C H A P T E R 16
• Describe the history of SDLC.
• Describe the types and topologies of SDLC.
• Describe the frame format of SDLC.
• Describe derivative protocols of SDLC.
Synchronous Data Link Control and Derivatives
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.
SDLC Types and Topologies
SDLC supports a variety of link types and topologies. It can be used with point-to-point and multipoint
links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and
circuit-switched and packet-switched networks.
SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the
operation of other stations, called secondaries. The primary polls the secondaries in a predetermined
order, and secondaries can then transmit if they have outgoing data. The primary also sets up and tears
down links and manages the link while it is operational. Secondary nodes are controlled by a primary,
which means that secondaries can send information to the primary only if the primary grants permissio