Editing Synchronous Data Link Control (section)

{{Short description|Computer communications protocol from IBM's Systems Network Architecture (SNA)}}
{{Infobox networking protocol
| title        = Synchronous Data Link Control
| logo         =
| logo alt     =
| image        =
| image alt    =
| caption      =
| is stack     =
| abbreviation = SDLC
| purpose      = Data framing
| developer    = IBM
| date         = {{Start date and age|1974|  |  }}<!--Fill in: Year (4 digits), month and day (2 digits)-->
| based on     = 
| influenced   =
| osilayer     = [[Data link layer]]
| ports        =
| rfcs         =
| hardware     =
}}

'''Synchronous Data Link Control''' ('''SDLC''') is a [[computer]] [[serial communication|serial]] [[communications protocol]] first introduced by [[IBM]] as part of its [[Systems Network Architecture]] (SNA). SDLC is used as layer 2, the [[data link layer]], in the SNA [[protocol stack]]. 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.<ref>{{harv|Odom|2004}}.</ref> 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.<ref>{{harv|ITS|2006}}.</ref> SDLC was released in 1975,<ref name=PCLT>[http://pclt.cis.yale.edu/pclt/COMM/SDLC.HTM PC Lube and Tune], accessed 15. October 2009.</ref> based on work done for [[IBM]] in the early 1970s.<ref name="hist">{{harv|Friend|1988|p=188}}.</ref> 

SDLC operates independently on each communications link in the network and can operate on [[point-to-point (telecommunications)|point-to-point]] [[Point-to-multipoint communication (telecommunications)|multipoint]] or [[Current loop|loop]] facilities, on switched or dedicated, [[Two-wire circuit|two-wire]] or [[Four-wire circuit|four-wire]] circuits, and with [[full-duplex]] and [[half-duplex]] operation.<ref>{{harv|Pooch|1983|p=302}}.</ref> 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.<ref>{{harv|Pooch|1983|p=303}}.</ref>

This [[de facto]] standard has been adopted by [[International Organization for Standardization|ISO]] as [[High-Level Data Link Control]] (HDLC) in 1979<ref name="hist"/> and by [[ANSI]] as [[Advanced Data Communication Control Procedures]] (ADCCP). The latter standards added features such as the [[Asynchronous Balanced Mode]], 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).<ref>{{harv|Friend|1988|p=191}}.</ref>

[[Intel]] used SDLC as a base protocol for [[BITBUS]], still popular in Europe as [[fieldbus]] 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 [[Zilog]], [[Motorola]], and [[National Semiconductor]].  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 [[DECnet]] with [[Digital Data Communications Message Protocol]] (DDCMP), Burroughs Network Architecture (BNA) with Burroughs Data Link Control (BDLC), and [[ARPANET]] with [[Interface Message Processors|IMPs]].<ref>{{harv|Pooch|1983|pp=309–321}}.</ref>