Cable modem - Inglês

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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.

Under a stricter definition, a cable modem is not a modem but a network bridge. ISPs may sometimes sell or rent hardware combining a bridge with network hub or router hardware as a cable modem.

Cable Internet Access

The term cable Internet access refers to the delivery of Internet service over this infrastructure. The proliferation of cable modems, along with DSL technology, has enabled broadband Internet access in many countries.

Bandwidth of business cable modem service typically range from 3 Megabits per second (Mbit/s) up to 30 Mbit/s or more. The upstream bandwidth on residential cable modem service usually ranges from 384 Kilobits per second (kbit/s) to 6 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 amount of bandwidth is shared by a population of users (in the case of cable internet, users in a neighborhood share the available bandwidth provided by a single coaxial cable line). Therefore, connection 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.
  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 $40/month for cable modem only access, than if one bundles it with a cable TV plan where it might be $30/month for cable modem service plus $20/month for cable TV service. This has ramifications similar to those of the lack of naked DSL. However, some cable internet access providers who resell access from cable companies, such as Earthlink, are generally not subject to these higher rates.


CDLP is a proprietary standard 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.

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.

An alternative to cable modems is the Embedded Multimedia Terminal Adapter (EMTA or E-MTA). An EMTA allows MSOs to offer both High Speed Internet and VoIP through a single piece of customer premise equipment.

Cable modem manufacturers