IBM developed the Synchronous Data Link Control (SDLC) protocol in the mid-1970s for use in Systems Network Architecture (SNA) environments. SDLC was the first link layer protocol based on synchronous, bit-oriented operation. This chapter provides a summary of SDLC's basic operational characteristics and outlines several derivative protocols. After developing SDLC, IBM submitted it to various standards committees. The International Organization for Standardization (ISO) modified SDLC to create the High-Level Data Link Control (HDLC) protocol. The International Telecommunication Union-Telecommunication Standardization Sector (ITU-T formerly CCITT) subsequently modified HDLC to create Link Access Procedure (LAP) and then Link Access Procedure, Balanced (LAPB). The Institute of Electrical and Electronic Engineers (IEEE) modified HDLC to create IEEE 802.2. Each of these protocols has become important in its domain, but SDLC remains the primary SNA link layer protocol for WAN links.