Digital Subscriber Line

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digital subscriber line

[¦dij·əd·əl səb′skrīb·ər ‚līn]
(communications)
A system that provides subscribers with continuous, uninterrupted connections to the Internet over existing telephone lines, offering a choice of speeds ranging from 32 kilobits per second to more than 50 megabits per second. Abbreviated DSL.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

Digital Subscriber Line

(communications, protocol)
(DSL, or Digital Subscriber Loop, xDSL - see below) A family of digital telecommunications protocols designed to allow high speed data communication over the existing copper telephone lines between end-users and telephone companies.

When two conventional modems are connected through the telephone system (PSTN), it treats the communication the same as voice conversations. This has the advantage that there is no investment required from the telephone company (telco) but the disadvantage is that the bandwidth available for the communication is the same as that available for voice conversations, usually 64 kb/s (DS0) at most. The twisted-pair copper cables into individual homes or offices can usually carry significantly more than 64 kb/s but the telco needs to handle the signal as digital rather than analog.

There are many implementation of the basic scheme, differing in the communication protocol used and providing varying service levels. The throughput of the communication can be anything from about 128 kb/s to over 8 Mb/s, the communication can be either symmetric or asymmetric (i.e. the available bandwidth may or may not be the same upstream and downstream). Equipment prices and service fees also vary considerably.

The first technology based on DSL was ISDN, although ISDN is not often recognised as such nowadays. Since then a large number of other protocols have been developed, collectively referred to as xDSL, including HDSL, SDSL, ADSL, and VDSL. As yet none of these have reached very wide deployment but wider deployment is expected for 1998-1999.

http://cyberventure.com/~cedpa/databus-issues/v38n1/xdsl.html.

2Wire DSL provider lookup.

["Data Cooks, But Will Vendors Get Burned?", "Supercomm Spotlight On ADSL" & "Lucent Sells Paradine", Wilson & Carol, Inter@ctive Week Vol. 3 #13, p1 & 6, June 24 1996].
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

DSL

(1) (Domain-Specific Language) See special-purpose language.

(2) (Digital Subscriber Line) A technology that increases the digital capacity of ordinary telephone lines (the local loops) into the home or office for Internet and TV service. Depending on the DSL version, speed is based on the distance between the customer and telco central office or telephone junction box.

DSL provides "always-on" operation. At the central office, DSL traffic is aggregated in a unit called the DSL Access Multiplexor (DSLAM) and forwarded to the appropriate ISP or data network. DSL arrived in the late 1990s with more version alphabet soup than most any other new transmission technology. See PPPoA and PPPoE.


Asymmetric: Fast Down - Slow Up



ADSL - (Asymmetric DSL)
ADSL shares ordinary telephone lines by using higher frequencies than human speech. To eliminate interference, the first DSL deployments required a technician from the phone company to install a splitter that divided the line into separate DSL and phone lines. Subsequent splitterless versions (known as G.Lite, Universal ADSL and ADSL Lite) eliminated the support visit but require the user to plug filters into every phone outlet.

ADSL is available in two modulation schemes: Discrete Multitone (DMT) or Carrierless Amplitude Phase (CAP). See CDSL, G.shdsl, ATU-C and ATU-R.


ADSL Transmission
The higher frequencies of DSL have to be filtered out for regular telephones, answering and fax machines.







The Filter
Low-pass DSL filters split the line between phone and DSL modem. When DSL is installed, every telephone and answering machine must plug into the filter.








RADSL (Rate Adaptive DSL)
RADSL is a version of ADSL that adjusts speeds based on signal quality. Many ADSL technologies are actually RADSL.


VDSL/VHDSL (Very High Bit Rate DSL)
VDSL is used as the final drop from a fiber optic junction point to nearby customers. VDSL lets an apartment or office complex obtain high-bandwidth services using their existing copper wires without having to replace them with optical fibers. Like ADSL, VDSL can share the line with the telephone.

Whereas VDSL achieves 50 Mbps transmission speed at 1,000 feet, VDSL2 extended the length of the path to 3,300 feet and delivers 100 Mbps at 1,600 feet. See Gfast.


Symmetric - Same Speed Both Ways



HDSL (High Bit Rate DSL)
HDSL provides T1 transmission over copper wires without the additional provisioning required for setting up T1 circuits, such as bridged tap removal and repeater installation. HDSL requires two cable pairs up to 12,000 feet, while HDSL-2 requires only one cable pair and spans 18,000 feet. HDSL does not allow line sharing with analog phones.


SDSL (Symmetric DSL)
SDSL is an HDSL variation that is rate adaptive, uses one cable pair and is offered in speeds from 144 Kbps to 1.5 Mbps. Like HDSL, SDSL does not share lines with analog phones.


IDSL (ISDN DSL)
IDSL is a slightly faster basic BRI ISDN service. It uses the 16 Kbps "D" channel for data rather than call setup to achieve 144 Kbps instead of 128 Kbps. It also offers the longest distance of 26,000 feet. Unlike standard ISDN, IDSL does not support analog phones, and signals are not switched through the telephone network. Since IDSL uses the same 2B1Q line coding as ISDN, ISDN customers can use existing BRI terminal adapters and routers. See ISDN.


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