ADSL modem

An asymmetric digital subscriber line transceiver, also known as an ADSL modem or DSL modem, is a device used to connect a single computer to a DSL 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: in this case, the unit is termed a DSL router or residential gateway. 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. Because a DSL modem is a bridge, it has no interface and the IP address that is configured to the computer it is attached to is assigned to the device.

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. 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, 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.

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)

Hardware components

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:

• Power: Transformer and capacitor
• Data connection and power circuitry (for example, USB, Ethernet, PCI)
• DSL digital data pump
• DSL analog chip and line driver
• Micro controller
• Filter

Service Features

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

• ADSL2 or ADSL2+ support
• Router functionality that uses Network Address Translation (NAT) to share the connection
• An 802.11b or 802.11g wireless access point
• A built-in switch (typically 4 ports)
• Virtual Private Network termination
• Dynamic Host Configuration Protocol (DHCP) server
• Dynamic DNS (Domain Name System) clients
• Voice over IP functionality including Quality of Service (priority control for data flows between users)

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.

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.

• 2Wire (United States)
• 3Com (United States)
• A-Link (Finland)
• Alcatel (France)
• AVM (Germany)
• Aztech (Singapore)
• Belkin (United States)
• Billion (Taiwan)
• D-Link (Taiwan)
• Draytek (Taiwan)
• Efficient Networks (Germany, related to Siemens)
• Fujitsu (Japan)
• Huawei (China)
• NetComm] (Australia)
• Netgear (United States)
• Netopia (United States)
• OvisLink (United States)
• Sagem (France)
• Siemens (Germany-United States)
• Symphony
• Thomson (France)
• U.S. Robotics (United States)
• Westell (United States)
• ZTE (China)
• ZyXEL (Taiwan)
• Zoom Technologies (United States)

Taken From http://en.wikipedia.org/wiki/DSL_modem

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