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xDSL, a term that encompasses the broad range of digital subscriber line (DSL) services, offers a low-cost, high-speed data transport option for both individuals and businesses, particularly in areas without access to cable internet.

xDSL provides data transmission over copper lines, using the local loop, the existing outside-plant telephone cable network that runs right to your home or office. DSL technology is relatively cheap and reliable.

While the quality of the signals in all xDSL technologies deteriorates as distance increases, some types of DSL are more suitable than others to a specific distance and bandwidth demands.

HDSL (High-Bit-Rate DSL)

Standardized in 1994, HDSL uses two pairs of 24 AWG copper wires to provide symmetric E1/T1 data rates to distances up to 3657 meters. Its successors are HDSL2 and HDSL4, the latter using four pairs of wire instead of two.

SDSL (Symmetric DSL)

SDSL succeeded HDSL as the two-wire (single-pair) type of symmetric DSL. SDSL is also known within ANSI as HDSL2.
Essentially offering the same capabilities as HDSL, SDSL offers T1 rates (1.544 Mbps) at ranges up to 10,000 feet and is primarily designed for business applications.

ADSL: Asymmetric DSL

ADSL provides transmission speeds ranging from downstream/upstream rates of 9 Mbps/640 kbps over a relatively short distance to 1.544 Mbps/16 kbps as far as 18,000 feet. The former speeds are more suited to a business, the latter more to the computing needs of a residential customer.

ADSL's substantial bandwidth accommodates large downstream transmissions, such as receiving data from a host computer or downloading multimedia files.

Its lopsided nature and various speed/distance options available within this range make ADSL attractive for use in high-speed internet access. Like most DSL services standardized by ANSI as T1.413, ADSL enables you to lease and pay for only the bandwidth you need.

SHDSL: Single-Pair, High-Speed Digital Subscriber Line

Also known as G.SHDSL, this type of DSL transmits data at much higher speeds than older types of DSL. It enables faster transmission and connections to the internet over regular copper telephone lines than traditional voice modems can provide. Support of symmetrical data rates makes SHDSL a popular choice for businesses using PBXs, private networks, web hosting and other services.

SHDSL can be used effectively in enterprise LAN applications. When interconnecting sites on a corporate campus, buildings and network devices are often beyond the reach of a standard Ethernet segment. Now you can use existing copper network infrastructure to connect remote LANs across longer distances and at higher speeds than previously thought possible.

Ratified as a standard in 2001, SHDSL combines ADSL and SDSL features for communications over two or four (multiplexed) copper wires. SHDSL provides symmetrical upstream and downstream transmission with rates ranging from 192 kbps to 2.3 Mbps. As a departure from older DSL services designed to provide higher downstream speeds, SHDSL specified higher upstream rates, too. Higher transmission rates of 384 kbps to 4.6 Mbps can be achieved using two to four copper pairs. The distance varies according to the loop rate and noise conditions.

For higher-bandwidth symmetric links, newer G.SHDSL devices for four-wire applications support 10-Mbps rates at distances up to 1.3 miles (2 km). Equipment for two-wire deployments can transmit up to 5.7 Mbps at the same distance.
SHDSL (G.SHDSL) is the first DSL standard to be developed from the ground up and to be approved by the International Telecommunication Union (ITU) as a standard for symmetrical digital subscriber lines. It incorporates features of other DSL technologies, such as ADSL and SDS, and is specified in the ITU recommendation G.991.2.

VDSL: Very-High-Bit-Rate DSL

Also approved in 2001, VDSL as a DSL service enables downstream/upstream rates up to 52 Mbps/16 Mbps. Extenders for local networks boast 100-Mbps/60-Mbps speeds when communicating at distances up to 500 feet (152.4 m) over a single voice-grade twisted pair. As a broadband solution, VDSL enables the simultaneous transmission of voice, data, and video, including HDTV, video on demand and high-quality video conferencing. Depending on the application, you can set VDSL to run symmetrically or asymmetrically.

VDSL2: Very-High-Bit-Rate DSL 2

Standardized in 2006, VDSL2 provides higher bandwidth (up to 100 Mbps) and higher symmetrical speeds than VDSL, enabling its use for Triple Play services (data, video, voice) at longer distances. While VDSL2 supports upstream/downstream rates similar to VDSL, at longer distances, the speeds don't deteriorate as much as those transmitted with ordinary VDSL equipment.

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