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DSL Advances by Peter J. Silverman, John M. Cioffi, Massimo Sorbara, Thomas Starr

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16.2. What is Voice-over DSL?

In voice-over DSL (VoDSL), the voice and the telephony signaling is digitized and carried over the DSL PHY along with the data traffic. Unlike the frequency division multiplex provided with ADSL over POTS (or over ISDN), the voice channels are multiplexed into the DSL path. Digitally multiplexed lines are sometimes known as “derived lines.” Although it is true that earlier digital technologies (e.g., T1, HDSL, and ISDN) were used to support telephony, VoDSL is distinguished from these voice environments in several ways. ADSL and SDSL (symmetric DSLs, both proprietary and the standard ITU-T G.991.2) are PHYs that were initially developed primarily to carry data. Unlike the telephony-oriented digital technologies that ...

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