Synchronous Data Link Control

Synchronous Data Link Control (SDLC) is a <a href="/facts/Computer/SQe1aK05">computer</a> <a href="/facts/Serial_communication/r5xP7P1c">serial</a> <a href="/facts/Communications_protocol/5uady7cB">communications protocol</a> first introduced by <a href="/facts/IBM/QszA7nwd">IBM</a> as part of its <a href="/facts/Systems_Network_Architecture/NlFaXaBC">Systems Network Architecture</a> (SNA). SDLC is used as layer 2, the <a href="/facts/Data_link_layer/K9yPufbt">data link layer</a>, in the SNA <a href="/facts/Protocol_stack/Btj9FtwJ">protocol stack</a>. It supports multipoint links as well as error correction. It also runs under the assumption that an SNA header is present after the SDLC header. SDLC was mainly used by IBM mainframe and midrange systems; however, implementations exist on many platforms from many vendors. In the United States and Canada, SDLC can be found in traffic control cabinets. SDLC was released in 1975, based on work done for <a href="/facts/IBM/QszA7nwd">IBM</a> in the early 1970s. 
SDLC operates independently on each communications link in the network and can operate on <a href="/facts/Point-to-point_(telecommunications)/RmGgN1Pj">point-to-point</a> <a href="/facts/Point-to-multipoint_communication_(telecommunications)/6xWWhgdn">multipoint</a> or <a href="/facts/Current_loop/FCnMkkWn">loop</a> facilities, on switched or dedicated, <a href="/facts/Two-wire_circuit/j0hbjBtD">two-wire</a> or <a href="/facts/Four-wire_circuit/C3InvEMe">four-wire</a> circuits, and with <a href="/facts/Full-duplex/LCULeVxr">full-duplex</a> and <a href="/facts/Half-duplex/LCULeVxr">half-duplex</a> operation. A unique characteristic of SDLC is its ability to mix half-duplex secondary stations with full-duplex primary stations on four-wire circuits, thus reducing the cost of dedicated facilities.
This <a href="/facts/De_facto/3Du6WQKh">de facto</a> standard has been adopted by <a href="/facts/International_Organization_for_Standardization/IacYR7Yg">ISO</a> as <a href="/facts/High-Level_Data_Link_Control/aTndRUQ3">High-Level Data Link Control</a> (HDLC) in 1979 and by <a href="/facts/ANSI/p3d1TQCb">ANSI</a> as <a href="/facts/Advanced_Data_Communication_Control_Procedures/hD1rD98Y">Advanced Data Communication Control Procedures</a> (ADCCP). The latter standards added features such as the <a href="/facts/Asynchronous_Balanced_Mode/aTndRUQ3">Asynchronous Balanced Mode</a>, frame sizes that did not need to be multiples of bit-octets, but also removed some of the procedures and messages (such as the TEST message).
<a href="/facts/Intel/SMF0gJJX">Intel</a> used SDLC as a base protocol for <a href="/facts/BITBUS/kna8UpU3">BITBUS</a>, still popular in Europe as <a href="/facts/Fieldbus/kna8UpU3">fieldbus</a> and included support in several controllers (i8044/i8344, i80152). The 8044 controller is still in production by third-party vendors. Other vendors putting hardware support for SDLC (and the slightly different HDLC) into communication controller chips of the 1980s included <a href="/facts/Zilog/sB6jH5vw">Zilog</a>, <a href="/facts/Motorola/ztqczx68">Motorola</a>, and <a href="/facts/National_Semiconductor/xeAD8doA">National Semiconductor</a>. As a result, a wide variety of equipment in the 1980s used it and it was very common in the mainframe-centric corporate networks which were the norm in the 1980s. The most common alternatives for SNA with SDLC were probably <a href="/facts/DECnet/9wnFzPvn">DECnet</a> with <a href="/facts/Digital_Data_Communications_Message_Protocol/je5sggJp">Digital Data Communications Message Protocol</a> (DDCMP), Burroughs Network Architecture (BNA) with Burroughs Data Link Control (BDLC), and <a href="/facts/ARPANET/LyMGWurJ">ARPANET</a> with <a href="/facts/Interface_Message_Processors/NNUx01pg">IMPs</a>.

Synchronous Data Link Control open-in-new

Synchronous Data Link Control