Digital Subscriber Line (xDSL)
by Peter R. Egli, Consultant on Dec 04, 2010
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Overview of DSL technologies. ...
Overview of DSL technologies.
DSL (Digital Subscriber Line) subsumes all layer 1 and 2 technologies for transporting digital data over (twisted pair) copper wires.
In traditional telephone networks, pairs of copper wire run from a central location (Central Office) to subscriber premises.
Although initially designed for 300Hz to 3.4kHz bandwidth, these copper wires could be upgraded to Internet access lines through the use of DSL technologies.
While early DSL technologies like HDSL required 4 copper wires, newer technologies like ADSL (Asymmetric Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line) and VDSL (Very High Datarate Digital Subscriber Line) only require a single local loop line (pair of copper wires).
Technological advances in digital signal processing and CPU speed increased possible data rates dramatically. ADSL2+ (ITU-T G.992.5) allows downstream data rates up to 24MBit/s on short distances (1-2km). When even higher speeds are required and more symmetric rates between up- and downstream, VDSL2 (ITU-T G.993.2) is the choice. It supports data rates up to 100MBit/s on distances lower than 1km.
Internet access on local loop lines also required upgrading the backend infrastructure of telephone networks. Additional gear like DSLAMs (Digital Subscriber Line Access Multiplexer) and access concentrators (LAC, LNS) had to be installed to extract and inject Internet traffic signals on the central office side.
A bevy of protocols like PPPoE, L2TP and MPLS is required to support Internet Access service. In order to reduce complexity, these protocols are typically replaced in new deployments by the Ethernet protocol.
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