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ADSL2 and ADSL2+

Described by standards G.992.3 and G.992.4, and approved by the International Telecommunications Union, these new technologies will soon start to become available. ADSL2 improves on the original ADSL implementation by increasing download rates from 8 to 12Mbps, as well as by extending the local-loop length (i.e. the distance that ADSL is available from the exchange). ADSL2 also offers improved diagnostics, better power management with stand-by mode, greater interoperability, faster start-up, and support for packet-based services (e.g. Ethernet). ADSL2+, a little further out, is based on ADSL2 and doubles the maximum frequency used for downstream data transmission from 1.1 MHz to 2.2 MHz. As a result, downstream data rates are increased to up to 24 Mbps on phone lines as long as 3,000 feet, and 20 Mbps on lines as long as 5,000 feet.

Note that widespread availability of this technology is dependent on upgrades to ADSL equipment at the telephone exchange.

SHDSL

Symmetric High Bit-rate DSL [=SHDSL] offers the same data transmission rate for both upload and download: 2.3MBps for a two-wire, or 4.6Mbps for a four-wire model. [A normal telephone service comprises two wires, while two telephone lines are needed to provide a four-wire connection.]

SHDSL conforms to the ITU standard G.991.2 and is also referred to as G.shdsl.

SHDSL uses the full spectrum of the line and therefore needs a dedicated line and cannot share a line being used for voice/fax traffic. It supports a greater local-loop distance than does ADSL, and is able to transmit data at 1.2Mbps over 6,100 meters wire-length from the exchange. SHDSL is rate adaptive and will automatically detect the maximum up and down speeds possible for a given line quality and length.

ADSL and SHDSL are the most common types of DSL service in the Australian market at time of writing, but for completion several other variants will be mentioned.

VDSL

Standing for ‘Very-high Bit Rate DSL', VDSL is an emerging technology that is intended to provide a high-speed link between homes and a nearby fibre-optic cable. The requirement for ‘nearby cable' means that deployment is probably not imminent for most users. VDSL offers up to 52Mbps over short distances and is being offered in some areas in the US.

RADSL

‘Rate Adaptive DSL' is an asymmetric technology that allows the modem to adapt the rate of transmission to 7Mbps downstream and 1MBps upstream, depending on variables such as file size and line quality. RADSL standards are certified but the protocol is yet to be widely adopted.

Other standards are IDSL, for ISDN over DSL; SDSL, an emerging enhancement to existing SHDSL; and HDSL, an older version of Symmetric DSL.

Approx xDSL Maximum Local-Loop Length (i.e. distance from exchange)

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