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Acable modem is a type ofnetwork bridge orgateway that provides bi-directionaldata communication viaradio frequency channels on ahybrid fiber-coaxial,radio frequency over glass andcoaxial cable infrastructure. Cable modems are primarily used to deliverbroadband Internet access in the form ofcable Internet. They are commonly deployed in theAmericas,Asia,Australia, andEurope.
Internet Experiment Note (IEN) 96[1] (1979) describes an earlyRF cable modem system. From pages 2 and 3 of IEN 96:
The Cable-Bus System
The MITRE/Washington Cablenet system is based on a technology developed at MITRE/Bedford. Similar cable-bus systems are in operation at a number of government sites, e.g.Walter Reed Army Hospital, and theNASAJohnson Space Center, but these are all standalone, local-only networks.
The system uses standardcommunity antenna television (CATV)coaxial cable and microprocessor based Bus Interface Units (BIUs) to connect subscribercomputers andterminals to the cable. ... The cable bus consists oftwo parallel coaxial cables, one inbound and the other outbound. The inbound cable and outbound cable are connected at one end, theheadend, andelectrically terminated at their other ends. This architecture takes advantage of the well developedunidirectional CATVcomponents.[2] Thetopology is dendritic (i.e.branched like a tree).
...The BIUs containRadio Frequency (RF) modems whichmodulate acarrier signal to transmitdigitalinformation using 1MHz of the availablebandwidth in the 24 MHz frequency range. The remainder of the 294 MHz bandwidth can be used to carry othercommunication channels, such asoff-the-airTV,FM,closed circuit TV, or avoicetelephone system, or, other digital channels. Thedata rate of our test-bed system is 307.2 kbps.
TheIEEE802 Committee defined10BROAD36 in802.3b-1985[3] as a10 Mbit/sIEEE 802.3/Ethernet broadband system to run up to 3,600 metres (11,800 ft) over CATV coax network cabling. The wordbroadband as used in the original IEEE 802.3 specifications implied operation infrequency-division multiplexed (FDM) channel bands as opposed to digitalbasebandsquare-waveformmodulations (also known asline coding), which begin near zeroHz andtheoretically consumeinfinitefrequency bandwidth. (In real-world systems, higher-ordersignalcomponents become indistinguishable from backgroundnoise.) In the market10BROAD36 equipment was not developed by many vendors nor deployed in many user networks as compared to equipment for IEEE 802.3/Ethernetbaseband standards such as10BASE5 (1983),10BASE2 (1985),10BASE-T (1990), etc.
TheIEEE 802 Committee also specified a broadband CATV digital networking standard in 1989 with802.7-1989.[4] However, like10BROAD36, 802.7-1989 saw little commercial success.
Hybrid Networks developed, demonstrated and patented the first high-speed, asymmetrical cable modem system in 1990. A key Hybrid Networks insight was that in the nascent days of the Internet, data downloading constitutes the majority of the data traffic, and this can be served adequately with a highly asymmetrical data network (i.e. a large downstream data pipe and many small upstream data pipes). This allowed CATV operators to offer high-speed data services immediately without first requiring an expensive system upgrade. 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 thepublic 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 U.S. LANcity, which was led by the Iranian-American engineerRouzbeh Yassini, was then acquired byBay Networks.[5] Bay Networks was subsequently acquired byNortel.[6] Nortel at the time had formed a joint-venture withAntec calledARRIS Interactive.[7] Because of contractual agreements with Antec involving this joint venture, Nortel spun the LANCity group out into the ARRIS Interactive joint-venture. ARRIS continues to make cable modems andcable modem termination system (CMTS) equipment compliant with theDOCSIS standard.
Zenith offered a cable modem technology using its own protocol which it introduced in 1993, being one of the first cable modem providers. TheZenith Cable Modem technology was used by several cable television systems in the United States and other countries, including Cox Communications San Diego, Knology in the Southeast United States,Ameritech's Americast service (later to be sold off toWide Open West after the SBC / Ameritech merger), Cogeco in Hamilton Ontario and Cablevision du Nord de Québec in Val-d'Or.[8] Zenith Homeworks used BPSK (Bi-Phase Shift Keyed) modulation to achieve 500 Kbit/sec in 600 kHz, or4 Mbit/sec in 6 MHz.[9]
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 aComController as the central bridge in CATV network head-ends, the ComPort cable modem in various models and the NMAPS management system usingHP OpenView as the 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 onAsynchronous 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 alsoFast Ethernet and ATM interfaces. The ATM interface became the most popular, as it supported the increasing bandwidth demands and also supportedVLANs.Com21 developed a DOCSIS modem, but the company filed for bankruptcy in 2003 and closed. The DOCSIS CMTS assets of COM21 were acquired byARRIS.
CDLP was a proprietary system manufactured byMotorola. CDLPcustomer premises equipment (CPE) was capable of bothPSTN (telephone network) and radio frequency (cable) return paths. The PSTN-based service was considered 'one-way cable' and had many of the same drawbacks assatellite Internet service; 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-directionaldigital subscriber line (DSL) service. The standard is in little use now as 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 peak10 Mbit/s downstream and1.532 Mbit/s upstream. CDLP supported a maximum downstream bandwidth of30 Mbit/s which could be reached by using several cable modems.
TheAustralian ISPBigPond employed this system when it started cable modem tests in 1996. For a number of yearscable Internet access was only available inSydney,Melbourne andBrisbane 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.
CDLP has also been rolled out at the French cable operatorNumericable before upgrading its IP broadband network using DOCSIS.
Digital Video Broadcasting (DVB) and Digital Audio Visual Council (DAVIC) are European-formed organizations that developed some cable modem standards. However, these standards have not been as widely adopted as DOCSIS.
In the mid-1990s theIEEE 802 committee formed a subcommittee (802.14)[10] to develop a standard for cable modem systems. IEEE 802.14 developed a draft standard, which wasATM-based. However, the802.14 working group was disbanded when North Americanmulti system operators (MSOs) instead backed the then-fledglingDOCSIS 1.0 specification, which generally usedbest-effort service and wasIP-based (with extensioncodepoints to supportATM[11] forQoS in the future).MSOs were interested in quickly deploying service to compete forbroadband Internet access customers instead of waiting on the slower, iterative, and deliberative processes of standards development committees. Albert A. Azzam was Secretary of the IEEE 802.14 Working Group,[12] and his book,High-Speed Cable Modems,[13] describes many of the proposals submitted to 802.14.
Although theInternet Engineering Task Force (IETF) generally does not generate complete cable modem standards, the IETF charteredWorking Groups (WGs) that produced various standards related to cable modem technologies (including 802.14, DOCSIS,PacketCable, and others). In particular, the IETF WGs on IP over Cable Data Network (IPCDN)[14] and IP overDigital Video Broadcasting (DVB)[15] produced some standards applicable to cable modem systems, primarily in the areas ofSimple Network Management Protocol (SNMP)Management Information Bases (MIBs) for cable modems and other networking equipment that operates over CATVnetworks.
In the late 1990s, a consortium of UScable 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 thephysical layer from theMotorolaCDLP system and theMAC layer from the LANcity system. When the initial specification had been drafted, the MCNS consortium handed over control of it toCableLabs which maintained the specification, promoted it in various standards organizations (notablySCTE andITU), developed a certification testing program for cable modem equipment, and has since drafted multiple extensions to the original specification.
While deployedDOCSIS RFI 1.0 equipment generally only supportedbest-effort service, the DOCSIS RFI 1.0 Interim-01 document discussedquality of service (QoS) extensions and mechanisms usingIntServ,RSVP,RTP, and Synchronous Transfer Mode (STM)telephony (as opposed toATM).[11]DOCSIS RFI 1.1[16] later added more robust and standardized QoS mechanisms to DOCSIS.DOCSIS 2.0 added support forS-CDMAPHY, while DOCSIS 3.0 addedIPv6 support andchannel bonding to allow a single cable modem to use concurrently more than one upstream channel and more than one downstream channel in parallel.
Virtually all cable modems operating in the field today are compliant with one of the DOCSIS versions. Because of the differences in the EuropeanPAL and US'sNTSC 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 US and 8 MHz for Europe. A third variant of DOCSIS was developed inJapan and has seen limited deployment in that country.
Although interoperability "was the whole point of the DOCSIS project,"[17] most cable operators only approve a very restricted list of cable modems on their network,[18][19][20][21] identifying the 'allowed' modems by their brand, models, sometimes firmware version and occasionally going as far as imposing a hardware version of the modem, instead of simply allowing a supported DOCSIS version.
In 2004, the Multimedia over Coax Alliance (MoCA) was established to develop industry standards for the connected home using the existing coaxial cabling. Initially developed for in-home networking with MoCA 1.0/1.1, the MoCA standards have continued to develop with MoCA 2.0/2.1 in 2010 and MoCa 2.5 in 2016.
In 2017, Multimedia over Coax Alliance introduced MoCA Access specification, based on the MoCA 2.5 standard, suitable for addressing broadband network access in-building using coaxial cabling.[22] MoCA Access extends MoCA 2.5 in-home networking to fit operators and ISPs that are installing fiber-to-the-basement/drop point (FTTB/FTTdp) and want to use the existing coax for connection to each apartment or house."
With the development ofvoice over Internet Protocol (VoIP) telephony,analog telephone adapters (ATA) have been incorporated into many cable modems for providing telephone service. An embedded ATA is known as anembedded multimedia terminal adapter (E-MTA).
Many cable TV service providers also offer VoIP-based telephone service via the cable infrastructure (PacketCable). Some high-speed Internet customers may use VoIP telephony by subscribing to a third-party service, such asVonage,MagicJack+ andNetTALK.
In network topology, a cable modem is anetwork bridge that conforms toIEEE 802.1D forEthernet networking (with some modifications). The cable modem bridges Ethernet frames between a customerLAN and the coax 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.
It implements anEthernet PHY on its LANinterface, and a DOCSIS-defined cable-specificPHY on its HFC cable interface. The termcable modem refers to this cable-specific PHY. TheNetwork Layer is implemented as an IP host in that it has its ownIP address used by the network operator to maintain the device. In thetransport layer the cable modem supportsUDP in association with its own IP address, and it supports filtering based onTCP and UDP port numbers to, for example, block forwarding ofNetBIOS traffic out of the customer's LAN. In theApplication Layer, the cable modem supports certain protocols that are used for management and maintenance, notablyDynamic Host Configuration Protocol (DHCP),SNMP, andTFTP.
Some cable modems may incorporate arouter 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 aresidential gateway. So, the cable modem function will have its ownIP address andMAC address as will the router.
Cable modems can have a problem known in industry jargon as "flap" or "flapping".[23] A modem flap is when the connection by the modem to the head-end has been dropped (gone offline) and then comes back online. The time offline or rate of flap is not typically recorded, only the incidence. While this is a common occurrence and usually unnoticed, if a modem's flap is extremely high, these disconnects can cause service to be disrupted. If there are usability problems due to flap the typical cause is a defective modem or very high amounts of traffic on the service provider's network (upstream utilization too high).[24] Types of flap include reinsertions, hits and misses, and power adjustments.[25]
In January 2020, a vulnerability affecting cable modems usingBroadcom chipsets was disclosed and namedCable Haunt. Security researchers say that the vulnerability affects hundreds of millions of devices. Exploits are possible because of the use of default credentials in thespectrum analyzer component of the modem (mostly used for debugging purposes) accessible through a networkport which is open by default in the vulnerable models.[26][27]
Amplifiers are one of the common components used in CATV system
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