SNA architecture overview

As a proven and robust technology in distributed enterprise computing, the SNA network protocol connects multiple enterprise-level hosts, which contain important corporate data and many legacy applications. The SNA JCA LU0 Connector and SNA JCA LU62 Connector provide Branch Transformation Toolkit applications with a way of connecting to these hosts and legacy applications using the SNA protocol. In this way, the legacy applications can play a role in the e-business model.

In an SNA network, all communication between users occurs through Logical Units (LUs), which hides the users from the network. A Logical Unit is a port into the network and is only one of the classes of communicating entities in the SNA network, generally called Network Addressable Units (NAUs). The SNA architecture does not define the interface between the end-user and the LU so that the interface is both product-specific and implementation-specific. LUs perform all session-specific functions. They interconnect through a Path Control network that performs all network functions that are not session-specific and transport messages from one LU to another. Other NAUs are Physical Units, which provide an interface to the control functions of a particular physical device, and System Services Control Points, which provide an interface for management and control of the network.

SNA uses Request Unit and Response Unit (RUs) messages to communicate between NAUs. An NAU adds information relevant to a particular layer (or a number of layers) to a Request or Response Unit through one or more of the following headers:

For SNA communication to occur, a session must exist between two LUs. The communication between applications occurs as a conversation within the LU-to-LU session. The requesting LU establishes a session using a particular mode, which is a set of networking characteristics that determine how the LUs use the session. As a System Services Control Point must approve the setup for each session, an LU must first be in a session with a System Services Control Point before it can be in a session with another LU. Therefore, the conversation between applications begins when the following happens:

During the conversation, the two applications exchange status and application data. Typically, the conversation ends when one of the applications instructs the Communication Server to de-allocate the conversation.

Note that an application can be part of multiple conversations simultaneously. There can be concurrent sessions with different partner LUs and parallel sessions with the same partner LU. During configuration, the system administrator determines how many sessions a particular LU supports and whether the LU supports parallel sessions.

Related concepts
SNA JCA LU0 Connector
SNA JCA LU62 Connector