SDLC Overview (Synchronous Data Link Control)
 

SDLC (Synchronous Data Link Control) is a transmission protocol developed by IBM in the 1970s as a replacement for its binary synchronous (BSC) protocol. SDLC is equivalent to layer 2 of the Open Systems Interconnection (OSI) model of network communication. This level of protocol makes sure that data units arrive successfully from one network point to the next and flow at the right pace.

SDLC uses the primary station-secondary station model of communication. Typically in IBM mainframe networks, the host mainframe is the primary station and workstations and other devices are secondary stations. Each secondary station has its own address. Typically, multiple devices or secondary stations are attached to a common line in what is known as a multipoint or multidrop arrangement. SDLC can also be used for point-to-point communication. SDLC is primarily for remote communication on corporate wide-area networks (WANs).

SDLC was a basis for the International Organization for Standardization (ISO) standard data link protocol, High-Level Data Link Control (HLDLC). SDLC essentially became one of several variations of HDLC, the normal response mode (NRM). While SDLC (and normal response mode) are efficient protocols for closed private networks with dedicated lines, other modes of HDLC serve X.25 and frame relay protocols that manage packets on shared-line switched networks like those used by the Internet.

SDLC became part of IBM's Systems Network Architecture (SNA) and the more comprehensive Systems Application Architecture (SAA). SDLC is still a commonly encountered and prevalent data link protocol in today's mainframe environment.