Citation :
HDSL stands for High bit-rate Digital Subscriber Line and,
like ADSL, provides high capacity digital transmission over
existing copper twisted pair. However, HDSL is a symmetrical
transport technology, which means that the full high capacity
bandwidth can be sent in both directions. In contrast, ADSL is
an asymmetrical technology, providing unidirectional high
capacity channels and separate, lower speed bi-directional
channels, as well as lifeline POTS.
The two technologies are designed for different applications.
HDSL is typically used to quickly and cost-effectively provision
high speed digital services, such as E1. ADSL, on the other hand,
is used to deploy interactive, multimedia services that are
asymmetrical by nature, such as movies-on-demand, home shopping
or Internet access. These asymmetrical applications require high
speed channels that can broadcast data or video from a central
location, as well as independent interactive channels that can
provide real-time interaction with the video or data service
provider. ADSL was specifically designed to provide this support.
Another difference between ADSL and HDSL involves the transmission
medium. While both HDSL and ADSL are transported over twisted
pair copper, HDSL requires two pairs, or four wires. In contrast,
ADSL only requires one pair, or two wires for transmission.
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