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. Each of these protocols has become important in its own 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.
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 permission. Intermediate secondaries pass messages through one another as they respond to the requests of the primary. The primary uses the outbound channel to communicate with the secondaries.