Basic SNA terms and concepts

A network addressable unit (NAU) is a resource managed by the communications system. NAUs provide a window through which users access the communications system network. An NAU is the origin and/or destination for information transmitted through the network. Each NAU has a unique network name that is translated into a unique address.

Note: Throughout this and related z/TPF information, the terms node and NAU have the same meaning.
The communications program must formally connect two NAUs before data may be exchanged. This is referred to as a session. The four types of NAUs are:
  • System service control point (SSCP) - The SSCP is a focal point within the SNA network. SSCPs manage the network, coordinate operator and problem-determination requests, and provide general support for users of the network. Multiple SSCPs divide the network into domains of control. Each SSCP controls the logical units and physical units within its domain.
  • Physical unit (PU) - A physical unit is associated with each network resource (host application, communications cluster, logical unit) defined to the SSCP.
  • Logical unit (LU) - A port through which users access the resources of the network. In SNA, both application programs and terminal operators can be defined as logical units. One or more logical units may be situated within a physical unit. SNA requires a hierarchical (primary-secondary) relationship between LUs in session. The primary logical unit (PLU) and the secondary logical unit (SLU) differ in the functions assigned to each. Primary logical units are responsible for:
    • Error recovery: After an error occurs, PLUs ensure communication is correctly restablished. SLUs may only request error recovery.
    • Establishing the LU to LU session: either an SLU or a PLU may request an LU-LU session.

    Logical units are further defined as either host node or non-host node. Host node LUs reside in the CPU; non-host node LUs reside in the cluster controller. A cluster controller is a device that controls input/output operations for multiple devices (terminals) attached to it. Host node LUs may be either primary or secondary.

  • Control point (CP) - A special LU that resides in each APPN node. CPs manage APPN networks and perform the tasks that the SSCPs perform in a PU 5 network. A control point in an APPN node communicates with an adjacent control point using special sessions called CP-CP sessions.

An adjacent link station (ALS) is the term used to describe a connection between the z/TPF system and a channel-attached NCP. With z/TPF’s support of PU 2.1, the NCP appears to z/TPF as an ALS. ALSs exchange identification (XID) to discover each others characteristics.

With SNA CTC, a channel contact procedure is implemented between z/TPF and channel-attached T5 nodes; that is, an explicit PU5 to PU5 session exists. This PU-PU session is comparable to the PU-PU sessions supported between NCPs. A formal channel protocol between the physical units is used. The session establishment and termination is initiated by the T5 node at the direction of its SSCP.

The z/TPF system connects to a PU 2.1 network as an Advanced Peer-to-Peer Networking (APPN) node.