DSL modem

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Westell Model 6100 ADSL modem
Westell Model 6100 ADSL modem

ADSL modem or DSL modem is a device used to connect a single computer or router to a DSL phone line, in order to use an ADSL service. The acronym NTBBA (network termination broad band adapter, network termination broad band access) is also common in various countries. Some ADSL modems also manage the connection and sharing of the ADSL service with a group of machines: in this case, the unit is termed a DSL router or residential gateway. Asymmetric digital subscriber line transceiver or ATU-R, as the telephone companies call it, is a functional block inside every ADSL modem which actually performs modulation, demodulation and framing, while other functional blocks perform Asynchronous Transfer Mode Segmentation and Reassembly, IEEE 802.1D bridging and/or IP routing. Typical user interfaces are Ethernet and USB. Although an ADSL modem working as a bridge doesn't need an IP address, it may have one assigned for management purposes.

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

  • Most DSL modems are external to the computer and wired to the computer's Ethernet port, or occasionally its USB port, whereas voiceband modems are usually built inside the computer. Internal DSL modems (with PCI interface) are rare but available.
  • Microsoft Windows and other operating systems do not recognize external DSL modems, and hence have no Property Sheet or other internal method to configure them. This is because the transceiver and computer are considered separate nodes in the LAN, rather than the transceiver being a device controlled by the computer (such as webcams, mice, keyboards etc.). Routers can be configured manually, using a Web page provided by the modem via the ethernet that the router connects 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).
  • For internal DSL modems, Microsoft Windows and other operating systems provide interfaces similar to those provided for voiceband modems. This is based on the assumption that in the future, as CPU speeds increase, internal DSL modems may become more mainstream.
  • 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 nominally 56K modems and actually 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.

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)

As technology advances, functions that are provided by multiple chips can be integrated onto one chip. Higher levels of integration have benefited DSL just as they benefited 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:

Inside component (chip) of a typical ADSL modem.
Inside component (chip) of a typical ADSL modem.

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.

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.

Retrieved from "http://en.wikipedia.org/wiki/DSL_modem"
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