(HDSL) A form of digital subscriber line, providing t1 or e1 connections over two or three twisted-pair copper 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.