One of the major uses of the DLC protocol today is connecting personal computers to SNA hosts, that is, IBM mainframe or midrange computers such as the AS/400. With the increased popularity of local area networks in the mid-1980s, IBM introduced two new connectivity options for its hosts. With the Token Ring Interface Connection (TIC), any SNA host can communicate with a token ring network. With the LAN Interface Connection (LIC), an AS/400 computer can communicate with an Ethernet network.
Figure 7.1 Mainframe Connectivity Path Using Token Ring
The SNA hosts already possessed a rich protocol stack in Systems Network Architecture (SNA). SNA provides equivalent functionality to the OSI Network, Transport, Session, and Presentation levels (although functionality might differ at each level). Because the DLC layer and the OSI Data Link layer are almost identical in functionality, a programming interface was developed for the DLC layer and exposed to programmers wanting to use this level of interface. The interface is described in the IEEE 802.2 standard.
Figure 7.2 Comparison of SNA and OSI Models
SNA Server uses the DLC protocol device driver when communicating to mainframes via the token ring interface. Detailed configuration and installation information is provided in the Microsoft SNA Server Installation Guide and the Microsoft SNA Server Administration Guide.