DSL modem

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Westell Model 6100 AXXDSL DSL router

A

PCI slot
.

The more common DSL router is a standalone device that combines the function of a DSL modem and a router, and can connect multiple computers through multiple Ethernet ports or an integral wireless access point. Also called a residential gateway, a DSL router usually manages the connection and sharing of the DSL service in a home or small office network.

Different DSL routers and modems support different DSL technology variants: VDSL, SDSL, and ADSL.

Description

A DSL router consists of a box with an

wall wart
transformer.

It usually has a series of LED status lights which show the status of parts of the DSL communications link:

  • Power light - indicates that the modem is turned on and has power
  • Ethernet lights - there is usually a light over each Ethernet jack; a steady (or sometimes flashing) light indicates that the Ethernet link to that computer or device is functioning
  • DSL light - a steady light indicates that the modem has established contact with the equipment in the local telephone exchange (DSLAM) so the DSL link over the telephone line is functioning; newer modems that support ADSL2+ bonding will have one light for each line[1]
  • Internet light - a steady light indicates that the
    DHCP
    protocol are initialized and working, so the system is connected to the Internet
  • Wireless light - (only in wireless DSL modems) indicates that the wireless network is initialized and working

Many routers provide an internal web page to the local network for device configuration and status reporting. Most DSL routers are designed to be installed by the customer for which a CD or DVD containing an installation program is supplied. The program may also activate the DSL service. Upon powering the router it may take several minutes for the local network and DSL link to initialize, usually indicated by the status lights turning green. There are also PCI DSL modems, which plug into an available PCI card slot on a computer.

Technology

DSL concept

The

megahertz.[2]
This capacity is unused in normal phone service. DSL uses these higher frequencies to send digital data between the DSL modem and the local switching center, without interfering with normal telephone service. At the local switching center the data is transferred directly between the customer's phone line and internet lines, so DSL signals do not travel through the telephone network itself. It is not necessary to dial a telephone number to initiate a connection; the DSL connection is "on" whenever the modem is on.

Data transmission

The device at the local switching center which communicates with the DSL modem is called a

Digital Subscriber Line Access Multiplexer (DSLAM), which is connected directly to the Internet.[2]
The local switching center must be equipped with these devices to offer DSL service.

With ADSL, the modem and the DSLAM communicate by a protocol called

error-correcting code to allow minor bit errors due to noise to be corrected at the receiving end. Most of the channels are unidirectional, carrying download data from the DSLAM to the modem, but some on the low frequency end are bidirectional, to carry the smaller quantity of upload traffic. The modem constantly monitors the transmission quality on each channel, and if it is too impaired it will shift the signal to other channels. The modem is constantly shifting data between channels searching for the best transmission rates.[2]
Thus interference or poor quality lines will generally not interrupt transmission, but only cause the data rate of the modem to degrade.

For example, when downloading a web page, the packets of web page data travel over

error correction, puts the data together again in the proper order, and sends it to the computer over the Ethernet line, or for a wireless (Wi-Fi
) network by radio signals.

Data rates and access

Most consumer DSL lines use one of several varieties of

Asymmetric DSL (ADSL).[2] The "asymmetric" means that more of the bandwidth of the line is dedicated to downstream (download) data than upstream (upload) data, so, download rates are faster than upload rates, because most users download much larger quantities of data than they upload. Because the telephone lines were never designed to carry such high frequency signals, DSL is distance-sensitive. The farther away from the switching center the modem is, the longer the telephone wires, the weaker the signal, and the lower the data rate that the modem can achieve. Users in cities, close to switching centers, may have access to higher rate service, up to 24 Mbit/s.[2] The distance limit for ADSL is 18 000 feet (5.5 km or 3.4 miles).[2] However, other devices installed in telephone lines by the phone company, such as loading coils and bridge taps
, block the signal, and may disqualify a given phone line from DSL service.

Filters

Main article: DSL filter

To prevent the DSL signal from entering the phone lines into telephones, answering machines, faxes and other devices where it could cause interference, DSL modems come with

low pass filters
which must be plugged into the phone lines going to all voiceband devices on the same line. The filter blocks all frequencies above 4 kHz, so it blocks the DSL signal while allowing voice frequency signals through. A filter must not be inserted in the phone line going to the DSL modem, because it would block the communication between the modem and switching center.

Comparison to voice-band modems

A DSL modem modulates high-frequency tones for transmission to a

voice-band modem
that was a mainstay in the late 20th century, but differs from it in important ways.

  • DSL modems transfer data at a rate which is at least 10 to 20 times that of a voice-band modem.
  • DSL does not interfere with normal telephone calls on the telephone line, and does not require dialing a telephone number to initiate a connection, it is always "on". A voice-band modem dials a telephone number to initiate a connection, and while it is connected the telephone line cannot be used for normal telephone service.
  • DSL routers, the most common form of DSL modem, are external to the computer and wired to the computer's Ethernet port or its USB port, whereas voice-band modems are usually internal devices installed in the computer itself in a PCI interface slot in the back. Internal DSL modems are rare but available.
  • webpage, accessed by typing an IP address given in the router's manual into the browser's address bar, with which various technical changes can be made, such as changing the wireless network's password, and adjusting the router's firewall
    .
  • For external DSL modems connected by USB, Microsoft Windows and other operating systems generally recognize these as a Network interface controller.
  • For internal DSL modems, Microsoft Windows and other operating systems provide interfaces similar to those provided for voice-band 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 1 MHz (see
    Asymmetric Digital Subscriber Line), in order not to interfere with voice service which is primarily 0–4 kHz. Voice-band modems use the same frequency spectrum as ordinary telephones, and will interfere with voice service - it is usually impossible to make a telephone call on a line which is being used by a voice-band modem. Because a single phone line commonly carries DSL and voice, DSL filters
    are used to separate the two uses.
  • DSL modems vary in data speed from hundreds of kilobits per second to many megabits, while voice-band 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 voice-band 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 voice-band modems use international standards and can "fall back" to find a standard that will work.[citation needed]

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.

Hardware components

ADSL modem router internals labeled

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:

Service features

Further information: Residential gateway

Apart from connecting to a DSL service, many modems offer additional integrated features, forming a residential gateway:

See also

References

  1. ^ "ADSL BONDING | Increase Broadband Speed". March 6, 2014.
  2. ^ a b c d e f g h Franklin, Curt (2011). "How DSL Works". How Stuff Works. Discovery Communications. Retrieved August 21, 2012.

External links

Media related to ADSL modems at Wikimedia Commons

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