Cable modem

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An outdated model of the Motorola Surfboard cable modem
An outdated model of the Motorola Surfboard cable modem

A cable modem is a type of modem that provides access to a data signal sent over the cable television infrastructure. Cable modems are primarily used to deliver broadband Internet access, taking advantage of unused bandwidth on a cable television network. There were 22.5 million cable modem users in the United States during Q1 2005, up from 17.4 million in Q1 2004. They are also commonly found in Australia, Canada and Europe.

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[edit] Cable internet access

The term cable Internet access refers to the delivery of Internet service over this infrastructure. Cable modems, along with DSL technology, are the two primary types of broadband Internet access.

Bit rate of business cable modem service typically range from 2 Megabits per second (Mbit/s) up to 50 Mbit/s or more. The upstream rate on residential cable modem service usually ranges from 384 Kilobits per second (kbit/s) to 20 Mbit/s or more. There are few attempts to offer different service tiers beyond the traditional 'home' and 'business' designations.

There are two potential disadvantages to cable internet:

  1. Like all residential broadband network technologies (e.g. DSL, FTTX, Satellite internet, WiMAX), a fixed channel capacity is shared by a population of users (in the case of cable internet, users in a neighborhood share the available capacity provided by a single coaxial cable line). Therefore, service speed can vary depending on how many people are using the service at the same time. This arrangement allows the network operator to take advantage of statistical multiplexing in order to provide an adequate level of service at an acceptable price. However, the operator has to monitor usage patterns, and scale the network appropriately, to ensure that customers receive adequate service even during peak usage times. Some operators also use a bandwidth cap.
  2. Many cable Internet providers are reluctant to offer cable modem access without tying it to a cable television subscription. They do this by charging higher rates, say $60/month for cable modem only access, than if one bundles it with a cable TV plan where it might be $40/month for cable modem service plus $40/month for cable TV service. This has ramifications similar to those of the lack of naked DSL. Cable internet access providers who resell access from cable companies, such as Earthlink, are generally not subject to these higher rates. However, they can not give you internet access if you don't have cable service.

[edit] Cable modems in the OSI model or TCP/IP model

From a network topology perspective, a cable modem is, strictly speaking, 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.

With respect to the OSI model, a cable modem is thus a data link layer (or layer 2) forwarder, rather than simply a modem.

A cable modem does however, also support functionalities at other layers. In physical layer (or layer 1), the cable modem supports the Ethernet PHY on its LAN interface, and a DOCSIS defined cable-specific PHY on its HFC cable interface. It is to this cable-specific PHY that the name cable modem refers. In the network layer (or layer 3), the cable modem is a IP host in that it has its own IP address used by the network operator to manage and troubleshoot the device. In the transport layer (or 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 5 or layer 7), the cable modem supports certain protocols that are used for management and maintenance, notably DHCP, SNMP, and TFTP.

Some cable modem devices may incorporate a router along with the cable modem functionality, to provide the LAN with its own 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. So, the cable modem function will have its own IP address and MAC address as will the router.

[edit] History

[edit] LANcity

LANcity was an early pioneer in cable modems, developing a proprietary system that saw fairly wide deployment 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.

[edit] CDLP

CDLP was a proprietary system that was made by Motorola. CDLP CPE was capable of both PSTN (telephone network) and RF (cable network) return paths. The PSTN return path cable modem 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 DSL which was bidirectional. The standard is more or less defunct now with new providers using, and existing providers having changed over to, the DOCSIS standard. The Motorola CDLP Proprietary CyberSURFR is an example of a modem 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 modems.)

The Australian ISP BigPond employed this system when it started cable modem trials in 1996. For a number of years cable Internet access was only available to Sydney, Melbourne and Brisbane via CDLP. This network ran parallel to the newer DOCSIS system for a number of years. In 2004 the CDLP network was switched off and now is exclusively DOCSIS.

[edit] 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 MSOs instead backed the fledgling DOCSIS specification.

[edit] DOCSIS

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. CableLabs took on maintenance of 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 version or another of DOCSIS.

[edit] Cable modems and VoIP

With the advent of Voice over IP telephony, cable modems can also be used to provide telephone service. Many people who have cable modems have opted to eliminate their Plain Old Telephone Service (POTS). Because most telephone companies do not offer naked DSL (DSL service without a POTS line), VoIP use is higher amongst cable modem users.

A cable modem subscriber can make use of VoIP telephony by subscribing to a third party service (e.g. Vonage or Skype). As an alternative, many cable operators offer a VoIP service based on PacketCable. PacketCable allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment, known as an Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA is basically a cable modem and a VoIP adapter (known as a Multimedia Terminal Adapter) bundled into a single device. PacketCable service has a significant technical advantage over third-party providers in that voice packets are given guaranteed Quality of Service across their entire path so that call quality can be assured.

[edit] See also

[edit] Cable modem manufacturers

[edit] External links

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Internet access methods
Wired: Dial-up, ISDN, DSL, Cable, Fiber optic, Power-line internet
Wireless: Wi-Fi, Bluetooth, WiBro, WiMAX, UMTS-TDD, HSPA, EV-DO, Satellite
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