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Data Link Control (DLC)
The Data Link Control (DLC) protocol does not provide a fully-functioning protocol stack. (Note in Figure 5.2 that DLC is not continuous with the upper layers.) In Windows NT systems, DLC is used primarily to access to Hewlett Packard JetDirect network-interface printers. DLC also provides some connectivity with IBM mainframes.

The Systems Network Architecture (SNA) Protocol Suite

Another important protocol suite is IBM’s Systems Network Architecture (SNA). The Microsoft BackOffice suite includes a product called SNA Server that provides connectivity with SNA networks. (The DLC protocol included with Windows NT is also sometimes used as an interface with certain SNA resources such as mainframes.)

SNA evolved when terminals were the devices usually used to interact with centralized computers. Early versions of SNA supported only hierarchical network systems designed for this centralized environment.

In 1984, SNA was updated to support distributed processing environments with a feature called Advanced Peer-to-Peer Networking (APPN). APPN can implement a distributed processing environment that can leverage the processing capabilities of mainframe hosts, minicomputers, and personal computers.

SNA wasn’t developed from a preconceived, carefully thought-out model from which protocols were developed. IBM literally was pioneering the development of computer networking, and new protocols were added to meet new needs and design criteria. One result of this is that multiple protocols can be present at any given layer. Each protocol serves a somewhat different purpose in the overall scheme of SNA. As such, SNA doesn’t consist of a protocol stack so much as it consists of multiple protocols that work together in different combinations to meet different needs.

SNA was a mature model by the time formulation of the OSI reference model began, and the SNA architecture had a significant influence on the definition of the OSI model. Figure 5.6 compares the layers of the OSI reference model to the layers of the SNA model.