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Cable modem

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Example of a cable modem installed in a home office.

Acable modemis a type ofnetwork bridgethat provides bi-directional data communication viaradio frequency channelson ahybrid fibre-coaxial(HFC),radio frequency over glass(RFoG) andcoaxial cableinfrastructure. Cable modems are primarily used to deliverbroadband Internet accessin the form ofcable Internet,taking advantage of the highbandwidthof a HFC and RFoG network. They are commonly deployed in theAmericas,Asia,Australia,andEurope.

History[edit]

MITRE Cablenet[edit]

Internet Experiment Note (IEN)96[1](1979) describes an earlyRFcable 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 cableand microprocessor based Bus Interface Units (BIUs) to connect subscribercomputersandterminalsto the cable.... The cable bus consists oftwo parallelcoaxial cables, one inbound and the other outbound. The inbound cable and outbound cable are connected at one end, theheadend,andelectrically terminatedat their other ends. This architecture takes advantage of the well developedunidirectionalCATVcomponents.[2]Thetopologyis dendritic (i.e.branched like a tree).
...

The BIUs containRadio Frequency(RF) modems whichmodulateacarrier signalto transmitdigitalinformationusing 1MHzof the availablebandwidthin 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 rateof our test-bed system is 307.2kbps.

IEEE 802.3b (10BROAD36)[edit]

TheIEEE802 Committeedefined10BROAD36in802.3b-1985[3]as a 10Mbit/sIEEE 802.3/Ethernetbroadband system to run up to 3,600 metres (11,800 ft) over CATV coax network cabling. The wordbroadbandas 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 zeroHzandtheoreticallyconsumeinfinitefrequency bandwidth.(In real-world systems, higher-ordersignalcomponentsbecome indistinguishable from backgroundnoise.) In the market10BROAD36equipment was not developed by many vendors nor deployed in many user networks as compared to equipment for IEEE 802.3/Ethernetbasebandstandards such as10BASE5(1983),10BASE2(1985),10BASE-T(1990), etc.

IEEE 802.7[edit]

TheIEEE802 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[edit]

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[edit]

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 withAnteccalledARRISInteractive.[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 theDOCSISstandard.

Zenith homeworks[edit]

Zenithoffered a cable modem technology using its own protocol which it introduced in 1993, being one of the first cable modem providers. TheZenith Cable Modemtechnology 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 Westafter 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, or 4 Mbit/sec in 6 MHz.[9]

Com21[edit]

Main article:Com21

Com21was another early pioneer in cable modems, and quite successful until proprietary systems were made obsolete by the DOCSIS standardization. The Com21 system used aComControlleras central bridge in CATV network head-ends, the ComPort cable modem in various models and the NMAPS management system usingHP OpenViewas 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 also Fast-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[edit]

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 Internetservice; 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 peak 10Mbit/sdownstream 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.

TheAustralianISPBigPondemployed this system when it started cable modem tests in 1996. For a number of yearscable Internet accesswas only available inSydney,MelbourneandBrisbanevia 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 been also rolled out at the French cable operatorNumericablebefore upgrading its IP broadband network using DOCSIS.

DVB/DAVIC[edit]

Digital Video Broadcasting(DVB) andDigital 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.

IEEE 802.14[edit]

In the mid-1990s theIEEE 802committee 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, the802.14working group was disbanded when North Americanmulti system operators(MSOs) instead backed the then-fledglingDOCSIS 1.0specification, which generally usedbest-effort serviceand wasIP-based(with extensioncodepointsto supportATM[11]forQoSin the future).MSOswere interested in quickly deploying service to compete forbroadband Internet accesscustomers 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.

IETF[edit]

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.

DOCSIS[edit]

Main article:DOCSIS

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 layerfrom theMotorolaCDLPsystem and theMAC layerfrom the LANcity system. When the initial specification had been drafted, the MCNS consortium handed over control of it toCableLabswhich maintained the specification, promoted it in various standards organizations (notablySCTEandITU), developed a certification testing program for cable modem equipment, and has since drafted multiple extensions to the original specification.

While deployedDOCSIS RFI 1.0equipment generally only supportedbest-effort service,the DOCSIS RFI 1.0 Interim-01 document discussedquality of servce (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 2.0added support forS-CDMAPHY,while DOCSIS 3.0 addedIPv6support andchannel bondingto 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 EuropeanPALand US'sNTSCsystems 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 inJapanand 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.

Multimedia over Coax Alliance[edit]

Main article:Multimedia over Coax Alliance

In 2004, the Multimedia over Coax Alliance (MoCA) was established to develop industry standard for the connected home, using the existing coaxial cabling. Initially developed for in-home networking with MoCA 1.0/1.1, the MoCA standards has 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. "

Multimedia terminal adapter[edit]

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.

Network architectural functions[edit]

In network topology, a cable modem is anetwork bridgethat conforms toIEEE 802.1DforEthernetnetworking (with some modifications). The cable modem bridges Ethernet frames between a customerLANand 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.

With respect to theOSI modelofnetwork design,a cable modem is both physical layer (layer 1) device and adata link layer(layer 2) forwarder. As anIP addressablenetwork node, cable modems support functionality at other layers.

Layer 1 is implemented in theEthernet PHYon its LANinterface,and a DOCSIS defined cable-specificPHYon its HFC cable interface. The termcable modemrefers to this cable-specific PHY. TheNetwork Layer(Layer 3) is implemented as an IP host in that it has its ownIP addressused by the network operator to maintain the device. In thetransport layer(layer 4) the cable modem supportsUDPin association with its own IP address, and it supports filtering based onTCP and UDP portnumbers to, for example, block forwarding ofNetBIOStraffic out of the customer's LAN. In theApplication Layer(Layer 7), 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 arouterand 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 addressandMAC addressas will the router.

Cable modem flap[edit]

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]

Known vulnerabilities[edit]

In January 2020, a vulnerability affecting cable modems usingBroadcomchipsets 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 analyzercomponent of the modem (mostly used for debugging purposes) accessible through a networkportwhich is open by default in the vulnerable models.[26][27]

See also[edit]

References[edit]

  1. ^IEN 96- TheMITRECablenet Project
  2. ^"RF Micro Devices, Inc. Whitepaper Describing Historical CATV Components"(PDF).Piedmontscte.org.Retrieved2016-08-03.Amplifiers are one of the common components used in CATV system
  3. ^IEEE 802.3b-1985 (10BROAD36)Archived2012-02-25 at theWayback Machine- Supplement to 802.3: Broadband Medium Attachment Unit and Broadband Medium Specifications, Type 10BROAD36 (Section 11)
  4. ^"IEEE SA - 802.7-1989 - Local Area Networks: IEEE Recommended Practice: Broadband Local Area Networks".Standards.ieee.org.1990-03-09.Retrieved2016-08-03.
  5. ^staff, CNET News."Bay Networks to acquire LANcity".CNET.Retrieved2019-09-05.
  6. ^Marshall, Jonathan; Writer, Chronicle Staff (1998-06-16)."Telecom Giants To Merge / Bay Networks bought by Nortel for $7.2 billion".SFGate.Retrieved2019-09-05.
  7. ^"Nortel ups stake in joint venture with Antec".CNET.Retrieved2019-09-05.
  8. ^Sallie Hofmeister (1996-08-23)."Americast Places $1-Billion Order for Set-Top Boxes".Los Angeles Times.Retrieved2010-08-28.
  9. ^Gilbert Held (2000).Network Design: Principles and Applications.Auerbach Publications. p. 765.ISBN978-0-8493-0859-8.
  10. ^"WalkingDog.com".Archived from the original on 1996-12-26.Retrieved2012-05-13.{{cite web}}:CS1 maint: bot: original URL status unknown (link)The IEEE 802.14 Working Group used WalkingDog.com as its web site.
  11. ^abDOCSIS RFI 1.0-I01 (March 26, 1997)ArchivedMay 25, 2011, at theWayback Machine(See section 6.2.3 for the DOCSISATMcodepoint. See sections 6.1.2.3, 6.2.5.3, 6.4.7, 9, and 9.2.2 for DOCSIS 1.0 QoS mechanisms.)
  12. ^"IEEE 802.14 WG Officers".Archived from the original on 1997-01-29.Retrieved2012-05-13.{{cite web}}:CS1 maint: bot: original URL status unknown (link)
  13. ^Azzam, Albert A. (1997).High speed cable modems: including IEEE 802.14 standards.New York, NY: New York: McGraw-Hill.ISBN978-0-07-006417-1.Retrieved7 April2024.
  14. ^"Ipcdn Status Pages".Tools.ietf.org.Retrieved2016-08-03.
  15. ^"Ipdvb Status Pages".Tools.ietf.org.Retrieved2016-08-03.
  16. ^DOCSIS RFI 1.1-I01 (March 11, 1999)(See section 8 and Appendix M.)
  17. ^"DOCSIS Modem Interoperability and Certification Overview"(PDF).Stuff.mit.edu.Retrieved2016-08-03.
  18. ^"Cable".TekSavvy.com.Archived fromthe originalon 2016-08-01.Retrieved2016-08-03.
  19. ^"Compatible Modems".vmedia.ca.Retrieved2021-10-27.
  20. ^"Unlimited Internet Plans Quebec | Cable, Fibre Optic | Acanac".Acanac.ca.Archived fromthe originalon 2015-05-12.Retrieved2016-08-03.
  21. ^"Fast Unlimited Download High Speed Cable 75 Internet Plus Home Phone Bundle".www.worldline.ca.Retrieved2018-04-23.
  22. ^KMCreative."MoCA Access™".www.mocalliance.org.Retrieved2017-10-03.
  23. ^"Flap List Troubleshooting for the Cisco CMTS"(PDF).Cisco.Retrieved26 July2016.
  24. ^"Cable modem flapping.. - RCN | DSLReports Forums".Dslreports.com.Retrieved2016-08-03.
  25. ^"CMTS Troubleshooting and Network Management Features Configuration Guide".Cisco.com.2016-01-27.Retrieved2016-08-03.
  26. ^"Hundreds of millions of cable modems are vulnerable to new Cable Haunt vulnerability".ZDNet.
  27. ^Goodin, Dan (2020-01-13)."Exploit that gives remote access affects ~200 million cable modems".Ars Technica.Retrieved2020-01-15.

Further reading[edit]

External links[edit]

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