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DSL: Glossary

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ATM ( Asynchronous Transfer Mode ) - the protocol used to "gather" DSL traffic from users and forward it to a DSLAM, which consolidates traffic across the backbone network. Carries data in fixed-length frames of 53 bytes each.

ATU-C (ADSL Termination Unit - Central Office) - the downstream channel.

ATU-R (ADSL Termination Unit - Remote) - the upstream channel.

CAP (carrierless amplitude/phase modulation) - the original ADSL modulation approach in which the signal frequency range is divided into voice (0-4 KHz), upstream data, and downstream data. DMT (discrete multitone) is now the preferred modulation alternative over CAP.

CDSL (Consumer DSL) - a trademarked version of DSL from Rockwell that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.

DMT (Discrete Multitone) - the leading method of signal modulation for DSL service in which the usable frequency range is separated into 256 frequency bands (or channels) of 4.3125KHz each. DMT uses the FFT (fast Fourier transform) algorithm as its modulator and demodulator. Dividing the frequency spectrum into multiple channels allows DMT is to function better when AM radio transmitters are present. Within each channel, modulation uses QAM. The number of bits per symbol within each channel may be independently selected allowing the modem to be rate adaptive. Both G.DMT and G.Lite use DMT.

DSL ( Digital Subscriber Line ) - a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL competes with the cable modem and satellite transmission for high-bandwidth information reception.

ADSL ( Asymmetric Digital Subscriber Line - the form of DSL most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. DSLAM (Digital Subscriber Line Access Multiplexer) - a device that interconnects multiple DSL users to a high-speed backbone network. Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections.

G.DMT - a form of ADSL that uses discrete multitone technology with a splitter. G.DMT is officially ITU-T standard G-992.1.

G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) - a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call "the truck roll." G.Lite, officially ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream.

HDSL (High bit-rate DSL) - used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).

IDSL (ISDN DSL) - somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.

QAM (quadrature amplitude modulation) - a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems. In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information. CAP is very similar to QAM.

RADSL (Rate-Adaptive DSL) - an ADSL technology in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.

SDSL (Symmetric DSL) - similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.

splitter-based DSL - any DSL service that requires that a signal splitter be manually installed at a home or business, usually requiring the expense of a phone company visit and installation.

splitterless DSL - any DSL service in which the signal plitting is provided remotely from the central office. G.Lite DSL is a splitterless service.

sync rate - the data rate or speed that the DSLAM negotiates with your DSL modem. For a given service, the service provider may mandate a given maximum data rate.

VDSL (Very high data rate DSL) - a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments is not yet determined. A number of standards organizations are working on it.

xDSL - a term for any of the various types of DSL.

This was last updated in June 2006

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