Sidebar: SNA and OSI Compared

A basic concept underlying any network design is the division of network functions into specific functional layers. Together these layers form what is known as a "network stack" and comprise a model by which all network functions operate. The Open Systems Interconnect (OSI) model is just such a model and contains definitions for interconnecting what might otherwise be incompatible systems. Systems Network Architecture (SNA), on the other hand, is a proprietary network model designed to allow IBM to optimize their hardware (controllers, printers, terminals, etc.) for large transaction-oriented networks. Although SNA is less concerned about multivendor interconnectivity than OSI, it still partitions the network functionality into multiple layers and protocols. Figure 1 shows both network stacks in a side by side comparison.

In an abstract sense, the layers in these two network models are quite similar -- even though in a practical sense they are totally incompatible. The following discusses the similarities between layers from an SNA point of view.

Layer 1, the physical layer, actually connects the device to the network cable, or medium (see the sidebar "The Physical Layer OSI vs. SNA" for more details). The physical layer has no explicit counterpart in the SNA model. It is assumed to be defined "outside the model."

Layer 2, the data link layer, exists in both OSI and SNA and is responsible for providing reliable transmission of data.

Layer 3, the network layer, exists in both but is referred to as path control in SNA. This layer refers to the "virtual connection" that exists between two machines. The actual connection between two machines may be through many physical connections but the bottom line is one "virtual connection," from point A or point B. On the OSI side, parts of this layer are typically implemented using the IP (or Internet Protocol). Layer 4, the transport layer, is referred to as transmission control in SNA. In this layer, SNA monitors the service quality of the lower three layers. This is very similar to the functions in the OSI model for this layer. On the OSI side, this layer is often implemented using TCP (or Transmission Control Protocol).

Layer 5, the session layer, is referred to as data flow control in SNA and has primarily the same purpose: overall data flow integrity.

Layer 6, the presentation layer, and Layer 7, the application layer, are actually combined into what is called function management in SNA. Except for UNIX network applications, which use a display manager such as X for presentation services, the functions in these two layers will be the application's responsibility.