Black Box Version 1.0 Technologies, XDSL technology, background, Asymmetric DSL Adsl technology

4 TECHNOLOGIES

4.1xDSL technology, background

xDSL technology appeared due to the growing user’s demand to high-speed digital stream transfer over telephone copper pairs. Operators had to organize the interconnection of backbone stations of cellular networks, Digital Loop Carrier systems, interstation connection and to provide high-speed Internet access at minimal expenses. In these circumstances, it was reasonable to use the existing telephone cables. The new technology acquired the name – xDSL (Digital Subscriber Line).

“x” key

x is a variable in the DSL technology, where every word has its own meaning. Thus the term “Digital” means, that not an analogue but digital signal, that was processed by one of line encoding methods, is transmitted over pairs. In fact, the term xDSL points at this or that line code the distance of data transfer and maximal connection speed depend on it. However, some technologies, for example ADSL, can use one of the two line codes: either Discrete Multi-Tone (DMT) or Carrierless Amplitude/Phase (CAP).

The term “Subscriber Line” is referred to physical copper pairs of telephone cable, or in simple words, to “direct wires”. The term DSL was originally referred to the ISDN technology, but later was borrowed by the developers of xDSL technologies.

4.1.1 Asymmetric DSL (ADSL) technology

The most popular DSL technology, ADSL, was developed in Bellcore laboratory in late 1980s. Standards Institutes assigned the use of carrier set modulation, which acquired the name DMT, to ADSL, while another leading method received the name of Rate-Adaptive DSL (RADSL). ADSL transmits downstream to the end user and upstream to the net. The ADSL technology does not use 25–30-kHz frequencies, used for a subscriber’s access to public switched telephone network (PSTN). This provides simultaneous subscriber’s access to data transfer networks and PSTN over the same copper pair. The original ADSL implies the presence of splitters both on LTU and NTU. However, ADSL without splitters found its use and acquired the name of ADSL Lite or G.lite. Later, it was standardized by ITU-T. This standard supports downstream speeds up to 1.5 Mbit/s and upstream speeds up to 512 Kbit/s.

4.1.1.1 ADSL in brief

Standard

•G.lite G.992.1 (G.DMT)

•T1 413-1998

•Interoperability between equipment of different manufacturers

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