Glossary

Asymmetrical Digital Subscriber Line: (ADSL) offers differing upload and download speeds and can be configured to deliver up to six megabits of data per second (6000K) from the network to the customer that is up to 120 times faster than dialup service and 100 times faster than ISDN. ADSL enables voice and high-speed data to be sent simultaneously over the existing telephone line. This type of DSL is the most predominant in commercial use for business and residential customers around the world. Good for general Internet access and for applications where downstream speed is most important, such as video-on-demand. ITU-T Recommendation G.992.1 and ANSI Standard T1.413-1998 specify full rate ADSL. ITU Recommendation G.992.3 specifies ADSL2 which provides advanced diagnostics, power saving functions, PSD shaping, and slightly better performance than G.992.1. ITU Recommendation G.992.5 specifies ADSL2Plus, which provides the benefits of ADSL2Plus twice the bandwidth so that bit rates as high as 20 Mb/s downstream can be achieved on relatively short lines.
DSL: (digital subscriber line) a technology that exploits unused frequencies on copper telephone lines to transmit traffic typically at multi-megabit speeds. DSL can allow voice and high-speed data to be sent simultaneously over the same line. Because the service is 'always available,' end-users don't need to dial in or wait for call set-up. With DSL you are wired for speed.
G.lite ADSL (or simply G.lite): The G.lite standard was specifically developed to meet the plug-and-play requirements of the consumer market segment. G.lite is a medium bandwidth version of ADSL that allows Internet access at up to 30 times the speed of the fastest 56K analog modems ~ up to 1.5 megabits downstream and up to 500 kilobits upstream. G.lite is an International Telecommunications Union (ITU) standard, globally standardized interoperable ADSL system per ITU G.992.2. G.lite has seen comparatively little use, but it did introduce the valuable concept of splitterless installation.
HDSL: (high data rate DSL) This variety created in the late 1980s delivers symmetric service at speeds up to 2.3 Mbps in both directions. Available at 1.5 or 2.3 Mbps, this symmetric fixed rate application does not provide standard telephone service over the same line and is already standardized through ETSI and ITU (International Telecommunications Union). Seen as an economical replacement for T1 or E1, it uses one, two or three twisted copper pairs.
HDSL2: (2nd generation HDSL) This variant delivers 1.5 Mbps service each way, supporting voice, data, and video using either ATM (asynchronous transfer mode), private-line service or frame relay over a single copper pair. This ATIS standard (T1.418) for this symmetric service gives a fixed 1.5 Mbps rate both up and downstream. HDSL2 does not provide standard voice telephone service on the same wire pair. HSDL2 differs from HDSL in that HDSL2 uses one pair of wires to convey 1.5 Mbps whereas ANSI HDSL uses two wire pairs.
HDSL4: HDSL4 is virtually the same as HDSL2 except it achieves about 30% greater distance than HDSL or HDSL2 by using two pairs of wire (thus, 4 conductors), whereas HDSL2 uses one pair of wires.
IDSL: (integrated services digital network DSL) This is a form of DSL that supports symmetric data rates of up to 144 Kbps using existing phone lines. It is unique in that it has the ability to deliver services through a DLC (Digital Loop Carrier: a remote device often placed in newer neighborhoods to simplify the distribution of cable and wiring from the phone company). While DLCs provide a means of simplifying the delivery of traditional voice services to newer neighborhoods, they also provide a unique challenge in delivering DSL into those same neighborhoods. IDSL addresses this market along with ADSL and G.lite as they are implemented directly into those DLCs. IDSL differs from its relative ISDN (integrated services digital network) in that it is an "always-available" service, but capable of using the same terminal adapter, or modem, used for ISDN.
RADSL: (rate adaptive DSL) A non-standard version of ADSL. Note that standard ADSL also permits the ADSL modem to adapt speeds of data transfer.
Symmetric flavors Symmetrical variations include: SDSL, SHDSL, HDSL, HDSL2 and IDSL The equal speeds make Symmetrical DSLs useful for LAN (local area network) access, video-conferencing, and for locations hosting their own Web sites.
SDSL: (symmetric DSL) SDSL is a vendor-proprietary version of symmetric DSL that may include bit-rates to and from the customer ranging of 128 kbps to 2.32 Mbps. SDSL is an umbrella term for a number of supplier-specific implementations over a single copper pair providing variable rates of symmetric service. SDSL uses 2B1Q HDSL run on a single pair with an Ethernet interface to the customer. The industry is expected to quickly move towards the higher performing and standardized G.shdsl technology developed by the ITU with support from T1E1.4 (USA) and ETSI (European Telecommunications Standards Institute).
SHDSL is state-of-the-art, industry standard symmetric DSL SHDSL equipment conforms to the ITU Recommendation G.991.2, also known as G.shdsl, approved by the ITU-T February 2001. SHDSL achieves 20% better loop-reach than older versions of symmetric DSL, it causes much less crosstalk into other transmission systems in the same cable, and multi-vendor interoperability is facilitated by the standardization of this technology. SHDSL systems may operate at many bit-rates, from 192 kbps to 5.7 Mbps, thereby maximizing the bit-rate for each customer. G.shdsl specifies operation via one pair of wires, or for operation on longer loops, two pairs of wire may be used. For example, with two pairs of wire, 1.2 Mbps can be sent over 20,000 feet of 26 AWG wire. SHDSL is best suited to data-only applications that need high upstream bit-rates. Though SHDSL does not carry voice like ADSL, new voice-over-DSL techniques may be used to convey digitized voice and data via SHDSL. SHDSL is being deployed primarily for business customers.
VDSL (very high bit rate DSL): Up to 26 Mb/s, over distances up to 50 Meters on short loops such as from fiber to the curb. In most cases, VDSL lines will be served from neighborhood cabinets that link to a Central Office via optical fiber. It is particularly useful for 'campus' environments - universities and business parks, for example. VDSL is currently being introduced in market trials to deliver video services over existing phone lines. VDSL can also be configured in symmetric mode.
VDSL2 (second generation VDSL): ITU Recommendation G.993.2 specifies 8 profiles that address a range of applications including up to 100 Mb/s symmetric transmission on loops about one hundred meters long (using a bandwidth of 30 MHz), symmetric bit-rates in the 10-30 Mb/s range on intermediate length loops (using a bandwidth of 12 MHz), and asymmetric operation with downstream rates in the range of 10-40Mb/s on loops of lengths ranging from 3km to 1km (using a bandwidth of 8.5 MHz). VDSL2 includes most of the advanced feature from ADSL2. The rate/reach performance of VDSL2 is better than VDSL.