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A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a cable television (CATV) 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 cable television network. They are commonly deployed in Australia, Europe, and North and South America. In the USA alone there were 22.5 million cable modem users during the first quarter of 2005, up from 17.4 million in the first quarter of 2004.


Hybrid Networks

Hybrid Networks developed, demonstrated and patented the first high-speed, asymmetrical cable modem system in 1990. A key Hybrid Networks insight was that highly asymmetrical communications would be sufficient to satisfy consumers connected remotely to an otherwise completely symmetric high-speed data communications network. This was important because it was very expensive to provide high speed in the upstream direction, while the CATV systems already had substantial broadband capacity in the downstream direction. Also key was that it saw that the upstream and downstream communications could be on the same or different communications media using different protocols working in each direction to establish a closed loop communications system. The speeds and protocols used in each direction would be very different. The earliest systems used the public switched telephone network (PSTN) for the return path since very few cable systems were bi-directional. Later systems used CATV for the upstream as well as the downstream path. Hybrid's system architecture is used for most cable modem systems today.


LANcity was an early pioneer in cable modems, developing a proprietary system that was widely deployed in the US. LANcity was sold to Bay Networks which was then acquired by Nortel, which eventually spun the cable modem business off as ARRIS. ARRIS continues to make cable modems and CMTS equipment compliant with the DOCSIS standard.


Com21 was another early pioneer in cable modems, and quite successful until proprietary systems were made obsolete by the DOCSIS standardization. The Com21 system used a ComController as central bridge in CATV network head-ends, the ComPort cable modem in various models and the NMAPS management system using HP OpenView as platform. Later they also introduced a return path multiplexer to overcome noise problems when combining return path signals from multiple areas. The proprietary protocol was based on Asynchronous Transfer Mode (ATM). The central ComController switch was a modular system offering one downstream channel (transmitter) and one management module. The remaining slots could be used for upstream receivers (2 per card), dual Ethernet 10BaseT and later also Fast-Ethernet and ATM interfaces. The ATM interface became the most popular, as it supported the increasing bandwidth demands and also supported VLANs.Com21 developed a DOCSIS modem, but the company filed for bankruptcy in 2003 and closed. The DOCSIS CMTS assets of COM21 were acquired by ARRIS


CDLP was a proprietary system manufactured by Motorola. CDLP customer premises equipment (CPE) was capable of both PSTN and radio frequency return paths. The PSTN-based service was considered 'one way cable' and had many of the same drawbacks as satellite Internet service and, as a result, it quickly gave way to two-way cable. Cable modems that used the RF cable network for the return path were considered 'two-way cable', and were better able to compete with the bi-directional Digital Subscriber Line (DSL) service. The standard is in little use now while new providers use, and existing providers having changed to, the DOCSIS standard. The Motorola CDLP proprietary CyberSURFR is an example of a device that was built to the CDLP standard, capable of a peak 10 Mbit/s downstream and 1.532 Mbit/s upstream. CDLP supported a maximum downstream bandwidth of 30 Mbit/s which could be reached by using several cable modems.

The Australian ISP BigPond employed this system when it started cable modem tests in 1996. For a number of years cable Internet access was only available in Sydneymarker, Melbournemarker and Brisbanemarker via CDLP. This network ran parallel to the newer DOCSIS system for several years. In 2004, the CDLP network was terminated and replaced by DOCSIS.

IEEE 802.14

In the mid-1990s the IEEE 802 committee formed a subcommittee (802.14) to develop a standard for cable modem systems. While significant progress was made, the group was disbanded when North American multi system operators instead backed the then-fledgling DOCSIS specification.


In the late 1990s, a consortium of US cable operators, known as "MCNS" formed to quickly develop an open and interoperable cable modem specification. The group essentially combined technologies from the two dominant proprietary systems at the time, taking the physical layer from the Motorola CDLP system and the MAC layer from the LANcity system. When the initial specification had been drafted, the MCNS consortium handed over control of it to CableLabs which maintained the specification, promoted it in various standards organizations (notably SCTE and ITU), developed a certification testing program for cable modem equipment, and has since drafted multiple extensions to the original specification. Virtually all cable modems operating in the field today are compliant with one of the DOCSIS versions. Because of the differences in the European PAL and USA's NTSC systems two main versions of DOCSIS exist, DOCSIS and EuroDOCSIS. The main differences are found in the width of RF-channels: 6 MHz for the USA and 8 MHz for Europe. Nearly all current cable modem systems use a version of this standard with the exception of those in Japanmarker.

Cable modems and VoIP

With the advent of Voice over Internet Protocol (VoIP) telephony, cable modems have been extended to provide telephone service. Some companies which offer cable TV service are still offering VOIP phone, allowing customers who already purchased cable TV to eliminate their Plain Old Telephone Service (POTS). Because many telephone companies do not offer naked DSL (DSL service without POTS line service), VoIP use is higher amongst cable modem users. Any high-speed Internet service subscriber can use VoIP telephony by subscribing to a third-party service (e.g., Vonage, Skype).

Many cable operators offer their own VoIP service, based on PacketCable. PacketCable allows Multi system operators (MSOs) to offer both high-speed Internet and VoIP through the same cable transmission system. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed Quality of Service across their entire transmission path, so call quality can be assured.

When using cable operator VoIP, a combined customer premises equipment device known as an Embedded Multimedia Terminal Adapter (EMTA) will often be used. An EMTA is a cable modem and a VoIP adapter (MTA, Multimedia Terminal Adapter) bundled into a single device.

Network architectural functions

In network topology, a cable modem is a network bridge that conforms to IEEE 802.1D for Ethernet networking (with some modifications). The cable modem bridges Ethernet frames between a customer LAN and the coax cable network. Technically, it is a modem because it must modulate data to transmit it over the cable network, and it must demodulate data from the cable network to receive it.

With respect to the OSI model of network design, a cable modem is both Physical Layer (Layer 1) device and a Data Link Layer (Layer 2) forwarder. As an IP addressable network node, cable modems support functionalities at other layers.

Layer 1 is implemented in the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. The term cable modem refers to this cable-specific PHY. The Network Layer (Layer 3) is implemented as a IP host in that it has its own IP address used by the network operator to maintain the device. In the Transport Layer (Layer 4) the cable modem supports UDP in association with its own IP address, and it supports filtering based on TCP and UDP port numbers to, for example, block forwarding of NetBIOS traffic out of the customer's LAN. In the Application Layer (Layer 7), the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.

Some cable modems may incorporate a router and a DHCP server to provide the LAN with IP network addressing. From a data forwarding and network topology perspective, this router functionality is typically kept distinct from the cable modem functionality (at least logically) even though the two may share a single enclosure and appear as one unit, sometimes called a residential gateway. So, the cable modem function will have its own IP address and MAC address as will the router.

Cable modem manufacturers

See also


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