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Two-binary, one-quaternary (2B1Q) is a line code used in the U interface of the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) and the high-bit-rate digital subscriber line (HDSL). [1] 2B1Q is a four-level pulse-amplitude modulation (PAM-4) scheme without redundancy, mapping two bits (2B) into one quaternary symbol (1Q).
Through the use of 2B1Q encoding, two bits are combined to one symbol, further reducing the symbol rate by a factor of two. For this two-pair 2B1Q variant of HDSL, framing increases the bitrate from 1.544 Mbit/s to 1.568 Mbit/s, resulting in a symbol rate of 392 kilobaud and a Nyquist frequency of 196 kHz.
2B1Q coding is defined in Appendix II of G.961, [9] ANSI T1.601, [7] and Annex A of ETR 080. [8] It can operate at distances up to about 18,000 feet ( 5.5 km ) with loss up to 42 dB . An internal termination impedance of 135 ohms is presented to the line at each end of the U-interface.
It is a variant of trellis coded modulation (TCM) which uses a one-dimensional pulse-amplitude modulation (PAM) symbol space, as opposed to a two-dimensional quadrature amplitude modulation (QAM) symbol space. [1] Compared to the 2B1Q scheme used in the older HDSL and SDSL standards, TC-PAM improves range at a given bit-rate and provides ...
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1 state 0 state NRZ–L: Non-return-to-zero level. This is the standard positive logic signal format used in digital circuits. forces a high level forces a low level NRZ–M: Non-return-to-zero mark forces a transition does nothing (keeps sending the previous level) NRZ–S: Non-return-to-zero space does nothing (keeps sending the previous level)