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A single-pair high-speed digital subscriber line (SHDSL) is a type of symmetric digital subscriber line (SDSL) that has symmetric upload and download speeds over conventional copper telephone lines, which is faster than what a conventional voiceband modem can provide. SHDSL uses trellis-coded pulse-amplitude modulation (TC-PAM) that operates at frequencies that encompass those used by analog voice POTS (plain old telephone service), meaning that a DSL splitter or frequency splitter cannot separate analog voice and data.
Single-pair high-speed digital subscriber lines allow for symmetrical data rates for both up and down streams, which range from 192 Kbps to 2312 Kbps in 8 Kbps speed increments for single pairs and 384 Kbps to 4624 Kbps in double pair mode. SHDSL is also spectrally compatible with other DSL technologies by using TC-PAM coding, allowing for the use of the technologies in parallel.
Because of the TC-PAM coding used for spectral compatibility with other DSL technologies, SHDSL cannot share the same telephone line with POTS, making it more suitable private branch exchange (PBX), web hosting, virtual private networks (VPN), leased lines (E1/T1) and other data services. SHDSL is described in the ITU-T G.991.1 standard and supersedes the older high bit-rate DSL (HDSL).