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Technologies known as numerical subscriber's loop , in English DIGITAL Subscriber Line or DSL or xDSL , gather the whole of the technologies installation for a numerical transport of information on a simple line of telephone connection.

Principle

It is about a wide mode of exploitation of existing copper lines on the basis of the following principle: a phone line makes it possible to diffuse waves included/understood in a certain frequency spectrum; however the voice uses only one very restricted part of this spectrum, and it is even possible to still reduce it without obstructing the communications. The idea is thus to make profitable the part not used of the spectrum to transport data.

Modulation

The methods of modulation most employed are DMT and CAPE, the first being used more.

Families of DSL

There exist several standards of the xDSL family:

IDSL ( ISDN DIGITAL Subscriber Line , directed alternative given of the ISDN)
ADSL ( Asymmetric DIGITAL Subscriber Line ), interesting when one receives more than one does not emit.
ReADSL (Reach Extended DIGITAL Subscriber Line ), makes it possible to increase the range of the ADSL

HDSL ( High Bit Misses DIGITAL Subscriber Line )

RADSL ( Rate Adaptive DIGITAL Subscriber Line )

SDSL ( Symmetric DIGITAL Subscriber Line , standardized version of HDSL)

VDSL ( Very high speed DIGITAL Subscriber Line )

G.SHDSL (Replacement standardized ITU-T for the first versions owners of SDSL)

G-Lite (ADSL with low flow not requiring filters ( filters ))

DSM (Dynamic Spectrum Management), competitor of the VDSL.

Type of DSL

One distinguishes various types of DSL like the ADSL and the SDSL. With for Asymmetrical and S for Symmetrical , this prefix describes the relationship between ascending band-width, from the user towards the network, ( upload ) and downward band-width, from the network towards the user, ( download ). The ADSL has a upload much weaker than the download while for the SDSL the two band-widths are equal. The first is thus appropriate well for a user of the Web (a line struck or clicked for a received file), while the second makes it possible to consider the creation of a Web server or interconnection of two sites undertaken, it allows also services of emulation of rented connection.

Deployment

At the time of the installation of technologies xDSL (mainly ADSL), several problems arose. The first was the presence of filters side customer preventing the high frequencies from passing on the telephone cables, that to make the voice clearer by avoiding the interferences. It is recommended to remove these filters at the time of the startup of a line ADSL. Another problem comes from the quality of the cables used, primarily of category 3, which much more quickly degrades the quality of the signal with the increase in the frequency than the cables category 5 usually currently installed. That very often limits the maximum frequency of the signals to 1 MHz. The distance on which the signal is propagated is limited. Beyond, the signal would be weakened too much and the Modem S do not make any more the distinction of with the Bruit. This is why, initially, the customers who lived with more than 6 km of the telephone centres could not have access to the ADSL.

In France, connections of the subscribers (whose local Loop), was until January 1st, 2001, the object of the Monopole of France Telecom. The end of this monopoly allowed the appearance of concurrent companies which, by installing their own equipment DSLAM (DIGITAL Subscriber Line Access To multiplex), connect their subscribers to the principal network - it is the Dégroupage.

See too

External bonds

teaching Card: technologies DSL
Weakening of the signals xDSL

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