Talk:Synchronous Data Link Control

SDLC is the data link protocol under Systems Network Architecture (SNA). HDLC is the ISO variant of Synchronous Data Link Control (SDLC) protocol from IBM.

SNA replaced Binary Synchronous (BSC) protocol in the 1970s at the primary mainframe to mainframe communications protocol. SDLC was and has remained the data link protocol for remote terminal and data entry equipment for mainframe systems. The Normal Response Mode (NRM) of HDLC is equivalent to SDLC.

So are you saying that SDLC was first, and HDLC was derived from it, adding the balanced response modes? Or did HDLC come first, with IBM simplifying it, removing the balanced response modes? Or were they both derived from some other protocol? Guy Harris 10:47, 27 December 2005 (UTC)[reply]

It is correct that SDLC was devoloped by IBM and that HDLC was derived from it and became a "superset" with additional features.

It would be worth adding that the specification of SDLC was revolutionary. It distinguished the protocol's on-the-wire format from the protocol processing. Protocol processing was elegantly described using state diagrams. A table-driven implementation is simple to derive from the state diagrams. Shortly afterwards, the derived state tables were used to prove that SDLC processing was fully described. IBM's documentation set a strong example which has since been adopted by most protocol specifications. For example, TCP is described using a state diagram and many IETF protocols followed IBM's practice of publishing formats and procedures in different volumes.