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). "DOCSIS RFI 1.1 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, most cable operators only approve a very restricted list of cable modems on their network, 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
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
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
Cable modems can have a problem known in industry jargon as "flap" or "flapping". 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). Types of flap include reinsertions, hits and misses, and power adjustments.
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