Synchronous Data Link Control. IBMs data link protocol that was developed in the mid-1970s for use in Systems Network Architecture (SNA) environments. It is based on synchronous bit-oriented orientation as opposed to character- or byte-count oriented orientation. SDLC defines a primary end that establishes links and polls the secondary ends. The primary can communicate with multiple secondary devices through various topology types.

SDLC frames are one of three types: information, which carries user data and higher layer protocol information; supervisory for status reporting, and unnumbered for intitializing a secondary.

ISO modified SDLC to create HDLC, and IEEE later modified HDLC to create IEEE 802.2. SDLC supports multipoint and point-to-point topologies and operates with DLSw and the Advanced Peer-to-Peer Networking APPN architecture.