HDSL2 headway: Vendors take one step closer to standard

The second incarnation of high bit-rate digital subscriber line reached a milestone on its way to standardization. The ANSI T1E1.4 technical subcommittee has settled a provisional agreement that provides a basis for line code, forward-error correction and spectral shaping for HDSL2 transmissions.

The committee, which includes representatives from ADC Telecommunications, Adtran, Level One Communications, PairGain and Siemens, has yet to decide on the framing structure and an overhead threshold for the technology, but members say the fully approved standard is set to be ratified at the end of 1998.

"The process has not been as contentious as [asymmetrical DSL]," said Mike Rude, editor of the HDSL2 standard and senior project engineer at ADC. "From this point forward, progress should be more rapid."

Carriers are interested in HDSL2 because it uses only one pair of copper wires to transmit 1.5 Mb/s in both directions. HDSL2 has been assigned a 6 dB noise margin, so transmissions must be able to bear an extra 6 dB beyond the noise generated by adjacent transmissions such as T-1 lines.

HDSL2's forward-error correction specification will enable users to program the encoder in HDSL2 modems so companies can write proprietary code beyond the limits of the standard.

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