The Free On-line Dictionary of Computing (19 January 2023):

Single-line Digital Subscriber Line
SDSL
S-HDSL
Single-pair High Speed Digital Subscriber Line

    (SDSL, or Single-pair High Speed
   Digital Subscriber Line, S-HDSL) A form of Digital Subscriber
   Line similar to HDSL but providing T1 or E1 connections
   over a single twisted-pair copper line.

   (1998-05-18)