Synchronous Data Link Control

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Synchronous Data Link Control (SDLC) is a computer communications protocol. It is the layer 2 protocol for IBM's Systems Network Architecture (SNA). SDLC supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header.^[1]. SDLC was mainly used by IBM mainframe and midrange systems; however implementations exist on many platforms from many vendors. The use of SDLC (and SNA) is becoming more and more rare, mostly replaced by IP-based protocols or being tunneled through IP (using AnyNet or other technologies).

In 1975, IBM developed the first bit-oriented protocol, SDLC. This de facto standard has been adopted by ISO as High-Level Data Link Control (HDLC) and by ANSI as Advanced Data Communication Control Procedures (ADCCP). The latter standards added features such as the Asynchronous Balanced Mode.

SDLC operates independently on each communications link, and can operate on multipoint or point-to-point links, on switched or dedicated circuits, and with full and half-duplex operation.

Intel used SDLC as a base protocol for BitBus, still popular in Europe fieldbus and included support in several controllers (i8044/i8344, i80152). 8044 controller is in production by third party vendors.