DSL Modems

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. more...

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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.

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. Therefore 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, which is available from a Web browser on the ethernet the router is connected to. Modems rarely need to be configured, hence 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.;

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 of the common mixed use of a single phone line for DSL and voice, filters are used to separate the two uses. (See DSL filter)

Features

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

ADSL2 or ADSL2+ support; Router functionality that uses NAT to share the connection; An 802.11b or 802.11g wireless access point; A built-in switch (typically 4 ports); VPN termination; DHCP server; Dynamic DNS client; VoIP functionality including QoS;

Read more at Wikipedia.org