ADSL transceiver

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Westell Model 6100 ADSL modem and router.
Westell Model 6100 ADSL modem and router.

An ADSL transceiver, also known as a ADSL modem or DSL modem, is a device used to connect one or more computers to a phone line, in order to use an ADSL service. Some ADSL modems also manage the connection and sharing of the ADSL service with a group of machines, making an ADSL router or ADSL modem/router a more accurate name for the device. A DSL modem acts as the ADSL Terminal Unit or ATU-R, as the telephone companies call it. The acronym NTBBA (network termination broad band adapter, network termination broad band access) is also common in various countries.

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[edit] Compared to voiceband modem

The two devices serve fundamentally the same purpose, but differ in important ways.

  • DSL modems are rarely internal to the computer but instead are wired to the computer's Ethernet port, or occasionally its USB port, while voiceband modems are usually built inside the computer.
  • MS Windows and other operating systems do not recognize DSL modems, hence have no Property Sheet or other internal method to configure them. Routers can be configured manually, for example using a Web page provided by the modem via the ethernet the router is connected to. DSL modems rarely need to be configured, because they are part of the physical layer of computer networks. They are only forwarding ones and zeros from one medium (CAT5) to another one (telephone line).
  • DSL modems use frequencies from 25 KHz to above 1MHz (see Asymmetric Digital Subscriber Line), in order not to interfere with voice service which is primarily 0-4 KHz. Voiceband modems use the same frequency spectrum as ordinary telephones, and will interfere with voice service.
  • DSL modems vary in data speed from hundreds of Kilobits per second to many Megabits, while voiceband modems are limited to approximately 50 Kbit/s.
  • DSL modems exchange data with only the DSLAM to which they are wired, which in turn connects them to the Internet, while most voiceband modems can dial directly anywhere in the world.
  • DSL modems are intended for particular protocols and sometimes won't work on another line even from the same company, while most voiceband modems use international standards and can "fall back" to find a standard that will work.
Inside component (chip) of a typical ADSL modem.
Inside component (chip) of a typical ADSL modem.

Most of these differences are of little interest to consumers, except the greater speed of DSL and the ability to use the telephone even when the computer is online.

Because a single phone line commonly carries DSL and voice, filters are used to separate the two uses. (See DSL filter)

[edit] Hardware components

As technology advances, functions that are provided by multiple chips can be integrated oto one chip. Higher levels of integration have benefitted DSL just as they benefitted other computer hardware. A DSL modem requires the following for its operation; exactly what is on the circuit card and how it is arranged can change as technology improves:

[edit] Service Features

Apart from connecting to an ADSL service, many modems offer additional integrated features:

Many ADSL modems can have their firmware upgraded to support additional features or fix existing problems. This can be done over the network, or using a dedicated serial connection.

[edit] Manufacturers

Currently, Taiwan manufactures most of the ADSL modems in the world, ODM plays a large role with designs from various parts of the world. Below lists an array of designers.

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