In addition to the open architectural models such as the OSI 7-layers model and TCP/IP architecture model, there exist a few popular vendor specific network communication models, such as IBM SNA (Systems Network Architecture), Digital Equipment Corporation (DEC; now part of HP) DNA (Digital Network Architecture). We are providing more details on the IBM SNA here.

Although it is now considered a legacy networking architecture, IBM SNA is still widely deployed. SNA was designed around the host-to-terminal communication model that IBM's mainframes use. IBM expanded the SNA protocol to support peer-to-peer networking. This expansion was deemed Advanced Peer-to-Peer Networking (APPN) and Advanced Program-to-Program Communication (APPC). Advanced Peer-to-Peer Networking (APPN) represents IBM's second-generation SNA. In creating APPN, IBM moved SNA from a hierarchical, mainframe-centric environment to a peer-based networking environment. At the heart of APPN is an IBM architecture that supports peer-based communications, directory services and routing between two or more APPC systems that are not directly attached.

SNA has many similarities with the OSI 7 layers reference model. However, the SNA model has only six layers and it does not define specific protocols for its physical control layer. The physical control layer is assumed to be implemented via other standards. The functions of each SNA component are described as follows:

• Data Link Control (DLC) – Defines several protocols, including the Synchronous Data Link Control (SDLC) protocol for hierarchical communication and the Token Ring Network communication protocol for LAN communication between peers. SDLC provided a foundation for ISO HDSL and IEEE 802.2.
• Path control – Performs many OSI network layer functions, including routing and datagram segmentation and reassembly (SAR)
• Transmission control – Provides a reliable end-to-end connection service (similar to TCP), as well as encrypting and decrypting services
• Data flow control – Manages request and response processing, determines whose turn it is to communicate, groups messages and interrupts data flow on request
• Presentation services – Specifies data transformation algorithms that translate data from one format to another, coordinate resource sharing and synchronize transaction operations
• Transaction services – Provides application services in the form of programs that implement distributed processing or management services

The following figure illustrates how the IBM SNA model maps to the ISO OSI reference model.

IBM SNA: Systems Network Architecture and SNA protocols