Leased lines were used to connect mainframe computers with terminals and remote sites, via IBM Systems Network Architecture ( created in 1974 ) or DECnet ( created in 1975 ).
These protocols included IBM Systems Network Architecture ( SNA ), Digital Equipment Corporation's DECnet, Open Systems Interconnection ( OSI ), and Xerox Network Systems ( XNS ).
While the PAD function defined by X . 28 and X . 29 specifically supported asynchronous character terminals, PAD equivalents were developed to support a wide range of proprietary intelligent communications devices, such as those for IBM System Network Architecture ( SNA ).