavelength standard is expected to be 1610 nm, but early deployments have used 1590 nm. Using 1590/1610 nm for the return path allows the fiber infrastructure to support both RFoG and a standards-based PON simultaneously, operating with 1490 nm downstream and 1310 nm return-path wavelengths.
Com21, Inc., was an early pioneer in developing cable modem networks in the era before the standard DOCSIS was introduced for Internet access via cable television networks. The company filed for bankruptcy in 2003.
EPON Protocol over Coax, or EPoC, refers to the transparent extension of an Ethernet passive optical network (EPON) over a cable operator's hybrid fiber-coax (HFC) network. From the service provider's perspective the use of the coax portion of the network is transparent to EPON protocol operation in the optical line terminal (OLT) thereby creating a unified scheduling, management, and quality of service (QoS) environment that includes both the optical and coax portions of the network. The IEEE 802.3 Ethernet Working Group initiated a standards process with the creation of an EPoC Study Group in November 2011. EPoC adds to the family of IEEE 802.3 Ethernet in the First Mile (EFM) standards.
Subisu Cablenet Ltd. is a Nepalese Internet Service Provider company located in Kathmandu, Nepal, and was established in 2001. Subisu employs over 1500 full-time employees, of which around 900 are technical and around 700 are non-technical. As of 2023, the company has over 235,000 customers. It has coverage in all 77 districts of Nepal. Subisu primarily provides cable and fiber internet and digital TV services through a hybrid fiber-coaxial (HFCC) network.
