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)


 * It is correct that SDLC was devoloped by IBM and that HDLC was derived from it and became a "superset" with additional features. —Preceding unsigned comment added by 194.237.142.10 (talk • contribs) 05:02, 9 January 2006 (UTC)

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. —Preceding unsigned comment added by 150.101.30.44 (talk) 03:53, 8 January 2008 (UTC)


 * Thank you for your suggestion. When you feel an article needs improvement, please feel free to make those changes. Wikipedia is a wiki, so anyone can edit almost any article by simply following the  link at the top. The Wikipedia community encourages you to be bold in updating pages. Don't worry too much about making honest mistakes — they're likely to be found and corrected quickly. If you're not sure how editing works, check out how to edit a page, or use the sandbox to try out your editing skills.  New contributors are always welcome. You don't even need to log in (although there are many reasons why you might want to).   Guy Harris (talk) 04:09, 8 January 2008 (UTC)

ICL C-03
Should also mention International Computers Limited (ICL) C-03 protocol. This was ICL's equivalent protocol, which formed part of the ICL Information Processing Architecture (IPA). See the IBM Systems Network Architecture (SNA) wikipedia article. 66.155.23.67 (talk) —Preceding undated comment added 12:51, 20 July 2016 (UTC)

Specification of SDLC was ground breaking
As well as being an influential protocol, the IBM's specification of SDLC was hugely influential. It was the origin of the notion that network protocols provide "services" to the next layer of protocol or application. It introduced the useful distinction betwen a protocol's over-the-wire "format" and the algorithmic "procedures" at each end-system, giving a specification for each. The use of state machines to specify the "procedures" was new.

This level of rigour meant that SDLC attracted the first academic treatment of protocols. For example, proofs that there were no undescribed states in the state machine, that the state machine couldn't loop without consuming incoming frames. — Preceding unsigned comment added by Gdt (talk • contribs) 03:54, 7 March 2017 (UTC)