High-bit-rate digital subscriber line 2 (HDSL2) is a standard developed by the American National Standards Institute (ANSI) Committee T1E1.4 and published in 2000 as ANSI T1.418-2000.[1][2][3] Like its predecessor HDSL, HDSL2 provides a symmetric data rate of 1,544 kbit/s in both the upstream and downstream directions at a noise margin of 5-6 dB.[3] Its primary purpose was also to provision a T-1 line, only this technology relies on fewer wires - two instead of four - and therefore costs less to set up.[3] The modulation technique used in HDSL2 is TC-PAM, which is also used in G.SHDSL, as opposed to 2B1Q in HDSL. Spectral shaping is applied to increase compatibility with ADSL and HDSL2 on the same bundle.[4]HDSL4 provides the same bitrate as HDSL2, but uses four wires instead of two, to increase robustness.[4] On an AWG26 local loop, the reach of HDSL2 is 9,000 feet (2.7 km), while that of HDSL4 is 11,000 feet (3.4 km).[4]
