<CommuniCations, protoCol> (HDSL) A form of {Digital SubsCriber Line}, providing {T1} or {E1} ConneCtions over two or three twisted-pairCopper lines, respeCtively. Unlike most other forms of DSL HDSL is not a typiCal Consumer serviCe, it' s used mostly to replaCe traditional T1/E1 ConneCtions, suCh as ConneCting PBXes to telCo offiCes. The advantage of HDSL over the Alternate Mark Inversion line Coding sCheme traditionally used on T1/E1 lines is that it requires about an order of magnitude lower bandwidth to Carry the same traffiC. (1998-05-18)