o The term refers to the technologies and equipments used in a telephone network to ensure the access to a high speed digital network on a twisted wire line; • there are two basic categories namely: SDSL – Symmetric DSL and ADSL-Asymmetric DSL; o SDSL ensures the same transfer rate in both directions, that is in: • " upstream " → subscriber – exchange • " downstream " → exchange – subscriber • due to the attenuation and crosstalk these systems can work only at medium frequencies; • SDSL is ideal for LAN, bidirectional-video, web servers. o ADSL ensures in " downstream " a large bandwidth channel, situated at high frequencies and a more narrow band channel in " upstream " situated at low frequencies; o this division of the frequency bands has two reasons: the information quantity transmitted in " downstream " is larger and it is reduced the crosstalk, which is higher at high frequencies; o ADSL variants include: ADSL, ADSL G.lite, RADSL and VDSL (can work both in symmetric and asymmetric mode). o The used transmission mode for different DSL techniques and the maximum length of the subscriber loop are presented in tab. 1 and fig. 1; it is specified if is possible the use of telephone service and the number of necessary pairs of wire.
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