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Main article: "Com21

"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 (telephone network) and "radio frequency (cable network) return paths. The PSTN-based service was considered 'one-way cable' and had many of the same drawbacks as "satellite 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-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 "Sydney, "Melbourne and "Brisbane 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 been also rolled out at the French cable operator "Numericable 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.

IEEE 802.14[edit]

In the mid-1990s the "IEEE 802 committee formed a subcommittee (802.14)[7] to develop a standard for cable modem systems. IEEE 802.14 developed a draft standard, which was "ATM-based. However, the "802.14 working group was disbanded when North American "multi system operators ("MSOs) instead backed the then-fledgling "DOCSIS 1.0 specification, which generally used "best efforts service and was "IP-based (with extension "codepoints to support "ATM[8] for "QoS in the future). "MSOs were interested in quickly deploying service to compete for "broadband 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,[9] and his book, High-Speed Cable Modems,[10] describes many of the proposals submitted to 802.14.


Although the "Internet Engineering Task Force (IETF) generally does not generate complete cable modem standards, the IETF chartered "Working 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)[11] and IP over "Digital Video Broadcasting (DVB)[12] produced some standards applicable to cable modem systems, primarily in the areas of "Simple Network Management Protocol (SNMP) "Management Information Bases ("MIBs) for cable modems and other networking equipment that operates over CATV "networks.



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.

While deployed "DOCSIS RFI 1.0 equipment generally only supports "best efforts service, the DOCSIS RFI 1.0 Interim-01 document discussed "QoS extensions and mechanisms using "IntServ, "RSVP, "RTP, and Synchronous Transfer Mode (STM) "telephony (as opposed to "ATM).[8] "DOCSIS RFI 1.1[13] later added more robust and standardized "QoS mechanisms to DOCSIS. "DOCSIS 2.0 added support for "S-CDMA "PHY, while DOCSIS 3.0 added "IPv6 support and "channel 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 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. A third variant of DOCSIS was developed in "Japan and has seen limited deployment in that country.

Although interoperability was the whole point of the DOCSIS project,[14] most cable operators only approve a very restricted list of cable modems on their network,[15][16][17] 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 as it should. From this point of view, the DOCSIS standard has failed to provide the promised practical interoperability for the end user.

Multimedia terminal adapter[edit]

With the development of "voice 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 an embedded multimedia terminal adapter (E-MTA).

Many cable TV service providers also offer VoIP-based telephone service via the cable infrastructure ("PacketCable). High-speed Internet access service subscriber may use VoIP telephony by subscribing to a third-party service, such as "Vonage, "MagicJack+ and NetTALK.

Network architectural functions[edit]

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 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 functionality 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 an "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 "Dynamic Host Configuration Protocol (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 flap[edit]

Cable modems can have a problem known in industry jargon as "flap" or "flapping".[18] 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).[19] Types of flap include reinsertions, hits and misses, and power adjustments.[20]

See also[edit]


  1. ^ IEN 96 - The "MITRE Cablenet Project
  2. ^ "RF Micro Devices, Inc. Whitepaper Describing Historical CATV Components" (PDF). Retrieved 2016-08-03. Amplifiers are one of the common components used in CATV system 
  3. ^ IEEE 802.3b-1985 (10BROAD36) - 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". 1990-03-09. Retrieved 2016-08-03. 
  5. ^ Sallie Hofmeister (1996-08-23). "Americast Places $1-Billion Order for Set-Top Boxes". "Los Angeles Times. Retrieved 2010-08-28. 
  6. ^ Gilbert Held (2000). Network Design: Principles and Applications. Auerbach Publications. p. 765. "ISBN "978-0-8493-0859-8. 
  7. ^ "". Archived from the original on 1996-12-26. Retrieved 2012-05-13.  The IEEE 802.14 Working Group used as its web site.
  8. ^ a b DOCSIS RFI 1.0-I01 (March 26, 1997) (See section 6.2.3 for the DOCSIS "ATM codepoint. See sections,, 6.4.7, 9, and 9.2.2 for DOCSIS 1.0 "QoS mechanisms.)
  9. ^ "IEEE 802.14 WG Officers". Archived from the original on 1997-01-29. Retrieved 2012-05-13. 
  10. ^ Albert A. Azzam, High-Speed Cable Modems "ISBN 978-0-07-006417-1
  11. ^ "Ipcdn Status Pages". Retrieved 2016-08-03. 
  12. ^ "Ipdvb Status Pages". Retrieved 2016-08-03. 
  13. ^ DOCSIS RFI 1.1-I01 (March 11, 1999) (See section 8 and Appendix M.)
  14. ^ "DOCSIS Modem Interoperability and Certification Overview" (PDF). Retrieved 2016-08-03. 
  15. ^ "Cable". Retrieved 2016-08-03. 
  16. ^ "Archived copy". Archived from the original on 2013-07-30. Retrieved 2013-06-17. 
  17. ^ "Unlimited Internet Plans Quebec | Cable, Fibre Optic | Acanac". Retrieved 2016-08-03. 
  18. ^ "Flap List Troubleshooting for the Cisco CMTS" (PDF). Cisco. Retrieved 26 July 2016. 
  19. ^ "Cable modem flapping.. - RCN | DSLReports Forums". Retrieved 2016-08-03. 
  20. ^ "CMTS Troubleshooting and Network Management Features Configuration Guide". 2016-01-27. Retrieved 2016-08-03. 

Further reading[edit]

External links[edit]

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