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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).
The first edition of ETR 152 specified the line code 2B1Q on either three pairs at 784 kbit/s each or two pairs at 1,168 kbit/s each. [3] A second edition of ETR 152, published in June 1995, specified trellis coded carrierless amplitude/phase modulation (CAP) as an alternative modulation scheme, running on two pairs at 1,168 kbit/s each. [4]
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.
Compared to the 2B1Q scheme used in the older HDSL and SDSL standards, TC-PAM improves range at a given bit-rate and provides enhanced spectral compatibility with ADSL. TC-PAM is also known as 4B1H, because it uses 16 levels to represents a 4 digit binary, 4 Binary 1 Hexadecimal.
The gross bit rate is 160 kbit/s; 144 kbit/s throughput, 12 kbit/s sync and 4 kbit/s maintenance. The signals on the U reference point are encoded by two modulation techniques: 2B1Q in North America, Italy and Switzerland, and 4B3T elsewhere. Depending on the applicable cable length, two varieties are implemented, U pN and U p0.
[2] [3] For HDSL, the American ANSI standard specifies 2B1Q rather than CAP, while the European ETSI ETR 152 and the international ITU-T G.991.2 standards specify both CAP and 2B1Q. [ 2 ] [ 4 ] [ 5 ] For ADSL deployments CAP was the de facto standard up until 1996, deployed in 90 percent [ citation needed ] of ADSL installs.
The copper pair between the EU and RU, which carries the 2B1Q signalling and the 140 V DC for powering the RU and subscribers' telephones. The 140 V DC is not applied to the line until an RU is detected so that engineers do not get a shock. It is also removed as soon as the RU is disconnected, again for safety.
MSDSL uses either 2B1Q [2] or carrierless amplitude phase modulation (CAP) transmission with a capacity of up to 2.32 Mbit/s. [1] The bandwidth is generally split between a full E-carrier E1 payload (2.048 Mbit/s), with the remaining bandwidth accommodating up to three voice channels or two ISDN channels. [1]