What are ADSL, HDSL and VDSL?
These are variations on the DSL theme:
ADSL - Asymmetric Digital Subscriber Line
Asymmetric means that a higher speed is available from the exchange to the user (downstream) and a slower speed from the user to the exchange (upstream) . For example the user can download web pages or videos at high speed, but can only send at a significantly lower speed. Capability varies with distance - up to 9 Mbps downstream can be supported up to one mile from an exchange and 2 Mbps up to three miles. 64 kbps is the usual upstream speed. Note that the distance is the length of the cable, because of the way networks are routed a 3 mile cable may mean only 2 miles physical distance. ADSL can operate in parallel with conventional analogue voice telephony over the same line.
HDSL - High speed Digital Subscriber Line
This has been in use for some years in some countries to provide broad band facilities to business premises (leased line), providing 2 Mbps across up to three miles of wire.
VDSL - Very high speed Digital Subscriber Line
VDSL is a new technology not expected to be in use in public networks for some years. Its expected to provide speeds as high as 52 Mbps downstream and between 1.5 and 2.3 Mbps upstream, but over shorter distances than ADSL - 1.3 km at 13 Mbps and 0.3 km at 52 Mps. Note that by deploying higher performance lines from exchanges to street cabinets, these speeds could be delivered to more homes.
DSL-Lite
This is a new focus among PC and modem manufacturers to apply the same principles as ADSL using DMT technology (see below). This could be an alternative way of increasing modem speeds and improving plug-and-play capabilities for Internet access, providing 1.5 Mbps downstream and 512 kbps upstream. DSL-Lite has also been called splitterless DSL because the cost of modems is reduced by omitting the splitter capability that is used in ADSL to provide voice telephony in parallel with data.
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