What are the various types of xDSL?

There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Some forms of xDSL are proprietary, some are simply theoretical models and some are widely used standards. They may best be categorized within the modulation methods used to encode data. Below is a brief summary of some of the known types of xDSL technologies. ADSL Asymmetric Digital Subscriber Line (ADSL) is the most popular form of xDSL technology. The key to ADSL is that the upstream and downstream bandwidth is asymmetric, or uneven. In practice, the bandwidth from the provider to the user (downstream) will be the higher speed path. This is in part due to the limitation of the telephone cabling system and the desire to accommodate the typical Internet usage pattern where the majority of data is being sent to the user (programs, graphics, sounds and video) with minimal upload capacity required (keystrokes and mouse clicks). Downstream speeds typically range from 768 Kb/s to 9 Mb/s Upstream speeds typically range from 64Kb/s to 1.5Mb/s. ADSL Lite (see G.lite) CDSL Consumer Digital Subscriber Line (CDSL) is a proprietary technology trademarked by Rockwell International. CiDSL Globespan's proprietary, splitterless Consumer-installable Digital Subscriber Line (CiDSL). EtherLoop EtherLoop is currently a proprietary technology from Nortel, short for Ethernet Local Loop. EtherLoop uses the advanced signal modulation techniques of DSL and combines them with the half-duplex "burst" packet nature of Ethernet. EtherLoop modems will only generate hi-frequency signals when there is something to send. The rest of the time, they will use only a low-frequency (ISDN-speed) management signal. EtherLoop can measure the ambient noise between packets. This will allow the ability to avoid interference on a packet-by-packet basis by shifting frequencies as necessary. Since EtherLoop will be half-duplex; it is capable of generating the same bandwidth rate in either the upstream or downstream direction, but not simultaneously. Nortel is initially planning for speeds ranging between 1.5Mb/s and 10Mb/s depending on line quality and distance limitations. G.lite A lower data rate version of Asymmetric Digital Subscriber Line (ADSL) was been proposed as an extension to ANSI standard T1.413 by the UAWG (Universal ADSL Working Group) led by Microsoft, Intel, and Compaq. This is known as G.992.2 in the ITU standards committee. It uses the same modulation scheme as ADSL (DMT), but eliminates the POTS splitter at the customer premises. As a result, the ADSL signal is carried over all of the house wiring which results in lower available bandwidth due to greater noise impairments. Often a misnomer, this technology is not splitterless per se. Instead of requiring a splitter at customer premises, the splitting of the signal is done at the local CO. G.shdsl G.shdsl is an ITU standard which offers a rich set of features (e.g. rate adaptive) and offers greater reach than many current standards. G.shdsl also allows for the negotiation of a number of framing protocols including ATM, T1, E1, ISDN and IP. G.shdsl is touted as being able to replace T1, E1, HDSL, SDSL HDSL2, ISDN and IDSL technologies. HDSL High Bit-rate Digital Subscriber Line (HDSL) is generally used as a substitute for T1/E1. HDSL is becoming popular as a way to provide full-duplex symmetric data communication at rates up to 1.544 Mb/s (2.048 Mb/s in Europe) over moderate distances via conventional telephone twisted-pair wires. Traditional T1 (E1 in Europe) requires repeaters every 6000 ft. to boost the signal strength. HDSL has a longer range than T1/E1 without the use of repeaters to allow transmission over distances up to 12,000 feet. It uses pulse amplitude modulation (PAM) on a 4-wire loop. HDSL2 High Bit-rate Digital Subscriber Line 2 was designed to transport T1 signaling at 1.544 Mb/s over a single copper pair. HDSL2 uses overlapped phase Trellis-code interlocked spectrum (OPTIS). IDSL ISDN based DSL developed originally by Ascend Communications. IDSL uses 2B1Q line coding and typically supports data transfer rates of 128 Kb/s. Many end users have had to suffice with IDSL service when full speed ADSL was not available in their area. This technology is similar to ISDN, but uses the full bandwidth of two 64 Kb/s bearer channels plus one 16 Kb/s delta channel. MDSL Usually this stands for multi-rate Digital Subscriber Line (MDSL). It depends on the context of the acronym as to its meaning. It is either a proprietary scheme for SDSL or simply a generic alternative to the more common ADSL name. In the former case, you may see the acronym MSDSL. There is also another proprietary scheme which stands for medium-bit-rate DSL. Confused yet? RADSL Rate Adaptive Digital Subscriber Line (RADSL) is any rate adaptive xDSL modem, but may specifically refer to a proprietary modulation standard designed by Globespan Semiconductor. It uses carrierless amplitude and phase modulation (CAP). T1.413 standard DMT modems are also technically RADSL, but generally not referred to as such. The uplink rate depends on the downlink rate, which is a function of line conditions and signal to noise ratio (SNR). SDSL Symmetric Digital Subscriber Line (SDSL) is a 2-wire implementation of HDSL. Supports T1/E1 on a single pair to a distance of 11,000 ft. The name has become more generic over time to refer to symmetric service at a variety of rates over a single loop. UDSL Universal DSL. See G.lite. VDSL Very High Bit-rate Digital Subscriber Line (VDSL) is proposed for shorter local loops, perhaps up to 3000 ft. Data rates exceed 10 Mb/s. Original Article here The best products and services in telecom and internet-working supply